ARI Workshop Psychrometrics

1
ARI Workshop - Psychrometrics

• Review of the Process
• Defining State Points
• Application of Mixed Air Calculation
• Defining Sensible Heat Ratio
• Application of Sensible Heat Ratio
• Defining Supply Airflow
• Re-World Example Exercises

2
Psychrometric Chart

3
Properties of Saturated Air

dry-bulbtemp.
humidityratio
dry-bulb temp.
humidityratio
25F 30F 35F 40F 45F 50F 55F
19.14 24.19 29.94 36.51 44.34 53.63 64.63
60F 65F 70F 75F 80F 85F 90F
77.61 92.89 110.82 131.83 156.38 185.03 218.42
4
Plotting Saturated 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
0
5
Saturation Curve

100 relative humidity curve
6
Relative Humidity Curves

relative humidity
100
90
80
70
60
50
40
30
20
10
0
7
Properties

dew point
humidity ratio
wet bulb
dry bulb
relative humidity
8
Specific Volume

specific volume lines (cubic feet / pound of
dry air)
14.5
14.0
13.5
13.0
9
Sensible Heating

10
Sensible Cooling

11
Adding Moisture

12
Removing Moisture

13
Sensible Latent Energy

14
State Point

78F
50
40
72F
95F
15
Mixed Air

mixture
outdoorair (OA)
C
supply fan
B
cooling coil
A
Re-circulated air (RA)
16
Outside Return Air

outdoor air
B
A
Re-circulated air
95F
80F
17
Determining Mix

C
B
4,000 cfmmixed air
1,000 cfmOA
1,000 cfm 4,000 cfm

0.25
A
3,000 cfmRA
OA RA mixture

25 75 100
18
Mixed Air Temperature

95F 0.25 23.75F 80F 0.75
60.00F mixture 83.75F
19
Mixed Air State Point

B
70F
C
95F
A
83.75F
80F
20
Space Sensible Latent Heat

supply air
return air
sensible heat
latent heat
21
Sensible Heat Ratio

Sensible Heat Gain Sensible Heat Gain Latent
Heat Gain
SHR
22
SHR Scale

78F DB 65F WB
index point
23
Drawing an SHR Line

0.80 SHR line
A
index point
24
Drawing an SHR Line

B
D
0.60 SHR line
C
index point
80F
25
Coil Curves

26
Determining Airflow

STEP 1 Calculate the sensible heat ratio (SHR)
80,000 Btu/hr sensible heat gain 20,000 Btu/hr
latent heat gain
80,000 Btu/hr 100,000 Btu/hr

SHR

0.80
27
Design Parameters

room 78F DB, 50 RH outdoor air (OA) 95F
DB, 78F WB ventilation 25 OA
28
Plotting Design Points
STEP 2 Plot room, outdoor, and entering
conditions

95F 0.25 23.75F 78F 0.75
58.50F mixture 82.25F
B
C
0.80 SHR
A
29
Determining Supply Airflow
STEP 3 Identify supply air conditions

B
C
0.80 SHR
A
D
56.5F
78F
30
Determining Airflow

STEP 4 Solve the supply airflow equation
SupplyAirflow
Sensible Heat Gain1.085 (Room DB Supply DB)


80,000 Btu/hr1.085 (78F 56.5F)
3,430 cfm
31
Completing the Loop

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