Civil Engineering Reference
In-Depth Information
Table 12.1
Physical properties of coolants
Physical properties
R717
R22
R12
R11
Boiling point
K
240
232
243
297
F
28
41.5
21.6
74.5
C
33.3
40.8
29.8
23.7
Freezing point
K
195.5
113
115
162
F
107.9
256
252
168
C
77.7
160
158
111
Critical temperature
K
406
369
385
471
F
271.5
204.8
233.6
388.4
C
133
96
112
198
Critical pressure
MPa
11.4
4.97
4.1
4.4
m
3
/kg
Specific volume at suction
0.51
0.07
0.09
0.7
Latent heat at boiling point
kJ/kg
1,367
234
234
182
Evaporator pressure
MPa
0.24
0.29
0.18
0.02
Condenser pressure
MPa
1.16
1.19
0.74
0.12
Compression ratio
4.94
4.03
4.07
6.25
Specific heat ratio
c
p
/
c
v
1.187
1.19
1.14
1.12
Net refrigeration effect
kJ/kg
1,100
163
117
156
Average evaporation temperature 258 K, 5
F,
15
C
Average condensation temperature 303 K, 86
F, 30
C
R717
ammonia, R22 monochlorodifluoromethane, R12
dichlorodifluoromethane, R11
trichlorofluoromethane
• The operating pressures, which are linked to the operating temperatures, should
avoid excessively low values in the evaporation process and excessively high
values in the condensation process (typical values are 0.1-0.2 MPa, 14.5-29 psi
for evaporator pressure and 0.8-2 MPa, 116-290 psi for condenser pressure).
The evaporator pressure should be higher than the atmospheric one to avoid air
input in the pipeline. Notice that a moderate pressure ratio should be guaranteed
for the compressor to achieve better performance.
• Other properties such as chemical stability, corrosiveness, toxicity, solubility in
water, compatibility with lubricant and other materials, and cost have also to be
considered when choosing refrigerants.
As a general rule, centrifugal compressors are best suited for low evapo-
rator pressures and refrigerants with relatively high specific volume at
low pressure. Reciprocating compressors work in higher pressure ranges
and with low specific volume refrigerants.
The commonest coolants are ammonia, which is a natural compound,
chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluor-
ocarbons (HFCs).
