HVAC Equations,
Concepts, and Definitions
Presented By:
David Sellers
Senior Engineer; Facility Dynamics Engineering
A Few Acronyms and Definitions
Acronyms
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AFD – Adjustable Frequency Drive
AHU – Air Handling Unit
ASHRAE – American Society of Heating Ventilating and Air
Conditioning Engineers
CV - Constant Volume
HVAC – Heating Ventilating and Air Conditioning
MOA – Minimum Outdoor Air
Psych Chart – Psychrometric Chart
VAV – Variable Air Volume
VFD – Variable Frequency Drive
VFD – (Adjustable Frequency Drive)
VSD – Variable Speed Drive
VFD – (Adjustable Frequency Drive)
AFD Acronym Definition
AFD A Few Days
AFD Abbreviated Functional Description
AFD Accelerated Freeze-Drying (food processing)
AFD Accident Free Discount (insurance)
AFD Acid Fractionator Distillate
AFD Acoustic Flat Diaphragm (electronics)
AFD Acrofacial Dysostosis
AFD Acrofacial Dysostosis, Catania Type
AFD Active Format Descriptor
AFD Adaptive Flexible Defense
AFD Adaptive Flight Display
AFD Adjustable Frequency Drive
AFD Advanced Full-screen Debugger
AFD African Development Foundation
AFD African Development Fund
AFD Aft Flight Deck
AFD Agence Française de Développement (French Development Agency)
AFD Air Force Depot
AFD Airfield Database
AFD Airport Facilities Directory
AFD Alarm Format Definition
AFD Albany Fire Department
AFD Alcohol Free Day
AFD Alexandria Fire Department
AFD All Friggin' Day
AFD Alt.fan.dragons (Usenet newsgroup)
AFD Alternative Forms of Delivery (Canada)
AFD Amarillo Fire Department (Amarillo, TX)
AFD American Funds Distributors, Inc.
AFD Amsterdam Fire Department
AFD Ancillary Function Driver
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AFD Angwin Fire Department (Angwin, CA)
AFD Anticipatory Failure Determination
AFD Apical Fibrobullous Disease
AFD Approved for Design
AFD Approximately Finite Dimensional
AFD April Fool's Day
AFD April Fools Day
AFD Arc Fault Detection
AFD Arc-Fault Detection
AFD Architecture Flow Diagrams
AFD Area Forecast Discussion (US National Weather Service)
AFD Armed Forces Division
AFD Arming & Fusing Device
AFD Arming-Firing Device
AfD Articles for Deletion (Wikipedia)
AFD Ask for Details
AFD assign fixed directory (US DoD)
AFD Assistant Flight Director
AFD Association Franèaise des Diabétiques
AFD Athletic Field Design
AFD Atlanta Fire Department
AFD Austin Fire Department (Texas)
AFD Autómata Finito Determinista
AFD Automated File Designator
AFD Automated Forging Design
AFD Automatic Fault Detection
AFD Automatic File Distribution
AFD Automatic Fire Detection
AFD Average Fade Duration
AFD Away from Desk
AFD Axial Flux Density
AFD Axial Flux Difference
AFD Active Format Description
AFD Adaptive Forward Differencing
AFD Adjustable Frequency Drives
AFD Asus Foundation Drivers
DEFINITIONS AND USEFUL EQUATIONS
2
AFA2D
Acronyms
•
•
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•
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•
AFD – Adjustable Frequency Drive
AHU – Air Handling Unit
ASHRAE – American Society of Heating Ventilating and Air
Conditioning Engineers
CV - Constant Volume
HVAC – Heating Ventilating and Air Conditioning
MOA – Minimum Outdoor Air
Psych Chart – Psychrometric Chart
VAV – Variable Air Volume
VFD – Variable Frequency Drive (Adjustable Frequency Drive)
VSD – Variable Speed Drive (Adjustable Frequency Drive)
DEFINITIONS AND USEFUL EQUATIONS
3
AFA2D
Definitions
• Sensible energy, QS (Btu’s, Btu’s/lb)
Energy that causes a temperature change we can
feel
• Dry bulb temperature, Tdb (°F)
An indication of sensible energy measured by a
standard thermometer exposed to air; increasing
dry bulb temperature = increasing sensible energy
DEFINITIONS AND USEFUL EQUATIONS
4
AFA2D
Definitions
• Latent energy, QL (Btu’s, Btu’s/lb)
Energy that is used to keep water in a vapor
state
• Wet bulb temperature, Twb (°F)
An indication of latent energy measured by a
standard thermometer with its bulb covered by a
wick that is saturated with water and exposed to
moving air; increasing wet bulb temperature =
increasing latent energy
DEFINITIONS AND USEFUL EQUATIONS
5
AFA2D
Definitions
• Dew point temperature, Tdp (°F)
The temperature at which water will begin to
condense out of a given sample of air. Also an
indication of moisture content; increasing dew
point = increasing latent energy.
At saturation Tdp = Twb = Tdb
DEFINITIONS AND USEFUL EQUATIONS
6
AFA2D
Definitions
• Enthalpy, h (Btu/lbdry air)
A measure of the total energy content of air including
both sensible and latent energy; increasing enthalpy
= increasing energy content
DEFINITIONS AND USEFUL EQUATIONS
7
AFA2D
Definitions
• Relative humidity, RH (%)
The amount of water vapor in the air at a given
temperature relative to what it could hold at that
temperature; 100% = saturation; increasing
specific humidity = increasing moisture content,
increasing dew point, and increasing wet bulb
temperature.
In Antarctica, the relative humidity approaches
100% much of the time, just like in Florida after a
thunderstorm
DEFINITIONS AND USEFUL EQUATIONS
8
AFA2D
Definitions
• Specific humidity, w (lbwater/lbdryair, grainswater/lbdryair)
The ratio of the mass of water to the mass of dry
air in a given sample of air; increasing specific
humidity = increasing moisture content, increasing
dew point, and increasing wet bulb temperature.
In Antarctica, the specific humidity at a relative
humidity of 100% is very low. In Florida, the
specific humidity at a relative humidity of 100% is
quite high relative to Antarctica.
DEFINITIONS AND USEFUL EQUATIONS
9
AFA2D
Definitions
• Psychrometrics
The field of engineering concerned with the
determination of physical and thermodynamic
properties of gas-vapor mixtures.
DEFINITIONS AND USEFUL EQUATIONS
10
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

0.8
SENSIBLE HEAT
TOTAL HEAT
0
300
0.4
Qs
Qt
50
55
.025
45
-
.024
2.0
-2000
-100
0
4.0
-8.0
-8 
.0
20
0
.0
-4 .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0.5
0.3
5000
0.6
0.2
0.1
AFA2D
0
.02355
80
40
0
85
.022
0
.021
500
h
W
.020
35
75
UN
D
PE
R
PO
70
0
5
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
DEFINITIONS AND USEFUL EQUATIONS
15
90
85
80
75
70
65
55
50
40
45
10
DRY BULB TEMPERATURE - °F
Y
IDIT
HUM
110
20
VE
ATI
REL
8%
6%
4%
2%
105
35
10
35
30
15
25
10
30
25
20
20
15
15
5
10
0
5
0
0
-5
-5
10
30
40
25
%
15
100
45
0
550
40
95
35
20
55
60
40
30
60
70
50
25
%
80
55
45
5
65
90
15
60
EN
TH
AL
PY
-B
TU
60
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
65
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
115
DR
Y
AI
R
75
25
20
5
50
.017
70
10
0
.018
30
O
F
• Psychrometric Chart
Scary, complicated
looking graph.
.019
80
.001
120
1000
150
0
Definitions
ENTHALPY
HUMIDITY RATIO
30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
11
AFA2D
Definitions
• Psychrometric Equations
The alternative to using the psych chart.
DEFINITIONS AND USEFUL EQUATIONS
12
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

0.8
SENSIBLE HEAT
TOTAL HEAT
0
300
0.4
Qs
Qt
50
55
.025
45
-
.024
2.0
-2000
-100
0
4.0
-8.0
-8 
.0
20
0
.0
-4 .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0.5
0.3
5000
0.6
0.2
0.1
AFA2D
0
.02355
80
40
0
85
.022
0
.021
500
1000
150
0
Definitions
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
75
.019
80
.018
• Psychrometric Chart
Graphical tool that allows the informed user to
determine multiple parameters like enthalpy, dew
point, relative humidity, specific humidity, dry bulb
temperature and wet bulb temperature for a
sample of air if any two of them are know.
30
0
5
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
DEFINITIONS AND USEFUL EQUATIONS
15
90
85
80
75
70
65
55
50
40
45
10
DRY BULB TEMPERATURE - °F
20
Y
IDIT
HUM
110
35
10
35
30
15
25
10
25
20
20
15
15
5
10
0
5
0
0
-5
-5
10
30
VE
ATI
REL
8%
6%
4%
2%
105
25
5
30
40
20
%
15
100
45
0
550
40
95
35
30
55
60
40
5
60
70
50
25
%
80
55
45
0
65
90
15
60
EN
TH
AL
PY
-B
TU
60
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
.001
120
70
20
PE
R
PO
UN
D
O
F
65
115
DR
Y
AI
R
75
25
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
.017
70
10
50
30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
13
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

SENSIBLE HEAT
TOTAL HEAT
0
300
20
00
0.2
0.1
5000
0.4
.025
45
-
.024
Qs
Qt
2.0
4.
-8.0 0
-8 
.0
-2000
-100
0
0
-4. .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0 .5
0.3
0.6
55
0
0
.02355
80
40
85
.022
.021
500
1000
150
0
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
75
.019
80
.018
30
PO
UN
D
O
F
65
70
PE
R
20
55
0
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
5
DEFINITIONS AND USEFUL EQUATIONS
90
85
80
75
70
65
55
50
45
40
35
30
Dry
Bulb Temperature
Axis
and Lines
5
1
1
2
8%
6%
4%
2%
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
115
10
0
DRY BULB TEMPERATURE - °F
20
110
20
25
15
20
10
15
5
10
0
5
0
0
-5
-5
5
30
25
35
Y
IDIT
HUM
E
ATIV
REL
105
25
15
30
40
20
%
15
100
45
0
550
40
95
35
30
25
%
80
5
55
60
40
10
Increasing
temperature
60
and energy content
70
50
45
0
65
90
15
60
EN
TH
AL
PY
-B
TU
60
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
75
25
DR
Y
AI
R
70
10
50
.017
.001
120
0.8
50
30
25
0
ENTHALPY - BTU PER POUND OF DRY AIR
14
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

0
300
20
00
0.2
0.1
5000
0.4
.024
Qs
Qt
2.0
4.
-8.0 0
-8 
.0
-2000
-100
0
0
-4. .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0 .5
.025
45
-
0
0
.02355
80
40
85
.022
.021
500
1000
150
0
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
75
.019
80
.018
30
PE
R
0
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
5
15
DEFINITIONS AND USEFUL EQUATIONS
90
85
80
75
70
65
55
50
45
40
10
DRY BULB TEMPERATURE - °F
8%
6%
4%
2%
110
20
Y
IDIT
HUM
E
ATIV
REL
105
35
10
35
30
15
25
10
30
25
20
20
5
15
15
0
10
0
5
5
0
-5
-5
10
30
40
25
%
15
100
45
0
550
40
95
35
20
55
60
40
30
60
70
50
25
%
80
55
45
0
65
90
15
60
EN
TH
AL
PY
-B
TU
60
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
PO
UN
D
O
F
65
Increasing
moisture and
70
energy content
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
115
DR
Y
AI
R
75
25
20
5
50
.017
70
10
Specific Humidity Axis and Lines
SENSIBLE HEAT
TOTAL HEAT
0.3
0.6
55
.001
120
0.8
50
30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
15
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

0.8
SENSIBLE HEAT
TOTAL HEAT
0
300
20
00
0.2
0.1
5000
0.4
2.0
4.
-8.0 0
-8 
.0
0
55
.025
45
-
.024
Qs
Qt
0
-4. .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0 .5
0.3
0.6
50
-2000
-100
0
0
.02355
80
40
85
.022
.021
500
1000
150
0
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
Increasing.019
relative
50
.018 energy
humidity and
.017
content
75
PO
UN
D
O
F
65
70
20
PE
R
Relative Humidity Lines
and Saturation
Curve
15
(100% RH)
90
0
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
5
25
%
20
15
DEFINITIONS AND USEFUL EQUATIONS
90
85
80
75
70
65
60
55
50
45
40
10
DRY BULB TEMPERATURE - °F
35
10
35
30
15
25
10
30
25
20
20
5
15
15
0
10
0
5
5
0
-5
-5
10
20
8%
6%
4%
2%
Y
IDIT
HUM
E
ATIV
REL
110
40
25
0
30
105
45
%
15
95
35
0
550
40
100
40
30
55
60
45
5
60
70
50
10
65
55
80
EN
TH
AL
PY
-B
TU
60
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
.001
120
75
25
DR
Y
AI
R
70
115
30
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
80
30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
16
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

SENSIBLE HEAT
TOTAL HEAT
0
300
20
00
0.2
0.1
5000
0.4
2.0
4.
-8.0 0
-8 
.0
0
.025
45
-
.024
Qs
Qt
0
-4. .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0 .5
0.3
0.6
55
-2000
-100
0
0
.02355
80
40
85
.022
.021
500
1000
150
0
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
75
.019
80
.018
30
PO
UN
D
70
20
PE
R
Wet Bulb temperature
Axis15 and Lines
90
0
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
5
15
DEFINITIONS AND USEFUL EQUATIONS
8
6%
4%
2%
ID
HUM
110
90
85
80
75
70
65
60
55
50
45
40
10
DRY BULB TEMPERATURE - °F
25
%
20
10
35
30
15
25
10
30
25
20
20
5
15
15
0
10
0
5
5
0
-5
-5
10
35
105
25
20
E
TIV
ELA
R
%
30
40
100
45
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
Increasing temperature
%
5
.00535
1and
energy
content
ITY
0
550
40
95
35
30
55
60
40
5
60
70
50
45
0
65
55
80
EN
TH
AL
PY
-B
TU
60
.004
.003
.002
115
O
F
65
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
75
25
DR
Y
AI
R
70
10
50
.017
.001
120
0.8
50
30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
17
50
Note that constant
enthalpy and constant wet
bulb temperature lines are
almost but not quite
parallel
SENSIBLE HEAT
TOTAL HEAT
2.0
4.
-8.0 0
-8 
.0
.024
-2000
-100
0
0
0
.02355
80
40
85
.022
.021
500
1000
150
0
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
75
Increasing
enthalpy or
80
energy content
30
.019
.018
75
DR
Y
AI
R
70
25
PO
UN
D
O
F
65
70
PE
R
20
90
0
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
5
15
DEFINITIONS AND USEFUL EQUATIONS
90
85
80
75
70
65
55
50
45
40
10
DRY BULB TEMPERATURE - °F
8%
6%
4%
2%
110
20
10
35
30
25
15
30
25
20
20
10
15
5
10
0
5
0
0
-5
-5
5
15
35
Y
IDIT
HUM
E
ATIV
REL
105
25
10
30
40
20
%
15
100
45
0
550
40
95
35
25
%
80
40
30
55
60
45
5
60
70
50
10
0
65
55
60
EN
TH
AL
PY
-B
TU
60
15
50
.017
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
0
300
20
00
0.2
0.1
5000
0.4
Qs
Qt
0
-4. .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0 .5
0.3
0.6
.025
45
-
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
115
0.8
55
.001
120
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
18
ALTITUDE: SEA LEVEL
BAROMETRIC PRESSURE: 29.921 in. HG
ATMOSPHERIC PRESSURE: 14.696 psia
1.0
1.0

0.8
SENSIBLE HEAT
TOTAL HEAT
0
300
0.4
Qs
Qt
50
55
.025
45
-
.024
2.0
-2000
-100
0
4.0
-8.0
-8 
.0
20
0
.0
-4 .0
-2
-1.0
.5
-0
-0.4
-0.3
-0.2
-0.1
0.5
0.3
5000
0.6
0.2
0.1
AFA2D
0
.02355
80
40
0
85
.022
0
.021
500
1000
150
0
Definitions
ENTHALPY
HUMIDITY RATIO
h
W
.020
35
75
.019
80
.018
• Psychrometric Chart
If you know the dry bulb temperature and relative
humidity entering and leaving a cooling coil, you can
plot those points on the chart and determine:
• The entering and leaving enthalpy (energy content)
• The load (how much energy was transferred)
• The amount of moisture condensed, if any
• The entering and leaving dew point temperature
• The entering and leaving wet bulb temperature
30
0
5
Chart by: AKTON PSYCHROMETRICS, www.aktonassoc.com
DEFINITIONS AND USEFUL EQUATIONS
15
90
85
80
75
70
65
55
50
40
45
10
DRY BULB TEMPERATURE - °F
20
Y
IDIT
HUM
110
35
10
35
30
15
25
10
25
20
20
15
15
5
10
0
5
0
0
-5
-5
10
30
VE
ATI
REL
8%
6%
4%
2%
105
25
5
30
40
20
%
15
100
45
0
550
40
95
35
30
55
60
40
5
60
70
50
25
%
80
55
45
0
65
90
15
60
EN
TH
AL
PY
-B
TU
60
.016
.015
.01445
.013
.012
.011
.010
.009
40
.008
.007
.006
.00535
.004
.003
.002
.001
120
70
20
PE
R
PO
UN
D
O
F
65
115
DR
Y
AI
R
75
25
HUMIDITY RATIO - POUNDS MOISTURE PER POUND DRY AIR
.017
70
10
50
30
20
25
ENTHALPY - BTU PER POUND OF DRY AIR
19
Learn More about Using a Psych Chart
HTML version at:
http://www.buildingcontrolworkbench.com
Downloadable .pdf at:
http://customer.honeywell.com/techlit/pdf/770000s/77-E1100.pdf
DEFINITIONS AND USEFUL EQUATIONS
20
AFA2D
Definitions
• Cooling
A process that removes energy. For a space,
this is often accomplished by circulating air
through it at a temperature below the required
set point. For an airstream, this is often
accomplished by passing it over a surface that is
below the required supply temperature. If the
surface is below the dew point of the air stream,
dehumidification (moisture removal) will also
occur.
DEFINITIONS AND USEFUL EQUATIONS
21
AFA2D
Definitions
• Heating
A process that adds energy. For a space, this is
often accomplished by circulating air through it at a
temperature above the required set point. For an
airstream, this is often accomplished by passing it
over a surface that is above the required supply
temperature.
DEFINITIONS AND USEFUL EQUATIONS
22
AFA2D
Definitions
• Freezing
A condition that occurs when water is cooled to the
point where it changes phase from a solid to a liquid.
DEFINITIONS AND USEFUL EQUATIONS
23
AFA2D
Definitions
• Water Damage
A condition that occurs after frozen water contained
in a HVAC coil changes back to the liquid phase.
DEFINITIONS AND USEFUL EQUATIONS
24
AFA2D
Definitions
• Expletive
A generic reference to the field terminology used to
describe and discuss water damage when it occurs.
DEFINITIONS AND USEFUL EQUATIONS
25
AFA2D
Definitions
• Preheat
A process that heats a fluid stream to prepare it
for a subsequent HVAC process. In air handling
systems, this process is used to raise
subfreezing air above freezing to protect water
filled elements down stream from damage due to
freezing.
See the Functional Testing Guide (www.peci.org/ftguide) Air
Handling System Reference Guide Chapter 5 – Preheat, Table
5.1 to contrast preheat, reheat and heating applications
DEFINITIONS AND USEFUL EQUATIONS
26
AFA2D
Definitions
• Reheat
A process that uses heat to warm air being delivered
to a zone to prevent over cooling. The temperature of
the air was set by the need to hit a dehumidification
target or by the requirements of another zone, so it
can not be raised at the central system. The volume
can not be reduced because it has been set to assure
proper ventilation (contaminant control). In the limit,
reheat will raise the supply temperature to the zone
temperature but not above it.
DEFINITIONS AND USEFUL EQUATIONS
27
AFA2D
Definitions
• Economizer Process
An HVAC process designed to minimize the energy
required to cool a building
DEFINITIONS AND USEFUL EQUATIONS
28
AFA2D
Definitions
• Constant Volume System
An air handling or pumping process that, in general
terms, is always moving the same amount of water
or air. Pump or fan energy is fairly steady state.
Supply and return temperature differences will tend
to vary with load. In water systems, the control
valves will tend to be three-way valves.
DEFINITIONS AND USEFUL EQUATIONS
29
AFA2D
Definitions
• Variable Volume System/Variable Air Volume System
(VAV)
An air handling or pumping process that varies the
flow of water or air to match the requirements of the
load.. Supply and return temperature differences
will tend to hold steady regardless of load. In water
systems, the control valves will tend to be two-way
valves.
DEFINITIONS AND USEFUL EQUATIONS
30
Benchmarks
Contrasting Utility Consumption with Your Peers
DEFINITIONS AND USEFUL EQUATIONS
31
Average Daily Consumption Analysis
Contrasting Utility Consumption with Other Metrics
Peak electrical consumption may
be driven by the need to cool
DEFINITIONS AND USEFUL EQUATIONS
32
Occupancy may also
be a driver for electrical
consumption, but
maybe not
FOCUSING OUR EFFORT
33
Gas consumption tends
to be driven by the
need to heat
FOCUSING OUR EFFORT
34
Gas consumption seems to be
unrelated to occupancy, which is
not always true for a hotel due to
the domestic hot water loads
FOCUSING OUR EFFORT
35
Interval Data Consumption Analysis
Looking at Variations in Hour by Hour Patterns
DEFINITIONS AND USEFUL EQUATIONS
36
Sensible Heating or Cooling Loads
DEFINITIONS AND USEFUL EQUATIONS
37
Where did the Units Conversion Constant
Come From?
DEFINITIONS AND USEFUL EQUATIONS
38
Where did the Units Conversion Constant
Come From?
DEFINITIONS AND USEFUL EQUATIONS
39
Specific Heat and Density of Air versus Temperature
0.260
0.24
0.250
0.20
0.240
0.16
0.230
0.12
0.220
0.08
0.210
0.04
0.200
-100
0
100
200
300
400
Density, lb/cubic foot
Specific Heat, btu/lb-°F
Physical Properties can Vary …
0.00
500
Temperature,°F
Specific Heat
Density of Dry Air
DEFINITIONS AND USEFUL EQUATIONS
Density of Saturated Air
40
… so Conversion Constants Only are Valid
for a Range of Conditions
Versus 1.08 in
the equation in
common use
DEFINITIONS AND USEFUL EQUATIONS
41
… so Conversion Constants Only are Valid
for a Range of Conditions
Virtually the
same as for
cold dry air
DEFINITIONS AND USEFUL EQUATIONS
42
… so Conversion Constants Only are Valid
for a Range of Conditions
Versus 1.08 in
the equation in
common use
DEFINITIONS AND USEFUL EQUATIONS
43
… so Conversion Constants Only are Valid
for a Range of Conditions
Significantly different
from hot saturated air
and the value in the
equation in common
DEFINITIONS AND USEFUL EQUATIONS
44
Latent Load
DEFINITIONS AND USEFUL EQUATIONS
45
Total Load
DEFINITIONS AND USEFUL EQUATIONS
46
Water Side Load
DEFINITIONS AND USEFUL EQUATIONS
47
The Relationship Between Flow and Velocity
DEFINITIONS AND USEFUL EQUATIONS
48
The Relationship Between Velocity and
Velocity Pressure
DEFINITIONS AND USEFUL EQUATIONS
49
Fan Power
DEFINITIONS AND USEFUL EQUATIONS
50
Unit Conversions for Working with SI Units
DEFINITIONS AND USEFUL EQUATIONS
51
Pump Power
DEFINITIONS AND USEFUL EQUATIONS
52
Unit Conversions for Working with SI Units
DEFINITIONS AND USEFUL EQUATIONS
53
Calculating Kw Into the Pump Motor
DEFINITIONS AND USEFUL EQUATIONS
54
Calculating Kw Into the Pump Motor
(Continued)
DEFINITIONS AND USEFUL EQUATIONS
55
Calculating Power Into the Fan Motor as kW
DEFINITIONS AND USEFUL EQUATIONS
56
Calculating Energy Use
What can cause the load to vary?
• Changes in ambient conditions
• Changes in internal conditions
• Changes in a production process
Just About Everything!
DEFINITIONS AND USEFUL EQUATIONS
57
The Square Law
DEFINITIONS AND USEFUL EQUATIONS
58
Conservation of Mass and Energy
The Goes Inta’s gotta equal the Goes
Outa’s
Dr. Al Black
The tee where the building return
meets the bypass line is a node in the
system
‒ Energy into and out of the
node must be equal
‒ Mass flow into and out of the
node must be equal
Re
Bypass Flow
at Bypass
Temperature
Chiller Flow at Chiller
Entering Temperature
DEFINITIONS AND USEFUL EQUATIONS
Return Flow at
Return Temperature
59
Conservation of Mass and Energy
The Goes Inta’s gotta equal the Goes
Outa’s
Dr. Al Black
The tee where the building return
meets the bypass line is a node in the
system
‒ Energy into and out of the
node must be equal
‒ Mass flow into and out of the
node must be equal
‒ This is a steady state, steady
flow process described by the
continuity equation
Re
Bypass Flow
at Bypass
Temperature
Chiller Flow at Chiller
Entering Temperature
DEFINITIONS AND USEFUL EQUATIONS
Return Flow at
Return Temperature
60
Conservation of Mass and Energy
The Goes Inta’s gotta equal the Goes
Outa’s
Dr. Al Black
The tee where the building return
meets the bypass line is a node in the
system
‒ Energy into and out of the
node must be equal
‒ Mass flow into and out of the
node must be equal
‒ This is a steady state, steady
flow process described by the
continuity equation
‒ For a tee in the pipe, the
continuity equation can be
simplified
Re
Bypass Flow
at Bypass
Temperature
Chiller Flow at Chiller
Entering Temperature
DEFINITIONS AND USEFUL EQUATIONS
Return Flow at
Return Temperature
61
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
62
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
63
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
64
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
65
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
66
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
67
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
68
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
69
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
70
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
71
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
72
Conservation of Mass and Energy
Doing the Algebra
DEFINITIONS AND USEFUL EQUATIONS
73