Pressure Enthalpy Diagram
(P-h Diagram)
LINES OF CONSTANT ENTHALPY
Pressure (psia)
P
R
E
S
S
U
R
E
LINES OF CONSTANT PRESSURE
P
R
E
S
S
HEAT CONTENT DECREASES
U
R
E
D
R
R HEAT CONTENT INCREASESO
I
P
S
S
E
S
Btu/lb
Heat Content
Pressure (psia)
SATURATION CURVE
Btu/lb
Btu/lb
Heat Content
THE SATURATION CURVE
• Under the curve, the refrigerant follows the
pressure-temperature relationship
• The left side of the saturation curve
represents 100% liquid
• The right side of the saturation curve
represents 100% vapor
• For non-blended refrigerants, one
pressure corresponds to one temperature
Pressure (psia)
LINES OF CONSTANT TEMPERATURE
Btu/lb
Heat Content
Pressure (psia)
LINES OF CONSTANT VOLUME (ft3/lb)
Btu/lb
Heat Content
Pressure (psia)
LINES OF CONSTANT ENTROPY
Btu/lb
Heat Content
Pressure (psia)
LINES OF CONSTANT QUALITY
Btu/lb
Heat Content
Pressure (psia)
PUT IT ALL TOGETHER…
Btu/lb
Heat Content
Pressure-Enthalpy (p-h) Diagram for R-12 (Simplified)
Pressure
(psia)
160°F
140°F
221
120°F
172
100°F
132
80°F
99
60°F
72
40°F
52
20°F
36
0°F
24
12
20
25 31
35
8
0
Enthalpy in btu/lb (Heat Content)
8 8
2 4
8 8 9 9
6 8 0 2
9 9 9
4 6 8
1 1 1
0 0 0
0 2 4
1 1
0 0
6 8
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
160°F
140°F
352
120°F
275
100°F
211
80°F
159
60°F
117
40°F
84
20°F
58
0°F
39
15
24 31
40
46
Enthalpy in btu/lb (Heat Content)
1
1
0
1
1
2
1
1
9
1
2
3
Example: An R-22 A/C System
•
•
•
•
•
Condenser saturation temperature 120°F
Condenser outlet temperature 100°F
Evaporator saturation temperature 40°F
Compressor inlet temperature 60°F
Compressor Horsepower: 4 hp
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
180°F
160°F
140°F
352
0.7
120°F
275
100°F
211
80°F
159
60°F
117
40°F
84
20°F
58
0°F
39
-20°F
25
-40°F
3 4 4 5
2 0 6 3
Enthalpy in btu/lb (Heat Content)
1 1 1 1 1
1 1 1 2 2
0 2 7 1 5
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
180°F
160°F
140°F
352
0.7
120°F
275
100°F
211
80°F
159
60°F
117
40°F
84
20°F
58
0°F
39
-20°F
25
-40°F
3 4 4 5
2 0 6 3
Enthalpy in btu/lb (Heat Content)
1 1 1 1 1
1 1 1 2 2
0 2 7 1 5
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
180°F
160°F
352
275
140°F
A
100°F
211
80°F
159
60°F
117
40°F
84
58
0.7
120°F
B
20°F
C
0°F
39
-20°F
25
-40°F
3 4 4 5
2 0 6 3
Enthalpy in btu/lb (Heat Content)
1 1 1 1 1
1 1 1 2 2
0 2 7 1 5
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
180°F
160°F
352
275
140°F
A
100°F
211
80°F
159
60°F
117
40°F
84
58
0.7
120°F
B
20°F
C
0°F
39
-20°F
25
-40°F
3 4 4 5
2 0 6 3
Enthalpy in btu/lb (Heat Content)
1 1 1 1 1
1 1 1 2 2
0 2 7 1 5
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
180°F
160°F
352
275
140°F
A
100°F
211
80°F
159
60°F
117
40°F
84
58
D
120°F
B
20°F
C
0°F
39
-20°F
25
-40°F
3 4 4 5
2 0 6 3
Enthalpy in btu/lb (Heat Content)
1 1 1 1 1
1 1 1 2 2
0 2 7 1 5
0.7
Pressure-Enthalpy (p-h) Diagram for R-22 (Simplified)
Pressure
(psia)
180°F
160°F
352
275
140°F
A
0.7
100°F
211
80°F
159
60°F
117
40°F
84
58
D
120°F
B
20°F
C
0°F
39
High: 275 psia
-20°F
25
-40°F
Low: 84 psia
“A”: 40 btu/lb
“B”: 40 btu/lb
“C”: 112 btu/lb
“D”: 125 btu/lb
3 4 4 5
2 0 6 3
Enthalpy in btu/lb (Heat Content)
1 1 1 1 1
1 1 1 2 2
0 2 7 1 5
High: 275 psia
Low: 84 psia
WORK INPUT
“A”: 40 btu/lb
= ENTHALPY AT “D” – HEAT CONTENT AT “C”
“B”: 40 btu/lb
= 125 btu/lb – 112 btu/lb = 13 btu/lb
“C”: 112 btu/lb
“D”: 125 btu/lb
COOLING EFFECT
= ENTHALPY AT “C” – HEAT CONTENT AT “B”
= 112 btu/lb – 40 btu/lb = 72 btu/lb
HEATING EFFECT
= ENTHALPY AT “D” – HEAT CONTENT AT “A”
= 125 btu/lb – 40 btu/lb = 85 btu/lb
Desired output Cooling effect
QL
COPR 


Required input
Work input
Wnet ,in
COPHP
Desired output Heating effect
QH



Required input
Work input
Wnet ,in