Page 440 - Mechanical Engineers' Handbook (Volume 4)

4 Refrigerants 429

larger condenser to reject heat energy from the cycle. Pressure losses in the evaporator and
condenser require the compressor to work harder to circulate refrigerant in the system. Pre-
dicting the performance of a real system must include these effects, or the system capacity
and efficiency will be overestimated.

4 REFRIGERANTS
No one refrigerant is capable of providing cost-effective cooling over the wide range of
temperatures and the multitude of applications found in modern refrigeration systems. Am-
4
monia is a popular refrigerant for many industrial and large warehouse applications. Both
chlorofluorocarbons (CFCs) and hydrochlorofluorocarbon (HCFC) refrigerants have histori-
cally been used in many refrigeration applications, ranging from domestic refrigerators to
supermarket and food storage applications. Most of these refrigerants are generally nontoxic
and nonflammable. Recent U.S. federal and international regulations 5–7 have placed restric-
tions on the production and use of CFCs and HCFCs. Hydrofluorocarbons (HFCs) and HFC
mixtures are now being used in some applications where CFCs and HCFCs have been used.
Regulations affecting refrigerants are discussed in the next section.
The chemical industry uses low-cost fluids such as propane and butane whenever they
are available in the process. These hydrocarbon refrigerants, often thought of as too hazard-
ous because of flammability, are suitable for use in modern compressors, and frequently add
no more hazard than already exists in an oil refinery or petrochemical plant. These low-cost
refrigerants are used in simple, compound, and cascade systems, depending on operating
temperatures.
A standard numbering system, shown in Table 1, has been devised to identify refrig-
erants without the use of their cumbersome chemical names. There are many popular refrig-
erants in the methane and ethane series. These are called halocarbons or halogenated
hydrocarbons because of the presence of halogen elements such as fluorine or chlorine. 8
Halocarbons include CFCs, HCFCs, and HFCs.
Numbers assigned to the hydrocarbons and halohydrocarbons of the methane, ethane,
propane, and cyclobutane series are such that the number uniquely specifies the refrigerant
compound. The American National Standards Institute (ANSI) and American Society of
Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 34-2001 de-
scribes the method of coding. 9
Zeotropes and azeotropes are mixtures of two or more different refrigerants. A zeotropic
mixture changes saturation temperatures as it evaporates (or condenses) at constant pressure.
This phenomena is called temperature glide. For example, R-407C has a boiling (bubble)
point of 44 C and a condensation (dew) point of 37 C, which means it has a temperature
glide of 7 C. An azeotropic mixture behaves much like a single-component refrigerant in
that it does not change saturation temperatures appreciably as it evaporates or condenses at
constant pressure. Some zeotropic mixtures, such as R-410A, actually have a small enough
temperature glide (less than 5.5 C) that they are considered a near-azeotropic refrigerant
mixture.
Because the bubble-point and dew-point temperatures are not the same for a given
pressure, some zeotropic mixtures have been used to help control the temperature differences
in low-temperature evaporators. These mixture have been used in the lowest stage of some
liquified natural gas (LNG) plants. 10
Refrigerants are grouped by their toxicity and flammability (Table 2). 9,11 Group A1 is
nonflammable and least toxic, while group B3 is flammable and most toxic. Toxicity is
quantified by the threshold limit value–time weighted average (TLV-TWA), which is the