psychrometry.ppt

A Trane Air Conditioning Clinic
Psychrometry
Air Conditioning Clinic TRG-TRC001-EN
© American Standard Inc. 1999

Psychrometry
period one
The Psychrometric Chart

Psychrometric
Chart

Properties Of Air
 Dry-bulb temperature
 Wet-bulb temperature
 Dew-point temperature
 Relative humidity
 Humidity ratio

Dry-Bulb Thermometer

Wet-Bulb Thermometer

Condensation Occurs At Dew Point

Fog Occurs When Air Is Saturated

Relative Humidity
…describes the degree of saturation
Relative
Humidity
Amount of moisture that a given
amount of air is holding
=
Amount of moisture that a given
amount of air can hold

50% 100%
(saturated)
Relative Humidity
…compares moisture content to saturation

Humidity Ratio
…compares water vapor to dry air, by weight

Knowing Two Properties
…lets you determine the remaining three
 Dry-bulb temperature
 Wet-bulb temperature
 Dew-point temperature
 Relative humidity
 Humidity ratio

Summer Design Conditions
 95°F DB (dry bulb)
 78°F WB (wet bulb)

“X Marks The Spot”…
95°F
78°F
72°F

25°F
30°F
35°F
40°F
45°F
50°F
55°F
19.14
24.19
29.94
36.51
44.34
53.63
64.63
60°F
65°F
70°F
75°F
80°F
85°F
90°F
77.61
92.89
110.82
131.83
156.38
185.03
218.42
DB
Temp.
Humidity
Ratio
DB
Temp.
Humidity
Ratio
Saturated Air
…grains of moisture per pound of dry air

30
Dry-Bulb Temperature (°F)
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
Plotting Saturation Points
218.42
185.03
156.38
131.83
110.82
92.89
77.61
64.63
53.63
44.34
36.51
29.94
24.19
19.14

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
Saturation Curve
100% Relative Humidity Curve

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
relationship between…
Dry Bulb, Wet Bulb And Dew Point
60°F
Dew Point
60°F
Wet Bulb
A B C D
Dry Bulb 60°F 62°F 75°F 90°F
Wet Bulb 60°F 60.8°F 65.2°F 70°F
Dew Point 60°F 60°F 60°F 60°F
Rel Humidity 100% 92% 60% 37%
D
C
B
A

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
moisture content doesn’t change by…
Adding Sensible Heat
100% Saturation
60°F
Dew Point
60°F
Wet Bulb

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
Relative Humidity Curves
Relative Humidity
0%

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
intersection represents…
Three Air Properties
Humidity Ratio
Dry Bulb
Relative Humidity

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
Determining Wet-Bulb Lines
A
BI
CI
DI
B C D
AI

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
intersection of two known air properties…
Determines The Remaining Three
Humidity Ratio
Dry Bulb
Relative Humidity
Wet Bulb
Dew Point

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
A Final Property…Specific Volume
Specific Volume Lines
(cubic feet / pound of dry air)

Effect Of Adding Sensible Heat

Effect Of Removing Sensible Heat

Effect Of Adding Moisture

Effect Of Removing Moisture

effect of changing…
Sensible Heat And Moisture Content

Psychrometry
period two
Air Mixtures

Air Conditioner
Supply Fan
Mixture Outdoor
Air (OA)
Recirculated Air
(RA)
A
C
B
Cooling Coil
Determining Entering Air Conditions

80°F
95°F
B
A
Outdoor
Air
Recirculated
Air

1,000 cfm
4,000 cfm
= 0.25
OA
RA
Mixture
=
=
=
25%
75%
100%
A
C B
4,000 cfm
Mixed Air
1,000 cfm
OA
3,000 cfm
RA

95°F × 0.25 = 23.75°F
80°F × 0.75 = 60.00°F
Mixture = 83.75°F
Mixed Air Temperature

83.75°F
70°F
B
80°F
95°F
C
A

Psychrometry
period three
Sensible Heat Ratio

Effect Of Removing Latent Heat

effect of…
Removing Sensible And Latent Heat

Supply Air
Sensible
Heat
Latent
Heat
Return Air
supply air conditions determine…
Heat And Moisture Transfer

sensible and latent heat…
Effects Of Improper Removal
Cooler
& Damper
Warmer
& Drier

A Look At “Heat Gain” . . .

SHR
Sensible Heat Gain
Sensible Heat Gain + Latent Heat Gain
=
determining the…
Sensible Heat Ratio (SHR)

78°F DB
65°F WB
Index Point
Sensible Heat Ratio Scale

Drawing An SHR Line
A
Index Point

Room SHR dictates…
Supply Air Conditions
A

C
D
B
80°F
drawing an SHR line for…
Other Room Conditions
Index Point

Psychrometry
period four
Air Quantity

Coil Curves
Coil Curves
…show changes in air properties through a cooling coil

80,000 Btu/hr sensible heat gain
20,000 Btu/hr latent heat gain
= 0.80
STEP 1: Calculate the sensible heat ratio (SHR)
80,000 Btu/hr
100,000 Btu/hr
SHR =
Determining Supply Airflow

Room — 78°F DB, 50% RH
OA — 95°F DB, 78°F WB
Ventilation — 25% OA
Design Conditions:

95°F × 0.25 = 23.75°F
78°F × 0.75 = 58.50°F
Mixture = 82.25°F
STEP 2: Plot room, outdoor, and entering conditions
A
C
B

56.5°F 78°F
STEP 3: Identify supply air conditions
A
C
B
D

Sensible Heat Gain
1.085 × (Room DB – Supply DB)
Supply
Airflow
=
STEP 4: Solve the supply airflow equation

80,000 Btu/hr
1.085 × (78°F – 56.5°F)
3,430 cfm
=
STEP 4: Solve the supply airflow equation

D
A
C B
3,430 cfm
56.5°F DB
55.2°F WB
RA
OA
82.2°F DB
68.6°F WB
Mixture
SA
56.5°F
D A
C
B

effect of…
Arbitrarily Using 55°F Supply Air
47,000 Btu/hr sensible heat gain
20,000 Btu/hr latent heat gain
= 0.70
47,000 Btu/hr
67,000 Btu/hr
SHR =

A
C
B
D
55°F
DI
AI
effect of…
Arbitrarily Using 55°F Supply Air
51°F

Psychrometry
period five
Tons Of Refrigeration

What is Enthalpy?
Enthalpy (h) = Sensible Heat + Latent Heat
The total heat energy in one
pound of air (Btu/lb) at its present
condition:

Determining Tons Of Refrigeration
STEP 1:
Find enthalpies
entering and
leaving coil
A
B
C
D

STEP 2: Solve the total refrigeration load equation
4.5 × Supply Airflow × (h1 – h2) Refrigeration
Load (Btu/hr)
=
where:
h1 = enthalpy of air entering coil (Btu/lb)
h2 = enthalpy of air leaving coil (Btu/lb)

= 11.8 tons of refrigeration
STEP 2: Solve the total refrigeration load equation
4.5 × 3,430 cfm × (32.7 – 23.5) = 142,000 Btu/hr
142,000 Btu/hr
12,000 Btu/hr/ton

determining cooling coil…
Sensible And Latent Loads
A
B
C
D
Sensible
Load
Latent
Load

Sensible Coil Load:
4.5 × 3,430 cfm × (29.6 – 23.5) = 94,150 Btu/hr
7.8 tons of refrigeration (sensible)
determining cooling coil…
Sensible And Latent Loads
Latent Coil Load:
4.5 × 3,430 cfm × (32.7 – 29.6) = 47,850 Btu/hr
4.0 tons of refrigeration (latent)

Psychrometry
period six
Psychrometric Analyses

SHR Changes Throughout The Day
Full-load conditions
56.5°F 78°F
D
A
B
C

Full Load:
80,000 Btu/hr
80,000 Btu/hr + 20,000 Btu/hr
SHR =
Part Load:
= 0.80
47,000 Btu/hr
47,000 Btu/hr + 20,000 Btu/hr
SHR = = 0.70
SHR Changes With Room Load

D
A
B
C
New SHR Requires New SA Condition
Part-load conditions

D
A
B
C
at part-load, with constant supply airflow …
Raising SA Temp. Increases Room RH
AI
DI
47,000 Btu/hr
1.085 × (78°F – Supply DB)
= 3,430 cfm
Supply DB = 65.4°F
65.4°F

constant quantity of variable-temperature air
OA
RA
SA
Constant-Volume System
B
D C
A
Modulating
Cooling Coil

Reheat
Coil
Cooling
Coil
Room
Thermostat
adding reheat improves
humidity control
B
E
D C
A
Constant-Volume With Reheat

D A
C
B
47,000 Btu/hr
1.085 × (78°F – Supply DB)
= 3,430 cfm
Supply DB = 65.4°F
Effect Of Adding Reheat…
E
65.4°F
56.5°F

Effect Of Adding Reheat…
D
A
C
B
E
65.4°F
56.5°F

mixed air bypass improves humidity control
Mixed Air Bypass
OA
RA
SA
B
D
C
A
Face-and-
Bypass
Dampers

Effect Of Mixed Air Bypass…
D
A
C
B
E
82.25°F × 0.455 = 37.4°F
51.40°F × 0.545 = 28.0°F
Mixture = 65.4°F
65.4°F
51.4°F

Effect Of Mixed Air Bypass…
A
C
B
AI
65.4°F
51.4°F
D
E

Variable-Air-Volume (VAV) System
OA
RA
SA
B
D
A
C
variable quantity of constant-temperature air
VSD

D A
C
B
Calculating Part-Load Airflow
47,000 Btu/hr
1.085 × (78°F – 56.5°F)
= 2,015 cfm
AI
55.5°F

PC-based software tools…
Simplify Psychrometric Analysis

Psychrometry
period seven
Review

30
Grains
Of
Moisture
Per
Pound
Of
Dry
Air
When
Saturated
(grains/lb
of
dry
air)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
0
40
60
80
100
120
140
160
180
200
220
review…
Five Properties Of Air
Humidity Ratio
Dry Bulb
Relative Humidity
Wet Bulb
Dew Point

80°F
95°F
83.75°F
70°F
C
B
A
review…
Determining Mixed Air Conditions
95°F × 0.25 = 23.75°F
80°F × 0.75 = 60.00°F
Mixture = 83.75°F

A
review…
Determining Sensible Heat Ratio
SHR =
Sensible Heat Gain
Sensible Heat Gain + Latent Heat Gain

A
C
B
D
Coil Curve
Sensible Heat Gain
1.085 × (Room DB – Supply DB)
Supply
Airflow
review…
Determining Coil Leaving Conditions
=
Supply DB

A
C
B
D
review…
4.5 × Supply Airflow × (h1 – h2) = Refrigeration Load

C
OA
RA
B
A
D
SA
review…
Psychrometric
Analysis
A
B
C
D

psychrometry.ppt

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