Fundamentals of
Psychrometries
Second Edition
Don Brandt
-
Inch-Pound
A Course Book for
Self- Directed or Group Learn ing
Learning Institute
Fundamentals of
Psxchr-ometrics
Second Edition
Don Brandt
A Course Book for Self-Directed or Group Learning
Atlanta
Fundamentals of pJychrometrics (I-P), Second Edition
A Course Book for Self- Directed or Group Learning
ISBN 978- 1-939200-09-9 (papcrtJUck)
ISBN 978- 1-939200-10-5 (PDF)
SOL Number: 00099
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•
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Don Brandt is an AS HRAE member fro m Phoenix, Arizona , who
spent 39 years (4 of them part time) with Trane Co., mostly as a Commercial Sales Engineer and Sales Manager. He was involved in thousands of projects over that time period, including many with large
industrial customers that had spec ial HVAC and process appli cations.
Brandt also taught thc Trane AIC Clinic scries many timcs to young
eng ineers starting out in the industry.
Brandt has al so been active in AS HRAE at all three levels of organization-Chapter, Regional. and Society-during thi s same peri od.
He is a charter member and past presiden t o f the Anthracite Chapter in
Northeast Pennsylvania. He was the Technical, Energy and Government Acti vities Committee (TEGA) Regional Vice Chair for Region X,
the TEGA Vice Chair and Chair in 2002, Region X Director and
Regional Chair from 2002- 2005, on the Board of Directors from 20022005, and a member of Standards Committee from 2005- 2009 as a
Standards Project Liaison Subcommi ttee (SPLS) Liaison. In 2001 he
became a member of Professional Development Committee, moving up
to Chair in 2013. He is currentl y a member of the Energy Targets Multidisc iplinary Task Group, a Nom inating Member for Regi on X, and
Member of the Appeals Board for Standards.
Brandt is a 1974 graduate of Penn State with a BS in Electrical
Engineering and an active member of the Alumn i Associati on. In retirement, he is an instructor for the successful AS HRAE HVAC Essentials
Course, both Leve ls I and 2, that is held both in the United States and
internationally. He also teac hes a portion of the Association of Energy
Engineers (AEE), Arizona Chapter, Certified Energy Manager (CEM)
preparation class held on an annual basis.
Contents
Preface . . . .............. _ ............. .. . . • .. .. • .. . . . . . . xi
Acknowledgments .
. . . . xiii
Chapter I: Introduction to Psychrometries. _. _. _. . _. . _ ....... . .
Introduction .. .
2
2
.. 2
..4
. . ... 5
·. 5
Enthalpy .... .
Air Density
Volumetric Airflow versus Mass Flow Calculations
Skill Development Exercises for Chapter I ...... .
Chapter 2: Properties of Moist Air ................ .
Introduction . ..
Temperature . . . . . . . . . . • .
Humidity. . . . .
5
. ............... . . . .
. . . _. . . . _. • .. _. ... _. . • .
. .... . .. 6
Enthalpy . . . . .
. ...•.... . •............
Specific Volume . . ...... . .... . .. . . . .. . . . .. . . . .
Using Appendix A
.... _ . . . . . . . . . . . . . . .
.. 6
..... 6
.. 6
T erminoiogy and Symbols for Psychrometries
.... 7
Skill Development Exercises for Chapter 2 ... . .. . . . , , . .. , , .
8
Chapter 3: Introducing the Psychrometric Chart .... . ... . . ... .. II
The Modern Age of Psychrometries. . . .
II
Creating the Psychrometric Chart. . . . .
II
Finding Seven Psychrometric Quantities .
14
Climatic Design Information. . . . . . . . . .
16
Psychrometric Chart for Extended Temperature and Altitude .
17
Skill Development Exercises for Chapter 3 . . . . . . . . . . . . . . . . .
18
Chapter 4: Air-Conditioning Processes
on the Psychrometric Chart .. . . . . . . . . . . . . . . . . • . . . . . . . . . . . . 21
The Power ofthe Psychrometric Chart. . . . . . . . • . . . . . . • . . . . . . 21
Sensible Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sensible Heating . . . . . . . . . . . . . . . . . . . . . . . . . .
Latent HeatAddition
. 22
. ......... 22
...... ..... .......
. 23
· 23
Latent Heat Removal .
Total Heat Content ........ .. .• . . .. .. . . . . • . . . .... . ..... 25
Cooling and Humidifying . . .. . . . .. . . . .. . . . .. . . . .. .
.26
Heating and Humidifying . . .. . . . .. . . . .. . . . .. . . . .. .
· 28
Dehumidification and Heating. . . . . . . . . . . .
. . .... . ..... 30
Skill Development Exercises for Chapter 4 . . . . . . . . . . . . . . .
. 32
viii
Contents
Chapter 5: HVAC Design and the Psychrometric Chart ..
35
Schematic of an Air-Conditioning System.
35
Mixing Airstreams- Cooling Systems. .
35
Mixing Airstreams- Heating System s . . . . . . . . . . . . . . . . .
37
Sensible Heat Ratio-Cooling. . . . . . . . . . . . . . . . .
38
Sensible Heat Ratio-Cooling with Outdoor Air . . .
. . .... 40
Psychrometric Process- Heating . . . . . . . . . . . . . . . . . . . .
Skill Development Exercises for Chapter 5. . . . . . . .
. . . 41
. . .... 44
Chapter 6: Psychrom etries in HVAC Equi pm ent ....... • .. . .... 47
The Air-Handling Unit:
Heart of the Commercial Air-Conditioning System ..
..47
..47
Psychrometries of a Cooling Coil .... .
Psychrometries of Fan Heat. . . . . . . . .
. ... . .•.
. . 49
Psychrometries of a Heating Coil . . .. . . . .. . . . .. . . . .. . . . .. . . 50
Humidification Equipment . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . 50
Skill Development Exercises for Chapter 6 .. . . . .. . . . .. . . . .... 54
Chapter 7: Psychrometri es in Zoned HVAC Systems. . . .
57
Constant-Volume and Variable-Air-Volume Systems. . . . . . . . .
57
Constant-Volume. Single-Zone System . . .. . . . .. . . . .. . . . .. . . 58
Constant-Volume, Single-Zone System with Reheat. . . .. . . . .. . . 59
Constant-Volume, Single-Zone System
with Face and Bypass Dampers on the Cooling Coil .
. . .... 60
Constant-Volume System with Terminal Reheat .. . . . .. . . . .. . . 62
Constant-Volume Multizone and Dual-Duct Systems. .
63
Variable-Air-Volume Systems for Multiple Zones. . . . .
65
Variable-Air-Volume Systems with Heating VAV Boxes . .. . . . .. . . 66
Skill Development Exercises for Chapter 7. . . . . . . . . .
. . .... 69
Chapter 8: Energy Conservation and Psychrometries ..... • • . .... 73
73
Heat Recovery Devices . . . ... . . . .. .. . . .... . . . . . . . . .... 73
Energy Recovery Devices . . .. . . • .
. . . . .. . . . .. . . . .. . . 78
Introduction
81
Air-Side Economizer . .
Water-Side Economizer.
..... ........... .......
Supply Air Temperature Reset .
81
. . . . . .. . . . .. . . . .. . . . .. . . 83
Skill Development Exercises for Chapter 8. . . . . . . . . . . . . . .
85
Chapter 9: Special Applications and Psychromet ric Considerations. 87
Introduction
Cooling Towers
Indoor Swimming Pools .
Cleanrooms
..... ...
. . .. . . . .. . . . .... 87
87
. . .. . . . .. . . . .... 89
90
Fundamentals of Psychrometries (I-P), Second Edition
ix
Direct Evaporative Cooling .. . . . .. . . . .. . . . .. .
. . . . 93
Indirect Evaporative Cooling ............. .
. .. 94
. .. 97
Skill Development Exercises for Chapter 9 ... .
Append ix A: Thermodynamic Properties of Moist Air ..
..99
Append ix B: Dimensions, Units, and Unit Conversion Factors.
103
Append ix C: Climatic Design Information . . . . . . . . . . . . . . . . . ..
105
Append ix D : Thermodynamic Properties of Water at Saturation.
135
Skill Development Exercises. . . . . . . . . . . . . . . . . . . . . . . . . . . ..
139
Preface
Psyc hrometries is a tool HV AC designers use to determine the
amount of moi sture in the air and to provide solutions to des igners for
the ultimate comfort of bui lding occupants. It can be used to size air
handling un its; optim ize energy performance; identify control sensors
for building automation; describe the performance of cooling coi ls,
cooling towers, and humidification equipme nt; and evaluate heat recovery strategies.
Yet the use of psychrometries and the psychrometric chart is different from designer to designer. Some do not use the psychrometric chart,
relying instead on simp lified formulas or complex computer simulations. Others use the chart onl y for special si tuations, such as indoor
swimming pool applications. And some use it as their primary system
evaluation tooL
This course is meant to cover all of these uses of the psychrometric
chart, to have something for all these different backgrounds, and to be
an introduction for the young designer who has yet to pick an approach.
The COUT::;e addresses the use of psychrome.trics and the psychrometric
chart fOT typical app li cations and systems and includes some theory.
This theory not on ly sets the basics but also gives students an appreciation of the si mp lification that the psychrometric chart provides. The
psychrometric chart gives a visual description of HVAC design, one
that could never be appreciated from fonnu las alone .
This second ed ition of the psychrometrics self-directed learn ing
(SOL) course was rewritten in an attempt to teach the fundamentals of
psychrometri cs in about half the time as the previous vers ion. The
author has used his 4 1 years of experience in the HV AC industry as the
expertise for the format and content.
Acknowledgments
I need to acknowledge a few folks who helped me out on this first
publishing adventure of my life. My wife JoAnn was the person who
put my scribble into a nice Word document. John Duren, Sa les Engineer for MPSW in Phoenix , did various eq uipment selections that are
used in the course. Mick Schwedler and John Murphy from Trane
Appli cations Engineering Department in La Crosse, Wisconsin, were
there when I need some technical help and review. And I wou ld like to
thank ASHRAE staff for their understanding and help to make this a
finished publication.
Introduction to
Psychrometries
Study Objectives
After completing this chapter, you should be able to
o understand the basic processes of psychrometries,
o understand enthalpy and vo lumetric airflow, and
o understand the basic formulas of HV AC design.
Instructions
Read the materia l in Chapter I. At the end of the chapter, complete the ski ll
development exercises without referring to the text.
Introduction
Psychrometries is an analysis tool that HV AC engineers use to provide
solutions to comfort issues. These issues can be related to human comfort or
process comfort depending on the applications.
If the air surrounding us were totally dry, our job as HVAC engineers
would be vel)' easy and probab ly boring. But, because all air we work with
contains some amount of moi sture in the form of water vapor, our jobs get
more complex.
The four ba sic processes that an HV AC system can perform on moist air
are as follows:
Cooling- Lowering the air temperature
Heating- Raising the air tempera ture
Humidification- Rai sing the moi sture content in the air
Dehumidification- Lowering the moisture content in the air
Note that HV AC processes can also be a combination of the above four
basic actions. These combinations include the fo llow ing:
Heating and humidification- Increasing the temperature and moisture content of the air at the same time
2
Chapter I
Introduction to Psychrometries
Heating and dehumidification- Increasing the temperature and decreas ing
the moisture content of the air at the sa me time
Cooling and humidification- Decreasing the temperature and increasing
the moi sture content of the air at the sa me time
Cooling and dehumid ifi cation- Decreasi ng the temperature and moisture
content of the ai r at the same time
These combination processes provide an infinite number of potential
actions. For example, consider the cool ing and dehumidification combination
process. We ha ve a solution that drops the temperature 25 degrees and only
drops the dew point (defined later) 1 degree. Another solution drops the temperature 10 degrees and the dew point 10 degrees. So, there are thousands of
solutions with too many temperature and dew-point combinations to li st.
Enthalpy
We will use the property of the enthalpy of air throughout this course.
Enthalpy is the s um of the interna l energy or the total heat content of the ai r. In
other words, warm and humid air can have the same heat content as hot and dry
air. So the energy required to coo l wannlhumid air in the Gulf Reg ion of North
Ameri ca might be close to the energy required to cool hot/dry air in the Southwest deserts. Enthalpy h is subdi vided into the follow ing:
hda
hs
has
enthalpy of dry air at 0% relative humidity
enthalpy of saturated air or 100% relative humidi ty
enthalpy d ifferences between hda and hs
Air Density
Elevation has an effect on psychrometric analyses. As elevation changes, so
does the ai r density. This means the constants used in equat ions will change
and different psychrometric charts (Chapter 3) are required for proper calculation.
For thi s course we will use sea level as atmospheric pressure for all calculations unless otherwise noted.
Volumetric Airflow versus Mass Flow Calculations
For easier understanding, we wi ll use vol umetric airflow in cubic feet per
minute (cfm) rather than mass flo w for our calculations throughout this course.
Standard air is defined as air at sea level or 29.92 in. Hg of barometric pressure and 69.5 °F 'db' The density of air at sea level is 0.075 Ib/ft) .
The three equations we will use in our calculations are for sensible heat
(q,), the total heat required (q,), and latent heat (q,):
qs = 1.085 >< cfm >< (I) - (2) temperature
(1·1 )
Fundamentals of Psychrometries (I-P), Second Edition
3
where the constant 1.085 is deri ved from the following:
0.075 Iblft3 x 0.24 Btullb ·oF x 60 mi nlh ~ 1.08
where 0.24 Btullb ·oF is the specific heat of air.
q, = 4.5 x cfm x (h. - "2) enthalpy
(1-2)
where the constant 4.5 is deri ved from the following:
0.075 Ib!ft3 x 60 minlh ~ 4.5
qf=4840 x cfm x (W1 - W 2) humidity ratio
(1-3)
where the constant 4840 is derived from the following:
0.075 Ib/ft3 x 1075 Btullb x 60 minlh ~ 4837.5 ~ 4840
where 1075 BtuJ1b is the enthalpy (h ) of air at 32°F for the evaporation of water
to vapor.
4
Chapter I
Introductio n to Psychro metri es
Skill Development Exercises for Chapter I
Complete these questions by writing your answers on the worksheets at the back a/this book.
I-I
How many basic processes of air conditioning can be performed on moist air?
a) Two
b) Three
c) Four
1-2
Wh ich combination process will increase both the temperature and the moi sture content?
a) Cooling and dehumidification
b) Heating and dehumidification
c) Heating and humidification
1-3
Enthalpy is the tota l heat content of the air.
a) True
b) Fa lse
1-4
Change in elevation has no effect on the air density_
a) True
b) False
Properties of
Moist Air
Study Objectives
After completing thi s chapter, you should be able to
o define some fundam ental properties used in psyc hrometries and
o understand how to use Appendix A.
Instructions
Read the material in Chapter 2. At the end of the chapter, complete the skill
development exercises without referring to the tex t.
Introduction
This chapter define s the properties that an HV AC engineer uses to do psychrometric analysis. The properties are temperature, humidity, enthalpy, and
specific volume.
Temperature
The temperatures we are concerned about in HV AC systems are the foll owing:
Dry-bulb temperature: The measure of the surrounding air temperature
with a standard thennometer in degrees Fahrenheit (OF) without infl uence
on the thennometer by heat sources or so lar heat gain.
Wet-bulb temperature; The measure of the moi st effect on the evaporation process in the air. Using a standard dry-bulb thermometer, place a cotton sock on the sensing portion. Next, soak the sock in amb ienttemperature water and, by rotation, move surrounding air across the sock.
The temperature will drop below ambient because o f the evaporative or
cooling effect on the sensing bulb. It will continue to drop until most of the
water is evaporated into the surrounding ai r. This is the wet-bulb temperature and may also be called wet-bulb depression. Any further drying of the
sock will result in the temperature goi ng back up to the ambient dry-bulb
temperature.
6
Cha pte r 2
Properties of Moist Air
Dew-point temperature: The measure of the dry-bulb temperature at the
poi nt where water vapor starts to condense to liq uid or be removed from
the air. This is also referred to as the condensation point, because it is the
temperature at which the water turn s to liquid from vapor in the airstream.
Satu ration temperature: The temperature at which the air cannot hold
any add itional water vapor. At the saturation temperature, the dry-bulb,
wet-bulb, and dew-point temperatures are identical.
Humidity
Humidity is the moisture in the air. We can talk about it in two ways:
Humid ity r a tio: The actual weight in pounds of water vapor per pound of
dry air (lb)j lbda ). Note that in some HV AC psychrometric ca lculati ons you
may see grains of moisture per pound of dry air (gr/lbda ), with 7000 grains
equal to I lb. Either calculation will result in the same answer .
Relative hu m idity: The ac tual amount of moi sture in the air at a given drybulb temperature versus the max imum amount of moisture in the air at the
same dry-bulb temperature. It is expressed in percentage because it is a partial moisturelmax imum moisture ratio. At constant moisture content, as
soon as the dry-bulb temperature changes, so does the relative humid ity .
Enthalpy
The energy content of a ir is defin ed as the enthalpy of the air or the total
heat content of the air. It is expressed in British the nnal uni ts (B tu) per pound
of dry air (B tullbda ). Aga in, warmlhumid a ir can have the same enthalpy as
hot/dry air, so it takes the sa me amount of energy to cool e ither a irstream to a
comfortable condition.
Specific Volume
Specific vol ume is the cubic fee t per pound of dry air (ft 3/1bda ). It is the
inverse of air de nsity (lb/ft 3).
Note that spec ifi c vo lume changes as the dry-bulb temperature changes, but
not nearl y as much as it changes with the effect of higher altitude.
Using Appendix A
The table in Appendix A li sts thermodynamic properties of moist air at
14.696 psia. Fo r each dry-bulb temperature in of, we have values for the following:
Humidi ty ratio at saturation
Specific volume at dry, saturated, and differentia l cond itions
Specific enthalpy at dry, saturated, and di fferential conditions
Fundamentals of Psychrometries (I-P), Second Edition
7
Specific entropy at dry, saturated, and differential conditions (not used in
this text)
We will use this table as we go through the text.
Terminology and Symbols for Psychrometries
h
enthalpy of moi st ai r, Btu/lb
ha
spec ifi c enthalpy of dry air, Btullb
hw
spec ifi c enthalpy of water vapor, Btullb
p
total pressure, usually barometric, in. Hg
Pw
partial pressure of water vapor, in. Hg
Pa
partial pressure of dry air, in. Hg
q
rate at which heat is transferred to a process, Btulh
tdb
dry-bu lb temperature of moist air, of
t \Vb
wet-bu lb temperature of mo ist air, of
tdp
v
dew-point temperature of moi st air, of
specific vol ume of moi st air, ft. 3/1b
va
specific vol ume of dry air, ft 3/1b
Vw
specifi c volume of water vapor, n3/1b
vs' Vg
W
spec ifi c volume of saturated water vapor, ft 3/1b
Ws
humidity ratio of moi st air at saturation,lb (water)l1 b (dry air)
$
re lati ve humidity the ratio of actual moi sture amount to maximum
humidity ratio of moi st air, Ib (water)llb (dry air)
moi sture amount, % rh
For dimensions and units used in air-conditioning applications and a table
of unit conversion factors for converting between Inch-Pound (l-P) and Systt~me International (SI) measurement units, see Appendix B.
8
Cha pte r 2
Properties of Moist Air
Skill Development Exercises for Chapter 2
Complete these questions by writing your answers on the worksheets at the back a/this book.
2-1
Dry-bu lb temperature is measured w ith a wet sock around the sensing bu lb.
a) True
b) Fa lse
2-2
Saturation temperature of ai r is the point at which the dry-bulb, wet-bu lb, and
dew-point temperatures are equa l.
a) True
b) Fa lse
2·3
Relat ive hum idity does not change as the dry-bulb temperature changes.
a) True
b) Fa lse
2-4
The dry-bu lb temperature can be above the dew-point temperature.
a) True
b) Fa lse
2-5
According to Append ix A, what is the spec ific enthalpy hs of saturated air at
40' F?
a) 15.23
b) 9.6
c) 5.8
d) None of the above
2-6
According to Appendix A, under the same condition cited in Exercise 2-5,
what is the spec ific volume v?
a) 12.69
b) 12.59
c) 0.105
d) None of the above
2·7
According to Appendix A, what is the spec ific en thalpy of dry air 17(1 at 100°F?
a) 29.27
b) 47. 73
c) 24.03
d) None of the above
Fundamentals of Psychrometries (I-P), Second Edition
2-8
9
According to Appendix A, under the same condition cited in Exercise 2-7,
what is the specific volume v?
a) 19.15
b) 19 .80
c) 15.45
d) None of the above
Introducing the
Psychrometric Chart
Study Objectives
After completing this chapter, you should be able to
o describe how the psychrometric chart was developed,
o understand how to read the psychrometric chart, and
o use climatic design information from tables publi shed by ASHRAE.
Instructions
Read the material in Chapter 3. At the end of the chapter, complete the skill
development exercises without referring to the text.
The Modern Age of Psychrometries
In this chapter, you wi ll learn about the psychro metric chart and how to use
it in HV AC problems. You wi ll see how to construct and then use the psychrometric chart.
The psychrometric chart was developed by Wi llis Carrier in the early
1900s. It has been refined over time for more accuracy to provide better resu lts.
The chart conta ins seven important psychrometric variables, represented on the
chart by the following symbol s:
'db
'wb
tdp
$
h
W
v
dry-bu lb temperature
wet-bu lb temperature
dew-point temperature
relative humidity
enthalpy
humidity ratio
spec ific volume
Creating the Psychrometrie Chart
We wi ll now discuss how to construct the psychrometric chart and plot the
seven important properties on the chart. Using Figure 3-1 , start with the x-axis
(the horizontal line across the bottom) and plot dry-bulb temperatures on a linear scale from low on the right to high on the left.
12
Chapte r 3
Introduci ng t he Psychrometric C hart
Next we go to Appendix A and get the saturated humidity ratio Ws values
from 32°F to 120°F. Then, put the y-axis (the vertical line on the ri ght side) on
a linear scale from Iowan the bottom to high on the top.
We develop the saturation line (the heavy dark curve shown in Figure 3-1)
by the intersection of the Ws value and the vertica l dry-bulb line. Remember,
the dry-bulb, wet-bu lb, and dew-point temperatures are eq ual on the saturat ion
line. Lines of constant humi dity ratio are all horizontal.
Figure 3-2 shows that the dew-point temperature is where the dry-bulb temperature intersects the saturation line. A line of constant dew point goes horizontall y on the psychrometri c chart.
Go ing to Figure 3-3, we can get the value of the enthalpy at saturation (h s )
and again plot that va lue on the saturation line for that dry-bulb temperature.
To find the other end of the enthalpy line, we simply take the hs value and
go to Appendix A and look fo r a very close value in the hda column. Once we
find it, that dry-bu lb temperature is the intersection point for that enthalpy li ne
with the x-axis.
For example, 60°F db has a value of26.4 Btu/lb, so we plot thi s value at the
60°F saturation temperature. We go to Appendix A and search for a value of
26.4 Btullb in the hda co lumn and find it at 100°F db. Those two points establi sh the 26.4 Btullbda enthalpy line. Lines of constant enthalpy are sloped and
are soli d dark.
90
60%
.028
.024
5
40%
45
.020 I
c
3
c:
.016 ~.
"
w
.012 0'
40
~
20%
,008
35
10
,004
30
so
"
"
Dry Bulb
Figure 3- 1
25
20
15
Beginning the psychrom etric chart.
"
".
'"
Fundamentals of Psychrometries (I-P), Second Edition
110
60%
.028
.024
"
~Q
is'
'v'"
5
40%
45
35
.020 I
c
3
c:
.016
30
~
:0
~
rl
40
.012
20%
.008
.008 Humidity ~tlo
15
o·
35
10
.004
30
"
Figure 3-2
60
25
20
15
70
80
90
'"
'00
Dry Bulb
Plotting intersection of dew-point temperature on the psychrometric chart.
90
110
60%
.028
.024
II
~Q
is'
'v'"
5
40%
45
3S
30
.020 I
c
3
.016
c:
~
:0
~
.012 "
0.
40
.008
35
.~~ ..-- Constant Wet
••••• Bulb = 60"
10
••.••.••
o
"
Figure 3-3
'"
80
90
Dry Bu lb
Plotting enthalpy on the psychrometric chart.
.004
30
.. ~..'.
'00
'"
13
14
Chapter 3
Introducing the Psychrometric Chart
Figure 3-3 also shows that li nes of constant wet-bulb temperature are
almost parallcl to lines of constan t enthalpy. Lines of constant wet-bulb temperature are sloped and dotted, as shown in Figure 3-4.
Figure 3-4 shows speci fic vo lume lines. We plot them by going to Append ix A and, at a given saturation temperature, getting the Vs value and inserting
it on the saturation line. Then we look at the table for the same value at a higher
temperature, and that is the x-axis intersection point for the oth er end of the
specific volume line.
Finding Seven Psychrometric Quantities
The psychrometric chart shown in Figure 3-4 is sufficient to provide immediate and comp lete characteristics of a moist air parcel, even if only a small
amount ofinfonnation is known about that parcel. More specifically, ifany two
of the seven important psychrometric variables (tdb, 'dp' 'wb, q" h, v, It) of a
moi st air sample are given (for a specific barometric pressure), then all of the
remaining ones can be determ ined immediately fro m the chart.
Example 3-1
Problem
Given a sample of air whe re {db = 70°F and ~ = 60% rh, determ ine its dew
point.
Solution
Using Figure 3-4, the location point is at the intersection of the conditions
stated in the problem. Moving to the left in a straight li ne indicates that the saturation curve is crossed at a temperature of 55°F. This is the dew-point temperature.
Example 3-2
Problem
What is the enthalpy of the 70°F and 60% rh parcel of air from Example 3-1?
Solution
Again using Figure 3-4, the locat ion point is the same. Following the line of
constant enthalpy up the enthalpy scale reveals that the enthalpy of this point is
27.0 BtuJlbdG'
Example 3-3
Problem
Find the wet-bulb temperature for the point in Example 3- 1.
Solution
Again using Fig ure 3-4, fo ll ow the constant wet-bulb line to the saturation line,
then drop stra ight down to read a temperature of6 1OF, the wet-bulb temperature.
Fundamentals of Psychrometries (I-P), Second Edition
-,
~ .
'.
! :
.~~
Figure 3-4
ASHRAE Psychrometric Chart No. I.
15
16
Chapter 3
Introducing the Psychrometric Chart
Example 3-4
Problem
Use Figure 3-4 to find the speci fi c volume of 'db ~ 70' F and $ ~ 60% rho
Solution
Finding this point on Figure 3-4 reveals that it is located between the values of
13.5 and 14.0 for specific volume. Further inspection of the figure ind icates
that there are more li nes of constant spec ific volume that are unmarked. It
appears that each of these represents an increase of 0.1 ft 3/lbdu . Therefore, it
can be determined that the intersection is a specific volume of 13.55 ft3 /1bdcl'
Example 3-5
Problem
Using the psychrometric chart in Figure 3-4, find the /db' Idp' Iwb. <p. and humidity ratio W of a parcel of air that has a specific volume of 13.9 ftllb da and an
entha lpy of 30 BtuJlbda .
Solution
Idb = 84°F, ldp = 54.3°F, ' wb = 65 °F, IP = 37% rh , W = 0.009 Ibl/1\J lbda
Notice that the dew point and relati ve humidity both needed interpolation.
There are many methods of interpolation. Most engineers simply "eyeball "
interpolate by doing a visual scaling between the lines of the chart. There is an
art to this that is learned by practice, but results in error by less than ± I % can
be achieved.
Climatic Design Information
Outdoor weather conditions ha ve a lot to do with the air conditioning and
heating processes described in this book. Climatic design information for the
United States, Ca nada, and other countries is provided in Appendix C.
For our examples, we will use the "2%" column under the "Cooling OBI
MCWS" heading in Appendix C as our design conditions. This means that
only 2% of the tota l hours, in an ave rage year, are above the li sted dry-bulb
temperature. Note that co lumns for 0.4% and 1% of the time are also shown.
Us ing a blank psychrometric chart and Appendix C, plot the outdoor
design conditions (tdb and mean coincident wet-bulb temperature) for summer
in the following cities (labe l them). You will use these outdoor design points as
we go further into this course.
Montreal, Quebec, Canada
Owen Sound, Ontario, Canada
Columbus, Ohio, USA
Boulder, Colorado, USA
Phoenix, Arizona, USA
Miami, Florida, USA
Fundamentals of Psychrometries (I-P), Second Edition
17
San Juan, Puerto Rico
Guadalajara, Mexico
Sao Paulo, Brazi l
Alice Springs, Australia
The plotted points are shown in Figure 3-5.
Psychrometric Chart for
Extended Temperature and Altitude
The psychrometric chart in Figure 3-4 is for sea level and nonnal temperatures (32°F db to 120°F db). It is availab le from ASHRAE as Psychrometric
Chart No. I.
Psychrometric charts are availab le at 5000 ft and 7500 ft elevations (Charts
No. 4 and No.5), at low temperatures of -40oF to 50°F (sea level, Chart No.2),
and at hi gh temperatures of 60°F to 250°F (sea level, Chart No.3).
If you do work at these elevations and temperature ranges, please use the
appropriate charts. Also, do not forget to correct the sensible heat formula,
enthalpy formu la, and humidity ratio formu la constants for air density changes
using the equations included at the end of Chapter 1.
Figure 3-5
Climatic design information plotted on the psychrometric chart (detail).
18
Cha pte r 3
Introducing t he Psychrometric C hart
Skill Development Exercises for Chapter 3
Complete these questions by writing your answers on the worksheets at the back a/this book.
3-1
On a psychrome tric chart, the y-axis is humidity ratio and the x-axis is:
a) Re lative hum idity
b) Dew-point temperature
c) Dry-bu lb temperature
d) Wet-bu lb temperature
3·2
Using the psychrometric chart in Figure 3-4, determine the re lative humidity of
an air parcel with W = 0.0 I0 and 'db = 60°F.
a) 60% rh
b) 70% rh
c) 80% rh
d) 90% rh
3·3
Using the psychrometric chart in Figure 3-4, determine the dew-point temperature of an air parce l with Idb = 70°F a nd IP = 50% rho
a) 52"F
b) 59°F
c) 70°F
d) 85°F
3-4
Using the psychrometric chart in Figure 3-4 , determine the humidity ratio Wof
an air parcel with a saturation temperature of tdb = 40°F.
a) 0.003
b) 0.005
c) 50%
d) 40°F
3-5
Using the psychrometric chart in Figure 3-4, determine the spec ific vo lume v
of an air parcel with 'db = 70°F and W = 0.010.
a)
b)
c)
d)
13.40
13.55
14.05
14.40
Fundamentals of Psychrometries (I-P) , Second Edition
3-6
19
According to the psychrometric cha rt in Figure 3-4, what is the enthalpy of
'db = 70°F dry air?
a) 45
b) 35
c) 26
d) 17
3-7
According to th e psychrometric chart in Figure 3-4, what is th e wet-bulb temperature of a moist air parce l with tdb = 70°F and $ = 50% rh air?
a) 70°F
b) 58°F
c) 50°F
d) 38°F
3-8
According to the psychrometric chart in Figure 3-4, what is th e dew point of
' db = 50°F saturated air?
a) 50°F
b) 40°F
c) 30°F
d) 20°F
3-9
According to the psychrometric chart in Figure 3-4, what is the wet-bulb tempe rature of tdb = 70°F dry air?
a) OaF
b) 22°F
c) 33°F
d) 44°F
3-10
Using the psychrometric chart in Figure 3-4, plot the points tdb = 70°F, h = 30,
and ' wb = 50°F, then connect the points with a line. Upon investigati on of the
line, which of the foll owing is the best descripti on?
a) The li ne is almost ve rtical.
b) The line has a slope of about 45° (angJe).
c) The line almost horizonta l.
Air-Conditioning
Processes on the
Psychrometric Chart
Study Objectives
After completing this chapter, you should be able to
o understand the air-conditioning processes shown on the psychrometric
o
chart and
understand the use of the HVAC eq uations prov ided.
Instructions
Read the material in Chapter 4. At the end of the chapter, complete the skill
development exercises without referring to the text.
The Power of the Psychrometric Chart
This chapter applies the processes discussed in Chapter 2 and the properties of moist air d iscussed in Chapter 3 to the psyc hrometric chart. But before
we start, we must first define two processes, sensible heat transfer and latent
heat transfer.
Sensib le heat transfer (qs) is changing only the dry-bulb temperature of the
air and can be sensible coo li ng (lowering the temperature) or sensible heating
(ra ising the temperature). On the psychrometric chart, it is pure hori zontal
movement, right to left or left to right only. We can use the following equation
for sensibl e heat change at sea level:
q, (Btulh) ~ 1.085 x cfm x (II - I,)
(4-1)
where cfm is the airflow in cubic feet per minute, '] is the initial temperature,
and '2 is the final temperature.
Latent heat transfer (q,) is changing only the moisture content of the air or
changing only the humidity ratio of the air. It is vert ical-only movement on the
psychrometric chart , top to bottom or bottom to top only. We can use the following equation for latent heat change at sea level:
(4-2)
22
Cha pte r 4
Air-Conditioning Processes on tne Psychrometric Chart
where cfm is the airflow in cubic feet per minute, WI is the init ial hum idity
ratio, and W2 is the final humidity ratio.
Sensible Cooling
We will first show the air conditioning process of sensib le coo ling. It is a
horizontal process on the psychrometric chart, moving from the ri ght to left.
For example, our entering temperature (D is at lOO°F db, = 10% rh, and we
coo l the air to 60°F db as shown in Figure 4- 1. The leaving 12 is at 60°F db and
4> = 37% rho Note the humid ity ratio of W = 0.004 did not change.
If we app ly our examp le cfm of 5000 to the problem, then
q, ~ 1.085 x cfm x (I, - 12)
~ 1.085 x 5000 cfm x (I OO' F -
60' F)
~ 1.085 x 5000 x (40) ~ 217,000 Btulh cooling
Sensible Heating
Next we wi ll review the air-conditioning process of sensible heating . It is
also a horizontal process on the psychrometric chart, but from left to right. In
this example, our entering temperature (D is 70°F db, 4> = 5 1% rh , and we heat
the air to 110°F db as shown in Figure 4-2. The leaving 12 is 110°F db and 4> =
14% rho Note the humid ity ratio of W = 0.008 did not change.
Dry Bulb
90
.028
.024
41"
.020 :J:
0
3
45
c:
.016 ~.
40
.012
2('" .008
10
:
f;I)' :
"
Figure 4-1
'"
.,
15
100" :
: 25
20
" Bulb'"
Dry
'"
".
Sensible coo ling shown o n t he psychrometric chart.
"
.004
30
,,,
'"g.
Fundamentals of Psychrometries (I-P), Second Edition
23
If we apply our example cfm of 5000 to our sensible heat equation, then
q, ~ 1.085 x cfm x (i, - /2)
~ 1.085 x 5000 cfm x (70°F -
110°F)
~ 1.085 x 5000 x (40) ~2 1 7, 000 Btulh heating
Latent Heat Addition
The addition of latent heat, or the add ition of moisture content to air, is the
next area of focus. It is a vert ica l movement, from bottom to top of the psychrometric chart. For example, the entering conditions of (db = 80°F and 4> =
18% rh have a W = 0.004 humidity ratio. The leaving conditions of 'db = 80°F
and 4> = 55% rh have a humid ity ratio of W = 0.012 , and the dry-bulb temperature did not change, as shown in Figure 4-3. The latent heat req uired with our
example of 5000 cfm can be calculated as fo llows:
ql ~ 4840 x cfm x (W t - W2)
~ 4840 x 5000 cfm x (0.004 - 0.01 2)
~ 4840 x 5000 x (0.008) ~ 193,600 Btulh
Latent Heat Removal
The remova I of latent heat, or the lowering of moisture content to air, is the
process shown in Figure 4-4. The entering conditions of tdb = 75°F and q. =
70% rh have a W = 0.01 3 humidity ratio. We remove moisture to the leaving
Dry Bulb
90
100
.028
..
,
"
5
.024
.020 :J:
c
3
a:
.016 ~.
~
•g.
.012
40
2. .
W .. .OO8
15
" ."'"
"0"
170"
to
.008
30
"
Figure 4-2
.,
15
'"
"
20
.,
"Dry Bulb
25
"
""
Sensible heating shown on t he psychrometric chart.
'"
24
Cha pte r 4
Air-Conditioning Processes on tne Psychrometric Chart
Dry Bulb
11 0
60%
90
.028
.024
5
40%
45
1;-«'" 3S
is'
<So
30
.020 :I:
c
3
a.
.0 16 ~.
.012
'0%
15
"
~
=.012 40
... ---W....
_---_ .. _-
o·
.008
35
W::.OO4
- --- -.- --- ---------------- -.-
10
.004
30
'0
15
32
Figure 4-3
"
50
'"
60
"
Dry Bulb
25
"
""
'"
Late nt heat add ition shown on the psychrome tric chart.
11 0
90
.028
.024
5
40%
45
.020 :I:
c
3
.0 16
c:
Q'
""a .
~
.012
'0%
.008
35
10
.004
..... ...... .........W=.OO3
................ 30
20
15
32
Figure 4-4
"
'"
"
25
'00
Dry Bulb
Late nt heat removal shown on the psychrome tric cha rt.
'"
Fundamentals of Psychrometries (I-P) , Second Edition
25
conditions of tdb = 75° F and q. = 17% rh, which have a humidity ratio of W =
0.003. The latent heat removed with our exa mpl e o f 5000 cfm is as foll ows:
q, ~ 4840 x cfm x ( WI - Wz)
~ 4840 x 5000 cfm x (0.0 13 - 0.003)
~ 4840 x 5000 x (0.0 10) ~ 242 ,00 0 Blu/h
It should be noted at thi s time that th e processes shown in both Figures 4-3
and 4-4 are nearly impossible to do in the real world o f HV AC as stand-alone
processes. When we humid ify the air, we generall y have to add heat to the air,
even if not desired (steam humidi fi er). When we d ehumidi fy the air, we need
to coo l the air dry-bu lb temperature bel ow the ente ring dew-po int temperature
to start the moisture removal process, so we end up with cooled and dehumidifi ed air.
Total Heat Content
We will now di sc uss the four ai r-conditioning processes that are combinations of two simple processes. For these combination processes, we use the
enthalpy equatio n to get the total heat requ ired (qt) at sea level:
(4-3)
where cfin is the airflo w in cubi c fe et per minute, h) is the initial enthalpy, and
h2 is the fin al enthalpy.
Let us start with the coo ling and dehumid ify in g process, because it is the
most common in the HVAC industry. Movement on the psychrometric chart is
to the left (sensible) and down (l aten t) fro m the initial conditi on. See Fi gure 4-5
fo r the actual m ovement of the air.
Also, note that a gradual slope indi cates a more sensible than latent load,
but a steeper s lope shows a more latent than sensible load. The following
example will explain thi s comb inatio n process.
The entering conditions to our co oling co il are 80°F db and 65 °F wb, with
hi = 30. The ai r is cooled and dehumidifi ed all the way down to 54°F db and
53°F wb with h2 = 22. We can find the total heat required by using our new
equation with our exa mple of 5000 cfm:
q, ~ 4. 5 x cfm x (h I - hz)
~ 4. 5 x 5000 cfm x (30 -
22)
~ 4.5 x 5000 x (S) ~ ISO,OOO Blu/ h
We can get the same answer by using the indi vidual sensi ble and latent heat
eq uati ons:
qs = 1.085 x cfm x (1) - 12)
~ 1.085 x 5000 cfm x (80°F - 54 OF)
~ 1.085 x 5000 x (2 6) ~ 14 1,050 Blu/h
26
Cha pte r 4
Air-Conditioning Processes on tne Psychrometric Chart
110
90
"'"
.028
.024
..
"
,..
,
.020
45
:z:
c
3
c:
.016 ~.
D
2S
..........•...
.012
35
..........
10
"
.
50
.
IS
········"20··...•.
" .,
Dry Bulb
.
.008
....•.., .... .
Figure 4-5
!'!
40
. .
,
.004
30
2S
n,
Psychrometric cha rt showing move me nt of a ir in the cooling and d e humidnying
process.
and
q, ~ 4840 x cfm x (WI - 11'2)
~ 4840 x 5000 cfrn x (0.0098 - 0.0082)
~ 4840 x 5000 x (0.0016) ~ 38,720 Btulh
q~+ ql = q,
~ 141 ,050+38,720
~ 179,770 Blulh
Same answer, but done in one less step by using the combination eq uation.
See Figure 4-6 fo r the breakdown of the sensi ble and latent components.
Cooling and Humidifying
Cooling and hum idify ing is most easi ly explained with the process ofevaporative cooling. Because evaporative coo ling is a constant wet-bulb or adiabatic r.;ooling process, the lola l heal or enthalpy eq uation dues nut wurk. The
cooli ng of the air at dry-bu lb temperature is done by the fact that the water in
the liquid form is evaporated to water vapor. See Figure 4-7 for an example,
and note that the movement on the psychrometric chart is to the left (sensible)
and upward (latent) to complete the process.
Our inlet co nd itions to the evaporative cooler are 100°F db and $ = I 0% rh,
resulting in a 63°F wb. We move upward and left on the 60°F wet-bulb li ne the
Fundamentals of Psychrometries (I-P), Second Editio n
90
27
110
.028
.024
5
.020 ::I:
o
' 0%
3
45
a:
.016 ~.
'0
.012
~
o·
.008
35
10
.004
30
15
"
Figure 4-6
2S
20
'"
'"
.,
Dry Bulb
"
'"
'00
Sensible and latent components of determining total heat content using the psychrometric chart.
90
Dry Bulb
100
110
60%
.028
.024
50 .020
:>:
c
'0%
45
3
2S
40
r.C twb =63"
i
10
'0%
.008
35
.004
66"
30
15
"
Figure 4-7
'"
so
'"
2S
'0
'"
so
Dry Bu lb
"
'00
Cooling and humidifying shown on the psychrometric chart.
a:
"'.
!'(
"
.012 o·
.0 16
'"
'<
28
Chapter 4
Air-Conditioning Processes on tne Psychrometric Chart
distance our evaporati ve media will provide. Our outlet cond itions will be
66°F db, 63°F wb, and ~ =85% rh. Note that we have dropped the dry-bulb
temperature from 100°F to 66°F by using only water. At our 5000 cfm example, we are able to obta in a sensible coo ling of
qs = 1.085 x cfm x (1 1 - 12)
~ 1.085 x 5000 cfm x (I OO°F - 66°F)
~ 1.085 x 5000 x (34) ~ 184,450 Btulh
So, how much water do we use in the evaporative process? We can use a
new equation to calculate the water usage in pounds of water per hour;
Ib,jh ~ cfm x Il v x (WI - W2) x 60
~ 5000 ft3/min x 1/1 3.5 ft 3/1bda
x (0.004 - 0.0 11 6) Ib,.,Ilb da x 60 minlh
~ 5000 x 1/13.5 x (0.0076) x 60
~ 169
Evaporative cooli ng should always be an option if you are doing a project
in the hot/dry climates of the world.
Heating and Humidifying
Heating and humidifying is a combination process that is frequently seen in
the HV AC industry when it is desirable to attempt to maintain a space at or
above a minimum rel ative humidity setpoint. The movement on the psychrometric chart is to the right and towards the top, as shown in Figure 4-8.
In our exampl e, we have an airstream at 65°F db and ~ = 20% rh, with
5000 cfm at sea level. We want to maintain a room at 75°F db and ~ = 50% rh .
We can use our equat ion from the evaporative cooling example to sol ve for the
pounds of water per hour needed to increase the re lative humidity of thi s airstream .
Ib"/h ~ cfm x Il v x (WI - W2) x 60
~ 5000 ft 3/min x 1/1 3.5 ft 3/1bda
x (0.0028 - 0.0094) Ib" /lb da x 60 min/h
~ 5000 x 1/ 13.5 x (0.0066) x 60
~ 147
So, we can select a steam humidifi er to provide a mmimum output of
l47lbll'/h to keep our space at up to 50% relati ve humidity.
We can also calcu late the energy required to complete thi s process with the
total heat equation and the enthalpy at the entering and room conditions;
65°F db, ~ ~ 20% rh, h ~ 18.4 Btullbda
70°F db, ~ ~ 50% rh, h ~ 27.7 Btu/lbda
Fundamentals of Psychrometries (I-P), Second Edition
29
Dry Bulb
90
11 0
.028
.024
.020 ::z::
o
3
45
0:
.0 16 ~.
~
40
"'"
.0 12
~
o·
.008
Dry Bulb
Figure 4-8
Heating and humidifying shown on the psychrometric chart.
ql ~4.5 x cfm x (it l -
it 2)
~ 4.5 x 5000 ctin x (l~.4 -
27.7)
~ 4.5 x 5000 x (9.3) ~ 209,250 Btulh
Another way to get thi s an swer is to break the problem into the sensibl e
portion and the latent portion. Th e sen sible portion is easy, 65 °F db to
75 °F db, or:
q, ~ 1.085 x cfm x (I, - '2)
~ 1.085 x 5000 cfm x (65 °F -
75°F)
~ 54,250 Btulh
For the latent portion , we need to go to Appendix D and look at the thermodynamic properties of water at saturation, or steam, tables. In the left-hand column, find 65°F temperature and fo ll ow that to the right until you get to the
column labeled " Evap. hi/ hfg " under the "Specific Enthalpy" heading and get
1056.5 Btu/lbw' The steam will reach equilibrium at 65 °F db soon after inj ect ion into the airstream . Because we know the pounds of water per hour of the
humidifier, the latent portion is
q, ~ 1471b",ih x 1056.5 Btu/ lb",
~ 155,306 Btu/h
30
Cha pte r 4
Air-Conditioning Processes on tne Psychrometric Chart
Now we add the sensible and latent portions together:
qs+ q{ = q,
~ 54,250 + 155,306
~ 209,556 Btulh
which is close to 209,250 Btu/h.
Remember, we are eyeballing a ll these values from the psychrometric
chart, so the actual va lues may be off ± ! %.
Dehumidification and Heating
The last comb ination process is dehumidification and heating, or dehumidification by desiccant moisture absorption. The desiccant material (contained
in a wheel) is either rotated through the airstream or sprayed into the airstream
and coll ected in a pan at the bouom.
The other portion of the desiccant cycle is the regeneration process that
heats up the desiccant to drive off the moi sture to the atmosphere and start the
cycle over again. The process is shown in Figure 4-9 and has movement to the
right and the bottom of the psychrometric chart.
For our examp le, we have our in let conditions of 80°F db and ~ = 27%. rh,
which gives a dew-point temperature of 42°F and whi ch a mechanical vapor
compression refrige ration can easily reach.
.028
.024
5
",,, .020 3c
a:
.016 ".
:I:
45
~
.012
2('"
tdp _ 26·
.008
3S
10
.004
30
lS
"
Fig ure 4-9
'"
2S
20
"
.,
Dry Bulb
' 00
'"gw
40
'"
De humid ific ation a nd heating shown on the psychrometric chart.
Fundamentals of Psychrometries (I-P), Second Edition
31
However, our leaving conditions require a dew-point temperature of 26°F,
less than freezing (32°F), so mechanical coo ling will not work.
Our leaving conditions with desiccant dehumidification are 96°F db and
4> = 8% rh, for a dew-point temperature of 26°F,
Note that these conditions are somethi ng you may not see in normal human
comfort cooling, but they may be used in an industrial process or candy manufacturing facility, You wi ll also have to contact a manufacturer for an exact
selection and the regeneration method they use.
32
Chapter 4
Air-Conditioning Processes on tne Psychrometric Chart
Skill Development Exercises for Chapter 4
Complete these questions by writing your answers on the worksheets at the back a/this book.
4.1
Moist air that is heated without humidi ficatiol1 has the following change in relative
humidity:
a) Increase
b) Decrease
c) Stays the same
d) Depends on the type of humidifier
4-2
What is the equation that converts enthalpy changes into capacity (Btu/h)?
a) 1.085 x cfrn x (I] - I,)
b) 4.5 x cfrn x (h] - h,)
c) 4840 x cfrn x (W] - W,)
d) None of the above
4·3
Which of the following is true concerning humidification by steam versus by
(cold water) atomization?
a) Atom ization always maintain s a constant relative humidity.
b) Steam humidification adds no net energy to the airstream.
c) Heat to make steam in the steam humidifier comes from the air
entering the humidifier.
d) Heat to evaporate water in the atomizer comes from the air
entering the humidifier.
4-4
A heating coil can provide for both heating and humidifi cation .
a) True
b) Fa lse
4·5
A cooling co il can provide fo r both cooling and dehumidification.
a) True
b) False
4-6
What is the change in enthalpy when dry air is heated from 50°F to 74°F?
a) 4.5
b) 5.5
c) 6.5
d) 7.2
Fundamentals of Psychrometries (I-P), Second Edition
4-7
33
What is the enthalpy change when saturated air at 50°F is conditioned to be
saturated air at 74°F?
a) 17
b) 21
c) 25
d) 32
4-8
One day in Phoeni x, Arizona, the temperature reaches lO5°F with 20% rho
Water is sprayed into the air to cool it. What will the temperature of the air be
when the relative humidity increases to 50% rh?
a) 87"F
b) 95°F
c) 105°F
d) 115°F
4-9
If the air entering a heating coil is dry and 70°F db and the leaving air is 110°F,
how many Btulh are supplied by the coil at 5000 cfm if the fan is located at the
coil inlet?
a) 200,000
b) 205,000
c) 209,000
d) 217,000
4-10
Air enters a cooling coi l at lOOoF and 40% rh and leaves saturated at a temperature of 45°F. What is the total Bluth of cooli ng required if a 5000 cfm fan
is located at the in let of the coo ling coil?
a) 565,000
b) 511,600
c) 460,600
d) 440,600
HVAC Design
and the
Psychrometric Chart
Study Objectives
After completing thi s chapter, you should be able to
o apply HVAC systems to the psychrometric chart,
o define and use sensibl e heat ratio for cooling, and
o show the heating and humidifi cat ion process on the psychrometric chart.
Instructions
Read the materia l in Chapter 5. At the end of the chapter, compl ete the skill
development exercises without referring to the text.
Schematic of an Air-Conditioning System
To understand what an air· conditioning system is or what components it
has, it is best to look firs t al the room, or space, th at it is to serve. This space is
to be occupied and maintained at some psychrometric conditi on (ldb and ~).
This stale is called the room des;gn condition. This condition of temperature
and humidity is being constantly defeated by heat fl owing through the building
envelope , coming in or go ing out. Furthennore, it is being changed by the
activities happe ning in side. Occupants are providing heat and moisture to the
space. There are machines and li ghts that tran sfer heat to the space as a byproduct of their operat ion. There may be things that are cooling the room, and there
are things that are adding humidi ty to the room. These tend to change the interior room conditi ons. It is the purpose of the air-conditioning system to offset
these changes by conditioning the room air to maintain the room at the desired
condition.
To do thi s, some air is taken out of the room, conditioned, and returned
back to the space, Thi s is done as depicted in the layout o f a typical air-conditioning system shown in Figure 5-1 .
Mixing Airstreams-Cooling Systems
The mixing of two airstream s is common in HVAC systems to ensure the
proper ventilati on amount in the occupi ed space . This involves mixing an
36
Chapter 5
HVAC Design and the Psychrometric Chart
Exhaust
Air
Return Fan
ROOM
Outdoor
Air
Figure 5-1
Air
Sche matic of a general air-conditioning system.
Dry Bulb
90
100
....
110
.028
.024
....
.020
45
40
:.:
0
3
.0 16
$
.012
g.•"
2l,,, .008
Ml""
Condition
tdboo79" F
35
, ,,
10
"
Figure 5-2
"
.
15
' 20
" Bulb"
Dry
twbot6 S· F
.
.004
30
2S
''''
110
Summer design conditions shown on the psychrometric chart.
amount (fi xed o r variable) of outdoor a ir with a di.fferent amount of return or
room ai r 10 mee t the ventilation code in your local area.
First. we wi ll look at summer design conditions for a cooling applicati on.
Assume our room des ign is 'db = 75°F and ~ = 50% rh , wi th an air outdoor
design temperature of Idb = 95°F and ' lI'b = 75°F. Sec the psychrometric chart in
Figure 5-2 for the plotted conditions.
Fundamentals of Psychrometries (I-P), Second Edition
37
Next, we draw a straight line between these two plotted points on the psychrometric chart. Our mixed condition will always be on this stra ight line. We
can locate the exact location by using the following formu la:
(5-1)
where
Ima
dIY-bu lb temperature of mixed air
cfmoa
volume of outdoor air
temperature of outdoor air
volume of return air
temperature of return air
volume of suppl y ai r
Also, assume our HVAC system has a supply air volume (cfmsa) of 10,000
cfm and a ventilation or outdoor air vo lume (cfmoa) of2000 cfm. Th is means
the return or room ai r vo lume (cfmm ) is the difference between the supply ai r
volume and the return air vo lume, or
(5-2)
In our examp le, then,
10,000 cfrnSfJ = 2000 cfm nQ + 8000 cfmm
Then, our mixed-air dIY bu lb temperature is
lum ~ [2000 cfmoo (95 °F) + 8000 cfm m (75 °F)] / 10,000 cfm,"
~ [190,000 + 600,000]11 0,000
~ 79°F Idb
Now go back to the psychrometric chart in Figure 5-2 and plot the mixedair condition on thi s straight line at the intersection with the 79°F dIY-bulb line
marked MA. So our mixed-air conditions for these two ai rstreams are tdb =
79°F and ' wb = 65°F. This is an importan t item to know because the cooling
coil will be sized using this cond ition as the enteri ng air to this heat exchanger.
Mixing Airstreams-Heating Systems
Now we wi ll look at the same example in the wi nter heating mode. Assume
a room design of tdb = 70°F and ~ = 40% rh with a ir outdoor design temperature of tdb = 32° F and ~ = 50% rho See Figure 5-2 for the plotted conditions.
We will again plot both points on our psychrometric chart, as shown in Figure 5-3, and connect these points wi th a new straig ht line. Usi ng the same formula from the Mixing Airstreams- Cooling Systems section with different
temperatures and the same vo lume, we get:
38
Cha pte r 5
HVAC Design and t he Psychrometric Chart
IlIIa = [cfmoa (too) + cfm ra (tra)] !cfmsa
~ [2000 cfm (32' F) + 8000 cfm (70' F)] /10,000 cfm
~ [64,000
+ 560,000 ]110,000
~ 62.4' F
Go back to the psychrometric chart in Figure 5-3 and plot this heating
mixed-air condition on the straight line at the intersection with the 62.4°F drybulb temperature line. Thi s re sults in a mixi ng of these two airstreams at tdb =
62.4°F and ' wb = 51 °F. This will be the entering air condition for the heating
coil used in our system.
One spec ial note on heati ng mixed airstreams is that we need only the drybulb temperature to select our heali.ng coil or heat exchanger. The wet-bulb
temperature becomes important only i f humidification is needed in the HVAC
system servi ng thi s area.
Sensible Heat Ratio-Cooling
Sensib le heat rat io (SHR) is a very important concept in HV AC psychrometric analysis. With the proper use ofSHR, we wi ll ensure that both the room
dry-bulb temperature and room re lative humidity are met in our design. It wi ll
ensure our room supply air dry-bulb temperature and relative hum idity are cold
and dry enough to achieve the room design conditions. Fail ure to do a proper
SHR analysis could result in not meeting one or both of the room design
parameters. Here 's an example to explain the concept.
90
.028
.024
.020 :t:
c
3
4S
.01 6
...
•
D
.012 is'
.008
"
35
.00<
. ..
30
25
20
II
Figure 5-3
.,
"
"
"
Dry Bulb
""
!
'"
Winte r design conditions shown on the psychrome tric chart.
Fundamentals of Psychrometries (I-P), Second Edition
39
For our examp le, we will use a space with a sensible heat gain of
80,000 Btuth and a latent heat gain of 20,000 Btu/h, for a total load of
100,000 Btulh. Our room SHR is
80,000
100,000
SHR
0. 8
We plot the rool11 cond itions of 1db = 75°F and q. = 50% rh on a psychrometric chart as shown in Figure 5-4. Then we draw a li ne from the center score
mark to the value of 0.8 on the left side of the half circle . This is now our SHR
slope line. Next we transfer it from the upper left corner to the room conditions
on the chart. Please make sure the slope of th is line is exactly the same as you
plotted it.
Note that any air condition along the SHR line will meet our room design
conditions of tdb = 75°F and ~ = 50% rho These air conditions are the leaving
air temperature off the cooling co il in the air handler. The only thing that
changes on these varying leaving air temperatures is the volume. To solve the
problem, we go to the sensible heat equation discussed in Chapter 4:
q, ~ 1.085 x cfm x (II - /2)
We plot the intersection of the SHR line and a condition around 90% rh to
the left of the room cond ition in Figure 5-4. We have selected a leaving air temperature of tdh = 55°F and ~ = 87% rh as the desired cooling coi l leaving air
temperature. It is best pract ice to contact a cooling coil manufacturer (or run
90
.028
.024
.020 I
c
3
c:
25
-
20
..
.016 ~.
~
~
"
~
.012 o·
''''
Condition
._7S"F
0.
.008
35
10
.00<
30
IS
"
Figure 5-4
0
~
M
25
20
~
~
~
'00
Dry Bulb
Drawing the SHR slope line on the psychrome tric cha rt.
'"
40
Chapter 5
HVAC Design and the Psychrometric Chart
their cooling coil select ion software) and confirm that they can provide a coil
that would perform to these conditions with a volu me of
q, = 1.085 x cfm x (II - 12)
80,000 = 1.085 x cfm x (75°F - 55°F)
cfm =
80,000 = 3690
1.085 x 20
So, our cooling coil needs to provide a leaving air temperature of 'db = 55°F
and $ = 87% rh at our entering air temperature of ' db = 75 °F and $ = 50% rh o
with 3690 cfm of airflow through the coi l.
We could also have picked a leaving air temperature of Idb = 60°F and $ =
76% rh , resulting in an volume of
q, = 1.085 x cfm x (II - 12)
80,000 = 1.085 x cfm x (75°F - 60°F)
cfm =
80,000 = 4915
1.085 x 15
We mayor may not have bee n able to find a cooling coil to perform this
duty, because the leaving relative humidity is not close to 90%. Coils that
dehumidify ty pically have a leaving relative humidity close to 90%.
By providing Ihis supply air quantity (cfm), supply air dry-bulb lemperature, and relativ e humidity. we wi ll e nsure that our room des ign conditions are
satisfied.
Sensible Heat Ratio-Cooling with Outdoor Air
This section covers the psychrome tric process that is added to the SHR process to account for the outdoor air (ventilation air) in ou r HVAC system.
We will use the same outdoor air design conditions of Idb = 95°F and ' \Vb =
75 °F and 20% outdoor air from our example of the Mixing Airstreams- Cool ing Systems section. We plot all the conditions on our psychrometric chart as
shown in Figure 5-5. The mixed-air conditions are Idb = 79°F and ' wb = 65°F
from our calculation of supply air as 3690 cfm with the values from the previous example of outdoor air as 738 cfm and return air as 2952 cfm.
til/a = [cfm oo (foa) + cfmra {fra)] /c fm sa
= [738 cfm (95°F) + 2952 cfm (75 °F) ]/3690 cfm
= [70, \I 0 + 221 ,900)13690 cfm
= [291 ,510[ /3690 cfm
= 79°F 'db
Again , go (0 the ' db = 79"F scale on the psychrometric chart and go up until
you intersect the mixed-air line. That is our entering air condition to the cooling coil, fdb = 79"F and ' wb = 65"F. We still need to cool the air down to ' lI'b =
Fundamentals of Psychrometries (I-P), Second Edition
41
Dry Bulb
90
100
.Q28
.024
.020 I
c
3
a:
.016 -Q.'
•
~
"
20
,
1
15
Figure 5-5
~
0·
,'" .008
, , 35
,,"
10
"
.012
~
~
"
'J
~
004
'l'!.
I ........
1 20 ............
~
,~
Dry Bulb
"
'"
SHR for cooling with out door air shown on the psychrome t ric chart.
55°F and q:. = 87% rh to meet our room conditions. To calculate the total cooling coil load, use the follow tota l heat equation:
q, = 4.5 x cfm x (hi - h2 )
~ 4.5 x 3690 cfm x (8 Btullb)
~ 132,840 Btulh
Note the increased coo ling requirement due to the addition of outdoor air
into the HV AC system. The room total load was 100,000 Btu/h and the outdoor
air is an additional 40,000 Btulh total. We can also calculate the room-only coil
load with the same tota l heat equation:
q, ~ 4.5 x cfm x (h, - h2)
~ 4.5 x 3690 cfm x (28 - 22)
~ 4.5 x 3690 cfm x (6.0 Btullb)
~ 99,630 Btulh
Notice that t hi s is not exact ly the same as the 100,000 Btulh tota l heat gai n,
but it is very close and within acceptable tolerance for HV AC calculations.
Psychrometric Process-Heating
This section uses the same HVA C system we've been discussing to show
how to hand le the heating requirements of our space. The air ha ndler has the
sa me 3690 cfm. Assume our space has a heat loss of 90,000 Btulh and all this
load is sensible load. Our sens ible heat is as follows:
42
Chapter 5
HVAC Design and the Psychrometric Chart
Dry Bulb
90
"0
.028
.024
.020 I
c
3
45
<i
.016 ~.
~
25
40
''''
_IngCoiI
tt "'"
"
"
.008
.00<
30
5
"
Figure 5-6
.012
•g.
~
25
'0
~
~
~
~
,~
Dry Bulb
'"
The heating process shown on the psychrometric chart.
q, ~ 1.085 x cfm x (I, - I,)
~ 1.085 x 3690 cfm x (I, - I,)
~ 90,000
The heating room des ign 'db = 70°F and <\l = 40% rho So, our I [ = 70°F and
q, ~ 90,000
90,000 ~ 1.085 x 3690 cfm x (70 - I, )
90,000 ~ 4003.7 x (70 - I,)
90,000 ~ (70 _ / )
4003.7
'
22.5°F ~ (70 - I,)
/, ~ 92YF db
So, if we supply 92.5°F warm air to our space on the coldest winter day, we
will keep the space at tdb = 70°F. See Figure 5-6 for how to show the heating
process on a psychrometric chart.
Now we add the need for humidification in the w inter to our space. Assume,
for example, that we need to add 15,000 Btu/ h of latent heating in the form of
moisture or water vapor. OUf outdoor design is (db = 32°F and <\l = 50% rh o We
add 20% outdoor air into our HV AC system and our new entering a ir conditions
are Idb = 62.4°F and ~ = 44% rh oThe new total heating required is
q, = 4.5 x efm x (h l - h2)
~ 4.5 x 3690 cfm x (2 1 - 29.5)
~ 4.5 x 3690 cfm x (8.5 B,ullb)
~ 141 , 143 B,ulh
Fundamentals of Psychrometries (I-P), Second Edition
90
43
Dry Bulb
100
110
"'"
.028
.024
'I"'
.,
.020 I
<
3
.016
~
~
tf!~ /=2:
:::::3£
~
t
10
J<)
2.
2S
'00
Dry Bulb
Figure 5-7
.008
.007
.0054
.DO<
HHting Coli line
"
~
.0 12 c)'
'"
Humidification need shown on the psychrometric chart.
An interesting part of thi s analysis is that the leaving ai r temperature from
the heating coil has been increased to approxi mately ldb = 96°F to account for
the temperature drop the humidi fied air will cause, as shown in Figure 5-7.
The amount of water vapor that must be added to the airstream is calculated
based on an entering air Ofldb = 62.4° F and $ = 44% rh with a humidity ratio of
WI = 0.0054 lb w/lbda and leaving cond itions of (db = 92.4°F and $ = 20% rh
with a humidity ratio of W2 = 0.0068 IbW/lbd(1' Use the following formu la:
Ib/h ~ cfm x INo lum e x (WI - W2) x 60 minlh
~ 3690 x 1/14 x (0.0068 - 0.0054 Ib"/lbda ) x 60 minlh
~ 22. 1
We will discuss humidification more in the next chapter as we differentiate
between steam and water spray humidifi cation.
44
Chapter 5
HVAC Design and the Psychrometric Chart
Skill Development Exercises for Chapter 5
Complete these questions by writing your answers on the worksheets at the back a/this book.
5·1
The definition of sensib le heat ratio (SHR) is the:
a) Ratio of se nsibl e to latent load
b) Ratio aflatent to sensible load
c) Ratio of tota l load to sensible load
d) Ratio of sensib le load to total load
5·2
If the sensible load on a bui lding is equa l to the latent load, the value ofSHR is:
a) 2
b)
c)
0.5
d) - 2
5-3
The psychrometric condition for supply air that will satisfy the requirements of
a room depends on:
a) The amount of outdoor air needed
b) The des ired room condition
c) Room SHR
d) All of the above
e) Answers band con ly
5-4
Why is it poss ible to sat isfy a room w ith a variety o f " assumptions" about the
temperature change across a coil (heati ng or cooling)?
a) Because there is a correspond ing cfm w ith every!Y.
b) Because the heatlcoo lload calculation is never accurate.
c) Because the comfort zone is large.
d) Because there is a wide variety of methods for heating and
cooling.
5-5
Wh ich condition below is not possib le to show on a psychrometric chart?
a)
tdb ~ 76' F, h ~ 30
b) tdb ~ 89'F, twb ~ 78'F
c) ' \Vb = 78°F, h = 44
d) tdb ~ 76' F, ~ ~ 50%
Fundamentals of Psychrometries (I-P), Second Edition
5-6
45
In a system, 200 cfm of air at 60°F a nd 30% rh is mixed with 800 cfm air at
80°F and 80% rho Find the mixed-air temperature using the mixing equation.
a) 74° F
b) 76° F
c) 78°F
d) 79° F
5-7
In Exercise 5-6, what is the mixed-air relative humidity?
a) 60% rh
b) 76% rh
c) 70% rh
d) None of these
5-8
In a system, 200 cfm of air at 40°F and 90% rh is adiabatically mixed with
moi st air at 80°F but unknown relative humidity. The fina l mixture is at 72°F
and 50% rh oWhat is the relative humidity and airflow rate (cfrn) of the second
airstream?
a) 40% rh, 800 cfm
b) 40% rh, 50 cfm
c) 80% rh, 800 cfm
d) 80% rh, 50 cfm
5-9
If the sensible load is 600,000 Btulh and the latent load is 300,000 Btuih, what
is the SHR?
a) 2.0
b) 1.0
c) 0.66
d) 0.76
5-10
If the room design is ldb = 75°F and 4J = 50% rh and we mix in 25% outdoor ai r
at Idb = 115°F and ~ = 10% rh, what is the mixed-air dry-bulb temperature?
a) 83°F
b) 85°F
c) IOsoF
d) Not poss ible
5-11
From Exercise 5-10, what is the mi xed-air re lati ve humidity?
a) 33% rh
b) 15% rh
c) 38% rh
d) 40% rh
Psychrometries
in HVAC Equipment
Study Objectives
After completing thi s chapter, you should be able to
o show the components of an air-handling unit and their psychrometric processes and
o explain two types of humidification.
Instructions
Read the material in Chapter 6. At the end of the chapter, complete the skill
development exercises without referring to the tex t.
The Air-Handling Unit:
Heart of the Commercial Air-Conditioning System
In Chapter 5, psychrometrie s was used to determ ine the technical characteristics of the a ir-conditioning system required to perform a specific function .
Psychrometries was used to convert thi s information into the necessary volume
and s upp ly air conditions for both heating and cooling. These conditions not
only determined the capacity of the unit in Btu/ h but also spec ifi ed the amount
of dehumidification and humidification by determining the entering and leaving dry-bulb and wet-bulb temperatures for both the heating and cooli ng coils.
A manufacturer will usua ll y package all (or most) of the components of an
HVA C system into one large enclosure called an air-handling unit (AHU).
AHUs (Figure 6-1) are almost custom-made for every design because the components are selected from an extensive li st of ava ila ble sizes and capabilities to
match the speci fi c application. So that the AHU manufacturer can deli ver the
proper unit for the application , the design engineer must provide a ll of the necessary information.
Psychrometries of a Cooling Coil
Let 's start th is di scussion on what actually happens in a dehumidi fying
cooli ng coil as the air goes through it. The entering side of the coi l is warmer
than the leaving side of the coil. Therefore, the first few rows o f the cooli ng
48
Chapter 6
Psychrometries in HVAC Equi pment
Mi x ing
C h amber
RClUrn Ai r
Duct
" l :il ! ;<i
""
....
,
L_
~uPplY Fan
,I ~ V
I I
O u t d OO l' A i r
DuCI
Figure 6-1
>f(ffi)"'- r==-->
Supply
A ir
0
0
Wa ter
C oo lin g
------
H eal ing
Flow path through a simple AHU.
90
.028
.024
50
40%
45
.020 I
c
".
'".
.012 "'
0
.016
40
,
20%
tatent CooIil'lg
-ll.
--+. ,'
"
Figure 6-2
so
60
70
so
~
.008
.004
30
o
15
'<
35
,,,
10
2.
c.
25
90
Dry Bu lb
""
'"
Cooling coil line shown on the psychrometric chart.
coil are doi ng sensible coo li ng only, as can be seen in Figure 6-2, the cooling
coil li ne. Note! it is horizontal and moving to the left side of the psychrometric
chart.
As the air moves further into the coi l, the dehumidification process is starting as the cooling co il starts curv ing downward and to the left. The maximum
dehumidification occurs just before the air exits the coi l and generall y leaves
the coil around $ = 90% rh. Again, refer to Figure 6-2 to see the fina l curve
showing the completed dehum idi fi cation and cooling process.
Fundamentals of Psychrometries (I-P), Second Edition
49
Assume a fan cfm of 3690, entering conditions of tdb = 82°F and ' \Vb = 67°F
(with outdoor air mixed), and desired leaving conditions of tdb = 52°F and
approximately q, = 90% rhoNow we can calculate the total cooling capacity of
the coo ing coil w ith the tota l heat equation discussed in Chapter 4:
q, = 4.5 x cfm x (11 1 - h2 )
~ 4.5 x 3690 x (31.9 - 20.4 Btullb)
~ 190,950 Btulh
We can now plot the coo li ng co il performance on the psychrometric chart
as shown in Figure 6-2. We can see the total heat is broken down into a sensible
component and a latent component, as also shown in Figure 6-2.
Again , the HV AC engineer must provide the above infonnation to the manufacturer of the coo ling co il s so they can provide an actual selection of either a
chilled-water coi l or a direct expansion (DX) refrigerant co il. Their output
would include the size, height, width, number of coil rows, pressure drop (ai r
pressure and water pre ssure, if a chilled-water coil), chill ed-water temperature
rise (you must supply the enteri ng chilled-water temperature), and the actual
leaving air conditions.
Psychrometries of Fan Heat
As a fan moves air through an HVAC system, the fan input energy is converted to heat as a result of the heat of compression. A ll the fan input energy
ends up as heat as the fan increases the air pressure to provide air motion.
Say, for example, a fan requires 10 bhp to move 10,000 cfm against of3 in.
of water of total pressure . We use two conversion factors to deri ve the actual
heat in British thermal units per hour (B tu/ h) added to the airstream:
1 bhp ~ 746 W or 0.746 kW
1 kW ~ 3413 Btulh
So, in this example, we can convert 10 bhp to BtuIh of fan heat as follows:
10 bh x 0.746 kW ~ 7.46 kW
P
bhD
and
7.46 kW x 34 13 B,ulh ~ 25,46 1 Btulh
kW
Because the fan is movi ng 10,000 cfm, we can use the sensibl e heat equation discussed in Chapter 4 to ca lculate the actual temperatures:
qs = 1.085 x cfm x (/1 - 12)
25,46 1 B,ulh ~ 1.085 x 10,000 cfm x (I, - '2)
(II - ( 2) = 2.3°F temperature rise
50
Chapter 6
Psychrometries in HVAC Eq ui pment
So, we have the add ition of 2.3 °F fan heat to account for in our psychrometri c ana lysis.
Fan heat is the addition of sensible heat, horizontal moving to the ri ght on
the psychrometr ic chart, either before the cooling coil (blow-through fan ) or
aft er the cooli ng coil (draw-through fan). Be careful with draw-through fa ns,
because with these fans the fan leaving air temperature is hi gher than the cooling coil1 eaving air temperature . It is an additional load that must be accounted
fo r in cooling heat gain ca lculations.
ASHRA E Handbook- Fundamentals (20 13) gi ves a general estimate of fan
heal as approximately O. 5°F per inch o r total fan press ure. In this example we
calculated a li ttle over 2°F, while thi s general estimate would h ave given us
1.5°F. Therefore , it is better to perfonn the calculations.
Psychrometries of a Heating Coil
The process of heating air is a sensible-heating -only psychrometric problem, whi ch means that the poi nt moves from left to right horizontally across
the psychrometr ic chart.
This examp le aga in uses 3690 cfm airflow and the entering cond itions to
the hot water coil of ' db = 60°F and approx imately $ = 30% rh (with outdoor air
mixed) and a leaving condition of ' db = 95°F. See F igure 6-3 for the process of
heating and use the follo wing equation :
q, ~ 1.085 x cfm x (I, - '2)
~ 1.085 x 3690 x (60' F -
95' F)
~ 1.085 x 3690 x (35' F)
~ 140, 128 Btu/ h
Even if you forget that the process is sensible heating only, using the total
heat equation d iscussed in Chapter 4 will give you the same result as shown
be low:
ql~ 4. 5 x c fm x (11 , - 112)
~ 4. 5 x 3690 x (2 6. 5 -
18)
~ 141 , 143 Btulh
Humidification Equipment
Humidifi cation equipment can be di vided into two groups: I) isothennal or
constant temperature and 2) adiabat ic or moisture evaporating (see Figure 6-4).
Isothennal humidifi cation genemll y in vul vt::s steam humidifiers, with many
ways of steam generation, and addition of mo isture to an airstream at a constant temperature.
Adiabatic humid ifi cation is mo isture evaporat ing and involves sprayed,
atomized, medi a, or ultrasonic humidifi ers. These humidifi ers lower the air
temperature as they add mo isture to th e airstream and are the same as evaporative coolers .
Fundamentals of Psychrometries (I-P) , Second Edition
51
Dry Bulb
90
100
110
60%
.028
.024
5
40%
45
.020 I
c:
3
.016
40
.012
20%
,
15
,
-""
0:
'<
'"o·
.008
35
10
.004
Heil~l ng Coil Line
, 15
32
40
50
60
30
25
20
70
80
90
100
11 0
Dry Bulb
Figure 6-3
Heating coi l line shown on the psychrometric chart.
Isothe rmal or constant temperature
Adiabatic or moisture evaporating
AIR
COllstallt 1ell/peratllre or Adiabatic Depend s on where Evaporative Heat is Added
Figure 6-4
Steam (constant-te mpe rature) and spray (adiabatic) humidifie rs.
52
Cha pte r 6
Psychrometries in HVAC Equipment
90
110
60%
.028
.024
SO
' 0%
4S
.020 :I:
c
3
a:
.016 ~.
"
_______40
"
~
:g~~5 g.
0%
.008
/" ____ __ '..lS
Humidific"lion
.006
UM
10
.00'
"
Figure 6-5
"
"
..
30
15
20
" Bulb
Dry
80
.,
''''
"
'"
Isothe rmal humidification shown on the psychrome tric cha rt.
In all methods of hum idification, the following formula can be used to calculate the amount of water that must be added to the airstream in pounds of
water per hour:
Ib/h = cfm x ..!.. x (w 1 - w 2 ) x 60 minlh
va
Isothermal humid ification is shown in Figure 6-5 and has entering air conditions of tdb = 90°F and ~ = 20% rh and leaving air conditions of 'db = 90°F
and $ = 40% rho The humidity ratio increases from 0.006 to 0.0 12. Note that
the dry-bulb temperature stays the same even though we have added moisture
to the airstream.
Adiabatic hum idification is shown in Figure 6-6 and has entering air conditions of (db = 90°F and $ = 20% rh and leavi ng air conditions of (db = 82°F and
$ = 36% rho The humid ity ratio is increased from 0.006 to 0.008. Note that the
dry-bulb temperature decreases in the process of adding moisture to the air.
Be care fu l to not hum idify the airstream at greater than $ = 90% rh o The
dew-point temperature of the interior surface of the ductwork is very important, and failure to observe th is rule wi ll result in condensation in the ductwork
that wi ll eventually leak out and cause a problem in the building. Please work
closely with a humid ifier supp lier to make sure you are fo ll owing all the application rules of that product and system.
Fundamentals of Psychrometries (I-P), Second Edition
53
Dry Bulb
90
110
100
60%
.028
.024
4()"
.020 :I:
c
3
45
c:
.016 Q'
'"
!'l
40
.012 6'
2....
-----_______ 15
15
.008
.006
10
.004
30
15
32
Figure 6-6
..
"
25
0
70
80
Dry Bulb
"
100
'"
Adiabat ic humidifi cation shown on the psychrometric chart.
Reference
AS HRAE. 2013. Chapter I. In ASHRAE handbook- Fundamentals. At lanta:
AS HRAE.
54
Chapter 6
Psychrometri es in HVAC Equipme nt
Skill Development Exercises for Chapter 6
Complete these questions by writing your answers on the worksheets at the back a/this book.
6·1
Which type of humidification requires the change to not exceed the temperature rise capacity ofa heating co il ?
a) Water spray
b) Steam
c) Both the same
d) Neither has an impact
6-2
From the di scussion of the psychrometries of cooling coils, wh ich "rule of
thumb" will best se lect the coo li ng coil conditions?
a) Temperature drop across a cooling coil should be about 20°F.
b) Re lative humidity off the coil should be 90%.
c) Volume of air (cfm) across a coo lin g coi l should be kept to a
minimum.
d) Coi l temperatures should be selected to be as low as possible.
6·3
Which of the following statements best describe why cooling coils cannot
accommodate large latent loads with small sensible loads?
a) Cooling coils rust if too much condensate form s.
b) Cooling coils will freeze up if the coil temperature gets too low.
c) Cooling co il s tend to dehumidify first, then drop the ai r
temperature.
d) Condensation requ ires a drop in air temperature to the dew point.
64
Consider a room heating load with a 700,000 Btu/ h sensible loss and 100,000
Btulh latent loss, with room design conditions of (db = 72°F and approximately
q, = 40% rho The air handler has an adiabatic humidifi er downstream from a
heating coi l without any outdoor air. If the leaving air temperature is (db =
lOO°F after the humidifier, what is the cfm required to satisfy the load?
a) 20,000
b) 23,040
c) 25,200
d) None of these
Fundamentals of Psychrometries (I-P), Second Edition
6-5
55
What is the leaving air temperature tdb from the heating coil for the conditions
li sted in Exercise 6-4?
a) 98° F
b) 104°F
c) lOO°F
d) None of these
6-6
What is the leaving relative humidity 4J from the heating coil for the conditions
li sted in Exercise 6-4?
a) 15% rh
b) 12%rh
c) 20% rh
d) 24% rh
6-7
What is the leaving relati ve humidity 4J from the adiaba tic humidifier for the
conditions listed in Exerc ise 6-4?
a) 15% rh
b) 25% rh
c) 19%rh
d) 28% rh
6-8
Using the air handler in Exercise 6-4 and 23,040 cfm, adding a cooling co il to
satisfy a room sensib le heat ga in of 500,000 Btu/h and a room latent heat gain
of50,000 BtU/h , and room conditions of tdb = 75°F and 4J = 40% rh and without
outdoor air, what is the required leaving air temperature (db and 4J from the
cooling coil ?
a) 55° F 'db, $ ~ 90% rh
b) 5r F ' db, $ ~ 80% rh
c) 55 of ' db, $ ~ 75 % rh
6-9
What is the room sensible heat ratio for the conditions li sted in Exercise 6-8?
a) 0.89
b) 0.95
c) 0.91
d) 1.0
6-10
Would you attempt to add humidity to the leav ing airstream for the conditions
li sted in Exercise 6-8 in the cooling mode with an adiabatic humidifier?
a) Yes
b) N o
c) Not sure
Psychrometries in
Zoned HVAC Systems
Study Objectives
After completing this chapter, you should be able to
o understand the most commonly used HVAC systems,
o understand the psychrometric ana lysis of these HV AC systems, and
o
explain why we use zoned systems for HVAC.
Instructions
Read the materia l in Chapter 7. At the end of the chapter, complete the skill
development exercises without referring to the text.
Constant-Volume and Variable-Air-Volume Systems
This chapte r covers the major types of HVAC air systems. They can be
divided into two major types : I) constant-vo lume, variabl e air temperature and
2) vari able-aiT-volume, constant air te mperature.
Constant-volume systems de liver the same volu me, or efm, at all load conditions and change the supp ly air dry-bulb temperature as the load changes.
The load changes as the time of day changes, as the time o f year changes, as
the occupancy changes, as the interna l loads change, and as the solar load
changes. The room thermostat senses these changes in load and adjusts the
supply air temperature accordingly to main tain the room at a constant temperature.
In a chill ed-water coo ling, constant- volume ai r handler, the chilled-water
coil has a contro l val ve, controlled by the room thermostat to vary the supply
air temperature. This type of system can provide acceptable comfort because
there is an infinite number of chi lled -water va lve positions to match the large
number of load conditions.
In a direct expansion (OX) system, w hich has refrigerant in direct contact
with the cooling coi l tubes, there are typically steps or stages of cooling capacity. This causes the supply air temperature to be delivered at a set temperature
between the design temperature and a few part-load temperatures. The room
thermostat must have multiple stages so it ca n bring on additional capacity if
the room temperature ri ses or remove capac ity if the room temperature drops.
58
Chapter 7
Psychrometries in Zoned HVAC Systems
This causes a compromise in room temperature control, as the setpoi nt is
almost never met; the room temperature actual ly flu ctuates around the setpoint ,
approxi mately ± 1.0°F or ±1.s oF. In most cases, this fl uctuation in room tem-
peratu res can sti ll provide an acce ptable comfort level, but not one as good as a
chi lled-water system.
Note that some newer styles of DX systems have variable-refrigerant-flo w
capacity, w hich ca n provide a greater level of comfort in the room . These systems can better match the refrigerant fl ow req uired to handle the room 's load
variations, thus providing better comfort than stepped OX systems.
Variable-air-volume (V A V) systems deliver a variable amount of volume ,
or cfm, at all load conditions at a constant supply air dry-bulb temperature ,
Agai n, the load changes with the fi ve variables mentioned above, The room
thermostat senses these load changes and adjusts the quantity of air delivered
to the s pace so the room a ir temperature re mains constant. Note that all VA V
systems do a very good job of part-load room humid ity control, as the main air
hand ler cools and dehumidifies the air at all load co nditions to a consta nt dewpOint temperatu re.
In most cases. with modern ai r-handli ng systems, the a ir vo lume (cfm) is
changed with a variable-frequency drive (V FO) wired to the electric fan motor.
Because input frequency determines the s peed of rotation in induction electric
moto rs as the load changes, the V FO frequency output matches the cfm
reqUired by adjusting the fan s peed.
VA V air hand lers can have chilled -water or OX cooling coils installed in
them . With chilled water, the chilled-water control valve is infinitely variable
and can be controlled to mai ntain a constant supply air temperature as the ai r
vo lume goes up and dow n in response to load changes . W ith OX cooling coils,
the supply air temperature changes in stages, up and dow n, as the load changes
and the airflow increases or decreases. Again , eve n with stages, you can still
obtain acceptable room com fa n .
One fi nal point: you can use many small fan -coils to provid e a lot of zone
control in a large bu ild ing. but each fan -coil is a constant-volume , sing le-zone
subsystem .
Constant-Volume, Single-Zone System
Constant-volume, Single-zone systems are the most commonly used in
H V AC and come in many fo rms. They include Sin gle-zone air-handling room
fan -coils, packaged DX sing le-zone rooftop systems, and residential split systems, to name a few. T hey have one common trait: the unit is controlled by one
roomtitermoslal only. T herefore, they G HI be applied to only one zone allll provide room comfort to on ly one zone.
A zone, by definition, has like occupancy and like thermal characteristics
bu t does not have a defi ned size. For example, a large interior cubical offi ce
area could be a zone as large as 5000 ft2 if the densit1' and usage are uniform
throug hout. It cou ld also be a zone as small as 150 ft with a row of exterior.
individual offi ces lini ng an ou tside wall .
Funda me ntals of Psychrometries (I-P), Second Edition
59
90
.028
.024
.020 I
c
3
45
a:
.016 ~.
"
~
40
.012 is-
_008
15
35
10
, L.
.1'0.
"
30
"
Figure 7-1
"
"
2S
20
15
ro
Dry Bulb
"
"
'00
'"
Psychrometries of single-zone system at full load.
The psychrometric chart of a single-zone system is determined by the
room 's sensibl e and latent loads, the room's sensibl e heat ratio (S HR), and
either the quantity of outdoor air needed or the code-required venti lation rate.
It is shown at full load in Figure 7-1. In all examples in this chapter, we will
use room design cond itions of tdb = 75°F and $ = 50% rh and 20% outdoor ai r
for ventilation.
Next, look at the psychrometric chart in Figure 7-2 for part-load operation.
The part-load s upply air temperature is higher than the design temperature
because the room thermostat is ca lling for less cooling. Depending on the partload SHR, the room relati ve humidity may not be met at this part-load condition. So you must determ ine if thi s deviation above the design ¢l = 50% rh is
acceptable. Also note that the mixed-air condition will usually change to a
lower value , as the outdoor air will typically be lower at part-l oad cond itions.
Constant-Volume, Single-Zone System with Reheat
Constant-volume, single-zone systems with reheat are used when we need to
control the room relative humidity at all load l:Ondilions. The con trol is simple;
the cooling coil supply air dry-bulb temperature is set to a constant leaving temperature. Therefore, the supply air is dehumidified regardless of the room partload conditions. The supply air is reheated purely to satisfy the room thermostat.
It should be noted that thi s type of system is an expensive one to operate, as
we pay to cool and dehumidify the supply air and then pay again to warm up,
or reheat, the same airstream. If you are forced to provide thi s de sign in a sys-
60
Chapter 7
Psychrometries in Zoned HVAC Systems
Dry Bulb
.028
.024
S
.020 I
c
3
0:
4S
40
"•g.
.008
3S
10
15
~
Figure 7-2
.012
'''''
15
.016 ~.
.004
,.
ro
~
Dry Bulb
3.
2S
~
'00
'"
Psychromet,.ics of single-zone system at part load.
tern , you shou ld look for ways to use recovered energy to provide the reheat.
Recovered energy sources could be condenser heat from the cooling syste m,
such as a heat recovery chiller or condenser reheat plus sensible heat recovery
from exhaust or return airstreams, among others.
The psychrometric chart for this system is shown in Figure 7-3 at part-load
conditions. Note that even with a SHR of less than the design SHR, we can
meet the room re lative humidity condition. The psychrometries of this system
at fuHload are exact ly the same as shown in Figure 7-1 because, at full load,
we are not doing any reheat.
Constant-Volume, Single-Zone System with
Face and Bypass Dampers on the Cooling Coil
Constant-volume, single-zone systems with face and bypass dampers on
the cooling coil are not as common as they were in the past, but we will analyze thi s type in our psychrometric training process nonetheless. This system is
basically a single-zone air handler with a coo ling coil plus an extra damper section; see Figure 7-4 for all the wmponents. The dampers are used to adjust the
amount of supply a ir that goes through the cooling coil or the amount of mixed
air that bypasses the cooling coi l, thus the name/ace and bypass.
The damper is controlled by the room thermostat as it maintains a constant
room dry-bu lb temperature. As the room temperature drops, the dampers are
adjusted to open the bypass sect ion and close down on the face section , raising
the air handler s uppl y dry-bulb temperature. The opposite is true if the room
Funda m e nta ls of Psychrome tries (I-P), Second Edition
61
110
"'"
.028
.024
...
.020
45
:.:
c
3
a:
.016 ~.
~
.0
.012
2. .
o·~
.008
3S
10
.004
20
15
"
Fig ure 7-3
"
'"
25
'"
Psychromet ries of constant-volume , sing le -zone syst e m with re heat at pa rt load.
- 1'%
-
Return Air'
Outdoor AIr
I'
_
A/V HE
It Mixed Air <: I V A
T
It
Figure 7-4
'"
Dry Bulb
. ,.
30
EI
,i' Bypass
, I'
®~
C
0
0
L
Fan
l eavAir
-
Supply Air
SA
Compone nts of constant-volume, sing le-zone syst e m w it h face and bypass
dampe rs on the cooling coil.
temperature rises: the damper adj usts for less bypass air and more coo ling coil
air to lower the ai r handler supply temperature.
The psychrometries of this system at part load are show in Figure 7-5. Note
that the cooling coi l supply air temperature decreases as we decrease the airflow through the cooling co il at part load. Therefo re, thi s system does a much
better job of maintain ing the room re lati ve humidity at part load than a constant-volume, vari able-temperature, single-zone system.
The psychrometries of the face and bypass system at fu ll load are the same
as shown in Figure 7- 1 because no air is being bypassed at fu ll load. The room
thermostat is ca lli ng for full coo ling, so 100% of the supply air is being cooled
in the cooling coil.
62
Chapter 7
Psychrometries in Zoned HVAC Systems
Dry Bulb
90
11 0
100
""
.028
.024
0 .020
c
40%
3
45
"
c:
.016
~.
D
!'(
40
.012 o·
20%
.008
35
10
.004
30
"
Figure 7-5
.,
'"
.,
20
15
"
.,
Dry Bul b
.,
25
".,
'"
Psychrometries of constant-volume, single-zone system with face and bypass
dampel'"S on the cooling coil at part load.
Constant-Volume System with Terminal Reheat
Now we shift our focus to HVAC air systems that arc designed to serve
many zones from one air handler. The constant-volume wi th tenninal reheat
system was the primary commercial office space system from the 19405 to the
mid-1970s. The system is fair ly simpl e: a single-zone air handler suppli es air
dueled throughout the bu il ding, and then reheat coil s are put in each duct
runoullo serve any individua l zones.
The air handl er ensures a constant leav ing supply air temperature all year
round, and each reheat coi l tempers the air to meet the room temperature
desired in each zone. A few things to note: zones can have different temperature setpoints, zone reheats can be at different stages of tempering as zone
loads change, and part-load humidity control by zone is very good because the
main supply ai r is constantl y be ing dehumidified. However, thi s system is very
expensive to operate because it is both cooling and reheating throughout the
day, month , and yea r. Also, most energy codes, such as ANSUAS HRAElIES
Standard 90. 1 (AS H RAE 20 13) and Cal iforni a's Title 24 (CBSC 20 13), restrict
the use of this system for obvious reasons. It was a popular way to get zone
control in buildings when energy was cheap and before VA V was in vented.
The psychrometries of this system at fu ll load are the same as those shown
in Figure 7-1.
The psychrometries of thi s system at part load are the same as those shown
in Figure 7-2, with the exception that each zone has a separate psychrometri c
Fundamentals of Psychrometries (I-P), Second Edition
PREH€ATCOIL
f UE R
COOI. ING COIL
(ALTERNATIVE
LOCATION)
HU:.A IDIF IER
63
,
"
>-'''-__
ZON~ 1
-\ -----(j)
_~
:.AgI\E 2...(!)
-~ -
~-
zo~l:i
--", _--0
ZONC DAMPeR
(TYPICAL)
Figure 7-6
Components of constant-volume, multizone system (ASH RAE 2016, Figure 12).
chart, because the amount of the reheat will vary by zone and the SHR can be
slightly different by zone. However, a word of caution: the worst zone, from a
lowest-SHR standpoint, sets the air handler supply air dry-bulb temperature for
the entire system.
Constant-Volume Multizone and Dual-Duct Systems
Constant-volume multizone and dual-duct systems are designed to provide
comfort to multip le zones by mixi ng cool air with warm air so that the discharge supp ly air temperature is sati sfied by the zone thermostat. The only difference between these two systems is where the mixing of the hot and cool air
occurs.
In a multi zone system, the blow-through air handler has the mi xing dampers mounted on the front or top of the air-handling unit (AHU). The dampers
are on a common shaft, but offset by 90°, so when the hot deck is fu ll open, the
cold deck is fu ll closed and vice versa. Control of the air handler is simply a
constant deck temperature fo r each, say tdb = 55°F for the cold deck and tdb =
JOsoF for the hot deck. A si ngle damper activator is then controlled by a room
thermostat to position the dampers to meet the room temperature setpoint. If
the zone is too cool, this actuator opens more to the hot deck to wann the air
and increase the supply air temperature, thus warming the room. If the zone is
too warm, this actuator repositions to open to the cold deck (closes down on the
hot deck) to cool the air and lower the supply air temperature. Supply ductwork
is run out from this central air handl er to each zone. Small units may have as
few as three zones and large units as many as 18 to 20 zones. Figure 7-6 shows
a constant-volume, multi zone system.
In the dual-duct system, the blow-through air handler again has a hot deck
and a cold deck on the discharge side of the supply fan, but no mixing dampers.
Two sets of suppl y ductwork are run around the building in parallel with each
other. At any location that a zone is required, a dual-duct mixing box is
install ed and dual taps are run to the coo l deck duct and the hot deck duct. The
mixing box has two dampers on a common shaft offset by 90° with a single
actuator. The room thermostat is connected to thi s actuator to provide zone
comfort. The air handler aga in has controls to maintain constant leaving cold-
64
Cha pte r 7
Psychrometries in Zoned HVAC Systems
deck and hot-dec k temperatures. Th e operation o f this system is identical to
that of the mu lt izone system. Figure 7-7 shows a constant-volume, dual-duct
system.
Both of these systems are very ex pensive to operate because both decks are
kept at a consta nt leaving air temperature and achieve comfort by mixing both
airstreams. Their use is restricted by most energy codes due to the high energy
usage requi red fo r proper operation.
The psychrometries of the multizone and dual-duct systems are shown in
Figure 7-8. Note the mi xing line fro m the cold-deck discharge at Idb = 55°F
AIRFlOW MONITOR ING STATION
COOliNG COIL
(ALTERNATIVli:
LOCATION)
HUMIDIfiER
VAR I A~lE· FR EQ (JHIC Y OR IVE
,
(oPTIONAL)
PR€HEAT COIL
fi LTER
COOL ING COIL
OUTOOOR A'
"'-----1- c-t
,
EXHAlJSTAIR _ _
i
Figure 7~7
,,
,:
H€ATING CO IL
SUP?\'Y AIR
FN-I (HOT)
'
~
~
I
(
_ _ _ :..J _ _ _ __ _____ _ ______ L _ ~
VARIABLE-FREOUENCY DR IVE
VARIA3LE·FREQU€NCY DRIVE
AIRFLOWMONITOR:NG
STATION
- -- -~
Y
RETU RN AIR FAN
Compone nts of constant-volume , dual-duct syst e m (ASHRAE 2016 , Figure 13)_
"
Figure 7-8
,
AIRFlOW MONITORING
SIAIION
'"
'"
"
.,
Dry Bulb
"
,,.
'"
Psychro metries of const ant-volume m ult izo ne a nd dua l-duct syst ems.
Funda m e nta ls of Psychrome tries (I-P), Second Edition
65
and $ = 87% rh to the hot-deck discharge at Idb = 105°F and $ = 22% rh oThe
actual di scharge temperature can be anyone of an infinite number of points
along this line, and each zone can be at a different di scharge point. Also note
that we can heat and cool with thi s mi xing of airstreams at each zone.
Exercise caution to make sure that the cold-deck dry-bulb temperature is
low enough to sati sfy the SHR for the worst zone and that the hot-deck temperature is warm enough to sati sfy the heat loss of the worst zone.
Figure 7-9 shows the psychrometrics of multi zone and dual-duct systems in
winter heating mode. In thi s example, the room is at ' db = 70°F and $ = 40% rh,
with an outdoor design of (db = 32°F and $ = 50% rho The mi xed condition is
'db = 62.4°F and $ = 45 % rh oThe cooling coi l line is sensible cooling only, or
horizontal , and ends at 'db = 55°F and $ = 57% rho The heating co il line is sensible heating only and ends at 'db = 105°F and $ = 12% rho Note that because
both the cooli ng and heating are sensibl e only (no latent), th e hot-deck and
cold-deck mixing line is the sum of the cooling coil line and the heating coil
line. Any zone wi ll require supp ly air that is mixed along this line.
Variable-Air-Volume Systems for Multiple Zones
In the early 1970s, the high energy usage of most of the constant-volume
systems forced designs and owners to look for systems with lower operational
costs. Variable-a ir-volume (VA V) systems made their debut and are still very
popular today, as they provide great fan horsepower savings for most of the
year.
Dry8ulb
90
110
.028
.024
.020
4S
"'"30:
.016 ~.
•o·
~
Heating Coif
Leavi r19
Outdoor
AI,
"
I
"
.012
Room Ai
.008
.00<
Mixing Line for Ai r
Entering the Room
20
"
30
2S
'00
n.
Dry 8ulb
Figure 7-9
Psychrometries of const ant-volume multizone a nd dual-duct syst e ms in winter
heat ing mode .
66
Cha pte r 7
Psychrometries in Zoned HVAC Systems
An air handler with a single supply duct and a VFD on the supply fan provides a consta nt disc harge air tempe rature of {db = 55°F to the building. As
zones are required, a V A V box is tapped into this main supply trunk.
The V A V box is a single-damper device that modulates the airflow to the
zone in response to the room thermostat. Said another way, it is an a ir-throttling device that provides comfort to the zone. A t fu1l1oad in the zone, the V A V
box is wide open in response to the room thermostat. At all part-load condi tions, the V A V box has its damper closed some amount in res ponse to lower
de mand for coo ling in the zone.
The psychrometries for VAV sys tems are the sa me as shown in Figure 7-1
for each zone on the AHU. Aga in , ma ke s ure the air handler leaving supply air
dry-bulb temperature is low enough to satisfy the worst zone's S HR. Now at
part load we simply go to the sensible heat equation , qs = 1.08 5 x cfm x (t1 t2)' to determine how we handle the V A V operation.
Because the supply air temperature is constant year round , we reduce the
se nsible heat by simply reduci ng the airflow delivered to the zon e. If we wa nt
half of the sensibl e load , we only s upply hal f the airflow to the zone. The
room th ermostat controls the actuator on the damper shaft to keep the zone
comfortable.
So , at most part-load conditions, the psychrometries do not c hange- only
the airflow changes to satisfy the reduced load , and Figure 7- 1 is still valid .
This assumes the room SHR stays close to the full - load SHR at part load.
This style of V A V box is a cooling-on ly box and can typically only be used
fo r the buildi ng interior or zones that are in cooling year round .
Variable-Air-Volume Systems with Heating VAV Boxes
Most exterior zones require heating for a portion of the year. This section
covers two different styles of heating V A V boxes: V A V reheat boxes a nd fanpowered V A V boxes that can have reheat as needed . Note that the cen tral air
handler does not change with this des ign- we have cooling-only boxes on the
interior and heating boxes on the exterior.
To build a YAV reheat box, we simply take a cooling-only box and put a
reheat coil on the discharge of it. The coil can be hot water, stea m, or electric
duct heater in design.
The psychrometries of a V A V reheat box at full load are the same as show n
in Figu re 7- 1, As the demand for cool ing drops, we use the same part-load psychrometric chart as shown in Figure 7 -1 but at lower airflow. But at some preset minimum airflow, say 35% of full airflow, we energize the reheat coil. T he
controls modulate the amount of reheat or temperature rise in response to the
room thermostat. The psychrometries are shown in Figure 7-10, with the maxi mum reheat to a dry-bulb temperature of 95 °F. Remember , only reheat to a discharge a ir tempe ratu re into the zone t hat will sa tisfy the roo m thermostat.
Fundamentals of Psychrometries (I-P), Second Edition
67
90
50
.028
-
40
5
.020 :I:
45
3
..
40
15
c
c:
.0 16 ~.
'"~.
.012 0
,
Room AIr
.024
.008
35
Ma~ Reheat
10
.004
Temp
30
"
Figure 7-10
.,
.,
15
"
" Bulb"
Dry
"
'"
"
'"
Psychrometl'"ics of a VAV reheat box at part load.
To construct a fan-po wered VA V box that can al so have reheat as needed,
add a sheet metal plenum on the side of a cooling-on ly box with a sma ll directdri ve centrifugal fan. The fan can draw plenum air through an air filter and di scharge it into the cooling box downstream of the cooling control damper. A
back-draft damper is required on the di scharge of the centrifugal fan. This is
considered a para ll el fan-powe red box.
The control is similar to that ofa VA V reheat box. At full cooling, we have
100% of the cooling air going to the zone (no fan operation). At part load, we
throttle down the supply air to a lower amount (no fan operation). At a preset
minimum airflo w, say 35% of the full-load airflow, we fi x the cooling damper
to that position and start the centrifugal fan. It draws air from the ceiling plenum and mixes it with the reduced flo w 'db = 55°F to discharge warmer air into
the zone and meet the room thermostat setpoint. This ceiling plenum air can be
3° F to 5°F hi gher than the room tempe rature as long as the building is occupi ed, because it has the heat of the li ghts added to it.
The psychrometrics of a fan-powered VAV box that can have reheat as
needed are shown in Figure 7-11. Note that you are mixing supply air at ' db =
55°F and $ = 87% rh with plenum air at fdb = 80°F and $ = 42% rh along the
mixing line. The location will be determined by the airflow of supply air and
the airflow of the plenum air provided by the small centrifugal fan .
If needed, another reheat coi l cou ld be mounted on the box di scharge section to provide additional heating capacity for wintertime zone heat losses. The
coil is shown as additional reheat (" Reheat if Needed") in Figure 7-11.
68
Chapter 7
Psychrometries in Zoned HVAC Systems
90
110
""
.028
.024
50
"'"
.020 :I:
c
3
0:
45
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~
!!(
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,.10
"
Figure 7-11
"
.
.008
.004
ixing line for Plenum Air and
Supply Air to Meet Zone Temperature3 0
20
15
"
Dry Bulb
.,
.,
o·
25
'00
'"
Psychrometries of a fan -powered VAV box with reheat at part load.
References
AS HRAE. 2013. ANS I/ASH RAE/IES Standard 90. 1, Energy standard Jor
building except low-rise residential bllildings. Atlanta: AS HRAE.
ASHRAE. 2016. Chapter 4, Air hand ling and distribution. In ASHRAE handbook- HVAC systems and equipment. Atlanta: AS H RAE.
esse. 2013. California building standards code. T itle 24 o f California Code
of Regulations. Sacrame nto, CA: Ca lifornia Building Standards CommissIOn.
Fundame ntals of Psychrometries (I-P), Second Edition
69
Skill Development Exercises for Chapter 7
Complete these questions by writing your answers on the worksheets at the back a/this book.
For all of the Ski ll Development Exerc ises for Chapter 7, consider three zones
in a small office build ing that we are going to heat and cool. The cooling and
heating loads are as follows:
Zone
Sensible Cooli ng
Latent Coolin g
HeatingSensible
36,000 Btulh
5,000Btu/h
20,000 Btulh
2
48,000 Btulh
6,000B tulh
25 ,000 Btulh
3
60,000 Btulh
10,000 Btu/h
30,000 Btulh
Assume room des ign cond itions of the follo wing:
Cooling 'db = 75°F and $ = 50% rh
Heating
' db = 70°F and $ = 40% rh
Use a sea-level psychrometric chart.
7-1
What is the sensib le heat ratio for all three zones in order I, 2, 3? (Round to
two decimal places.)
a) 0.S8, 0.89, 0. 86
b) 0.S7, 0.85, 0. 89
c) 0.S5, 0.84, 0.87
7-2
If we provide 25% outdoor air for code-required venti lation to all three zones,
what is the mixed ai r condition in the summer if the outdoor air is 'db = 100°F
and $ = 25% rh?
a) 'db ~ 79' F and ~ ~ 48% rh
b) 'db ~S5 ' Fand~~40%rh
c) 'db ~ SI.2' F and ~ ~ 42% rh
7-3
For Zone I only, if we use indi vidual fan-coi ls for each zone, what is the
required supp ly airflow?
a) airflow = 1600 cfm
b) airflow ~ 1750 cfm
c) airflow = 2000 cfm
70
Cha pte r 7
7-4
Psychrometries in Zoned HVAC Systems
For Zone I only, what are the leaving air conditions from the cooling co il
assuming we use 25% outdoor air from Exerc ise 7-2 and the correct supply
cfm?
aj tdb ~ 54' F and ~ ~ 90% rh
bj tdb ~ 56' F and ~ ~ 88% rh
cj 'db ~ 60'F and ~ ~ 80% rh
7·5
For Zone 1 on ly, what is the total cooling capaci ty, q" of the coo ling coi l with
the correct cfm and leaving air conditions?
aj 41 ,000 Btu/h
bj 52,300 Btuth
cj 48,825 Btuth
7.6
If all three zones were put on a central ai r handler with a constant- volume terminal reheat system , what would the cfrn of all three zones be, in order 1,2, 3?
(Same outdoor design and percent outdoor ai r.)
aj 1750, 2000, 2500
bj 1600, 1800,2200
cj 1750, 2460,2765
7-7
If all three zones were put on a central air handler with a variable-air-volume
reheat VA V box and 25% outdoor air, what are the required leaving air conditions from this air handler?
aj tdb ~ 55"Fand~ ~ 91 % rh
bj tdb ~ 57' F and ~ ~ 88% rh
cj tdb ~ 60' F and ~ ~ 82% rh
7·8
With the system in Exerci se 7-7, what are the new required ai rflows by zone in
order 1,2,3 with the new leaving conditions?
aj 1660, 2110,2765
bj 1750, 2460,2750
cj 1700,2300,2600
7·9
What is the reheat required by zone in order 1, 2,3 to meet the tota l reheat load
plus the winter heat loss load? (Use ' db = 70°F for room condition.)
aj 49,000 Btulh, 61 ,000 Btulh, 79,000 Btuth
bj 47,000 Btulh, 59,300 Btulh, 75,000 Btuth
cj 56,000 Btulh, 73,000 Btulh, 90,000 Btulh
Fundame ntals of Psychrometries (I-P), Second Edition
7-10
71
From Exercise 7-7, with the correct leaving conditions and cfm, what is the
total cooling capacity of the central air-handler coo ling coi l?
a) 180,000 Btulh
b) 167,000 Btu/h
c) 194,100 Btulh
7-11
If the system in Exe rcise 7-7 were a constant-volume, dual-duct system, what
would be the heat capacity of the hot-deck coi l used in the central air handler?
(Room at 'db ~ 7SOF.)
a) 219,000Btulh
b) 199,000 Btulh
c) 212,500Btulh
Energy Conservation
and Psychrometries
Study Objectives
After completing this chapter, you should be able to
o understand e nergy saving systems and strategies and their effects on the
o
psychrometric analysis and
understand why lower energy costs result from the use of these systems and
strategies.
Instructions
Read the materia l in Chapter 8. At the end of the chapter, complete the skill
deve lopment exercises without referring to the text.
Introduction
This chapter covers energy conservation principles and strategies and how
they affect the HVAC system design. There are many devices and strategies
that can conserve energy, but thi s text focuses on only the most commonly used
in HV AC: heat recovery devices, energy recovery devices, air-side economizers, water-side economizers, and supply air temperature reset. We will exam ine
the psychrometric processes and the energy-saving effects of these fi ve systems.
Heat Recovery Devices
Heat recovery is the exchange of dry-bulb air temperature only between
two airstreams. In an HVAC system, thi s is typica lly between the outdoor air
used for venti lation and the common building exhaust airstream. The greater
the temperature difference that exists between the two airstreams, the more we
can affect the HV AC performance. This is also called sensible heal recovery,
because we only change the dry-bu lb temperature.
The four most common types of heat recovery devices are heal wheels, airto-air heat exchangers, heat pipes, and glycol run-around loops. Figure 8-1
shows three of these devices, and Figure 8-2 shows a schematic of a coi l runaround loop.
74
Chapter 8
Energy Conservatio n and Psychrometries
Figure 8-1
Hea r Pipe
Air-ro-Air
He.r W heel
Heat and e ne rgy re covery devices.
OUTDOOR SUPPLYAIR 0:::::::::;> AIR COIL
8--··
TOBUILDING,-_ _,
EXHAUST AIR
EXPANSION TANK
r:.::-l
~
EXHAUST-
=:>
AIR CO IL TO
OUTSIDE
PUMP
(
r
Figure 8-2
THREE -WAY VALVE
Run-around loop (ASHRAE 2012, Figure 14).
A heat whee l is a large-diameter, deep whee l consisting of a honeycomb
sty ling of metal pockets. Ha lf of the wheel is located in the exhaust airstream
and the other half is located in the inc oming outdoor airstream. In the winter, as
the wheel rotates slowly, the co ld outdoor air is preheated by the wanner
exhaust airstream that is being dumped outdoors, The indi vidual pockets
change temperature rapid ly as the wheel rotates from one airstream to the other
and back again.
In the summ er, the wheel also precools the hot outdoor air with indoor
room-temperature exhaust air from the building. Note that thi s device must be
ducted and positioned in such a way that the airstream s are next to each other
somewhere in the system. A lso, a small amount of cross-contamination occurs
between the airstreams, so care must be taken depending on the application.
For example, exhaust ai r fro m an office buildi ng (toilet, break room,janitori al
Fundamentals of Psychrometries (I-P), Second Edition
75
closets, etc.) is genera ll y acceptable to use, but exhaust from hospita l isolation
rooms is never acceptable.
One final note on rotary heat wheels is that the loss of air from the supply
side (outdoor air) to the exhaust air s ide can be as high as 10% of the total airflow. This is the cause of the cross-contamination, but you must also increase
the airflow hi gher than the design required amount to cover th is loss or leakage.
An air-to-air heat exchanger uses parallel plates of metal (or other material)
to separate the exhaust air from the outdoor air. The plates are packed tightly
next to each other in the heat exchanger, and the heat transfer goes across each
plate. So every other plate has indoor or outdoor air flowing through it in opposite directions. This heat exchanger design does not have any cross-contamination between the two airstreams, and both airstreams must be side by side
somewhere in the system.
Heat pipes look like one big chilled-water coil or heat exchanger. The difference is that each tube go ing across the coil is a separate chamber filled with
a very small refrigerant charge. Eac h half of the coil, split side to side, sits in
one of the airstreams. In winter, the outdoor air side of the heat pipe condenses
the refrigerant in the tube and rejects the heat to the outdoor air, warming it.
The other side sits in the wa rm exhaust air, wh ich vaporizes the refrigerant,
absorbing the heat from the warm airstream. By natural pressure d iffe rence, the
warm refrigerant vapor mi grates to the colder side, where it condenses. The
coil is tilted sli ght ly so the liqu id refrigerant flow s back to the warm side on the
bottom of each small tube. When the season changes to summer, the tilt must
be reversed so the heat pipe can work in the reverse and cool the warm entering
outdoor air.
A glycol run-around loop is two large coils or heat exchangers placed in the
two airstreams that arc connected by two pipes and one pump to move the glycol-water solution from one coi l to the other. They transfer heat from the
exhaust airstream to the outdoor airstream by warm ing up and then cooling
down the pumped glyco l-water so lution. The advantage of thi s system is that
the airstreams can be located great di stances from each other. There is no
cross-contamination with th is dev ice.
Psychrometric Effects and Savings of Heat Recovery
Figure 8-3 shows winter temperature-change-only heat recovery. The
exhaust airstream is at 'db = 70°F and $ = 40% rh and 1500 cfm. The outdoor
airstream is at [db = 32°F and $ = 50% rh and 2000 cfm. N ote: Typically more
outdoor air is brought in than exhausted to posi ti ve ly pressurize the building to
keep the indoor env ironment clean and dust free.
However, we must first introduce the concept of heat exchanger effecti veness. Counterflow air-to-air heat exchangers can achieve close to 100% effectiveness. But, the range of 50% to 70% effectiveness for cost and air pressure
drop considerations is generally selected. For this example, we wi ll use 60%
effectiveness.
76
Cha pte r 8
Energy Conservation and Psychrometries
90
110
"'"
.028
.024
5
"'"
.020 :I:
c
3
4S
a:
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~
:':
40
"'"
Air
"
Fi gure 8-3
.008
.004
Room Air
~
'"
30
.
25
20
15
ro
"
o·
35
ct-
10
.012
"
Dry Bul b
'''''
'"
Heat recove ry in the winter.
If the exhaust a irstrea m were coo led from {db = 70°F to 'db = 32°F the maximum amount of sensible heat transfer wou ld be
qs = qmar = 1.085 x cfm x (I [ - 12)
~ 1.085 x
1500 x (70 - 32)
qmar = 6 1,845 Stulh
Because the effective ness is 60%, the transferred heat is
qmax x effectiveness = q'rallsjerral
6 1,845 B,uIh x (0.6) ~ 37, 107 B,ulh
Then the outdoor air is warmed to
q, ~ 37, 107 B,uIh ~ 1.085 x 2000 cfm x (32 - ',)
37, 107 ~ 17' F ~ (32 _ , )
2170
'
'2 = 49°F
Likewise, the exhaust air is cooled to
q, ~ 37, 107 B,uIh ~ 1.085 x 1500 cfm x (70 - ',)
37, 107 ~ 22.8 ' F ~ (70 - , )
1625
2
12 = 47.2°F
Funda me ntals of Psychrometries (I-P), Second Edition
77
110
90
"'"
.028
.024
5
"'"
.020 :J:
c
3
45
0:
.016 ~.
25
40
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1-
15
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Exhaust
Air
10
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.004
30
15
"
Figure 8-4
'"
25
20
.,
"Dry Bulb
"
""
,,.
Heat recove ry in the summe r.
So 37, 107 Btulh of energy has been conserved by preheating the outdoor
air by 17°F and therefore lowering the operating cost in the process.
Next, we will look at the process in the summer with o utdoor des ign conditions of Idb = 95°F and Ql = 40% rh and room conditions of Idb = 75°F and Ql =
50% rh, as shown in Figure 8-4.
q, ~ q",a, ~ 1.085 x 1500 cfm x (95 - 75)
~ 1.085 x
1500 cfm x (20)
qmar = 32,550 Btulh
Us ing the same effectiveness of 60%,
q~ = qmar X (0.60) = qlrallsjerral
q'rallsjerral = 32,550 x (0.60)
~ 19,53 0 Btulh
So we cool the outdoor air sen sibly by
q, ~ 19,53 0 Btulh ~ I. 085 x 2000 cfm x (95 - ',)
19,530 ~ gOF ~ (95 _ , )
2 170
'
= 86°F
'2
78
Cha pte r 8
Energy Conservation and Psychrometries
And we wann the exhaust air by
q, ~ 19,53 0 Bluth ~ 1.085 x 1500 cfm x (75 - (2)
19,530 ~ 12 ' F ~ (75 - 1)
1628
2
12 = 87°F
An air-te-air heat exchanger was used in thi s ex ample, but the same procedure applies to the other three heat recovery devices-only the effecti veness
will change.
Condensation and Frost Formation
We must consider two other item s in the use of heat recovery devices, condensation and frost formation. Condensation can occur on a heat exchanger if
the exhaust air dew-point temperature is reached. For example, in Figure 8-3 , if
the exhaust air temperature were lowered below tdb = 45°F, then condensation
would occur on a small portion of th e heat transfer surface. Be sure to specify
these devices w ith a condensate drain to properly collect this water.
Frost can fo rm on the leaving side of the exhaust air in the heat recovery
device when the outdoor air gets to temperatures o f + 10°F or less. In the winter, because the exhaust air dew-point temperature is almost always above
35°F, this is a design consideration in C limate Zones 4 to 8 (ASHRAE 2013).
Frost formation in the heat recovery device decreases the exhaust airflow and
reduces the de vice 's effectiveness.
The methods used to preven t frost formation on heat recovery devices are
as fo llows:
Preheat the outdoor airstream to some preset temperature entering the
device (e.g., + IO°F) (a ll types of heat recovery devices).
Install a set of bypa ss dampers around the device to bypass a portion of the
outdoor airstream so the heat exchanger does not get so cold (heat wheel,
air-to-air heat exchanger, heat pipe).
Install a three-way contro l va lve in the glycol piping to control the glycol
solution inlet temperature on the exhaust coil to somewhere around +30°F,
thus pre venting fro st formation (run-around loo p).
Increase the rotationa l speed of the heat wheel so the outdoor air does not
cool the heat transfer part of the wheel to below around + 30°F (heat
wheel).
Energy Recovery Devices
Energy recovery is the transfer of sensible heat and latent heat from the
exhaust airstream to the outdoor airstream . At first glance, the whee l looks
identical to a heat recovery whee l that transfers temperature only. However, the
heat transfer material in an energy recovery device is coated with a des iccant
Funda me ntals of Psychrometries (I-P), Second Edition
79
materi al that absorbs water vapor and then rejects the water vapor to the other
airstream. So in winter, the moisture in the wanner building air is transferred to
the very dry outdoor ai r, thus helping maintain the indoor relative hum idity.
And in the summer, the coo l and dryer indoor air that is exhausted absorbs
some of the moisture in the hot, humid outdoor air.
Energy recovery devices come in two types: I) rotary energy wheels and
2) plate air-to-air heat exchangers with moisture transfer plates that are not
solid metal in construction. Fo ll owing are performance examples of an energy
recovery rotary wheel. Summer perfonnance with a total energy effecti veness
of 0.87 is shown in Figure 8-5 and is as follows:
Outdoor air cond itions :
'db = 95°F. ' wb = 74.6°F, outdoor air = 2296 cfm
Supply air conditions:
'db = 82°F. ' wb = 67°F, supply air = 2000 cfm
Return air conditions:
tdb = 75°F. twb = 62.4°F, return air =
Exhaust air condit ions:
tdb = 92.4° F, twb = 73.2°F, ex haust air =
1500 cfm
1796 cfm
Note the leakage airfl ow is 296 cfm and the outdoor air total cooling load
reduction is
q,= 4.5 x cfrn x (11\ - 112)
~ 4.5 x 2000 cfm x (38.4 - 31.8)
~ 59,400 Btulh
90
.028
.024
5
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.020 J:
c
3
45
a:
".
'"
40
.012 ".
.0 16
~
~
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.008
35
10
.004
30
"
Figure 8-5
'"
'"
.,
25
20
15
"
.,
Dry Bulb
'"
""
'"
Energy recove ry rota ry w heel s umme r p erformance example.
80
Cha pte r 8
Energy Conservation and Psychrometries
The coo ling system on ly needs to cool the outdoor air from tdb = 82°F and
' \Vb = 67°F to Idb = 75 °F and $ = 50% rh instead of from ldb = 95°F and 'wb =
74.6°F, which reduces the cooli ng energy costs.
Winter performance with the sa me effectiveness is shown in Figure 8-6 and
is as fo llows:
Outdoor air conditions:
'db = 32°F, {wb = 27. 1°F, outdoor air = 2296 cfm
Supply air conditions:
tdb = 56. 8°F, 'wb = 47.2°F, supply air = 2000 cfm
Return air conditions:
'db = 70°F, ' wb = 55.6°F, return air = 2000 cfrn
Exhaust air cond itions:
'db = 37°F, ' wb = 31.s F, ex haust air = 1796 cfrn
o
The outdoor air heat ing load is reduced by
ql~4.5 x cfm x (h, - h2)
~ 4.5 x 2000 cfm x ( 18.8 -
9.8)
~ 4.5 x 2000 cfm x (9)
~ 8 1,000 Btulh
The outdoor air now has only to be heated from 'db = 56. 8°F to 'db = 70°F
instead of from 'db = 32°F, which reduces the cooling energy costs, as with the
winter operation.
Note that in both cases of heating and coo ling to the outdoor air, the humidity ratio changes so you are not on ly transferring sensible heat, but also latent
11 0
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Fundame ntals of Psychrometries (I-P), Second Edition
81
heat. This late nt heat helps mai ntain the indoor room relative humidi ty at the
design condition.
Air-Side Economizer
An air-side economizer is an HVAC system option that allows cooling
without the use of mechanical refrigeration , thus making the cooling energy
equal to zero. For instance, to satisfy the cooling load, we need the supply ai r
{db = 55°F. Then, any time of the year that the outdoor air temperature is {db =
55°F or less, we can shut the mechanized refrigeration off and open our outdoor air dampers to 100%, pulling in 55°F air. Now this 55°F outdoor air provides all the cooling to the building. This is called full economizer mode and
should be considered any place where there is a cooling need in the fall, winter,
and spring.
You can also implement partial air-side economizing in your HVAC system. For examp le, the outdoor air temperature is Idb= 65°F and your room is at
{db = 75°F. Instead of cooling the air from 75°F down to 55°F for the supply air,
you only have to cool the air from 65°F to 55°F with mechanical refrigeration.
In this example , the refrigeration load is reduced approximately 50%. So parti al economizing can be used any time the outdoor air is less than the room
cooling condition.
A word of caution when using partial air-side economizing on constantvolume, variable- temperature systems and it is very humid or raining outdoors
is that the humid outdoor air will cause the room relative humidity to go above
the room design condition of ~ = 50% rho In this instance, use the outdoor air
enthalpy instead of the dry-bulb temperature to initiate partial economizing.
The outdoor en thalpy should be at least 5 Btu/lb da less than the room enthalpy
condition before you allow partial economizing. This is less of a concern on
V AV systems since they control to a constant leaving cooling ai r temperature at
all load conditions.
Note that with the air-side economizer option the air-handling system must
be capable of bri ngi ng in up to 100% outdoor air. This mea ns the outdoor ai r
weather louvers. outdoor air duct. and outdoor air dampers must all be sized
and selected for the fu ll ai rflow of the ai r handler.
Also, becau se yo u are bringing up to 100% outdoor air into the building,
you must provide a way to relieve, or exhaust, this additional air and provide
building pressure control to prevent overpressurization of the building. Failure to do th is will result in th e exterior doors staying partially open, not fu lly
closing.
Water-Side Economizer
The water-side economizer system was developed to provide cooling via
the chilled-wa ter system to air-handling systems that do not have any outdoor
air connection or have minimum outdoor air capability. For this energy-saving
82
Cha pte r 8
Energy Conservation and Psychrometries
option, you must have a chi lled-water cooling delivery system, a cooling tower,
and a heat exchanger piped between the chilled- and condenser-water systems.
Let's look at the performance of a cooling tower from the psychrometric
side. A cooling tower produces cool water dependent on the ambient wet-bulb
temperature onl y. For examp le, the amb ient is (db = 95°F and twb = 75°F, as in
our previous examp les. The cooling tower has a 7°F approach temperature at
fu ll load or heat rejection. Thi s means the cooling tower can produce 82°F
leaving cooling tower water, or 'wb = 75°F + 7°F = 82°F. In the process of cooling the water down to 82°F, a portion of the recirculating water is evaporated
by slightly cooling the air (lower 'db) and greatly adding moisture to the air. It
is not uncommon that the air leaving a cooling tower is between 90% and 95%
relative humidity.
As the outdoor air coo ls in the fa ll, winter, and spring, so does the outdoor
wet-bulb temperature. Also, the building sensible load decreases in these nonpeak cooling seasons. For examp le, the ambient temperature is 'db = 40°F and
' \Vb = 35°F. We also have the same approach temperature at part load of 7°F (if
the tower is at fu ll load and the same ambient conditions, the approach would
be around 12°F).
So this tower will make 42°F leaving condenser water. If our heat
exchanger has a 2°F approach temperature, then we can make 44°F chilled
water to be distributed throughout the building to provide cooling where
needed. See Figure 8-7 fo r a system schematic of a water-side economizer.
This water-side economizer is most commonly used on systems that have
chilled-water fan-coils or small air handler type systems. Typically, these systems have no outdoor air or a small amount of outdoor air, and it could be difficult or impossible to duct 100% outdoor air capability to them.
Condenser Water with Economizer
r-- ,
I
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I
I
I
-r--- I
1"" - -
_ _ _ _ _ .J
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Wate r-side economize r schematic.
Funda me ntals of Psychrometries (I-P), Second Edition
83
The heat exchanger between the condenser water system and the chilledwater system must be cleanab le. So, because both can have the condenser
water debris eas ily removed, plate-and-frame and shell -and-tube (tube-side
condenser water) are the two heat exchangers most commonly used.
Supply Air Temperature Reset
Supply ai r temperature reset works because in almost all comfort cooling
system app lications, the sensib le heat gain decreases in the fall, winter, and
spring. So, if the sensible heat gain to a zone is half the summer peak gain by
the sensible heat equation di scussed in Chapter 4, qs = 1.085 x cfm x (II - '2)'
and if our ai rflow" is constant, then the fjJ can be ha lf to produce half the sensible cooling. Consider: if we have cfm = 2000, a summer peak supply air temperature of tdb = 55° F, and a room condition of tdb = 75° F, then
q, full ~ 1.085 x 2000 cfrn x (75' F - 55' F)
Full sensible cooling:
~ 43,400 Btulh
q, ha lf ~ 1.085 x 2000 cfrn x (75' F - 55' F) x 0.5
Half sensib le cooling:
~ 2 1,700 Btu/h
Simply by having the air hand ler supply tdb = 65°F, we can provide the
cooling necessary. See Figure 8-8 for the psychrometric analysis of this con-
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84
Cha pte r 8
Energy Conservation and Psychrometries
cept. A lso note that the sensib le heat ratio will change as the heat gain
decreases in the off-peak load times of the year.
A few words of caution are in order before you apply thi s strategy:
Data centers, IT rooms, and te lecom rooms may have only slight decreases
in the sensible load throughout the year and , therefore, cannot use supply
air temperature reset.
Process or industrial applications may never change in sensible load and so
cannot use supp ly air temperature reset.
VA V systems are very economica l to nm because the airflow varies as the
sensible load goes down. But if you reset the supply air temperature
upward too much , you will eat into or eliminate the fan horsepower savings
deri ved from th is system. Granted, you can probably reset the supply air
tdb = 55°F to 58°F or 60°F, but not up to 65 °F to 70°F. This is a great case
fo r energy model ing of the V A V system to see how high in reset temperature you can go at the expense of fa n energy usage. Remember, the higher
the supply ai r temperature, the more refrigeration or cooli ng energy you
can save.
Be ve ry carefu l about how much supply air temperature reset you do in
very humid areas of the world. Remember, the supply air temperature from
the cooling coi l sets the req uired dew point to maintain the room relative
humidity via the sensib le heat ratio calculation. Many times in the olT-peak
cooling season you cou ld reset the supply air temperature, but the outdoor
hum idity cond itions force the cooling coil to always be in dehumidification
mode. And be ca reful of ra iny days in the off-peak cooling season, as the
outdoor moisture content may take precedent over supply air temperature
reset. All modem HVAC contro l systems should do some indoor relative
humidity sensing as a standard benefit, so the building manager/operator
has the ability to make the right decision in this event.
Many other energy-conserving measures can be used on a building that do
not involve the psychrometric process in the HV AC system and, therefore, are
not discussed in thi s course.
References
AS HRA E. 2013. Figure 8 1-1 , Normative Appendix B, Bui lding envelope climate criteria. In ANS I/ASHRAEnES Standard 90.1-2013, Energy standard/or buildings except low-rise residential buildings. At lanta: ASH RA E.
ASHRAE. 2012. Chapter 26, Air-to-air energy recovery equipment. In
ASHRAE handbook- HVAC systems and equipment.
Fundame ntals of Psychrometries (I-P), Second Edition
85
Skill Development Exercises for Chapter 8
Complete these questions by writing your answers on the worksheets at the back a/this book.
8-1
A heat wheel with a desiccant coating is a:
a) Sensible heat recovery device
b) Total entha lpy heat recovery device
c) Sensible-to-total heat recovery device
d) Total-to-sensible heat recovery device
8-2
When is preheating of the outdoor airstream necessary on a heat recovery
device?
a) When the outdoor air temperature is below O°F.
b) When the outdoor air dew point is below 32°F.
c) When the exhaust airstream has a dew point above 32°F and the
leaving air temperature is below 32°F.
d) All of the above.
8-3
Heat recovery effect iveness is the actual amount of heat transferred versus the
max imum amount that could be transrerred.
a) True
b) Fa lse
8-4
Energy recovery invo lves the trans rerof sensi ble heat from one airstream to the
other airstream.
a) True
b) False
8-5
An air-side economizer shou ld be considered on any/all air systems that have
100% outdoor air capability and high operation hours with an ambient air temperature below 60°F and a demand for cooling.
a) True
b) Fa lse
8-6
Water-side economizers can be used on a chilled-water system with all terminal fan-coils and an air-cooled water chiller.
a) True
b) False
86
Cha pte r 8
8-7
Energy Conservation and Psychrometries
There is a sensible heat recovery system between eq ual outdoor air and exhaust
airstreams in Phoenix , Arizona, and the summer design outside is tdb = 115°F
and q, = 10% rho (fthe effectiveness is 75% of the heat recovery device and the
exhaust ai rstream is ldb = 75°F and $ = 40% rh, what are the dry-bulb temperature and relative humid ity of the outdoor airstream leaving the recovery
device?
a) 'db ~ 90°F and ~ ~ 15% rh
b) 'db ~ 95°F and ~ ~ 12% rh
c) 'db ~ 70°F and ~ ~ 25% rh
d) 'db ~ 85°F and ~ ~ 20% rh
8.8
From Exercise 8-7, what are the leaving air conditions of the exhaust airstream
with everything else being the same?
a) 'db ~ 85°F and ~ ~ 30% rh
b) 'db ~ 95°F and ~ ~ 25% rh
c) 'db ~ 100°F and ~ ~ 20% rh
d) 'db ~ 105°F and ~ ~ 16% rh
8·9
If the entering air cond itions to a cooling tower are {db = 115 °F and tll'b = 65 °F
and the cooling tower has a full-load approach temperature of 8°F, what is the
leaving water from cooling tower (at full load)?
a) 107°F
b) 95 ° F
e) 73 ° F
d) 84° F
8-10
Supply air temperature reset can be used on all air-conditioning systems, any
time of the year in all parts of the world, regardless of the amb ie nt air cond itions.
a) True
b) Fa lse
Special Applications and
Psychrometric Considerations
Study Objectives
After completing this chapter, you should be able to
o understand the five special cases of psychrometric appli cations m the
o
o
HV AC industry and the psychrometric analysis of each,
select equipment for each of these systems, and
understand the effect of indirect and direct evaporative coolin g in series.
Instructions
Read the materia l in Chapter 9. At the end of the chapter, complete the sk ill
development exercises without referring to the text.
Introduction
This chapter discusses five special cases of psychrometric applications in
the HVA C industry: cooling towers, c1eanrooms, indoor swimming poo ls,
direct evaporative coo ling, and indirect evaporative cooling.
Cooling Towers
Starting with coo ling towers may seem strange because the function of a
cooling tower is to cool water. However, it cools the water by rejecting the
heat, through an evaporative/sensible process cooling, to the ambient or outdoor air.
The cooling tower approach temperature is the difference between the leaving water temperature and the ambient air wet-b ulb temperature.
An example shows what happens to the ambient air and the entering water
as they pass through the cooling tower. Consider a 300 ton cooling tower that
can cool 900 gpm of water from 95°F to 85°F, The heat being rejected by the
water is
q = 500 x 1:11 x gpm for water
~ 500 x 10' F x 900
~ 4,500,000 Btulh
88
Chapter 9
Special Applications and Psychro metric Considerations
The entering amb ient air to the cooling tower is Idb = JOsoF and t ll'b = 78°F.
This tower moves 60,300 efm of air, which leaves the tower almost at the saturation line on the psychrometri c chart. Therefore, the air must pick up
4,500,000 Btulh, as shown by the total heat required equation discussed in
Chapter 4:
q, ~ 4.5 x cfm x (h, -
h2)
where hI is the entha lpy at tdb = JOs oF and ' wb = 78°F, or h = 42 BtuJlb dw
Therefore,
4,500,000 ~ 4.5 x 60,300 x (42 - h 2)
"2 ~ 58.6 BtuJlbda
So, the leaving air te mperature is tdb = 92.5°F and ' \Vh = 92.3 °F, or almost
saturated air. Note that the cooli ng tower approach is the difference between
the leaving water temperature (85 °F) and the 78°F entering wet-bulb ambient
temperature, or ]OF.
See Figure 9-1 fo r the detai ls of the air condition as it flows through the
cooli ng tower. Note that part of the process is sensible cooli ng, but the maj ority
is latent heat be ing added to the ambient air as the tower water is cooled.
Also note that the entering ambient air can be anywhere on th e Idb = 78°F
wet-bulb line and we will get the same results. The only difference is the
amount of latent heat and the amount of sensible heating or coo ling that takes
place as the air moves through the coo li ng tower.
Leaving Cooling Tower ---j.
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Psychrometries of air through a cooling towe r.
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Fundamentals of Psychrometries (I-P), Second Edition
89
Indoor Swimming Pools
From a design prospective, the indoor air dry-bulb temperature for indoor
swimming pools or natatoriums used for recreational purposes should be the
same temperature as the pool water temperature. That way, the amount of pool
water lost to evaporation into the pool enclosure is reduced. However, ifthis is not
possible, do not allow tdb to be greater than +4°F above the water temperature.
The range for recreational pool water temperature is 75°F to 85°F, with a
recommended $ = 50% rh to 60% rh. As an example in this section, we wi ll
design around ldb = 800F and ~ = 55% rh. A specially designed unit called a pool
dehumidifier is used to provide dehumidification, reheat, and the proper amount
of outdoor ventilation air as shown in Figure 9-2. The unit also has the capabil ity
to provide auxi liary heat (of wintertime outdoor air), pool water heat (energy
saver), an external refrigerant condenser (reject heat outdoors), and energy or
heat recovery devices as explained in Chapter 8. It is also acceptable to return
the water condensed by the dehumidi fyi ng coil back to the swimming pool.
The amount of outdoor ventilation a ir required is 0.48 cfm/ ft 2 of total area,
which comes from ANSVASHRAE Standard 62. 1 (ASH RAE 2013). Total area
is defined as the pool surface plus the deck area around the pool if it gets wet
during normal operat ion. I f this indoor pool area is connected to or is part of a
larger building, then it should be at a sli ghtly negative pressure to the rest of the
building (-0.05 in. of water). This wi ll ensure that the chlorine odor and the
high-moisture-content air do not get into the rest of the bui lding.
Take care to ensure that the building envelope is designed to handle the
high-dew-point indoor air. Exterior windows are discouraged in cold winter
design areas, as they will sweat excessively and cause damage.
For our example, the indoor pool is at tdb = 80°F and ~ = 55% rh . Note that
the indoor dew point is ldp = 62°F. The pool dehumidification unit cools and
OUTDOOR AIR
AIR FILTER
REFRIGERANT REHEAT COIL
AUXILIARY HEATI NG
BLOWER
COIL
DAMPER
AIR--,
FILTER
COMPRESSOR
o/'-;{----',
==:>
ENT ERING
SUPPLY
AIR=:>
DRIP PAN
AIR
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COIL
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Figure 9-2
Single -blowe r pool dehumidifier.
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WATER
POOL WATER HEATER
90
Cha pte r 9
Special Applications and Psychrometric Considerations
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dehumidifies the air first and then reheats the air to meet the psychrometric
needs. From a load ca lculation, the room sensi ble heat ratio (S HR) for this
example is 0.5. So the air is cooled from tdb = 80°F down to ldb = 55°F and
room mo isture is removed in the process. Then the air is reheated from 'db =
55°F up to 'db = 65°F to intersect the SHR line on the psychrometric chart and
balance the sensible and latent cooling processes. See Figure 9-3 for the actual
pool dehumidification and reheat process.
Also note that to provide for the fu ll heating load at winter design, both the
refri geration reheat coil and the aux ili ary heat wi ll be used to warm the air to
the design supply air temperature. Conversely, for the summer design, dehumidification will be needed. So the supply air temperature of Idb = 55°F will
cover the cooling des ign load without any reheat. This then requires a second
refrigerant condenser to reject the hea t to the ambient air.
Cleanrooms
The need fo r c\eanrooms has expanded greatly over time. They are used in
manufacturing facilities for microprocessors, pharmaceuti cals, medical products, and variuus electronic devices. The commun requirement uf these fac ilities is a clean area using high-effic iency particle arrestor air fi ltration with
precise dry-bulb temperature and rel ative humidity control.
A c1eanroom HVAC system is divided into two subsystems with different
functions. First is the makeup air system, which provides preconditioned outdoor air to the c1eanroom area because a large amount of ex haust air is typicall y removed in the manufacturi ng process. The second subsystem is the
Funda me ntals of Psychrometries (I-P), Second Edition
HEPA
Air
Filter
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91
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recirculating room air handlers that slightl y cool and filter the room air, at the
same time maintaining an airflow rate in the room.
As an example, a cleanroom is designed at summer conditions of tdb =
70°F and $ = 45% rh, which is means a tdp of 48 °F. The design outdoor air is
(db = 105°F and ' \Vb = 78°F. Therefore, the makeup air handler must cool this
hot/humid summer design air to something less than (dh = 48°F, because the
recirculation air hand lers pe rform on ly sensible cooling. Or, sa id another way,
the makeup air has to remove all the outdoor air latent load plus any room
latent load pri or to the air being mixed into the c1eanroom .
Because most c1eanrooms have very few people working in them at any
given time and the manufacturing tool load is mostly a sensible load, the latent
load from the c1eanroom is typica ll y small. For thi s example, by cooling the ai r
down to 'db = 45°F, we ca n handle the c1eanroom latent load from our latent
load calculations.
The makeup air hand ler must also be able to add humidity to the air when
the outdoor air is dry, as we ll as heat the air to near room conditi on in the winter. These are typi cally very large and long ai r handl ers, because they perform
many funct ions on the outdoor airstream. Note in Figure 9-4 all the components necessary to prov ide preconditioned outdoor air.
Let's look at the psychrometries of thi s make-up air handler at the summer
des ign conditions. The psychrometric chart for this unit is shown in Fi gure 9-5.
All three cooling coils and the reheat coil are used to precondition the air
before mixing it with the c1eanroom recirculation air. A summary of the fou r
coils follows:
I. Precool coil takes the 100% outdoor air from Idb = 105°F down to (db =
95°F with a process coo ling loop water supply a t 86°F.
92
Cha pte r 9
Special Applications and Psychrometric Considerations
90
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Make up air handler psychrometries at summe r design conditions.
2. Chilled-water coi l then takes the air from 'db = 95 °F down to 'db = 52°F
with the chilled-water loop at 42° F.
3. Glycol (antifreeze) cooling coi l then takes the air from Idh = 52°F down to
ldb = 45 °F with a water/g lycol solution at 35°F.
4. Reheat coil then heats the air from {db = 45 °F up to 'db = 66°F with the
return water from the process coo ling loop that is at about 95 °F. This is a
huge energy-saving feature to use the return water as a heating source and
thus cool the return water in the process.
This precond itioned outdoor air is then mixed with the c1eanroom return air
that is at Idb = 700F and IP = 45% rh at the inlet of the recirculation air-hand ling
un its. If the relative hum idity gets to below 45%, then the glycol coil raises its
supply air temperature a degree or two to bring the c1eanroom back up to q, =
45 % rho
At any outdoor condition , th is makeup air handler has the components to
bring the outdoor a ir to the de sired mixed condition before it is introduced into
the cleanroom.
The c1eanroom rec ircu lation units are very si mple: they consist of a fan , a
small chilled-water coil, and a set of air prefilters. The actual c1eanroom higheffic iency particle arrestor filters are in the cei ling of the c1eanroom and provi de airflow at a high ve locity through the room. Because they are sensiblecooling-only units and have a very small temperature drop, Idb = 70°F to tdb =
66°F, or only 4°F of cooling, there is no temperature deviation in the cleanroom. The psychrometries of the rec irculation c1eanroom air handler are shown
in Figure 9-6.
Funda me ntals of Psychrometries (I-P), Second Edition
93
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Recirculation clean room air handler psychrometries.
The mixing of the preconditioned makeup air and the return air from the
cLeanroom is int eresting in that the ratio of tile room/makeup air is typically 20/ 1
to 50/ 1, depending on the process in the c1eanroom. Therefore, the mixed condition is only reduced a fe w tenths of a degree in the dry bulb temperature. The
room temperature sensor controls the chilled-water valve in the recirculation air
handler to change the leav ing air temperature slightly if needed.
Direct Evaporative Cooling
Direct evaporati ve cooling can be used very effectively in the hot and dry
climates of the world to provide for human comfort. In direct evaporative cooling, the airstream is 100% outdoor air and in contact with water. As some water
evaporates, it lowers the dry-bu lb temperature of the airstream, cooling the air.
The process of direct evaporative cooling is a constant-wet-bulb-temperature
process, as shown in Figure 9-7.
For example, consider outdoor air conditions of tdb = 100°F and $ = 5% rh,
in which the process goes up and to the left on the 61 °F wet-bu lb line. If the
direct evaporative coo li ng has a 95% efficiency, then the leaving air temperature will be tdb = 63°F and ~ = 90% rho We can also calculate the condition as
fo llows:
Evaporative effect = (EATdb - EAT wb ) x Efficiency
37°F ~ (lOO°F - 61 OF) x 0.95
94
Cha pte r 9
Special Applications and Psychrometric Considerations
....
II •
.028
.024
....
.020
:r
c
3
;;:
4S
.016 ~.
...
~
.012
'0%
~
o·
.008
3S
"
,.
,.
'" lS
~
'00
1S
"
Dry Bulb
Figure 9-7
.004
30
'"
Psychrometries of d irect evaporative cooling.
LA T = EATdb - Evaporative effect
~ 100' - 37'
~ 63 ' F
where
EAT
entering air temperature
LAT
leav ing air temperature
So, if the room has a high sensible load and a very low latcnt load, we can
keep the room conditions at 'db = 75°F and 4> = 60% rh with a fairly flat SHR line.
Indirect Evaporative Cooling
Indirect evaporative coo ling is simply cooling the air with a cooling co il and
then using the evaporative process to cool the water that goes through the cooling co il. By definition, then, indirect evaporative cooling is not as effic ient as
direct evaporative cooling because two heat transfers take place in the process.
Take the direct evaporative cooling example: we can make 63 °F water in this
process and we waste the cool air back to ambient. We take this 63 °F water to a
cooling coil and we can make tdb = 68°F air with Idb = 75 °F air entering the
coil. Again, if our cooling load is mostly/all sensi bl e and our airflow is high
enough, we can mainta in the room at ' db = 75°F and $ = 50% rho See Figure 9-8
fo r the psychrometries of the indirect evaporative cooling process.
Funda me nta ls of Psychrome tries (I-P), Second Edition
95
Dry Bulb
90
100
11O
.028
.024
.020
"3
c
c:
.016 ~.
•o·
~
"
.012
20
2<," .008
"
35
.,
10
20
15
"
Figure 9-8
~
~
ro
~
~
Dry Bulb
,~
.GO<
" '"
Psychrometries of indirect evaporative cooling.
However, w hen used together with 100% outdoor air, the leaving air temperature can be lowered by 10°F. We use the same outdoor conditions of (db =
I GO°F and $ = 5% rh , but our efficiency is onl y 50% at best.
We can use the same formula as be fore so our ind irect section can de li ver
Evaporative effect = (EATdb - EAT wb ) x Effi ciency
19' F ~ (I 00' F - 61 ' F) x 0.50
LAT = EATdb - Evaporati ve effec t
~ 100' F -
19' F
= 8 1°F
Indirect evaporati ve cooling can be used in seri es with direct evaporati ve
coo ling. Air from the indi rect sect ion can now enter the direct evaporati ve secti on at (db = 81 °F and $ = 10% rh and move up the wet-bulb line of 53°F with a
leaving air condition of tdb = 55°F and $ = 90% rhoThi s is now a much better
leaving air cond ition, as we can easil y maintain room conditions of (db = 75°F
and $ = 50% rh o
Evaporative effect = (EATdb - EAT wb ) x Efficiency
26' F ~ (8 1' F - 53' F) x 0.95
LAT = EATdb - Evaporative effec t
~ 81' F ~ 55' F
26'F
96
Cha pte r 9
Special Applications and Psychrometric Conside rations
Dry Bulb
90
100
.028
.024
....
.020 I
c
3
4S
0:
.016 ~.
...
,
~
..
25
..,
~.
.012 0
.008
Outodoof 35
Di"~t
10
15
"
Figure 9-9
~
~
Indirect
~
w
'00
25
20
ro
w
.004
30
Dry Bulb
'"
Psychrometries of indirect a nd d irect evaporat ive cooling in series.
Waste Air
I
Air
Outdoor Air
Indirect
~
Evaporative
Filters
Section
Direct
->
Evaporative
Section
->
,,,
Supply Air
~
Supply
I
Fig ure 9-10
Air ha nd ler with indirect and d ir ect eva porative cooling sectio ns.
So, as you can see, the combination of both indirect and direct evaporative
coo ling in series can deliver ai r that ca n provide for a comfortable room without mechani cal refrigeration.
See Figure 9-9 fo r a plot of indirect and direct evaporative coo ling in seri es.
Figure 9- 10 shows the component arrangement for an air hand ler with both
indirect and direct evaporative cooling. The waste air is the air that provides
cooling to the one side of the indirect heat exchanger.
Reference
ASHRAE. 20 13. ANSIIAS HRAE Standard 62. 1-20 13, Ventilation/or acceptable indoor air quality. Atlanta: ASH RAE.
Fundame ntals of Psychrometries (I-P), Second Edition
97
Skill Development Exercises for Chapter 9
Complete these questions by writing your answers on the worksheets at the back a/this book.
9-1
A cooling tower needs to reject heat from 1200 gpm of water entering at 95°F
and leaving at &5°F. What is the total heat req uired to be rejected?
a) 6,000,000 Btulh
b) 600,000 Btulh
c) 5,400,000 Btulh
d) 4,5 00,000 Btulh
9-2
From Exercise 9-1 , if the cooling tower has an airflow of 100,000 cfm and
ambient air conditions of tdb = 85° F and twb = 75°F, what are th e leaving air
conditions of the tower?
a) tdb ~ 85'F, twb ~ 84.8'F
b) tdb = 89°F, twb = 88°F
c) tdb = 87°F, tll'b = 86.8°F
d) 'db = 86° F, lll'b = 84°F
9-3
What is the cooling tower approach temperature fo r the cooli ng tower in Exercise 9-2?
a) 7' F
b) 12'F
c) 8' F
d) 10'F
9-4
In the des ign of an indoor sw imming pool, it is best to keep the swimming pool
water temperature and the room temperature as far apart as comfortab ly possibl e.
a) True
b) False
9-5
In a cleanroom with design conditions of tdb = 68°F and $ = 40% rh , the
makeup air must be cooled to what dry-bulb temperature or the relative humidity will not be met?
a) tdb ~ 55' F
b) tdb ~ 68 ' F
c) tdb ~ 43'F
d) tdb ~ 40' F
98
Chapter 9
9-6
Special Applications and Psychrometric Considerations
If we cool the air via direct evaporative cooling from Idb = llOoF and q, =
2% rh, what is the lowest leavi ng air temperature we can achieve?
a) 'db ~ 62°F
b) 'db ~ 68 ° F
c) 'db ~ 64°F
d) 'db ~ 55°F
9-7
In Exercise 9-6, if our evaporative efficiency is 80%, what are the leaving air
conditions?
a) 'db ~ 68°F and ~ ~ 70% rh
b) 'db ~ 70°F and ~ ~ 70% rh
c) 'db ~ 7I.soFand~ ~ 58%rh
d) 'db ~ 79°F and ~ ~ 50% rh
9-8
In Exercises 9-6 and 9-7, if the room sensible heat ratio is 0.9, what is the
expected room relative humidity i f the room is at (db = 75°F?
a)
~ ~ 53% rh
b) ~~60%rh
c)
~~5 0 %r h
d) Cannot maintain room at 'db = 75 °F with thi s leaving condition
9·9
If we use the same outdoor conditions of {db = 11 0°F and tV = 2% rh from Exercise 9-6 and an indirect evaporati ve cooling section of 40% efficiency, what are
the leaving air conditions from thi s secti on?
a) 'db ~ 91°F and ~ ~ 3% rh
b) 'db ~ 88°F and ~ ~ 20% rh
c) 'db ~ 95°F and ~ ~ 5% rh
d) 'db ~ 85°F and ~ ~ 5% rh
9-10
!fwe add a direct evaporative cooling section in series downstream of the indirect section in Exerci se 9-9 and the direct secti on has an efficiency of 70%,
w hat are the leaving air conditions?
a) 'db ~ 61°F and ~ ~ 95% rh
b) 'db ~ 60°F and ~ ~ 65 % rh
c) 'db ~ 65°F and ~ ~ 60% rh
d) 'db ~ 65°F and ~ ~ 52% rh
Appendix A.Thermodynamic
Properties of Moist Air
Table A-I
Th ermodyna mic Properties of j\:l oist Air a l Standard Atmospheric Press ure, 14.696 psia
,
Humidit y Ratio
Specific Volum e, ft~/lbdQ
11"$' Ib ~jlbdQ
" Ja
"a.
".
II JQ
- 80
O. {)()()()()4 9
9553
9553
- 79
- 77
0.0000053
0.0000057
0.0000062
9.578
9.603
9.6 29
9.578
9.604
9.629
19.218
- 18.977
- 18.737
- 18.497
0 .005
0 .005
0 .006
0 .006
19.213
- 18.972
- 18.731
- 18.490
-0.04593
-0.04530
-0.04467
-0.04404
-0.04592
-0.04528
-{).04465
-0.04402
- 76
0.0000067
9.654
9.654
- 18.256
0.007
- 18.250
-0.0434 1
-0.04339
- 76
- 75
- 74
- 73
- 72
- 71
0.0000072
0.0000078
0.0000084
9.680
9.705
9.730
9.756
9.78 1
- 18.01 6
- 17.776
- 17.295
- 17.055
0 .007
0 .008
0 .009
0 .009
0 .010
- 18.009
- 17.768
- 17.527
- 17.286
- 17.045
-0.04279
-0.042 16
-0.04 154
-0.04092
-0.04030
-0.04277
-{).042 14
-0.04 152
-{).04090
-{).04027
- 75
0.0000097
9.680
9.705
9.730
9.756
9.781
0.0000104
0.0000112
0.0000120
0.0000 129
0.0000139
0.0000149
0.0000 160
0.0000 172
0.0000184
0.0000 198
9.806
9.832
9.857
9.882
9.908
9.933
9.958
9.984
10.009
10.034
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0000
0.000
0000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0000
0.000
9.806
9.832
9.857
9.882
9.908
9.933
9.959
9.984
10.009
10.035
- 16.815
- 16.574
- 16.334
- 16.094
- 15.853
- 15.613
- 15.373
- 15.132
- 14.892
- 14.652
0 .0 11
0 .0 12
0 .0 12
0 .0 13
0 .0 14
0.01 5
0 .0 17
0 .018
0 .019
0 .020
- 16.804
- 16.563
- 16.321
- 16.080
- 15.839
- 15.598
- 15.356
- 15.115
- 14.873
- 14.632
-0.03968
-0.03907
-0.0384:5
-0.03784
-0.03723
-0.03662
-0.03601
-0.0354 1
-0.03480
-0.03420
-{).03966
-{).03904
-{).03842
-{).03781
-{).037 19
-{).03658
-{).03597
-{).03536
-{).03475
-{).0341 4
0.00002 12
0.0000227
0.0000243
0.0000260
0.0000279
0.0000298
0.00003 19
0.000034 1
0.0000365
0.0000390
10.060
10.085
10. 11 0
10.136
10. 161
10.186
10.212
10.237
10.262
10. 288
0.000
0000
0.000
0000
0.000
0.000
0.00 1
0.00 1
0.00 1
0.00 1
10.060
10.085
10.1 11
10.136
10.16 1
10.187
10.212
10.237
10.263
10.288
- 14 .41 2
- 14.171
- 13.931
- 13.691
- 13.451
- 13.2 10
- 12.970
- 12.730
- 12.490
- 12.249
0 .022
0 .023
0 .025
0 .027
0 .029
0 .031
0 .033
0 .035
0 .038
0 .040
- 14.390
- 14.148
- 13.906
- 13.664
- 13.422
- 13.180
- 12.937
- 12.695
- 12.452
- 12.209
-0.03360
-0.03300
-0.03240
-0.03 180
-0.03 120
-0.0306 1
-0.03002
-0.02942
-0.02883
-0.02825
-{).033S 4
-{).03293
-{).03233
-{).03 173
-{).03113
-{).03053
-{).02993
-{).02933
-{).02874
-{).028 14
- 51
0.00004 16
0.0000445
0.0000475
0.0000507
0.000054 1
0.0000577
0.00006 15
0.0000656
0.0000699
0.0000744
10.3 13
10.338
10.364
10.389
10.41 4
10. 439
10.465
10.490
10.5 15
10.54 1
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.00 1
0.00 1
0.00 1
10.3 14
10.339
10.364
10.390
10.4 15
10.440
10.466
10.49 1
10.517
10.542
- 12.009
- 11.769
- 11.529
- 11.289
- 11.048
- 10.808
- 10.568
- 10 .328
- 10.087
- 9 .847
0 .043
0 .046
0 .049
0 .053
0 .056
0.060
0 .068
0 .073
0.078
- 11.966
- 11.723
- 11.479
- 11.236
- 10.992
- 10.748
- 10 .504
- 10.259
- 10.0 15
- 9 .770
-0.02766
-0.02707
-0.02649
-0.0259 1
-0.02532
-0.02474
-0.024 17
-0.02359
-0.0230 1
-0.02244
-{).02755
-{).02695
-{).02636
-{).02577
-{).025 18
-{).02459
-{).02400
-{).0234 I
-{).02283
-0.02224
-49
-48
-47
-46
-45
-44
-43
-42
-4 1
0.0000793
0.0000844
0.0000898
0.0000956
0.000 10 17
0.000 108 1
0.000 11 50
0.000 1222
0.000 1298
0.0001379
10.566
10.591
10.617
10.642
10.667
10.693
10.718
10.743
10.769
10.794
0.00 1
0.00 1
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
10.567
10.593
10.6 18
10.644
10.669
10.695
10.720
10.745
10.77 1
10.796
- 9.607
- 9.367
- 9 .127
-8.886
- 8.646
-8.406
- 8.166
- 7.926
- 7.685
- 7.445
0 .083
0 .088
0 .094
0 .1 00
0 .1 06
0 .113
0 .120
0 .128
0 .136
0 .144
- 9.524
- 9.279
- 9.033
- 8.787
- 8.540
- 8.293
- 8.046
- 7.798
- 7.550
- 7.301
-0.02187
-0.02 129
-0.02072
-0.020 15
-0.0 1959
-0.0 1902
-0.01846
-0.01789
-0.0 1733
-0.0 1677
-{).02166
-{).02107
-0.02049
-{).01990
-{).01932
-{).01874
-{).018 16
-{).01757
-{).01699
-{).01641
Temp .• of
- 78
- 70
-69
-68
- 67
-66
- 65
-64
-6]
-62
-61
-60
- 59
- 58
- 57
- 56
- 55
- 54
- 53
- 52
- 51
- 50
-49
-48
-47
-46
-45
-44
-43
-42
-41
-40
- 39
- 38
- ]7
- 36
- 35
- 34
- ]]
- 32
- ll
0.0000090
Spec ific [nlh a lp)'. Blull bda
- 17535
a
" •
0 .064
h.,
Specific Entropy. Blullbda ,o F Temp., of
~'JQ
'.
-80
- 79
- 78
- 77
- 74
- 73
- 72
- 71
- 70
- 69
-<i'
- 67
-66
- 65
-64
- 63
- 62
-6 1
-w
- 59
- 58
- 57
- 56
- 55
- 54
- 53
- 52
- 50
-40
- 39
- 38
- 37
- 36
- 35
- 34
- J]
- 32
- ll
100
Appendix A Thermodynamic Properties of Moist Air
Table A-I
Temp., ° F
Thermody namic Properties of Moist Air al Standard Atmospheric Ilrcssurc, 14.696 psia (Colllilflled)
Hu midit y Rat io
Ws' I b ~JlbdQ
SpK ific Vol u me. ftJll bJQ
" Ja
"a.
S pocilic [ nth a lll}', Blufl bda
",
" at
h,
- 7.205
-6.965
- 6.725
- 6.485
- 6.244
- 6.004
- 5.764
- 5.524
- 5.284
- 5.044
0.153
0.163
0.173
0.184
0.195
0.207
0.219
0.233
0.246
0.261
- 7.052
-6.802
-6.552
-6.301
-6.050
- 5.797
-0.01621
-0.01565
-0.01509
- 5.545
- 5.291
- 5.037
-4.782
-0.01288
-0.01233
-0.01178
-0.01 123
-0.01234
-0.01176
-0.01118
-0.01060
-4.803
-4.563
--4.323
--4.083
- 3.843
- 3.602
- 3.362
- 3.122
- 2.882
- 2.642
0.277
0.293
0.310
0.329
0.348
0.368
0.389
00412
00436
00460
00487
0.514
0.543
0.574
-4.527
-4.270
-4.013
- 3.754
-0.01068
-0.01014
-0.00959
-0.00905
-0.00851
-0.00797
-0.00743
-0.00689
-0.00635
-0.00582
-0.00528
-0.00475
-0.00422
-0.00369
-0.00316
-0.00263
-0.002 10
-0.00157
-0.00105
-0.00052
-0.01002
-0.00943
-0.00885
-0.00826
-0.00768
-0.00709
-0.00650
-0.00591
-0.00532
-0.00473
-0.00414
-0.00354
-0.00294
-0.00234
-0.00174
-0.00114
-0.00053
0.00008
0.00069
0.00130
"
- 30
- 29
- 28
- 27
- 26
- 25
- 24
- 23
- 22
- 21
- 20
- 19
- 18
- 17
- 16
- 15
- 14
- lJ
- 12
- II
- 10
-9
-8
-7
-6
-5
-4
-J
-2
-I
0
1
2
J
4
5
6
7
8
9
\0
II
12
lJ
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
32
33
34
35
36
37
38
39
0.0001465
0.0001555
0.0001650
0.0001751
0.0001857
0.0001970
0.0002088
0.0002213
0.0002345
0.0002485
10.819
10.845
10.870
10.822
10.847
10.873
10.920
10.946
10.971
0.003
0.003
0.003
0.003
0.003
0.003
0.004
10.975
10.996
11.022
11.047
0.004
0.004
0.004
11.000
11.026
11.05 I
10.895
0.0002632
0.0002786
0.0002949
0.0003121
0.0003302
0.0003493
0.0003694
0.0003905
0.0004127
0.000436 1
0.0004607
0.0004866
0.0005138
0.0005425
0.0005725
0.000604 1
0.0006373
0.0006721
0.0007087
0.000747 1
11.072
11.098
11.123
11.148
11.174
11.199
11.224
11.249
11.275
11.300
11.325
11.351
11.376
11.401
11.427
11.452
11.477
11.502
11.528
11.553
0.0007875
0.0008298
0.0008741
0.0009207
0.0009695
0.0010207
0.0010743
0.0011306
0.0011895
0.0012512
10.898
10.924
10.949
0.007
0.007
0.007
0.008
0.008
0.009
0.009
0.010
0.010
0.011
0.012
0.012
0.013
0.014
11.077
11.103
11.128
11.154
11.179
11.205
11.231
11.257
11.282
11.308
11.334
11.360
11 .385
110411
11.437
11.463
11.489
11.515
11.541
11.567
11.578
11.604
11.629
11 .654
11.680
11.705
11.730
11.755
11.781
11.806
0.015
0.015
0.016
0 ,017
0.018
0.019
0.020
0.021
0.022
0 ,024
11.593
11.619
11.645
11.671
11.698
11.724
11.750
11.777
11.803
11.830
0.000
0.0013158
0.00 13835
0.0014544
0.0015286
0.0016062
0.0016874
0.0017724
0.00186 13
0.00 19543
0.00205 15
11.831
11.857
11.882
11.907
11.933
11.958
11.983
12.008
12.034
12.059
0.025
0.026
0.028
0.029
0.031
0.032
0.034
0.036
0.038
0.0021531
0.0022593
0.0023703
0.0024863
0.0026075
0.0027340
0.0028662
0.0030042
0.003 1482
0.0032986
12.084
12.110
12.135
12.160
12.185
12.211
12.236
12.261
12.287
12.312
0.0034555
0.0036192
0.0037900
0.003790
0.003947
0.004109
0.004278
0.004452
0.004633
0.004821
0.005015
12.337
12.362
12.388
12.3877
12.4130
1204382
1204635
1204888
12.5141
12.5394
12.5647
0.005
0.005
0.005
0.006
0.006
0.006
Specifi c Entropy, Blull bdu·o F Tem p_, ° F
l1a
- 30495
'"
--{).O1454
-0.01398
-0.01343
"
-0.01583
-0.01525
-0.0]467
-0.01409
-0.01351
--{l.OI293
0.639
0.675
0.712
0.751
0.792
- 3.234
- 2.973
- 2.710
- 2.446
- 2.181
- 1.915
- 1.647
- 1.378
- 1.107
-0.835
-0.561
-0.286
-0.009
0.271
0.552
0.835
1.121
10408
1.698
1.991
2.286
2.583
2.884
3.187
3.494
0.00000
0.240
00480
0 .720
0.961
1.201
1.441
1.681
1.921
2.161
0.835
0.880
0.928
0 .978
1.030
1.085
1.142
1.203
1.266
1.332
0.00052
0.00104
0.00156
0.00208
0.00260
0.00311
0.00363
0.00414
0.00466
0.00192
0.00254
0.00317
0.00379
0.00443
0.00506
0.00570
0.00635
0.00700
0.00766
11.856
11.883
11.910
11.936
11.963
11.990
12.017
12.044
12.071
12.099
20402
2.642
2.882
3.122
3.362
3.603
3.843
4.083
4.323
4.563
1.401
10474
1.550
1.630
1.714
1.801
1.892
1.988
2.088
2.193
3.803
4.116
4.752
5.076
50403
5.735
6.071
60411
6.756
0.00517
0.00568
0.006 19
0.00670
0.00721
0.00771
0.00822
0.00872
0.00923
0.00973
0.00832
0.00898
0.00965
0.01033
0.01102
0.01171
0.01241
0.01312
0.01383
0.01455
4.803
0.048
0.051
0.054
0.056
0.059
0 ,062
0.065
12.126
12.153
12.181
12.209
12.236
12.264
12.292
12.320
12.349
12.377
2.303
20417
2.537
2.662
2.793
2.930
3.073
3.222
3.378
3.541
7.106
7.461
7.821
8.186
8.557
8.934
9.317
9.707
10.103
10.506
0.01023
0.01073
0.01 123
0.01 173
0.01222
0.01272
0.01321
0.01371
0.01420
0.01469
0.01528
0.01602
0.01677
0.01753
0.01830
0.01908
0.01987
0.02067
0.02148
0.02231
0.068
0.072
0.075
0.0753
0.0786
0.0820
0.0855
0.0892
0.0930
0.0969
0.1010
120405
12.434
12.463
1204630
1204915
12.5202
12.5490
12.5780
12.6071
12.6363
12.6657
3.711
3.888
4.073
4.073
4.244
40420
4.603
4.793
4.990
5.194
50405
10.916
11.334
11.759
11.759
12.169
12.586
13.009
130439
13.877
14.321
14.772
0.015 18
0.01567
0.01616
0.016 16
0.01665
0.017 14
0.01762
0.018 11
0.01859
0.01908
0.01956
0.02315
0.02400
0.02486
0.02486
0.02570
0.02654
0.02740
0.02827
0.02915
0.03004
0.03095
0.040
0.042
0.044
0.046
- 20402
- 2.161
- 1.921
- 1.681
- 1.441
- 1.201
-0.961
- 0.720
- 0.480
-0.240
5044
5.284
5.524
5.764
6004
6.244
60485
6.725
6.965
7.205
70445
7.686
7.686
7.926
8.166
80406
&.646
8.887
9.127
9.367
0606
40432
,
- 30
- 29
- 28
- 27
- 26
- 25
- 24
- 23
- 22
- 21
- 20
- 19
- 1&
- 17
- 16
- 15
- 14
- lJ
- 12
- II
- 10
-9
-8
-7
-6
-5
-4
-3
-2
-I
0
1
2
;
4
5
6
7
&
9
\0
II
12
lJ
14
15
16
17
1&
19
20
21
22
23
24
25
26
27
2&
29
30
31
32
32
33
34
35
36
37
3&
39
Fundamentals of Psychrometries (I-P), Second Edition
Table A- I
Temp., OF
40
101
Thermodynamic Properties of Moist Air at Standard Atmospheric Pressu re, 14.696 psia (COlllilflled)
Humi dil y Ralio
Ws , Ib ~JlbJQ
SpK ific Volume. n Jll bJQ
" Ju
"a.
Spocific [ nlh a lll}" Bluflb JQ
",
Specifi c Enlropy, BluflbJu·o F Temp., OF
at
h,
"
"
'"
"
I1Q
,
0.005216
0.005425
0.005640
0.005864
0.006095
0.006335
0.006582
0.006839
0.007 104
0.007379
12.5899
12.6152
12.6405
12.6658
12.6911
12.7163
12.7416
12.7669
12.7922
[2.8[75
0.1053
0.1097
0.1143
0.1191
0.1240
0.1292
0.1345
0.1400
0.1457
0.15[6
12.6952
12.7249
12.7548
12.7849
12.8151
12.8455
12.8761
12.9069
12.9379
12.9691
9.607
9.848
10.088
10.328
10.568
10.808
11.049
11.289
11.529
1 [.769
5.625
5.852
6.087
6.331
6.583
6.844
7.115
7.395
7.685
7.985
15.232
15.699
16.175
16.659
17.151
17.653
18.164
18.684
19.2[4
[9.755
0.02004
0.02052
0.02100
0.02 148
0.02 196
0.02243
0.02291
0.02338
0.02386
0.02433
0.03187
0.03280
0.03375
0.03472
0.03570
0.03669
0.03770
0.03873
0.03978
0.04084
40
0.007663
0.007956
0.008260
0.008574
0.008899
0.009235
0.009582
0.009940
0.010311
0.010694
0.011089
0.01 1498
0.011921
0.012357
0.0 12807
0.013272
0.013753
0.014249
0.014761
0.0 15289
12.8427
12.8680
12.8933
12.9186
12.9439
12.9691
12.9944
13.0197
13.0450
13.0702
13.0955
13.1208
13.1461
13.1713
13. 1966
13.2219
13.2472
13.2724
13.2977
13.3230
0.1578
0.1641
0.1707
0.1776
0.1847
0.1920
0.1996
0.2075
0.2156
0.2241
0.2328
0.2418
0.2512
0.2609
0.2709
0.2813
0.2920
0.3031
0.3146
0.3265
13.0005
13.0322
13.0640
13.0962
13.1285
13.1611
13.1940
13.2272
13.2606
13.2943
13.3283
13.3626
13.3973
13.4322
13.4675
13.5032
13.5392
13.5755
13.6123
13.6494
12.010
12.250
12.490
12.730
12.971
13.211
13.451
13.691
13.932
14.172
14.412
14.653
14.893
15.133
15.373
15.614
15.854
16.094
16.335
16.575
8.296
8.617
8.950
9.294
9.650
10.018
10.399
10.792
11.199
11.620
12.055
12504
12.968
13.448
13.944
14.456
14.986
15532
16.097
16.680
20.306
20.867
21.440
n024
22.621
23.229
23.850
24.484
25.131
25.792
26.467
27.157
27.861
28581
29.3[8
30.070
30.840
31.626
32.431
33.255
0.02480
0.02527
0.02574
0.0262 [
0.02668
0.027 15
0.02761
0.02808
0.02854
0.02901
0.02947
0.02993
0.03039
0.03085
0.03 131
0.03177
0.03223
0.03268
0.03314
0.03360
0.04192
0.04302
0.04414
0.04528
0.04645
0.04763
0.04884
0.05006
0.05132
0.05259
0.05389
0.05522
0.05657
0.05795
0.05936
0.06080
0.06226
0.06376
0.06529
0.06685
SO
0.015835
0.016398
0.016979
0.017578
0.0 18 197
0.018835
0.019494
0.020173
0.020874
0.02 1597
13.3482
[3.3735
13.3988
13.4241
13.4493
13.4746
13.4999
13.5251
13.5504
13.5757
0.3388
0.35[5
0.3646
0.3782
0.3922
0.4067
0.4217
0.4372
0.4533
0.4698
13.6870
13.7250
13.7634
13.8022
13.8415
13.8813
13.9216
13.9624
14.0037
14.0455
16.815
17.056
17.296
17536
17.776
18.017
18.257
18.498
18.738
18.978
17.282
17.903
18.545
19.208
19.892
20598
21.327
22.079
22.855
23.656
34.097
34.959
35.841
36.744
37.668
38.615
39.584
40576
41.593
42.634
0.03405
0.03450
0.03496
0.03541
0.03586
0.03631
0.03676
0.03720
0.03765
0.038 10
0.06844
0.07007
0.07173
0.07343
0.07516
0.07694
0.07875
0.08060
0.08250
0.08444
0.022343
0.023112
0.023905
0.024723
0.025566
0.026436
0.027333
0.02825 7
0.02921 1
0.030193
13.6010
13.6262
[3.6515
13.6768
13.7020
13.7273
13.7526
13.7778
13.8031
13.8284
0.4869
05046
05229
0 .5418
0.5613
0.5814
0.6022
0.6237
0.6459
0.6688
14.0879
14.1308
14.1744
14.2185
14.2633
14.3087
14.3548
14.4015
14.4490
14.4972
19.219
19.459
19.699
19.940
20.180
20.420
20.661
20.901
21.142
21.382
24.482
25.335
26.2[5
27.122
28.059
29.025
30.021
31.049
32.109
33.202
43.701
44.794
45.9[4
47.062
48.239
49.445
50.682
51.950
53.250
54584
0.03854
0.03899
0.03943
0.03988
0.04032
0.04076
0.04 120
0.04 164
0.04208
0.04252
0.08642
0.08845
0.09052
0.09264
0.09481
0.09703
0.09930
0.10163
0.10401
0.10645
99
0.03 1206
0.03225 1
0.033327
0.034437
0.035581
0.036760
0.037976
0.039228
0.040520
0.04185 1
13.8536
13.8789
13.9042
13.9294
13.9547
13.9800
14.0052
14.0305
14.0558
14.0810
0.6925
0.7170
0.7422
0.7683
0.7952
0.8230
0.8518
0.8814
0.9120
0.9436
14.5462
14.5959
14.6464
14.6977
14.7499
14.8030
14.8570
14.9119
14.9678
15.0247
21.622
21.863
22.103
22.344
22584
22.825
23.065
23.305
23.546
23.786
34.329
35.492
36.691
37.928
39.203
40.518
41.874
43.272
44.714
46.201
55.952
57.355
58.795
60.272
61.787
63.343
64.939
66.578
68.260
69.987
0.04296
0.04340
0.04383
0.04427
0.04470
0.045 14
0.04557
0.04600
0.04643
0.04686
0.10895
0.11150
0.11412
0.11681
0.11955
0.12237
0.[2525
0.12821
0.13124
0.13434
100
101
102
10]
104
105
106
101
108
109
0.043222
0.044636
0.046094
0.047596
0.049145
0.050741
0.052386
0.054082
0.055830
0.057632
14.1063
14.1316
14.1568
14.1821
14.2074
14.2326
14.2579
14.2831
14.3084
14.3337
0.9763
1.0100
1.0448
1.0807
1.1178
1.1561
1.1957
1.2365
1.2787
1.3222
15.0826
15.1416
15.2016
15.2628
15.3252
15.3887
15.4535
155[96
15 .5871
15.6559
24.027
24.267
24.508
24.748
24.989
25.229
25.470
25.710
25.951
26.191
47.734
49.315
50.945
52.626
54.359
56.146
57.989
59.889
61.849
63.870
71.76 1
73582
75.453
77.374
79.348
81.375
83.459
85.600
87.800
90.061
0.04729
0.04772
0.048 15
0.04858
0.04901
0.04943
0.04986
0.05028
0.05071
0.05 113
0.1376
0.1408
0.1442
0.1476
0.1511
0.1547
0.1584
0.1622
0.1661
0.1701
41
42
4l
44
45
46
47
48
49
SO
51
52
53
54
55
56
57
58
59
6iJ
61
62
6J
64
6S
66
67
68
69
10
11
12
13
14
15
16
11
18
19
80
81
82
8]
84
85
86
81
88
89
9Q
91
92
9]
94
95
96
91
98
41
42
4l
44
45
46
47
48
49
51
52
53
54
55
56
57
S&
59
60
61
62
63
64
6S
66
67
68
69
10
11
12
13
14
15
16
17
18
19
80
81
82
8]
84
8S
86
87
88
89
90
91
92
9]
94
9S
96
91
9&
99
Il)()
101
102
10]
104
105
106
107
108
109
102
Appendix A Thermodynamic Properties of Moist Air
Table A-I
Temp., ° F
Thermodynamic Properties of Moist Air al Standard Atmospheric Ilrcssurc, 14.696 psia (Colllilflled)
Hu midit y Ratio
Ws' I b ~JlbdQ
110
III
112
III
114
115
116
117
118
119
120
III
122
III
124
115
126
127
118
129
SpKific Vol u me. ftJll bJQ
" Ja
"a.
S pocilic [ntha lll}', Blufl bda
",
l1a
" at
h,
26.432
26.672
26.913
27.154
27.394
27.635
27.875
28.116
28.356
28.597
65.954
92.386
68.104
94.777
97,234
99.762
102.362
105.036
107.787
110.617
113.530
116.529
119.615
122.792
126.064
129.434
132.905
136.481
140.166
143.964
147.879
151.915
"
0.059490
0.061405
0.063380
0.065416
0.0675 16
0.06%80
0.071913
0.074215
0.076590
0.079040
0.081566
0.084173
0.086863
0.089638
0.092503
0.095459
0.098510
0.10 166 1
0. 104914
0.108273
14.3589
14.3842
14.4095
14.4347
14.4600
14.4852
14.5105
14.5358
14.5610
14.5863
14.6116
14.6368
14.6621
14.6873
14.7126
14.7379
14.7631
14.7884
14.8136
14.8389
1.3672
1.4136
1.4615
I.S III
1.5622
1.6150
1.6696
1.7259
1.7842
1.8443
1.9065
1.9707
2.0370
2.1056
2.1765
2.2498
2.3255
2.4038
2.4848
2.5686
(Source: ASHRAf: I/""dbook- Fw/ikmlema(s, C~ap1er I. Table 2)
15.7261
15.7978
15.8710
15.9458
16.0222
16.1003
16.1801
16.2617
16.3452
16.4306
16.5180
16.6075
16.6991
16.7929
16.8891
16.9876
17.0886
17.1922
17.2985
17.4075
28.838
29.078
29.319
29.559
29.800
30.041
30.281
30.522
30.763
31.003
70 .321
72.608
74.967
77.401
79.911
82.502
85.174
87.932
90.777
93.714
96.746
99.875
103.105
106.441
109.885
113.442
117.116
120 .912
Specific Entropy, Blullbdu·o F Te m p_, ° F
'"
0.05 155
0.05197
0.05240
0.05282
0.05324
0.05365
0.05407
0.05449
0.05491
0.05532
0.05574
0.05615
0.05657
0.05698
0.05739
0.05781
0.05822
0.05863
0.05904
0.05945
"
0.1742
0.1784
0.1828
0.1872
0.1918
0.1965
0.2013
0.2062
0.2113
0.2165
0.2219
0.2274
0.2331
0.2389
0.2449
0.2510
0.2574
0.2639
0.2706
0.2776
,
110
III
112
III
114
115
116
117
118
119
120
III
122
12l
114
125
126
127
128
129
Appendix BDimensions, Units, and
Unit Conversion Factors
Table B-1
Dimension
Dimensions and Units Used in Air-Conditioning Applications
SI Unit
I-P U nit
Acceleration
ftis L
Area
ft2
Density
kglm 3
Ib,,/ tr'
Energy
N om, joule (J)
Btu, tUb
Force
(kg-m)/s2, newton (N)
pound (Ib! )
Length
m t:tre (m)
[UUI (ft)
Mass
ki logram (kg)
pound mass (Ibm)
Power
J/5, watt (W)
Btuth
Pressure
N / m2, pascal ( Pa)
pounds per sq uare inch (psi)
Specific heat
J/(kg °C)
Btu/lbm "oF
Time
second (5)
second (s)
Temperature (absolute)
kelvin (K)
degree Rankine (OR)
Temperature
degree Celsius (0C)
degree Fahrenheit (OF)
Thennal cond uctivity
Btulh-ft·oF
Thermal flu x de nsity
W /(m·°C)
W /m 2
Ve loc ity
m/s
ft/s, ftlm in, fpm
Volume
m3
ft3
Volume flow rate
m 3/s
n3/s, 0 3/ min, cfrn
Btulll' ft 2
104
Appe ndix B Dimensions, Units, and Unit Conversion Factors
Table 8-2
Unit Conversion Factors
Dimens ion
SI Un it
I-P Un it
Length
1m
1ft
Area
Volume
I m' ~ 10.76 ft2
I m 3 ~ 35.32 ft3
I ft 2 ~ 0.0929 m 2
I ft3 ~ 0.02 84 m 3
Mass
I kg = 2.205 Ibm
I Ibm = 0.454 kg
Force
I N ~ 0.2248 Ibj
I I bj ~ 4.448 N
Energy
I kJ ~ 0.9478 Btu
I J ~ 0.7376 ft ·lbj
I kWh = 3.412 x 10 3 Btu
I Btu ~ 778.2 ft ·lbr 1.055 kJ
I ft ·lbr 1.356 J
I Btu = 2.930 x 10-4 kWh
I kJ/kg = 0.4298 Btu/Ibm
I Btu/Ibm = 2.3 26 kJ/kg
Power
I W ~ 3 .412 Btu/h
I kW ~ 1.341 hp
I kW = 0. 2844 ton refri geration
I Btull, ~ 0.293 W
I hp ~ 2545 Btu/h ~ 0.746 kW
I ton = 12,000 Btu/h = 3.5 17 kW
Pressure
I Pa = 1.450 x 10-4 psi
I atm = 101 kPa
I psi = 6.89 7 x 103 Pa
I atm = 14.7 psi = 29.92 in. Hg
Specific energy,
Specific enthalpy
3.28 1 ft
0. 305 m
1°C 6.T = 915°F /1T
1°F 6.T = 519°C 6.T
yOC ~ ((9/5)y + 32r F
K = °C + 273. 15
yOF ~ (y - 32)(5/9)OC
OR = of + 459.67
Ve loc ity
I m /s = 1.969 x 102 ftlmin
I ftlmi n = 5.079 x 10- 3 mls
Mass density
I kg/m 3 = 6.243 x 10- 2 Ib,,!ft 3
I Ib,,/ ft ' ~ 16.02 kg/m'
Mass fl ow rate
I kg/s = 2 .205 lbmls
I kg/s = 7. 937 x 103 1bm/h
Volume flow rate
I m 3/s = 2.119 x 103 cfm
I m 3/s = 1.585 x 104 gal/min
I Ib,,/ s ~ 0.4535 kg/s
I Ib,J h = 1.260 x 10-4 kg/s
I cfm = 4.7 19 x 10-4 m 3/s
I gal/mi n = 6. 309 x 10- 5 m3/s
Thermal conductivity
Heat transfer coeffi cient
I W/(m'°C) ~ 0.5 778 Btu/h·ft ·oF
I W /(m 2.oC) = 0. 1761 BtU/h ·ft 2.oF
I Btulh ·ft ·oF ~ 1.73 1 W /(m '°C)
I Btulh ·ft 2 .oF ~ 5.679 W /(m 2.0 C)
Specific heat
I J/(kg'°C) = 2.389 x 10-4 Btu/lbm·oF
I BtU/lbm ·oF = 4.1 86 x 10 3 J/(kg '°C)
Temperature
Appendix CClimatic Design
Information
The climatic design informati on in th is appendix is from Chapter 14 of the
201 3 ASHRAE Handbook- Fundamentals.
106
Appendix C
Climatic Design Information
55
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Appendix C
Climatic Design Information
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Appendix 0 Thermodynamic Properties
of Water at Saturation
T a ble D-I
Te m p ..
OF
Abso lut e
Sal. So lid
E,'ap.
P....·, psia
"ill'l
Vigli'f,
0.01732
0.01732
0.01732
1953807
- 79
- 78
0.000116
0.000125
0.000135
- 77
0.000145
- 76
- 15
- 74
- 73
- 72
- 71
- 70
- 69
-68
-67
-66
-65
-6<
-6]
- 62
-61
-6iJ
- 59
- 58
- 57
- 56
- 55
- 54
- 53
- 52
- 51
- 50
-49
-48
-47
-46
-45
-44
-4]
-42
-41
-40
- 39
- 38
- 37
- ]6
- 35
- ]4
- 3l
- 32
- 31
Sp«ifi c Enth al p)'. 8lul lb~.
Specific Volume. flJll b ...
Press ur('
-SO
Thermodynamic Propert ies of Waler al Satura tion
Saf. Vapo r
[,'ap.
Specifi c E ntropy. Bt u/Ill,., ,o F
OF
2.8048
2.7903
- 80
- 79
- 78
2.7831
- 77
2.7759
2.7688
2.7617
2.7547
2.7477
2.7408
- 76
- 75
- 74
- 73
- 72
- 71
- 70
I,.
Evap.
sjls/
S;iSfg
1219.19
1219.23
1219.28
1025.81
-0.4064
-0.4054
-0.4043
3.2112
3.2029
3.1 946
- 192.19
1219.33
1027.13
3.]864
1457224
1355519
1261390
1174239
1093518
1018724
- 191.80
- 191.40
- 191.00
- 190.60
- 190.20
- 189.80
1219.38
1219.42
1219.4 6
1219.51
1219.55
1219.59
1027.58
1028.02
1028.46
1028.90
1029.35
1029.79
-0.4033
-0.4023
-0.4012
-0.4002
-0.3992
-0.398 1
-0.3971
3. 1782
3.1700
3. 1619
3.1539
3. 1458
3. 1379
949394
885105
825469
770128
718753
671043
626720
585529
547234
511620
949 394
885105
825469
770128
718753
671043
626720
585529
547234
511620
- 189.40
- 189.00
- 188.59
- 188.19
- 187.78
- 187.38
- 186.97
- 186.56
- 186.15
- 185 .74
1219.63
1219.67
1219.71
1219.75
1219.78
1219.82
1219.85
1219.89
1219.92
1219.95
1030.23
1030.67
1031.11
1031.56
1032.00
1032.44
1032.88
1033.33
1033.77
1034.21
-0.3961
-0.3950
-0.3940
-0.3930
-0.39 19
-0.3909
-0.3899
-0.3888
-0.3878
-0.3868
3.1299
3.1220
3.1141
3.1063
3.0985
3.0907
3.0830
3.0753
3.0677
3.0601
2.7338
2.7270
2.7201
2.7133
2.7065
2.6998
2.6931
2.6865
2.6799
2.6733
0.01734
0.01734
0.01735
0.01735
0.01735
0.01735
0.01735
0.01735
0.01735
0.01736
478487
447651
418943
392207
367299
344086
322445
302263
283436
265866
478487
447651
418943
392207
367299
344086
322445
302263
283436
265866
- 185.33
- 184.92
- 184.50
- 184.09
- 183.67
- 183.26
- 182.84
- 182.42
- 182.00
- 181.58
1219.98
1220.04
1220.07
1220.09
1220.12
1220.15
1220.17
1220.19
1220.21
1034.65
1035.09
1035.54
1035.98
1036.42
1036.86
1037.30
1037.75
1038.19
1038.63
-0.3858
-0.3847
-0.3837
-0.3827
-0.3816
-0.3806
-0.3796
-0.3785
-0.3775
-0.3765
3.0525
3.0449
3.0374
3.0299
3.0225
3.0151
3.0077
2.9931
2.9858
2.6667
2.6602
2.6537
2.6473
2.6409
2.6345
2.6282
2.6219
2.6156
2.6093
0.000978
0.001045
0.001115
0.00119 1
0.001270
0.001355
0.001445
0.001540
0.001641
0.001749
0.01736
0.01736
0.01736
0.01736
0.01736
0.01736
0.01736
0.01737
0.01737
0.01737
249464
234148
219841
206472
193976
182292
171363
161139
151570
142611
249464
234148
219841
206472
193976
182292
171363
161139
151570
142611
- 181.16
- 180.74
- 180.32
- 179.89
- 179.47
- 179.04
- 178.62
- 178. 19
- 177.76
- 177.33
1220.24
1220.26
1220.28
1220.29
1220.31
1220.33
1220.34
1220.36
1220.37
1220.38
1039.07
1039.52
1039.96
1040.40
1040.84
1041.28
1041.73
1042.17
1042.61
1043.05
-0.3755
-0.3744
-0.3734
-0.3724
-0.3713
-0.3703
-0.3693
-0.3683
-0.3672
-0.3662
2.9786
2.9714
2.9642
2.9571
2.9500
2.9429
2.9359
2.9288
2.9219
2.9149
2.6031
2.5970
2.5908
2.5847
2.5786
2.5726
2.5666
2.5606
2.5546
2.5487
0.001862
0.001983
0.002111
0.002246
0.002389
0.002541
0.002701
0.002871
0.003051
0.003241
0.01737
0.01737
0.01737
0.01737
0.01738
U.0 1738
0.01738
0.01738
0.01738
0.01738
134222
126363
118999
112096
105624
99555
93860
88516
83500
78790
134222
126363
118999
112096
105625
99555
93860
88516
83500
78790
- 176.90
- 176.47
- 176.04
- 175.60
- 175.17
- 174.73
- 174.30
- 173.86
- 173.42
- 172.98
1220.39
1220.41
1220.4 1
1220.42
1220.4 3
1220.44
1220.4 4
1220.45
1220.45
1220.45
1043.49
1043.94
1044.38
1044.82
1045.26
1045.70
1046.15
1046.59
1047.03
1047.47
-0.3652
-0.3642
-0.3631
-0.3621
-0.3611
-0.3600
-0.3590
-0.3580
-0.3570
-0.3559
2.9080
2.9011
2.8942
2.8874
2.8806
2.8739
2.867]
2.8604
2.8537
2.847 1
2.5428
2.5370
2.5311
2.5253
2.5196
2.5138
2.5081
2.5024
2.4968
2.4911
1I;,lhfg
1814635
1686036
1953807
1814635
1686036
193.38
- 192.98
- 192.59
0.01732
1567159
1567159
0.000157
0.000169
0.000182
0.000196
0.00021 1
0.000227
0.01732
0.01733
0.01733
0.01733
0.01733
0.01733
1457224
1355519
1261390
1174239
1093518
1018724
0.000244
0.000263
0.000283
0.000304
0.000326
0.000350
0.000376
0.000404
0.000433
0.000464
0.01733
0.01733
0.01733
0.01733
0.01734
0.01734
0.01734
0.01734
0.01734
0.01734
0.000498
0.000533
0.000571
0.000612
0.000655
0.000701
0.000749
O.ooogOI
0.ooog57
0.0009 16
"
mom
Sa l. Va por
1026.25
1026.69
31JOO.1
Te mp ..
Sa l. Vapo r
Sal. Solid
Sat. Solid
II;II'f
'.
2.7975
- 69
-68
- 67
- 66
-65
-M
-6]
-62
-<>1
-60
- 59
- 58
- 57
- 56
- 55
- 54
- 53
- 52
- 51
- 50
-49
-48
-47
-46
-45
-44
-4]
-42
-41
-40
- 39
- 38
- 37
- 36
- 35
- ]4
- 3l
- 32
- 31
136
Appe ndix D Thermodynamic Properties of Water at Saturation
Table 0- 1 T hermodyna mic Properties of Water at Satura tion (Comilll/cd)
.,
T emp.,
- 30
- 29
- 28
- 27
- 26
- 25
- 24
- 23
- 22
- 21
- 20
- 19
- 18
A bsolute
Prt'Ssure
Spec ific Volume, fI ]/lb ...
P ..... , psia
Sat. Solid
"jl,'I
0.003442
0.003654
0.003878
0.004115
0.004365
0.004629
0.004908
0.005202
0.005512
0.005839
0.01738
0.01738
0.01739
0.01739
0.0]739
0.01739
0.01739
0.01739
0.01739
0.01740
[,'a p.
Spce ific En th al py, Btullb",
Sa l. Vapo r
ViII/vIII
"
74366
70209
66303
62631
59179
55931
52876
74366
70209
66303
62631
59179
55931
52876
50000
50001
47294
44745
47294
44745
0.0 [740
0.01740
0.01740
0.01740
0.01740
0.0 1740
0.01741
Sat. Solid
";!",
- 172.54
- 172.10
- 171.66
- 171.22
- 170.77
- 170.33
- 169.88
- 169.43
- 168.99
- 168.54
- 168.09
42345
42345
40084
40084
- 167.64
37953
37953
- 167.19
35944
35944
- 166.73
- 17
- I.
34050
34050
- 166.28
- 165.82
- IS
32264
32264
- 14
- 165.37
30580
30580
No/e: Subscript i denotes ,·alues for I "; 32"F and subscripl j denole< valuc< for' ;" J2"F.
- 13
0.009177
0.01741
- 164.91
28990
28990
- 12
0.009700
0.01741
27490
27490
- 164.46
- II
0.010249
0.01741
- 164.00
26073
26073
- 10
0.010827
0.01741
24736
24736
- 163.54
-9
om 1435 0.01741
23473
23473
- 163.08
22279
- 162.62
0.0[2075
-8
0.01741
22279
-7
0.012 747
0.01 742
21151
21152
- 162.15
0.013453
0.01742
20086
20086
- 161.69
- 161.23
-5
U.U14194
0.01741
1907H
1907H
-4
0.014974
0.01742
18125
181Z5
- 160.76
0.015792
0.0 1742
17223
17223
- 160.29
-3
0.016651
0.01742
16370
16370
- 159.83
-2
-I
0.017553
0.01742
15563
15563
- 159.36
0.018499
0.01743
14799
14799
- 158.89
0
1
0.019492
0.01743
14076
14076
- 158.42
2
0.020533
0.01743
13391
13391
- 157.95
0.021625
0.01743
12742
12742
- 157.48
3
4
12127
- 157.00
0.01743
12127
0.022770
5
0.02397 1
0.01743
11545
11545
- 156.53
0.01743
10992
10992
- 156.05
6
0.02 5229
7
0.02654 7
0.01744
10469
10.+69
- 155.58
8
0.027929
0.01744
- 155.10
9
0.0293 75
0.01744
9501
9501
- 154.62
10
0.030890
0.01744
9055
9055
- 154.15
II
0.032476
0.0 1744
8631
8631
- 153.67
12
0.034136
0.01744
8228
8228
- 153.18
0.035874
0.01744
7846
- 152.70
13
7846
0.01745
7484
- 152.22
14
0.037692
7484
IS
0.039593
0.01 745
7139
7139
- 151.74
- 151.25
0.041582
0.01 745
16
6812
6812
17
0.043662
0.01745
6501
6501
- 150.77
18
0.04583 7
0.01745
6205
6205
- 150.28
19
0.048109
0.01745
5925
5925
- 149.79
20
0.050485
0.01746
5658
5658
- 149.30
0.052967
0.01746
- 148.81
21
5404
5404
22
0.055560
0.01746
5162
5162
- 148.32
0.01746
4932
4932
- 147.83
23
0.058268
24
0.06 1096
0.01746
4714
4714
- 147.34
- 146.85
25
0.064048
0.01746
4506
4506
0.067 130
0.01746
4308
4 308
- 146.35
26
0.070347
0.0174 7
4119
4119
- 145.86
27
0.01747
- 145.36
28
0.073704
3939
3939
0.01747
- 144.86
29
0.077206
3768
3768
0.Og0ll58
0.01747
- 144.36
30
3605
3605
- 143.86
0.01747
3450
3450
31
0.084668
32
0.088640
0.01747
3302
3302
- 143.36
0.006184
0.006548
0.006932
0.007335
0.00 776 1
0.008209
0.008681
-.
"'''
"'''
[,'a p .
" it:'hfll
Sat. Vapo r
1220.46
1220.4 6
1220.46
1220.4 6
1220.45
1220.4 5
1220.4 5
1220.4 4
1220.4 3
1220.4 3
1047.91
1048.36
1048.80
1049.24
1049.68
1050.12
1050.56
1051.0[
1051.45
1051.89
1220.4 2
1220.4 1
1220.4 0
1220.39
1220.38
1220.36
1220.35
1220.33
1220.32
1220.30
1220.28
1220.26
1220.24
1220.22
1220.20
1Z20.17
1Z20.15
1220.12
1220.1 0
1220.07
1220.04
1220.01
1219.98
1219.95
1219.92
1219.88
1219.85
1219.81
1219.77
1219.74
1219.70
1219.66
1219.61
1219.57
1219.53
1219.48
1219.4 4
1219.39
1219.34
1219.29
1219.24
1219.19
1219.14
1219.09
1219.03
1218.98
1218.92
1218.86
1218.80
1218.74
1218.68
1218.62
1218.56
",
Specific Entropy, Bt u/III", ,o F
Sat. Solid
I>Jy
E,'ap.
Sal. Vapo r
"'il/~fll
"
-0.3498
-0.3488
-0.3477
-0.3467
-0.3457
2.8405
2.8339
2.8273
2.8208
2.8143
2.8078
2.8013
2.7949
2.7885
2.7821
2.4855
2.4800
2.4 744
2.4689
2.4634
2.4580
2.4525
2.4471
2.4418
2.4364
1052.33
1052.77
1053.21
1053.65
1054.10
1054.54
1054.98
-0.3447
-0.3436
-0.3426
-0.34 16
-0.3406
-0.3396
-0.3385
2.7758
2.7694
2.7632
2. 7569
2.7506
2.7444
2.7382
2.431 [
2.4258
2.4205
2.4153
2.4101
2.4049
2.3997
1055.42
1055.86
1056.30
1056.74
1057.18
1057.63
1058.07
1058.51
1058.95
1059.39
1059.83
1060.27
1060.71
1061.15
1061.59
1062.03
1062.47
1062.91
1063.35
1063.79
1064.23
1064.67
1065.11
1065.55
1065.99
1066.43
1066.87
1067.31
1067.75
1068.19
1068.63
1069.06
1069.50
1069.94
1070.38
1070.82
1071.26
1071.69
1072.13
1072.57
1073.01
1073.44
1073.88
1074.32
1074.76
1075.19
-0.3375
-0.3365
-0.3355
-0.3344
-0.3334
-0.3324
-0.3314
-0.3303
-0.3293
-0.3283
-0.3 273
-0.3263
-0.3252
-0.3242
-0.3232
-0.3222
-0.3212
-0.3201
-0.3191
-0.3181
-0.3171
-0.3160
-0.3150
-0.3140
-0.3130
-0.3120
-0.3109
-0.3099
-0.3089
-0.3079
-0.3069
-0.3058
-0.3048
-0.3038
-0.3 028
-0.3018
-0.3007
-0.2997
-0.2987
-0.2977
-0.296 7
-0.2957
-0.2946
-0.2936
-0.2926
-0.2916
2.7321
2.7259
2.7198
2.7137
2.7077
2.7016
2.6956
2.6896
2.6H37
2.6777
2.6718
2.6659
2.6600
2.6542
2.6483
2.6425
2.6368
2.6310
2.6253
2.6196
2.6139
2.6082
2.6025
2.5969
2.5913
2.5857
2.5802
2.5746
2.5691
2.5636
2.5581
2.5527
2.5473
2.5418
2.5364
2.5311
2.5257
2.5204
2.5151
2.5098
2.5045
2.4992
2.4940
2.4888
2.4836
2.4784
2. 3946
2.3895
2.3844
2.3793
2.3743
2.3692
2.3642
2.3593
2.3543
2.3494
2.3445
2.3396
2.3348
2.3300
2.3251
2.3204
2.3156
2.3109
2.3062
2.3015
2.2968
2.2921
2.2875
2.2829
2.2783
2.2738
2.2692
2.2647
2.2602
2.2557
2.2513
2.2468
2.2424
2.2380
2.233 7
2.2293
2.2250
2.2207
2.2164
2.2121
2.20 78
2.2036
2.1994
2.1952
2.1910
2.1868
-0.3549
-0.3539
-0.3529
-0.3518
-0.3508
.,
Temp.,
- 30
- 29
- 28
- 27
- 2.
- 25
- 24
- 23
- 22
- 21
- 20
- 19
- IS
- 17
- I.
- IS
- 14
- 13
- 12
- II
- 10
-9
-8
-7
-6
-5
-4
-3
-2
-I
0
1
2
3
4
5
•
7
8
9
10
II
12
13
14
15
I.
17
"
19
20
21
22
23
24
25
2.
27
28
29
30
31
32
Fundame ntals of Psychrometries (I-P), Second Edition
Tab le 0- 1
.,
Temp.,
Absolute
Prt'Ss ure
P ..... , psia
T h e rmodyna m ic Properties of Water at Sat u ration (Comilll/cd)
Specific Volume, fI ]/lb ...
Sat. Solid
E,·a p.
"jll'!
Vjll/vIII
137
Spce ific En thal py, Btu/lb",
Sal. Vapo r
Sat. Solid
E,·a p .
Sa t. Vapo r
"
" j'''1
" ;II/"fll
",
3302.04
3178.08
3059.32
2945.52
2836.46
2731.92
2631.70
2535.59
2443.41
2354.98
2270.15
2188.74
2110.60
2035.59
1963.58
1894.42
1828.00
1764.20
1702.90
1643.99
-0.02
1075.21
1074.64
1074.07
1073.5 0
1072.93
1072.37
107 1.80
107l.23
1070.67
1070.10
1069.53
1068.97
1068.40
1067.84
1067.27
1066.70
1066.14
1065.57
1065.01
1064.44
1075.19
1075.63
1076.07
1076.51
1076.95
1077.38
1077.82
1078.26
1078.70
1079.14
1079.57
1080.01
1080.45
1080.89
1081.33
1081.76
1082.20
1082.64
1083.07
1083.51
I
I
Specifi c Entropy, Bt u/lb" .. oF
Sat. Solid
E,·ap.
Sat. Va po r
1>;1.'1
"';II/'~/II
"
0.0000
2.1869
2.1813
2. 1757
2.1701
2.1646
2.1591
2.1536
2. 1482
2. 1427
2.1373
2.1319
2.1266
2.1212
2.1159
2.1106
2.1053
2.1001
2.0948
2.0896
2.0844
2.1868
2.1833
2.1797
2.1762
2.1727
2.1693
2.1658
2.1624
2.1590
2.1556
2.1522
2.1488
2.1454
2.1421
2.1388
2.1355
2.1322
2.1289
2.1257
2.1225
.,
Temp.,
TransiliOl' from S<lmrulml solid 10 s,,,,,raled liquid
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
0.08865
0.09229
0.09607
0.09998
0.10403
0.10823
0.11258
0.11708
0.12173
0.12656
0.13155
0.13671
0.14205
0.14757
0.15328
0.15919
0.16530
0.1 7 161
0.17813
0.18487
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.0 1602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
0.01602
3302.02
3178.06
3059.30
2945.51
2836.45
2731.91
2631.68
2535.57
2443.39
2354.97
2270.13
2188.72
2110.58
2035.58
1963.56
1894.41
1827.99
1764.19
1702.88
1643.98
iii
(Source: ASIlRA£ lIa",lbook- FlImJ<lmem<lls. Chapter I. Table 3)
0.99
2.00
3.00
4.01
5.02
6.02
7.03
8m
904
10.04
11 .05
12.05
13.05
14.06
15.06
16.06
17.06
18.07
19.07
0.0020
0.0041
0.0061
0.0081
0.0102
0.0122
0.0142
0.0162
0.0182
0.0202
0.0222
0.0242
0.0262
0.0282
0.0302
0.0321
0.0341
0.0361
0.0381
II
J2
33
l4
J5
3.
J7
38
39
40
41
42
43
44
45
46
47
48
49
50
51
Skill Development
Exercises
To receive full continuing education credit, all questions must be answered
and submitted at www.as hrae.org/sd lo nl ine. Please log in using your student
10 number and the SOL number. Your student 10 number is composed of the
last fi ve digits of your Soc ial Security Number or another un ique fi ve-digit
number you create when first registering online. The SOL number for this
course can be located near the top of the copyright page of this book.
Skill Development Exercises
Chapter I
Skill Development Exercises for Chapter I
Total number of questions: 4
~
C
QJ
I-I
E
C-
a) Two
o
~
How many basic processes of air conditioning can be performed on moist air?
b) Three
c) Four
1-2
.
i.
Wh ich combination process will increase both the temperature and the moi sture content?
a) Cooling and dehumidification
b) Heating and dehumidification
c) Heating and humidification
J:.
U
1-3
Enthalpy is the total heat contcnt of the air.
a) True
b) False
14
Change in elevat ion has no effect on the air density.
a) True
b) False
Fundamentals of Psychrometries (I-P), Second Edition
Skill Development Exercises for Chapter 2
Total number of questions: 8
2-1
Dry-bu lb temperature is measured with a wet sock around the sensing bu lb.
a) True
b) False
2-2
Saturation temperature of air is the point at which the dry-bulb, wet-bulb, and
dew-point temperatures are equa l.
a) True
b) Fa lse
2-3
Relative humidity does not change as the dry-bulb temperature changes.
a) True
b) Fa lse
2-4
The dry-bulb temperature can be above the dew-point temperature.
a) True
b) False
2-5
According to Append ix A, what is the specific enthalpy hs of saturated air at
40°F?
a) 15.23
b) 9.6
e) 5.8
d) None of the above
2-6
According to Appendix A, under the same condition cited in Exercise 2-5,
what is the specific volume v?
a) 12.69
b) 12.59
e) 0.105
d) None of the above
2·7
According to Appendix A, what is the specific enthalpy of dry air h(l at 100°F?
a) 29.27
b) 47.73
e) 24.03
d) None of the above
n
.
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1...
....
o
~
o
"3
CD
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m
x
CD
ri
~.
~
Skill Development Exercises
2-8
~
QI
·e
According to Appendix A, under the same condition cited in Exercise 2-7,
what is the specific volume v?
~
a) 19.15
QI
b) 19.80
W
c) 15.45
x
~
C
QI
E
cO
QI
>
QI
0
.>£
Vl
.
N
...
QI
D-
IU
J:.
U
Chapter 2
d) None of the above
Fundamentals of Psychrometries (I-P), Second Edition
Skill Development Exercises for Chapter 3
Total number of questions: 10
3-1
On a psychrometric chart, the y-ax is is humidity ratio and the x-axis is:
o
~
Using the psychrometric chart in Figure 3-4, determine the relative humidity of
an air parcel with W = 0.0 10 and 'db = 60°F.
a) 60% rh
o
"3
CD
::J
rl
m
x
CD
b) 70% rh
ri
c) 80% rh
~
Using the psychrometric chart in Figure 3-4, determine the dew-point temperature of an air parce l with ldb = 70°F a nd 4t = 50% rho
a) 52° F
b) 59° F
c) 70° F
d) 85° F
Using the psychrometric chart in Figure 3-4, determine the humidity ratio Wof
an air parcel with a saturation temperature of 'db = 40°F.
a) 0.003
b) 0.005
c) 50%
d) 40° F
3-5
w
V>
b) Dew-point temperature
d) 90% rh
3-4
1...
io.
d) Wet-bu lb temperature
3-3
.
a) Re lative humidity
c) Dry-bu lb temperature
3-2
n
::r
Using the psychrometri c chart in Figure 3-4, determine the specific vo lume v
of an air parcel with ldb = 70°F and W = 0.0 I O.
a) 13.40
b) 13.55
c) 14.05
d) 14.40
~.
Skill Deve lopm ent Exercises Chapter 3
3-6
According to the psychrometric cha rt in Figure 3-4, what is the enthalpy of
'db = 70°F dry a ir?
a) 45
b) 35
c) 26
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d) 17
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3-7
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According to th e psychrometric chart in Figure 3-4, what is th e wet-bulb temperature of a mo ist air parce l with tdb = 70°F and $ = 50% rh air?
a) 70° F
b) 58° F
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c) 50° F
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d) 38° F
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3-8
According to the psychrometric chart in Figure 3-4, what is th e dew point of
' db = 50°F saturated air?
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a) 50° F
b) 40° F
c) 30° F
d) 20° F
3·9
According to the psychrometric chart in Figure 3-4, what is the wet-bulb tempe rature of tdb = 70°F dry air?
a) OaF
b) 22° F
c) 33° F
d) 44° F
3-10
Using the psychrometri c chart in Figure 3-4, plot the poi nts 'db = 70°F, h = 30,
and ' wb = 50°F, then connect the points with a line. Upon investigati on of the
line, which of the fo ll owing is the best descripti on?
a) The li ne is almost ve rtical.
b) The line has a slope of about 45° (angJe).
c) The line almost horizonta l.
Fundamentals of Psychrometries (I-P), Second Edition
Skill Development Exercises for Chapter 4
Total number of questions: 10
4-1
Moist air that is heated without humidi fication has the following change in relative
humidity:
a) Increase
b) Decrease
c) Stays the same
d) Depends on the type of humidifier
4-2
What is the equation that converts enthalpy changes into capaci ty (Btu/h)?
a) 1.085 x cfm x (I, - I, )
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Which of the follo wing is true concerning humidification by steam versus by
(cold water) atomization?
c) Heat to make steam in the stea m humidifi er comes from the air
entering the humidifi er.
d) Heat to evaporate water in the atomizer comes from the air
entering the humidifi er.
A heating coil can provide for both heating and humidifi cation .
a) True
b) Fa lse
A cooling co il can provide for both cooling and dehumidification.
a) True
b) False
4-6
...
c) 4840 x cfm x (W, - W, )
b) Steam humidifi cat ion adds no net energy to the airstream.
4-5
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a) Atomi zation always maintains a constant relative humidity.
4-4
.
b) 4.5 x cfm x (hi - h,)
d) None of the above
4-3
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What is the change in entha lpy when dry air is heated from 50°F to 74°F?
a) 4.5
b) 5.5
c) 6.5
d) 7.2
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Skill Development Exercises
4-7
Chapter 4
What is the enthalpy change when saturated air at 50°F is conditioned to be
saturated air at 74° F?
a) 17
b) 21
c) 25
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d) 32
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a) 87"F
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b) 95°F
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One day in Phoenix, Arizona, the temperature reaches JOSo F with 20% rh o
Water is sprayed into the air to cool it. What will the temperature of the air be
when the relative humidity increases to 50% rh?
c) 105°F
d) 115°F
4-9
U
If the air entering a heating coil is dry and 70°F db and the leaving air is 110°F,
how many Btulh are supplied by the coil at 5000 cfm if the fan is located at the
coil inlet?
a) 200,000
b) 205,000
c) 209,000
d) 217,000
4-10
Air enters a cooling coi l at lOOoF and 40% rh and leaves saturated at a temperature of 45°F. What is the total Bluth of cooling required if a 5000 cfm fan
is located at the in let of the coo ling coil?
a) 565,000
b) 511,600
c) 460,600
d) 440,600
Fundamentals of Psychrometries (I-P), Second Edition
Skill Development Exercises for Chapter 5
Total number of questions: II
5-1
The definition of sensib le heat ratio (SHR) is the:
a) Ratio of sensib le to latent load
b) Ratio of latent to sensible load
c) Ratio of tota l load to sensible load
d) Ratio of sensib le load to total load
5-2
If the sensible load on a building is equa l to the latent load, the value ofSHR is;
a) 2
b) I
c) 0.5
d) - 2
5-3
The psychrometric condition for supply air that will satisfy the requirements of
a room depends on:
a) The amount of outdoor air needed
b) The des ired room condition
c) Room SHR
d) All of the above
e) Answers band con ly
5-4
Why is it poss ible to satisfy a room w ith a variety of "assumptions" about the
temperature change across a coil (heating or cooling)?
a) Be cause there is a corresponding cfm w ith every IJJ.
b) Be cause the heatlcoo lload calculation is never accurate.
c) Be cause the comfort zone is large.
d) Be cause there is a wide vari ety of methods for heating and
cooling.
5-5
Wh ich condition below is not possible to show on a psychrometric chart?
a) tdb ~ 76' F, h ~ 30
b) ldb = 89°F, twb = 78°F
c) I l1'b = 78°F, h = 44
d) tdb ~ 76' F, ~ ~ 50%
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Skill Deve lopm ent Exercises Chapter 5
5-6
In a system, 200 cfm of air at 60°F and 30% rh is mixed with 800 cfm air at
80°F and 80% rho Find the mixed-air temperature using the mi xing equation.
a) 74° F
b) 76° F
c) 78° F
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d) 79° F
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a) 60% rh
b) 76% rh
c) 70% rh
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In Exercise 5-6. what is the mixed-air relative humidity?
d) None of these
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5-8
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In a system , 200 cfm of air at 40°F and 90% rh is adiabatically mixed with
moi st air at 80°F but unknown relative humidity. The fina l mixture is at 72 °F
and 50% rhoWhat is the relative humidity and airflow rate (cfrn) of the second
airstream?
a) 40% rh, 800 cfm
b) 40% rh, 50 cfm
c) 80% rh, 800 cfm
d) 80% rh, 50 cfm
5-9
If the sensible load is 600,000 Btulh and the latent load is 300,000 Btuih, what
is the SHR?
a) 2.0
b) 1.0
c) 0.66
d) 0.76
5-10
If the room design is ldb = 75°F and 4J = 50% rh and we mix in 25% outdoor ai r
at Idb = 115°F and ~ = 10% rh, what is the mixed-air dry-bulb temperature?
a) 83°F
b) 85°F
c) IOsoF
d) Not poss ible
5-11
From Exercise 5-10, what is the mi xed-air re lati ve humidity?
a) 33% rh
b) 15% rh
c) 38% rh
d) 40% rh
Fundamentals of Psychrometries (I-P), Second Edition
Skill Development Exercises for Chapter 6
Total number of questions: 10
6-1
Which type of humidification requires the change to not exceed the temperature rise capacity of a heating co il ?
a) Water spray
b) Steam
c) Both the same
d) Ne ither has an impact
6-2
From the discussion of the psychrometrics of cooling coils, wh ich "rule of
thumb" will best se lect the coo li ng coil conditions?
a) Temperature drop across a coo ling coil should be about 20°F.
b) Relati ve humidity off the coil should be 90%.
c) Volume of air (cfm) across a coolin g coi l should be kept to a
min imum.
d) Co il temperatures shou ld be selected to be as low as possible.
6-3
Which of the following statements best describe why cooling coils cannot
accommodate large latent loads with small sensible loads?
a) Cooling coils rust if too much condensate form s.
b) Cooling coils will free ze up if the coil temperature gets too low.
c) Cooling co il s tend to dehumidify first, then drop the ai r
temperature.
d) Condensation requires a drop in air temperature to the dew point.
6-4
Consider a room heating load with a 700,000 Btu/ h sensib le loss and 100,000
Btulh latent loss, with room design conditions of tdh = 72°F and approximately
q, = 40% rh o The air handler ha s an adiabatic humidifier downstream from a
heating coil without any outdoor air. I f the leavi ng air temperature is 'db =
100°F after the hum idifier, what is the cfm required to satisfy the load?
a) 20,000
b) 23,040
c) 25,200
d) None of these
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Skill Development Exercises
6-5
Chapter 6
What is the leaving air temperature tdb from the heating coil for the conditions
li sted in Exercise 6-4?
a) 98° F
b) 104°F
c) lOO°F
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d) None of these
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6-6
What is the leaving relative humidity 4J from the heating coil for the conditions
li sted in Exercise 6-4?
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a) 15% rh
b) 12%rh
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c) 20% rh
d) 24% rh
6-7
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What is the leaving relati ve humidity $ from the adiaba tic humidifier for the
conditions listed in Exerc ise 6-4?
a) 15% rh
b) 25% rh
c) 19%rh
d) 28% rh
6.8
Using the air handler in Exercise 6-4 and 23,040 cfrn , adding a cooling co il to
satisfy a room sensib le heat ga in of 500,000 Btulh and a room latent heat gain
of50,000 BtU/h , and room conditions of tdb = 75°F and q. = 40% rh and without
outdoor air, what is the required leaving air temperature (db and $ from the
cooling coil ?
a) 55° F 'db, $ ~ 90% rh
b) 5r F ' db, $ ~ 80% rh
c) 55 of ' db, $ ~ 75 % rh
6-9
What is the room sensible heat ratio for the conditions li sted in Exercise 6-8?
a) 0.89
b) 0.95
c) 0.91
d) 1.0
6-10
Would you attempt to add humidity to the leav ing airstream for the conditions
li sted in Exercise 6-8 in the cooling mode with an adiabatic humidifier?
a) Yes
b) N o
c) Not sure
Fundamentals of Psychrometries (I-P), Second Edition
Skill Development Exercises for Chapter 7
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Total numbe r of que stions: II
....
For all of the Ski ll Development Exercises for Chapter 7, cons ider three zones
in a small office building that we are going to heat and cool. The cooling and
heating loads are as follows:
Zone
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Sensible Cooling
Latent Coolin g
Heati ngSensibl e
36,000 Btulh
5,000B tuih
20,000 Btulh
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2
48 ,000 Btulh
6,000Btuih
25,000 Btuih
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60,000 Btulh
10,000 Btulh
30,000 Btuih
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Assume room design conditions of the following:
Cooling [db ~ 75°F and $ ~ 50% rli
Heating
Idb = 70°F and.p = 40% rh
Use a sea-level psychrometric chart.
7·1
What is the sensib le heat ratio for all three zones in order I, 2, 3? (Round to
two decimal places.)
a) 0,88,0,89, 0.86
b) 0.87,0.85, 0.89
c) 0.85,0.84, 0.87
7·2
If we provide 25% outdoor air for code-required ventilation to all three zones,
what is the mixed ai r condition in the summer if th e outdoor air is 'db = 100°F
and .p = 25% rh?
a) [db ~ 79°F and $ ~ 48% rh
b) [db ~ 85°F and $ ~ 40% rh
c)
7·3
[db ~ 81.2°F and $ ~ 42% rh
For Zone I only, if we use individual fan-coils for each zone , what is the
required supp ly airflow?
a) airflow = 1600 cfm
b) airflow ~ 1750 cfm
c) airflow = 2000 cfm
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Skill Development Exercises
7-4
Chapter 7
For Zone I on ly, what are the leaving air conditions from the cooling co il
assuming we use 25% outdoor air from Exercise 7-2 and the correct supply
cfm?
aj tdb ~ 54' F and ~ ~ 90% rh
bj tdb ~ 56' F and ~ ~ 88% rh
cj tdb ~ 60' F and ~ ~ 80% rh
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7·5
the correct cfm and leaving air conditions?
aj 41 ,000 Btu/h
bj 52,300 Btuth
cj 48,825 Btuth
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For Zone 1 on ly, what is the tota l cooling capacity, q" of the coo ling coi l with
7.6
If all three zones were put on a central air handler with a constant- volume terminal reheat system, what would the cfm of all three zones be, in order 1,2, 3?
(Same outdoor design and percent outdoor air.)
J:.
aj 1750, 2000, 2500
bj 1600, 1800,2200
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cj 1750, 2460,2765
7-7
If all three zones were put on a central air handler with a variable-air-volume
reheat VA V box and 25% outdoor air, what are the required leaving air conditions from thi s air handler?
aj tdb ~ 55"Fand~ ~ 91%rh
bj tdb ~ 57' F and ~ ~ 88% rh
cj tdb ~ 60' F and ~ ~ 82% rh
7·8
With the system in Exerci se 7-7, what are the new required ai rflows by zone in
order 1, 2, 3 with the new leaving conditions?
aj 1660, 2110,2765
bj 1750, 2460,2750
cj 1700, 2300,2600
7·9
What is the reheat required by zone in order 1, 2, 3 to meet the tota l reheat load
plus the winter heat loss load? (Use 'db = 70°F for room conditi on.)
aj 49,000 Btu/h, 61 ,000 Btu/h, 79,000 Btuth
bj 47,000 Btu/h, 59,300 Btu/h, 75,000 Btuth
cj 56,000 Btu/h, 73,000 Btu/h, 90,000 Btu/h
Fundamentals of Psychrometries (I-P), Second Edition
7-10
From Exercise 7-7, with the correct leaving conditions and cfm, what is the
total cooling capacity of the central ai r-handl er coo ling coi l?
a) 180,000 Btulh
b) 167,000 Btu/h
c) 194,100 Btulh
7-11
I f the system in Exe rcise 7-7 were a constant-volume, dual-duct system, what
would be the heat capacity of the hot-deck coi l used in the central air handler?
(Room at ' db ~ 7SO F.)
a) 219,000Btulh
b) 199,000 Btulh
c) 212,500Btulh
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Skill Development Exercises
Chapter 8
Skill Development Exercises for Chapter 8
Total number of questions: 10
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8-1
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b) Total enthalpy heat recovery device
c) Sensible-to-total heat re covery device
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A heat wheel with a desiccant coating is a:
a) Sensible heat recovery device
d) Total-to-sensible heat recovery device
8-2
When is preheat ing of the outdoor airstream necessary on a heat recovery
device?
a) When the outdoor air temperature is below O°F.
b) When the outdoor air dew point is below 32°F.
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c) When the exhaust airstream has a dew pOint above 32°F and the
leaving air temperature is below 32°F.
d) All of the above.
U
8-3
Heat recovery effectiveness is the actual amount of hea t transferred versus the
maximum amount that could be transferred.
a) True
b) False
8-4
Energy recovery involves the transfer of sensible heat from one airstream to the
other airstream.
a) True
b) False
8-5
An air-side economizer shou ld be considered on any/all air systems that have
100% outdoor air capability and high operation hours with an ambient air temperature below 60°F and a demand for cooling.
a) True
b) False
8-6
Water-side economizers can be used on a chilled-wa ter system with all terminal fan-coils and an air-cooled water chiller.
a) True
b) False
Fundamentals of Psychrometries (I-P), Second Edition
8-7
There is a sensible heat recovery system between equa l outdoor air and exhaust
airstreams in Phoenix , Arizona, and the summer design outside is tdb 11 5°F
and q, 10% rho If the effectiveness is 75% of the heat recovery device and the
exhaust ai rstream is tdb 75 °F and q, 40% rh , w hat are the dry-bulb temperature and relative humidity of the outdoor airstream leaving the recovery
device?
0=
0=
0=
0=
a) Idb = 90°F and ~ = 15% rh
b) Idb = 95 °F and ~ = 12% rh
c)
From Exercise 8-7, what are the leaving air conditions of the exhaust airstream
with everything else being the same?
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b) IdV 95 °F and ~ = 25% rh
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d) Idb = I05°F and ~ = 16% rh
If the entering air conditions to a cooling tower are tdb 11 5°F and t wb 65 °F
and the cooling tower has a full -load approach tempera ture of SOF, w hat is the
leaving water from cooling towe r (at full load)?
=0
0=
a) IOrF
b) 95 °F
c) 73 °F
d) 84 °F
8-10
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c) Idb = IOooF and ~ = 20% rh
8·9
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Idb = 700F and ~ = 25% rh
d) Idb = 85 °F and ~ = 20% rh
8-8
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Supply air temperature reset can be used on all air-conditioning systems , any
time of the year in all parts of the world, regardless of the ambie nt air conditions.
a) True
b) False
Skill Development Exercises
Chapter 9
Skill Development Exercises for Chapter 9
Total number of questions: 10
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9-1
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A cooling tower needs to reject heal from 1200 gpm of water entering at 95°F
and leaving a185°F. What is the total heat required to be rej ected?
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a) 6,000,000 Btulh
b) 600,000 Btu/h
c) 5,400,000 Btulh
d) 4,500,000 Btu/h
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9-2
From Exercise 9-1, if the cooling tower has an airflow of 100,000 cfm and
ambient air conditions of 'db = 85 °F and twb:=O 75°F, what are the leaving air
i.
conditions of th e tower?
a) tdb = 85°F, twb = 84.8°F
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b) tdb = 89°F, twb = 88°F
c) tdb = 87°F, twb = 86.8°F
d) tdb = 86°F, t wb = 84 of
9-3
W hat is the coo ling tower approach temperature for the cooling tower in Exer-
cise 9-27
a) r F
b) 12°F
c) SOF
d) 10°F
9-4
In the design of an indoor swimmi ng pool , it is best to keep the swimming pool
water temperature and the room temperature as far apart as comfortably possible.
a) True
b) False
9·5
In a cleanroom w ith design conditions of (db = 68"F and q, = 40% rh, the
makeup air must be cooled to w hat dry- bulb temperature or the relative humid ity w ill not be met?
a)
tdb = 55°F
b) tdb = 68 °F
c)
tdb = 43°F
d) tdb = 40°F
Fundamentals of Psychrometries (I-P), Second Edition
9-6
9-7
If we cool the air via direct evaporative cooling from (db = 110°F and q, =
2% rho what is the lowest leaving air temperature we can achieve?
a)
(db = 62°F
b)
c)
d)
(db = 6SOF
(db = 55°F
In Exercise 9-6 . if our eva porative efficiency is 80%, what are the leaving ai r
conditions?
a)
b)
c)
d)
9-8
(db = 64°F
(db = 6SOF and ~ = 70% rh
(db = 70°F and ~ = 70% rh
(db = 71.5°F and ~ = 58% rh
(db = 79°F and ~ = 50% rh
In Exercises 9-6 and 9-7. if the room sensible heat ratio is 0.9 . what is the
expected room relative hum idity if the room is at (db = 75 °F?
a) ~ = 53% rh
b) ~ = 60% rh
c) ~ = 50% rh
d) Cannot maintain room at {db = 75 °F w ith this leaving condition
9·9
If we use the same outdoor conditions of (db = 110°F and q, = 2% rh from Exercise 9-6 and an indirect evaporative cooli ng section of 40% efficiency , w hat are
the leaving air cond itions from this section?
a)
b)
c)
d)
9-10
(db = 91°F and ~ = 3% rh
(db = 88°F and ~ = 20% rh
(db = 95°F and ~ = 5% rh
(db = 85°F and ~ = 5% rh
If we add a direct evaporative cooli ng section in series downstream of the indi rect section in Exercise 9-9 and the direct section has an efficiency of 70%.
what are the leaving air conditions?
a)
b)
c)
d)
(db = 61°F and ~ = 95% rh
(db = 60°F and ~ = 65% rh
(db = 65°F and ~ = 60% rh
(db = 65°F and ~ = 52% rh
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ASHRAE LEARNING INSTITUTE
Self-Directed Learning Course Evaluation Form
Course Title: Fundamentols of Psychrometries (I-Pl, Second Edition (201 6)
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Strongly
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Fax: 404·321·5478
Flexible and Effective Continuing Education
for HVAC&R Professionals
This revision of ASHRAE'S Fundamentals af Psychrometries self-directed
learning course book addresses the use of psychrometries and the
psychrometric chart for typical applications and systems. It is intended
for HVAC designers of various backgrounds and to be an introduction for
those new to psychrometries.
This second edition of the course was rewritten in an attempt to teach
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www.ashrae.org/ali
learning Institute
ISBN 978-1-939200-09-9 (paperback)
ISBN 978-1-939200-1 0-5 (PDF)
It
200099
Product Code: 98048
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