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Refrigeration Systems 351

ware may require modification or replacement to accom- charge costs. Reciprocating compressors are preferred for
modate the “new” refrigerants. small tonnages. As a general rule, ammonia is not used in
systems handling air conditioning applications.
Flammability and Toxicity (Tables 11-3, 11-4, and 11-5) Refrigerant 12 *** is a versatile material for a wide range of
applications and will often result in lower first costs due to
Most of the chloro-fluoro-refrigerants are nonflammable fewer stages of compression. Refrigerants 114 and 11 *** are
and nontoxic. Ammonia does not require explosion-proof considered for higher temperature levels and lower tonnage
equipment, but it will burn and is toxic and somewhat diffi- loads than refrigerant 12 *** ( *** = to be phased out).
cult to handle. The hydrocarbons propylene, ethylene, and Propane, propylene, and ethylene are used in large refrig-
propane are explosive and somewhat toxic and must receive eration tonnage and very low temperature applications.
proper attention to safety, as in the design of a light hydro-
carbon plant.
Materials of Construction
Refer to Tables 11-4 and 11-5 and the ANSI/ASHRAE
Standards 15-1994 and ANSI/ASHRAE 34-1992, latest
The chloro-fluoro-refrigerants and hydrocarbons use any
editions. Also refer to the discussion under “Process Perfor-
reasonable material satisfactory for the pressure-copper (or
mants—Refrigerants,” earlier in this chapter.
alloys), galvanized steel, steel, aluminum, tin, etc. Ammonia
requires an all steel and/or cast iron system with no copper
Action with Oil and Water or its alloys in any part. On ammonia centrifugal compres-
sors, the interstage labyrinths are aluminum, and the associ-
When water comes in contact with the chloro-fluoro- ate rotating part is free machining stainless steel. The wheels
refrigerants, an acid condition is established. This mois- are steel forging with a lead coating. The shaft seal is
ture may be in the form of water vapor coming in with air mechanical carbon ring.
and is more likely if the suction side is lower than atmos-
pheric pressure. These systems must be checked for leaks Standard Ton Conditions. These are taken by industry to
and moisture content. The descending order of reactivity represent the refrigeration tonnage of a system when
**
*
with water is refrigerants 11, 12, *** 114, 22, and 113. operating with an 86°F condenser temperature and a 5°F
Water vapor does not affect ammonia, except to modify evaporator temperature. This is a comparative reference
the pressure-temperature relationship. When this becomes condition and does not need interpolation for effective
noticeable, the charge must be dried. Water must be evaluation of other tonnage requirements and conditions.
purged from hydrocarbon systems, because emulsions or
two-phase conditions may develop. Refrigerating Effect. This is the heat absorbed in the evapo-
Oil is miscible with all the refrigerants except ammonia. rator per lb of refrigerant. It is determined by the difference
This may create foaming in the crankcase and an unsatis- in enthalpy of a lb of refrigerant vapor leaving the evapora-
factory compression condition for reciprocating compres- tor and that of a lb of liquid just upstream (ahead) of the
sors. expansion valve at the evaporator. From Figure 11-48A,
Each of the refrigerant manufacturers has determined
the proper lubricant to use in a system. They should be con- RE h 1 h 3 (11-3)
sulted for recommendations. (It is beyond the scope of this
chapter to provide all of the detail necessary to utilize each and from Figure 11-48,
refrigerant.)
Oil is not a real problem in centrifugal machines, except RE h 1 h 6 (11-3A)
that its carry-through affects condensation in the condenser.
Coefficient of Performance. COP is the ratio of refrigerating
In an ammonia system, the oil will settle out and can be
effect to work of compression. The higher the value of COP,
purged from low points of the system, as receiver, evapora-
the higher the efficiency of the cycle. Referring to Figure
tor, etc.
11-48A,
Generalized Comments Regarding Refrigerants T 4 h 1 h 3
COP (11-4)
T 3 T 4 h 2 h 1
Each system and its particular requirements must be evalu-
Work of Compression. This is the enthalpy of a lb of refrig-
ated from a composite of the conditions affecting the refrig-
erant at compressor discharge conditions minus the
erant. After a refrigerant is selected, the accepted design
enthalpy of a lb of refrigerant at compressor suction condi-
procedure and materials of construction can be applied.
tions,
Where ammonia can be accepted as the refrigerant, it is
recommended due to the lower initial equipment and h 2 h 1 , Btu>lb, (Figure 11-48A) (11-5)