Page 443 - Mechanical Engineers' Handbook (Volume 4)

432 Refrigeration

be used as a replacement refrigerant in an R-22 system. However, manufacturers have begun
designing new systems that use R-410A, and these systems can be applied in situations
where R-22 systems were used. The higher operating pressures as well as lubricant incom-
patibility with mineral oils has required manufacturers to completely redesign systems with
R-410A.
Refrigerant 502 is an azeotropic mixture of R-22 and R-115. Its pressure characteristics
are similar to those of R-22, but it is has a lower discharge temperature.
The B1 refrigerants are nonflammable, but have lower toxicity limits than those in the
A1 group. Refrigerant 123, an HCFC, is used in many new low-pressure centrifugal chiller
applications. Industry standards, such as ANSI/ASHRAE Standard 15-1994, provide detailed
guidelines for safety precautions when using R-123 or any other refrigerant that is toxic or
flammable. 11
One of the most widely used refrigerants is ammonia, even though it is moderately
flammable and has a class B toxicity rating. Ammonia liquid has a high specific heat, an
acceptable density and viscosity, and high conductivity. Its enthalpy of vaporization is typ-
ically 6–8 times higher than that of the commonly used halocarbons. These properties make
it an ideal heat-transfer fluid with reasonable pumping costs, pressure drop, and flow rates.
As a refrigerant, ammonia provides high heat transfer, except when affected by oil at
temperatures below approximately 29 C, where oil films become viscous. To limit the
ammonia-discharge-gas temperature to safe values, its normal maximum condensing
temperature is 38 C. Generally, ammonia is used with reciprocating compressors; although
relatively large centrifugal compressors ( 3.5 MW), with 8–12 impeller stages required by
its low molecular weights, are in use today. Systems using ammonia should contain no copper
(with the exception of Monel metal).
The flammable refrigerants (groups A3 and B3) are generally applicable where a flam-
mability or explosion hazard is already present and their use does not add to the hazard.
These refrigerants have the advantage of low cost. Although they have fairly low molecular
weight, they are suitable for centrifugal compressors of larger sizes. Because of the high
acoustic velocity in these refrigerants, centrifugal compressors may be operated at high im-
peller tip speeds, which partly compensates for the higher head requirements than some of
the nonflammable refrigerants.
Flammable refrigerants should be used at pressures greater than atmospheric to avoid
increasing the explosion hazard by the admission of air in case of leaks. In designing the
system, it also must be recognized that these refrigerants are likely to be impure in refrigerant
applications. For example, commercial propane liquid may contain about 2% (by mass)
ethane, which in the vapor phase might represent as much as 16–20% (by volume) ethane.
Thus, ethane may appear as a noncondensable. Either this gas must be purged or the com-
pressor displacement must be increased about 20% if it is recycled from the condenser;
otherwise, the condensing pressure will be higher than required for pure propane and the
power requirement will be increased.
Refrigerant 290, propane, is the most commonly used flammable refrigerant. It is well
suited for use with reciprocating and centrifugal compressors in close-coupled or remote
systems. Its operating temperature range extends to 40 C.
Refrigerant 600, butane, occasionally is used for close-coupled systems in the medium
temperature range of 2 C. It has a low-pressure and high-volume characteristic suitable for
centrifugal compressors, where the capacity is too small for propane and the temperature is
within range.
Refrigerant 170, ethane, normally is used for close-coupled or remote systems at 87
to 7 C. It must be used in a cascade cycle because of its high-pressure characteristics.