Psychrometrics
Scott Laurila ~ Senior Application Engineer
Greenheck Fan Corporation
Schofield, Wisconsin
1
A
Agenda
d



Properties of Air
The Psychrometric
y
Chart
Applications
 Mixed
Air
 Cooling
 Dehumidification
 Energy Recovery
2
C
Course
Obj
Objectives
ti

At the completion of this session, you should be able to use a
Psychrometric Chart to:
 Identify
Id if the
h properties
i off air
i at a given
i
state condition
di i
 Illustrate the performance characteristics of basic heating
and cooling processes
 Demonstrate how the use of Energy Recovery devices can
reduce heating and cooling loads in building designs
3
P
Psychrometrics
h
ti

Psychrometrics
h
i is
i the
h science
i
dealing
d li with
i h the
h physical
h i l
laws or air – water mixtures

Important in the design of HVAC systems as numerous air
properties can be found over a broad range of conditions
4
P
Properties
ti off Air
Ai

Properties of Air







Dry-Bulb Temperature
Wet-Bulb Temperature
Dew-Point Temperature
Relative Humidity
Humidity Ratio
Enthalpy
If any two of the above properties are known, the
Psychrometric Chart can be used to find the remaining
properties
5
Properties of Air

Dry-bulb temperature is the
temperature read from a
standard thermometer


Wet-bulb
Wet
bulb temperature is read
from a thermometer with the
bulb covered by a wet wick


Representative of the sensible
heat energy at the given
condition
"Sling Psychrometer"
Difference between dry-bulb
and wet
wet-bulb
bulb represents the
dryness of air
6
P
Properties
ti off Air
Ai


Dew-point temperature is the
temperature at which
moisture leaves the air and
condenses on objects
When the dry-bulb, wet-bulb,
and dew-point temperatures
are equal, the air is saturated
 Fog occurs when the air is
saturated
7
P
Properties
ti off Air
Ai

Relative humidity is the measure of how much moisture the air
is holding versus how much moisture the air can hold at a
given dry-bulb
dry bulb temperature
 Expressed as a percentage
 As the dry-bulb
dry bulb temperature increases, the amount of
moisture the air can hold increases
Amountt off moisture
A
it
the
th air
i is
i holding
h ldi
Relative Humidity =
Amount of moisture the air can hold
8
P
Properties
ti off Air
Ai

Humidity Ratio is a measure of the weight of water in a given
amount of air
 Also
Al referred
f
d to as S
Specific
ifi Humidity
H idi
 Can be expressed in grains
 7,000
7 000 grains equals one pound of water
Pounds
P
d off moisture
it
Humidity Ratio =
Pounds of dry air
9
Th P
The
Psychrometric
h
t i Ch
Chartt
10
S t ti Curve
Saturation
C
Saturation Curve
11
D B lb T
Dry-Bulb
Temperature
t
12
W t B lb T
Wet-Bulb
Temperature
t
13
D
Dew-Point
P i tT
Temperature
t
14
R l ti H
Relative
Humidity
idit
15
Humidity Ratio
16
S
Sensible
ibl and
dL
Latent
t tE
Energy



Sensible Energy is the heat that causes changes in the air's dry
bulb temperature
L
Latent
Energy
E
i the
is
h heat
h associated
i d with
i h phase
h
change.
h
It
I is
i
representative of changes in the air's moisture content with no
g to the dry-bulb
y
temperature
p
change
Enthalpy is the total energy in a given amount of air at its
present conditions
Enthalpy (h) = Sensible Energy + Latent Energy
17
E th l
Enthalpy
18
P
Psychrometric
h
t i Ch
Chartt R
Reconstructed
t t d
19
Example:
93°F DB, 65°F WB
E th l
Enthalpy
Find:
Relative Humidity = 24.0%
Dew-Point = 50.9°F
Hum. Ratio = 66.8 grains/lb dry air
Enthalpy = 32.9 Btu/lb
Wet-Bulb
Dew-Point
Humidity Ratio
Relative Humidity
Dry-Bulb
20
Ai D
Air
Density
it C
Corrections
ti



Do NOT forget about elevation!
Above 2,000 feet elevation, the air density is reduced by
approximately
i t l 3
3.6%
6% per every 1
1,000
000 ffeett
A change in air density also changes the physical and
thermodynamic properties of air-water
air water mixtures
21
P
Psychrometric
h
ti P
Processes

Air Conditioning
 Mixed
Air
 Cooling Coils
 Reheat
 Calculations

Energy Recovery
 Sensible
Devices
 Total Enthalpy Devices
22
Psychrometric Processes
Example Processes


Evaporative
Cooling
Steam
Humidification
Sensible: Side
Side-to-side
to side
Latent: Up & down
Coolingg &
Dehumidification
180
160
80
140
Desiccant
Dehumidification
120
70
100
60
80
50
60
40
Heat
eat
30
40
20
20
30
40
50
60
70
80
90
100
110
Dry Bulb Temp.
23
Use the ASHRAE indoor
d i conditions
design
di i
to help
h l with
ih
process calculations
ASHRAE IIndoor
d
D
Design
i C
Conditions
diti
Summer Indoor Design
75°F, 50% RH
Winter Indoor Design
72°F, 35% RH
24
Mi d Ai
Mixed
Air C
Conditions
diti



Step 1: Identify outdoor condition, indoor condition, and
ventilation rate (% outside air)
Step 2: Draw a straight line between the outdoor and
indoor conditions
Step 3: Draw a straight line between points 1 and 2
25
Mi d Ai
Mixed
Air E
Example
l
Outdoor Air: 95°F DB/75°F WB
Room Air: 75°F DB, 50% RH
Ventilation: 25% Outdoor Air
95°F x 00.25
25 = 23
23.75°F
75°F
75°F x 0.75 = 56.25°F
Mixture = 80°F
26
C li P
Cooling
Process E
Example
l
Condenser
High
g Pressure
Compressor
Expansion Valve
E
Evaporator
t
Low Pressure
27
C li C
Cooling
Coilil P
Processes
Capacity (Btu/hr) = 4.5 x SCFM x Δh
 1 Cooling
C li Ton
T = 12,000
12 000 Btu/hr
B /h
Capacity (Tons) = 4.5 x SCFM x Δh / 12,000
28
C li E
Cooling
Exercise
i



Exercise 2: A customer wants to cool 4,000 SCFM of
outside air from 93°F DB / 65° WB to 50°F saturated air
Find the following parameters for 50°F saturated air:
Property
Value
Units
Relative Humidity
100
%
Dew Point
50
°F
Humidity Ratio
64.8
Enthalpy
22
grains / lb dry air
Btu / lb dry air
What is the change
g in enthalpy
py from 93DB/65WB to
50DB/50WB?
 Δh = 10.9 Btu / lb dry air

D t
Determine
i the
th cooling
li tonnage
t
required
i d in
i this
thi process.
 Capacity = 16.4 Tons
29
Deh midification w// Reheat
Dehumidification



Introducing cold, saturated air directly into an occupied
space may be too cool for occupant comfort
Reheat can be used to bring the air to a space-neutral
condition
Reheat Methods
Electric Heat
 Hot Water Coil
 Hot Gas Reheat

30
C li P
Cooling
Process E
Example
l
Condenser
High
g Pressure
Compressor
Expansion Valve
E
Evaporator
t
Low Pressure
31
H t Gas
Hot
G Reheat
R h t
Discharge Line
Reheat
Coil
Air Conditions After Reheat Coil:
65°F DBT/56°F WBT (50% RH)
Air Conditions After Evaporator:
50°F DBT / 50°F WBT
Evaporator
Supply Air
32
E
Energy
R
Recovery
33
Traditional HVAC System
34
35
Wh E
Why
Energy R
Recovery?
?


Economic Benefits
 Reduced Initial Costs
 Reduced
R d d Operating
O
ti Costs
C t
ASHRAE Standards & Guidelines
 ASHRAE 62.1 – 2010 – Ventilation for Acceptable
p
indoor
air quality
 ASHRAE 90.1 – 2010 – Energy Standard for Buildings
 ASHRAE 189
189.1
1 – Standard for Design of High
Performance Green Buildings
36
ASHRAE Standard 62
“Ventilation for Acceptable Indoor Air Quality”


Must supply fresh outdoor air to occupied spaces to
minimize the potential for adverse health effects.
Typical ventilation rates:
 15 CFM per person for Classrooms
 17 CFM per person for Offices
Steven Taylor – Taylor Engineering
37
ASHRAE 90
90.1-2010
1 2010


Exhaust Air Energy Recovery (6.3.6)
 Supply air is greater than 5000 CFM
 70% or more of supply is outdoor air (3500 CFM)
Energy recovery system shall have a total effectiveness
of greater than 50%
38
ASHRAE Climate Zones
39
ASHRAE 90.1-2010
Table 6.5.6.1 Energy Recovery Requirement (IP)
40
ASHRAE 189.1
“Standard for Design of High-Performance Green Buildings”
www.ashrae.org/greenstandard
Table 7.4.3.8 Energy Recovery Requirement (IP)
41
E
Energy
R
Recovery B
Basic
i T
Terms



Sensible Energy Recovery transfers only sensible energy
(no moisture)
 Also referred to as heat recovery
y
Total Energy Recovery transfers both sensible and latent
energy
Effectiveness refers to the efficiency of the energy
recovery device (expressed as a percentage)
 Sensible effectiveness
 Total effectiveness
42
S
Sensible
ibl H
Heatt R
Recovery


Sensible Heat Recoveryy
 Aluminum fixed plate
 Run-around coils
 Heat-pipe heat
exchangers
 Sensible
S ibl wheels
h l (no
(
desiccant)
Transfers sensible energy
gy
only (no moisture transfer)
43
S
Sensible
ibl Pl
Plate
t HRV
44
Sensible Energy Recovery

Sensible Energy
gy recoveryy
moves the outdoor air dry
bulb temperature laterally
toward the room dry bulb
temperature
180
160
80
140
120
70
Outdoor Air
Heat
Exchangers
60
100
80
50
60
Room Air
40
40
30
20
20
30
40
50
60
70
Dry Bulb Temp.
80
90
100
110
45
T t l Energy
Total
E
Recovery
R



Total Energy
gy Recoveryy
 Enthalpy wheels
 Enthalpy plates (or cores)
Transfers sensible energy
through the media
T
Transfers
f latent
l t t energy
through a desiccant (wheel) or
molecular transfer (core)
46
E
Energy
R
Recovery
Summer Operation w/ Rotary Wheel
47
Total Energy Recovery


Total energy recovery
shifts OA point towards
RA point
Total effectiveness 60-80%
60 80%
180
160
80
140
120
70
Outdoor
OutdoorAir
Air
60
100
80
50
60
Room Air
40
40
30
20
20
30
40
50
60
70
80
90
100
110
Dry Bulb Temp.
48
P tL dC
Part-Load
Conditions
diti
49
Energy
gy Recovery
y and Cooling
g
Potential
Energy Savings
50
Winter Operation
p
Supply Air
Dry Bulb 54°F
Humidity 31 grains/lb.
Room Air
(to be exhausted)
Dry Bulb 72°F
Humidity 41
grains/lb
grains/lb.
Outdoor Air
Dry Bulb 0°F
Humidity 2
grains/lb.
Exhaust Air
Dry Bulb 18°F
Humidity 12
grains/lb.
51
Winter Operation
Sensible vs. Total ERV
52
Energy Recovery and Frost


Possibility of condensation forming whenever a warm, moist
air stream comes in contact with a cold surface
Always consider frost control methods in when winter design
temps drop below 55°F
F
53
Winter Exhaust Air Process
Sensible vs. Total ERV
Sensible
Total
54
Question?
55
R i
Review

Psychrometric Charts are useful tool for
understanding
g HVAC p
processes
 Easily
find properties of air across numerous
conditions
 Allows us to plot, predict, and calculate the
heating and cooling capacities
 An understanding of psychrometrics is the
foundation of energy recovery
56
THANK YOU!
57