Page 444 - Mechanical Engineers' Handbook (Volume 4)

4 Refrigerants 433

Refrigerant 1150, ethylene, is similar to ethane but has a slightly higher-pressure, lower-
volume characteristic, which extends its use to 104 to 29 C. Like ethane, it must be used
in the cascade cycle.
Refrigerant 50, methane, is used in an ultralow range of 160 to 110 C. It is limited
to cascade cycles. Methane condensed by ethylene, which is in turn condensed by propane,
is a cascade cycle commonly employed to liquefy natural gas.
Refrigerant 744, carbon dioxide, is currently receiving attention as a possible refrigerant
for use in cooling and refrigeration applications. It has the appeal of being a natural sub-
stance. Systems can be designed with R-744, but must operate at elevated pressures. Solid
carbon dioxide (dry ice) is commonly used in the food industry for chilling and freezing
applications.
Table 3 shows the comparative performance of different refrigerants at conditions more
typical of some freezer applications. The data show the relatively large refrigerating effect
that can be obtained with ammonia. Note also that for these conditions, both R-11 and R-
123 would operate with evaporator pressures below atmospheric pressure.

4.1 Regulations on the Production and Use of Refrigerants
In 1974, Molina and Rowland published a paper where they put forth the hypothesis that
CFCs destroyed the ozone layer. 13 By the late 1970s, the United States and Canada had
banned the use of CFCs in aerosols. In 1985, Farmer noted a depletion in the ozone layer
4
of approximately 40% over what had been measured in earlier years. This depletion in the
ozone layer became known as the ozone hole. In September 1987, 43 countries signed an
7
agreement called the Montreal Protocol in which the participants agreed to freeze CFC
production levels by 1990, then to decrease production by 20% by 1994 and 50% by 1999.
The protocol was ratified by the United States in 1988 and, for the first time, subjected the
refrigeration industry to major CFC restrictions.
Recent regulations have imposed restrictions on the production and use of refriger-
14
ants. 4,6,14 Production of CFCs in the United States was prohibited after January 1, 1996. A
schedule was also imposed that started a gradual phase-out of the production of HCFCs in
2004 and will end complete production by 2030. Refrigerants are divided into two classes:

Table 3 Comparative Refrigeration Performance of Different Refrigerants at 23 C Evaporating Temperature and 37 C
Condensing Temperature a
Net
Evaporator Condenser Refrigerating Refrigerant Compressor Power
Refrigerant Pressure Pressure Effect Circulated Displacement Input
Number Refrigerant Name (MPa) (MPa) (kJ/kg) (kg/h) (L/s) (kW)
11 Trichlorofluoromethane 0.013 0.159 145.8 24.7 7.65 0.297
12 Dichlorodifluoromethane 0.134 0.891 105.8 34.0 1.15 0.330
22 Chlorodifluoromethane 0.218 1.390 150.1 24.0 0.69 0.326
123 Dichlorotrifluoroethane 0.010 0.139 130.4 27.6 10.16 0.306
125 Pentafluoroethane 0.301 1.867 73.7 48.9 0.71 0.444
134a Tetrafluoroethane 0.116 0.933 135.5 26.6 1.25 0.345
502 R-22/R-115 azeotrope 0.260 1.563 91.9 39.2 0.72 0.391
717 Ammonia 0.166 1.426 1057.4 3.42 0.67 0.310

a Reference 12, reprinted by permission from 2001 ASHRAE Handbook of Fundamentals. American Society of Heating, Refrigerating and
Air-Conditioning Engineers, Inc., www.ashrae.org.