Handbook of air conditioning and refrigeration / Shan K

1 1s superheat in evaporator
1s1v pressure drop in suction line
1v1� pressure drop from suction valve and passage to cylinder
1�2� compression process in cylinder
2�2v pressure drop from discharge valve and passage
2v2� pressure drop in discharge line
2�3� condensing and subcooling processes in condenser
3�3x pressure drop in liquid line
3x4 throttling process in expansion valve
4 1s evaporating process in evaporator
11.2 RECIPROCATING COMPRESSORS
In a reciprocating compressor, shown in Fig. 11.5, a single-acting piston in a cylinder is driven by a
crankshaft via a connecting rod. At the top of the cylinder are a suction valve and a discharge valve.
There are usually two, three, four, or six cylinders in a reciprocating compressor.
Vapor refrigerant is drawn through the suction valve into the cylinder until the piston reaches its
lowest position. As the piston is forced upward by the crankshaft, it compresses the vapor refrigerant
to a pressure slightly higher than the discharge pressure. Hot gas opens the discharge valve and
discharges from the cylinder. The gaseous refrigerant in a reciprocating compressor is compressed
by the change of internal volume of the compression chamber caused by the reciprocating motion
of the piston in the cylinder.
The cooling capacity of a reciprocating compressor ranges from a fraction of a ton to 200 tons
(1 to 3.5 kW to 700 kW). Refrigerant HCFC-22, HFC-134a, HFC-404A, HFC-407A, and HFC-
407C in comfort and process air conditioning and R-717 (ammonia) in industrial applications are
the currently used refrigerants in reciprocating compressors because of their zero ozone depletion
factors, except HCFC-22 which will be restricted from the year 2004 due to its ozone depletion.
The maximum compression ratio p dis/p suc of a single-stage reciprocating compressor is about 7.
Volumetric efficiency of a typical reciprocating compressor � v decreases from 0.92 to 0.65 when
p dis/p suc increases from 1 to 6, and the isentropic or compressor efficiency � isen decreases from 0.83
to 0.75 when p dis/p suc increases from 4 to 6. Methods of capacity control during part-load operation
include on/off, cylinder unloader, and hot-gas bypass controls.
Reciprocating compressor design is now in its mature stage. There is little room for significant
improvement. Reciprocating compressors are still widely used in small and medium-size
refrigeration systems. However, they will be gradually replaced by rotary, scroll, and screw
compressors.
11.3 RECIPROCATING REFRIGERATION SYSTEM
COMPONENTS AND ACCESSORIES
Compressor Components and Lubrication
REFRIGERATION SYSTEMS: RECIPROCATING, ROTARY, SCROLL, AND SCREW 11.5
The housing of a semihermetic reciprocating compressor, shown in Fig. 11.5, is the enclosure that
contains the cylinder, pistons, crankshaft, main bearing, oil sump, crankcase, and hermetic motor,
including both the rotor and the stator. The crankcase is the bottom part of the housing of the reciprocating
compressor in which the crankshaft is housed.