HVAC
NC STATE UNIVERSITY
MECHANICAL & AEROSPACE ENGINEERING
MAE - 406
Douglas L. Gunnell, P.E., PEM, CEM
Gunnell Engineering Services
Clemmons, NC
2010-3
1
Outline
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Definitions
Heat Transfer/Heat Generation
Heating and Cooling Loads
Indoor Air Quality-ASHRAE STD. 62.1
Vapor Compression Refrigeration Cycle
Energy Usage
Psychrometrics
HVAC Systems
Air-to-Air Heat Recovery
Control Strategy
2
DEFINITIONS
3
Air Conditioning

Process of treating air so as to control
simultaneously its temperature, humidity,
cleanliness, and distribution to meet the
environmental requirements of the conditioned
space

Environmental requirements of the conditioned
space may be determined by human occupancy
as related to comfort and health, a process, or a
product
4
Air Conditioning Processes

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Heating: transfer of energy to the air in a space
Cooling: transfer of energy from the air in a space
Humidifying: transfer of water vapor to the air in a
space
Dehumidifying: removal of water vapor from the air in
the space
Cleaning: removal of particulate and biological
contaminants from the air in a space
Air Motion (Circulation): movement of air through the
spaces in a building to achieve the proper ventilation and
facilitate the energy transfer, humidification (or
dehumidification), and cleaning processes described above
5
Energy

The capacity for producing an effect

Either stored or transient, and can be
transformed from one to another

Forms include: thermal (heat), mechanical
(work), electrical, chemical
6
Heat

Energy in transit from one mass to another as a
result of a temperature difference between two
masses

A basic law of thermodynamics states that heat
always flows from a higher temperature to a
lower temperature
7
Sensible Heat

Heat which changes the temperature of a
substance without changing its state
8
Latent Heat

Heat which changes the state of a substance
without changing its temperature

Two familiar examples: latent heat of fusion
(changing ice to water) and latent heat of
vaporization (changing water to vapor)
9
British Thermal Unit - BTU

A measure of the quantity of heat energy

Heat energy in a BTU is that required to raise
the temperature of a pound of water 1º
Fahrenheit
10
Heat Energy Flow Rate

Rate of heat loss/heat gain associated with
buildings

Also associated with applied heating and air
conditioning equipment

Normally stated in the terms BTU/hr
11
HEAT TRANSFER/
HEAT GENERATION
12
Heat Transfer

Movement of heat through surfaces and
openings of a building

Usually assumed to be steady state (various
temperatures throughout a system remain
constant with respect to time during heat
transmission)

Based upon predetermined temperature
differences
13
Heat Loss/ Heat Gain

Heat Loss – heat transferred from the interior
of a building to its exterior

Heat Gain – heat transferred from the exterior
to the interior of a building
14
Heat Transfer Modes

Three modes of heat transfer: Conduction,
Convection, Radiation

Usually all three modes occur simultaneously

In some instances, the methods can be
separated, but in others, only the combined
effect can be determined
15
Conduction
Conduction is the transmission of heat through
solids and composite sections such as structural
components
 Conduction does not occur only within one
object or substance, it also occurs between
different substances that are in contact with
one another
 By building the walls and roofs of a building of
materials having known conductive
characteristics, the heat flow rate for the
building can be controlled

16
Convection

Convection is the transfer of heat due to the
movement of a fluid: gases, vapors, and liquids

If the fluid moves because of a difference in
density resulting from temperature changes, the
process is called natural convection or free
convection

If the fluid is moved by mechanical means
(pumps or fans), the process is called forced
convection
17
Radiation

Radiation is the transfer of heat through space by
energy carrying electromagnetic waves

Radiant heat passing through air does not warm the
air through which it travels

All objects absorb and radiate heat

The amount of radiant heat given off in a specified
period of time is dependent on both the
temperature as well as the extent and nature of the
radiating object
18
19
HEATING & COOLING
LOADS
20
Factors that Determine Building
HVAC Energy Use

Building configuration and orientation

Building envelope construction

Interior space arrangement

Design temperature and humidity, indoor and outdoor

Zoning criteria

Equipment application and sizing

Control methodologies
21
Building Cooling and Heating
Requirements

A function of three heat transfer
components:
◦ Heat gains or losses through the building
surfaces [walls, fenestration, roof, etc.]
◦ Heat gains from internal heat producing
sources [lights, people, appliances, etc.]
◦ Heat gains or losses from infiltration of
outdoor air through window and door cracks,
floors, walls, etc.
23
Indoor Design Conditions
The primary purpose of the heating and airconditioning system is to maintain the space in
a comfortable and healthy condition
 This is generally accomplished by maintaining
the dry-bulb temperature and the relative
humidity within an acceptable range
 The HVAC Applications Volume of the ASHRAE
Handbook gives recommendations for indoor
design conditions for specific comfort as well as
industrial applications

24
Indoor Design Conditions…cont’d

ANSI / ASHRAE Standard 55-2004
◦ “Thermal Environmental Conditions for Human
Occupancy” specifies the combinations of indoor
thermal environmental factors and personal factors
that produce acceptable conditions to a majority of
the occupants
25
ANSI / ASHRAE Std. 55-2004
Acceptable range of operative temperature and humidity
26
Outdoor Design Conditions

Climate Design Information for 4422 locations
throughout the world are included on the CD
that accompanies the HVAC Fundamentals
Volume of the ASHRAE Handbook

Appropriate outdoor design conditions can be
selected from this ASHRAE document in most
applications; however some states dictate the
outdoor design conditions in their building
codes
27
Typical Brick Veneer Wall Section
28
Calculation of Overall Heat Transfer Coefficient - U


U = overall heat transfer coefficient, BTU/ hr· sf· ºF
R = thermal resistance, hr· sf· ºF /BTU
Wall Component
Thermal Resistance
Stud Path (~10%)
Cavity Path (~90%)
Outside air film (15 mph)
0.17
0.17
Face brick (4”)
0.44
0.44
Air space (1”)
0.85
0.85
Styrofoam insulation (1”, 90ºF)
5.00
5.00
Batt insulation
0
11.00
Wood studs
4.38
0
Foil-back gypsum board
0.56
0.56
Inside air film (still air)
0.68
0.68
Total R (R+)
12.08
18.70
R = (0.10)(12.08)+(0.90)(18.70) = 18.04
U = 1 / R = 1 / 18.04 = 0.055
29
Transmission Heat Loss Through Walls, Roofs, and Glass
H = A x U x TD
H = heat loss, BTU/hr
A = surface area of element, sf
U = overall heat transfer coefficient, BTU/ hr· sf· ºF
TD = design dry bulb temperature difference between
indoors and outdoors, ºF
30
Transmission Heat Gain Through Walls and Roofs
H = A x U x CLTD
H = heat gain, BTU/hr
A = surface area of element, sf
U = overall heat transfer coefficient, BTU/ hr· sf· ºF
CLTD = cooling load temperature difference, ºF
31
Conduction Heat Gains Through Glass
H = A x U x CLTD
H = heat gain, BTU/hr
A = surface area of element, sf
U = overall heat transfer coefficient, BTU/ hr· sf· ºF
CLTD = cooling load temperature difference, ºF
Solar Heat Gain Through Glass
H = A x SC x SCL
H = heat gain, BTU/hr
SC = shading coefficient
SCL = solar cooling load factor
32
Infiltration Heat Gain and Heat Loss

The uncontrolled leakage of outdoor air into a
building through window and door cracks,
floors, walls, etc., as well as the flow of outdoor
air into a building through the normal use of
exterior doors
[Exfiltration is the leakage of indoor air out of
the building. The amount of exfiltration equals
the amount of infiltration]
33
Heat Gain from Occupants
Activity
Typical
Application
Sensible Latent
(BTU/hr) (BTU/hr)
Seated at rest
Theater
210
140
Seated, very light work
Hotels, Apartments
230
190
Seated, eating
Restaurant
255
325
Seated, light work
Offices
255
255
Standing, walking slowly
Retail store, bank
315
325
Light bench work
Factory
345
435
Walking, light machine work Factory
345
695
Bowling
Bowling alley
345
625
Heavy work, lifting
Factory
565
1035
Heavy work
Gymnasium
635
1165
34
Heat Gains from Lights
•
Each watt of lighting load (including both lamp and ballast) releases
3.413 BTU/hr
Heat Gain from Motors
•
Each brake or net horsepower of motor load divided by the
efficiency (including both motor and drive) releases 2545 BTU/hr
H = 2545 BTU/hr x Bhp / EffM x EffD
H = heat gain, BTU/hr
Bhp = brake horsepower
EffM = motor efficiency, decimal fraction, 0 – 1.0
EffD = drive efficiency, decimal fraction, 0 – 1.0
35
Heat Gains from Appliances and Equipment



Appliances and equipment (including food prep.,
hospital, lab, office, etc.) normally produce significant
sensible heat, and may also produce significant latent
heat.
To estimate the cooling load, specific heat gain data
obtained from the manufacturer is preferred. However,
if it is not available, recommended heat gains are
published by ASHRAE and other sources.
Evaluation of the operating schedule and the load factor
for each piece of equipment is essential.
36
Energy Saving Opportunities

Change indoor temperature and/or humidity
set-points

Improve building thermal envelope
◦ Apply additional thermal insulation
◦ Improve fenestration
◦ Reduce infiltration

Improve lighting system efficiency
37
INDOOR AIR QUALITYASHRAE STD. 62-1
38
39
AHSRAE Standard 62.1
Ventilation for Acceptable Indoor Air Quality

Acceptable Indoor Air Quality:
Air in which there are no known contaminants at
harmful concentrations as determined by cognizant
authorities and with which a substantial majority (80%
or more) of the people exposed do not express
dissatisfaction
40
The Purpose of Standard 62

The purpose of the Standard, first published in
1973 – “Standards for Natural and Mechanical
Ventilation”, has remained consistent:
“To specify minimum ventilation rates and other
measures intended to provide indoor air quality that
is acceptable to human occupants and that minimizes
adverse health effects.”
41
Under Continuous Maintenance…

The standard is updated on a regular bases
using ASHRAE’s Continuous Maintenance
Procedures
◦ Continuously revised addenda are publicly reviewed
and approved by ASHRAE
◦ Published in a Supplement approximately 18 months
after each new edition of the Standard
OR
◦ A new, complete edition of the Standard is published
every three years
42
Significant Changes to ASHRAE Standard 62
1981 Edition:
◦ Reduced the minimum outdoor air requirements for ventilation
 Office – 15 cfm/person to 5 cfm/person
1989 Edition:
◦ Increased minimum outdoor air requirements for ventilation
[Response to growing number of buildings with apparent IAQ
problems]
 Office – 5 cfm/person to 20 cfm/person
2004 Edition:
◦ Changed the ventilation rate procedure to include the
summation of two components: the occupant-density related
component, and the area related component
◦ Changed the ventilation rates in Table 6-1 to apply to nonsmoking spaces
43
Significant Changed … cont’d
2004 cont’d:
◦ Added classification of air with respect to contaminant and odor
intensity, and established guidelines for recirculation
2007 Edition:
◦ Updated information in Table 4-1 – “National primary ambient
air quality standards for outdoor air as set by the U.S.
Environmental Protection Agency”
◦ Added Section 5.18 – Requirements for buildings containing ETS
areas and ETS-free areas (ETS-Environmental Tobacco Smoke)
44
ASHRAE Standard 62.1

Two alternative procedures for determining
outdoor air intake rates:
◦ Ventilation Rate Procedure
 This is a prescriptive procedure in which outdoor
air intake rates are determined based on space
type/application, occupancy level, and floor area
◦ IAQ Procedure
 This is a design procedure in which outdoor air
intake rates and other system design parameters
are based on an analysis of contaminant
concentration targets, and perceived acceptability
targets
45
62.1-2007
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62.1-2007
47
62.1-2007
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62.1-2007
49
62.1-2007
50
62.1-2007
51
62.1-2007
52
North Carolina State Building Code
Mechanical Code
[International Mechanical Code]
Energy Code
[International Energy Conservation Code]
53
NCSBC
54
NCSBC
55
NCSBC
56
Noteworthy Energy Conservation
Considerations

CO2 based demand controlled ventilation

Air-to-air energy recovery
[Exhaust air stream – outdoor ventilation air stream]
57
Energy Conservation Imperative

Ongoing effective maintenance program for
equipment and controls
58
Commentary
ASHRAE Standard 62.1 – Code Adoption
◦ Standard 62.1 is voluntary until adopted by code or
other regulation
◦ Code adoption is often delayed due to time required
to be accepted and integrated into the model codes,
as then accepted and adopted by the local codes
59
Energy Saving Opportunities
Optimize the energy requirements associated
with outdoor ventilation air

Apply CO2 based demand control

Apply air-to-air energy recovery equipment
60
VAPOR
COMPRESSION
REFRIGERATION
CYCLE
61
Vapor Compression
Refrigeration Cycle
Evaporation: Low pressure liquid absorbs heat
(heat source) and changes state to a low
pressure vapor
 Compression: Low pressure vapor is
compressed to high pressure vapor
 Condensation: High pressure vapor is cooled
(heat sink) and changes state to a high pressure
liquid
 Expansion: High pressure liquid is reduced to
low pressure liquid via throttling

62
Vapor Compression
Refrigeration Cycle Components
EVAPORATOR
63
Basic Liquid Chiller - Water Cooled
EVAPORATOR
CHILLED WATER
64
ENERGY USAGE
HVAC SYSTEM ENERGY USE
The energy use in a Heating, Ventilating and AirConditioning System is that associated with:
The generation of heating and cooling medium – steam,
hot water, chilled water, and dx refrigeration (through
boilers, chillers, and dx refrigeration assemblies utilizing
fossil fuels and electricity)
 The movement of heat transfer fluids – air and water
(through fans and pumps utilizing electricity)
[As in the previous sections, energy saving opportunities
will be identified and discussed throughout this seminar]

PSYCHROMETRICS
67
Psychrometrics

The science that deals with the thermodynamic
properties of moist air – dry air/water vapor
mixture, and the utilization of these properties
to analyze conditions and processes involving
moist air

For the accuracy required in the majority of air
conditioning problems, the perfect gas relations
can be applied when calculating the
thermodynamic properties of moist air
68
Psychrometrics

Atmospheric air consists of a large number of gases
(including oxygen, nitrogen, argon, and carbon dioxide)
as well as water vapor and various contaminants

Dry air exists when all water vapor and contaminants
have been removed from atmospheric air

Moist air is a binary (two-component) mixture of dry
air and water vapor. The amount of water vapor in
moist air can vary from zero (dry air) to a maximum
(saturation) which depends on temperature and
pressure
69
Properties of Moist Air

Psychrometrics deals with the following defined
properties of moist air:
◦ Dry Bulb Temperature –the temperature of a gas or mixture
of gases indicated by an accurate thermometer after correction
for radiation. (ºF)
◦ Wet Bulb Temperature – thermodynamic wet bulb,
temperature is the temperature at which liquid or solid water, by
evaporating into the air, can bring the air to saturation
adiabatically at the same temperature. (ºF)
◦ Dew Point Temperature – the temperature at which the
condensation of water vapor begins for a given state of humidity
and pressure as the temperature of the vapor is reduced. (ºF)
70
◦ Relative Humidity–the ratio of the mol fraction of water
vapor present in the air, to the mol fraction of water vapor
present in saturated air at the same temperature and
barometric pressure. (%RH)
◦ Humidity Ratio– Ratio of the mass of water vapor (steam)
associated with one pound mass of dry air. (Lbmw/Lbmda) or
(Grainsw/Lbmda ) [Also referred to as Specific Humidity]
◦ Enthalpy– thermodynamic property of a substance defined as
the sum of its internal energy plus the quantity PV/J, where
P=pressure of the substance,V=it’s volume, J=mechanical
equivalent of heat. (Btu/Lbmda) [Also referred to as total heat
and heat content]
71
◦ Specific Volume–the ratio of the volume of the mixture to one
pound mass of dry air (Ft³/Lbmda)
◦ Vapor pressure– the pressure exerted by a vapor. If a vapor is
kept in confinement over its liquid so that the vapor can
accumulate above the liquid, the temperature being held
constant, the vapor pressure approaches a fixed limit called the
maximum, or saturated vapor pressure, dependent only on the
temperature and the liquid. (In. Hg)
72
PSYCHROMETRIC CHART
The Psychrometric Chart is a graphic representation of
the thermodynamic properties of moist air. To minimize
the complexity of the chart, the air component is a fixed
value - per pound of dry air.
Dr. Willis Carrier is credited with the development of the
psychrometric chart in 1911. There are a number of
psychrometric charts, defined by dry-bulb temperature
ranges.
74
75
76
Sling Psychrometer
Battery Operated Psychrometer
Digital Psychrometer
80
81
82
83
84
85
PSYCHROMETRIC CLASS PROBLEM
In an office, the following measurements were made with a
sling psychrometer, 70ºF dry-bulb, and 58.5ºF wet-bulb. From
a normal temperature psychrometric chart, determine the
following thermodynamic properties:
Dew Point Temperature
Humidity Ratio
Relative Humidity
Enthalpy
Specific Volume
CLASS PROBLEM SOLUTION
Dew Point Temperature: 50.6ºF
Humidity Ratio: 54.6 Grains/Lbm
Relative Humidity: 50% RH
Enthalpy: 25.33 Btu/Lbm
Specific Volume: 13.5 Ft³/Lbm
88
89
90
91
92
93
94
PSYCHROMETRIC CLASS PROBLEM
Field measurements from an industrial air handling unit cooling and
dehumidification section (spray-type air washer) indicated the
following:
CFM: 71,000
Entering Air Conditions: 84.4ºF dry-bulb, 71.4ºF wet-bulb
Leaving Air Conditions: 63.5ºF dry-bulb, 63.0ºF wet-bulb
Determine the following:
Entering Air Enthalpy (h): Btu/Lbm
Leaving Air Enthalpy (h): Btu/Lbm
Total Cooling Capacity: Btu/Hr & Tons
Psychrometric Equations
HT = HS + HL
HT = Total Heat, BTU/Hr
HS = Sensible Heat, BTU/Hr
HL = Lateral Heat, BTU/Hr
HS = (1.08)(CFM)(∆t)
HL = (0.68)(CFM)(∆W)
HT = (4.45)(CFM)(∆h)
HT = (500)(GPM)(∆t)
CFM - Volumetric airflow rate, Ft2/Min
∆t - Temperature differential, ˚F
∆W - Humidity ratio differential, Grains/Lb.
∆h - Enthalpy differential, BTU/Lb.
GPM - Volumetric water-flow rate
97
CLASS PROBLEM SOLUTION
Entering Air Enthalpy (h) = 35.1 Btu/Lbm
Leaving Air Enthalpy (h) = 28.5 Btu/Lbm
(From Psychrometric Chart)
HT = (4.45) (71,000) (35.5 – 28.5) = 2,085,270 Btu/Hr
HT = 2,085,270 Btu/Hr ∕ 12,000 Btu/Hr Ton = 173.8 Tons
Continuity Equation
Q = VA
Q = Volumetric flow rate, Ft3/Min
V = Average velocity, Ft/Min
A = Cross-sectional Area, Ft2
99
HVAC SYSTEMS
100
Developing an HVAC System
Basic System Requirements
Provide heating
 Modulate heating to satisfy variations in load
 Provide cooling
 Modulate cooling to satisfy variations in load
 Provide adequate ventilation
 Provide air cleaning (filtration)
 Control humidity (humidify/dehumidify)
 Integrate with other building systems

101
Developing an HVAC System
Critical Consideration Issues

Environmental Control Requirements
◦ Occupant Comfort
◦ Clean Air / Ventilation
◦ Product / Process Requirements

Equipment Fuctionality
◦ Reliability while meeting requirements

Economics
◦ Initial Cost
◦ Operating Cost
◦ Maintenance Cost
102
HVAC System General Classification

All-Air Systems

Air-and-Water Systems

All-Water Systems

Unitary Air Conditioners
103
HVAC System Definitions
All-Air System
◦ Provides complete sensible and latent cooling capacity in the
cold air supplied by the system
◦ No additional cooling is required at the zone
◦ Heating can be accomplished by the same airstream, either in
the central system or at a particular zone
Air-and-Water System
◦ Conditions the spaces by distributing air and water sources to
terminal units installed in habitable space throughout a building
◦ The air and water are cooled or heated in central mechanical
equipment rooms
◦ The air supplied is called primary air, the water supplied is called
secondary water
104
HVAC System Definitions
All-Water System
◦ Heats and/or cools a space by direct heat transfer between
water and circulating air
Unitary System
◦ Packaged air conditioning units with integral refrigeration cycles
105
All-Air Systems

Single zone draw-through

Constant volume terminal reheat

Dual-duct

Multizone

Variable air volume (VAV)
106
General Air Handling System Layout
107
Air Side Economizer
Modes:
 Free Cooling
 Economy Refrigeration
108
Constant Volume System with Terminal
Reheat
109
Dual Duct System
110
Variable Air Volume System (VAV)
111
True VAV Terminal Unit
112
Parallel Fan-Powered VAV Terminal Unit
113
Series Fan-Powered VAV Terminal
Unit
114
Fan Volume Modulation for VAV
Systems
116
Basic Fan Laws
1.
Volume varies directly with speed ratio
CFM2 = CFM1 (RPM2 / RPM1)
2.
Pressure varies with square of speed ratio
P2 = P1 (RPM2 / RPM1)2
3.
Horsepower varies with cube of speed ratio
HP2 = HP1 (RPM2 / RPM1)3
117
Fan Problem
An existing centrifugal supply air fan serving a central
station air washer delivers 90,000 cfm @ 2” s.p. (wg), 825
rpm and 47.3 bhp.
It has been established that the volumetric air flow rate
(cfm) can be reduced 20% because of excessive design
safety factors and plant production equipment
modifications.
Determine: 1) new air volume, 2) rpm @ new cfm, 3) bhp
@ new cfm, and 4) annual electrical savings
Electricity cost:
Demand charge - $6.00/kw (avg)
Energy charge - $0.031/kwh (avg)
118
FanNewProblem
Air Volume = 0.8 Solution
x 90,000 = 72,000 cfm
1.
2.
3.
4.
New RPM = 825 (72,000/90,000) = 660 rpm
New HP = 47.3 (660/825)3 = 24.2 hp
Annual electrical savings…
HP reduction = 47.3 – 24.2 = 23.1 hp
KW reduction = (23.1 hp)(0.746 kw/hp) = 17.2 kw
Energy:
(23.1 hp)(0.746 kw/hp)(8760 hr/yr)($0.031/kwh)(1.03 tax)
= $4,820 /yr
Demand:
(17.2 kw/mo)(12 mo/yr)($6.00/kw)(1.03 tax)
= $1,276 /yr
Annual Electrical Savings:
$4,820 + $1,276 = $6,096 /yr
119
Air and Water System

Induction

Fan Coil
120
Induction Unit
Induction Nozzle
121
Fan Coil Unit

Note: Conditioned outdoor ventilation air is delivered
into the space through an independent de-coupled
system
122
All-Water Systems

Unit Ventilator

Fan Coil
123
Unit Ventilator
124
Fan Coil Unit

Note: Outdoor ventilation air provided through
infiltration
125
Unitary Air Conditioners

Rooftop

Split System

Through-the-wall
126
Packaged Rooftop Air Conditioning
Units
127
Water-Source Heat Pumps
128
Water Loop Heat Pump System
129
Energy Saving Opportunities

Convert air-handling systems from constant volume to
variable-air-volume (VAV) airflow: employ VAV boxes and fan
motor variable speed drives. [Typical target systems:
constant volume systems with terminal reheat, & dual
duct systems]

Convert traditional multi-zone units to by-pass multi-zone
units

Install air-side economizers – maximize the use of
outdoor air for cooling: “free-cooling” and “economy
refrigeration”
Eliminate the air-side economizer cycle on multi-zone units
 Install water-side economizers

130
Energy Saving Opportunities… cont’d





Optimize/balance volumetric airflow rates and eliminate
excess by fan speed adjustments
Implement occupied/unoccupied scheduling
Employ air-to-air heat exchangers – exhaust air heat
recovery
Develop and implement an effective Preventative
Maintenance (PM) program
Replace equipment with higher efficiency equipment.
[Evaluate the employment of evaporative condensers in lieu
of air-cooled condensers]
Air-to-Air Heat Recovery
Properly applied air-to-air energy recovery equipment, which
transfers energy between supply and exhaust airstreams, will
reduce building and/or process energy usage in a costeffective manner.
Air-to-air energy recovery applications fall into three categories
(ASHRAE):
◦ Comfort-to-comfort
◦ Process-to-comfort
◦ Process-to-process
[Because of time constraints in this workshop, we will limit our
discussion to comfort-to-comfort applications]
132
Comfort-to-Comfort Applications

Sensible Heat Devices - only transfer sensible heat
between the supply and exhaust airstreams, except
when the exhaust airstream is cooled to below its dew
point.

Total Heat Devices - transfer both sensible and latent
heat between the supply and exhaust airstreams
133
Performance Rating of Air-to-Air EnergyRecovery Equipment
ASHRAE Standard 84-1991, “Method of Testing Air-to-Air
Heat Exchangers”, was developed to establish a uniform
testing and rating standard.
e=
Actual transfer for the given device
Maximum possible transfer between airstreams
e = effectiveness
134
Air-to-Air Heat Recovery
Equipment

Rotary (Heat Wheel)

Heat Pipe

Static Heat Echanger

Runaround System
135
Rotary (Heat Wheel)
136
Heat Pipe
137
Static Heat Exchangers
138
Runaround System
139
CONTROL STRATEGY
140
Control Strategy
Optimize the operation of the HVAC systems
[To minimize the fan, heating and cooling energy requirements]
 Develop and implement system scheduling –
occupied/unoccupied
 Implement optimal start/stop
 Optimize the temperature and/or humidity setpoints in both
the occupied and unoccupied periods
 Introduce outdoor ventilation air only when the building is
occupied
 Provide control system override
141