Handbook of air conditioning and refrigeration / Shan K

REFRIGERATION SYSTEMS: RECIPROCATING, ROTARY, SCROLL, AND SCREW 11.7
Cylinder Block and Piston. In most compressors, the cylinder block is integrated with the
crankcase and forms a single casting. The crankcase and the cylinder block are usually made of
high-grade cast iron. In medium and large reciprocating compressors, premachined cylinder liners
or sleeves are often inserted in the crankcase, and they can be replaced when worn. It costs far less
to replace cylinder liners or sleeves than to replace the cylinder block.
The pistons are generally made of aluminum, aluminum alloy, or cast iron. For aluminum
pistons, piston rings are often used to prevent leakage of gas refrigerant from the cylinder and to
prevent oil from entering the cylinder. The upper piston rings are called compression rings, and the
lower rings are called oil rings. For cast-iron pistons, a running clearance of 0.0004 in./in. (cm/cm)
cylinder diameter provides an effective seal without oil rings.
Suction and Discharge Valves. The suction valve controls the vapor refrigerant entering the
cylinder, and the discharge valve controls the hot gas discharging from the cylinder. Suction and
discharge valves are usually made of high carbon-alloy steel or stainless steel. Spring-action ring
valves are most extensively used in medium and large compressors. Ring valves are usually heattreated
to the resilience of spring steel and must be precisely ground to a perfectly flat surface. A
deflection of 0.001 in. (0.025 mm) may cause leakage.
In addition, reed valves, either free-floating or clamped at one end, are used. A reed valve opens
under discharged pressure and is closed by spring action. Reed valves are usually limited to small
valve ports. A sufficient valve opening area must be provided to reduce pressure loss. A valve may
open up to 0.01 in. (0.25 mm) during suction or discharge. The maximum velocity of HCFC-22
flowing through valves may reach 150 ft/s (45 m/s).
Oil Lubrication. Oil lubrication is necessary to form a fluid film separating the moving surfaces
to protect them from wear and corrosion. Oil is also used as a coolant to carry heat away and cools
the refrigerant. Oil provides an oil seal between the piston and cylinder and between the valve and
valve plates. In refrigeration systems, mineral and synthetic oils are used for lubrication. HCFC-22
is partly oil-miscible so that the refrigerant-oil mixture can return to the compressor. HFCs and
HFC blends are not miscible with mineral oil. Polyolester-based synthetic oil should be used for
lubrication. Oil must be free of solids such as wax, must be chemically stable, and must have a
suitable viscosity. In reciprocating compressors using HCFC-22 and halogenated refrigerants, a
Saybolt seconds universal (SSU) viscosity between 150 and 300 is recommended by ASHRAE
Handbook 1994, Refrigeration Systems and Applications. Kinematic viscosity of 1 ft 2 /s (92,900
mm 2 /s) is approximately equal to 4.3 � 10 5 SSU.
In medium and large reciprocating compressors, forced-feed lubrication through a positive
displacement vane or gear oil pump is usually used. Lubricant is fed to the main bearing surfaces and
crankshaft through oil passages. An oil mist formed by the crankshaft rotation lubricates the cylinder
wall and piston pins. The oil pump is often mounted on one end of the crankshaft. An oil strainer
submerged directly in the oil sump is connected to the suction intake of the oil pump. Because the
vapor refrigerant in the crankcase is always at the suction pressure, the pressure lift of the oil pump is
equal to the difference of the discharge pressure at the oil pump and the suction pressure.
In small reciprocating compressors, splash lubrication may be used. Splash lubrication uses the
rotation of the crankshaft and the connecting rod to splash oil onto the bearing surfaces, cylinder
walls, and other moving parts. Often, oil passages, oil rings, and other devices are used to increase
oil splashing. An adequate oil level in the crankcase is necessary for satisfactory operation. Splash
lubrication is simple. Its disadvantage is that bearing clearance must be larger than normal, which
increases noise during operation.
For HCFC-22 and other halocarbon refrigerants, lubrication of suction and discharge valves is
achieved because the refrigerant is miscible with the lubrication oil. Refrigerant piping design must
accommodate the return of oil from evaporators and condensers. Oil should be separated from the
suction vapor at the compressor and returned to the crankcase.
Crankcase Heater. When the reciprocating compressor is installed outdoors, refrigerants may migrate
to the compressor from the indoor evaporator during the shutdown period of the refrigeration