Reciprocating Compressors

Reciprocating compressors are used for applications with capacity requirements of up to 400 tons (1,400 kW^), based on ARI standard conditions. They are generally used for smaller-capacity applications ranging from about 5 to 150 tons (18 to 525 kW^), most commonly in air-cooled systems. The major components of a reciprocating compressor are the body, cylinder head, valve plates, and pistons driven by a connecting rod from the crankshaft. The design has many similarities to a reciprocating engine.

In the basic design, much like a reciprocating engine, the pistons provide compression by reducing the volume of refrigerant vapor in a cylinder. The cylinder head serves as a pressure plate to support and hold the valves and valve plate in position. It also provides the vapor passage into and out of the compressor. As the piston moves downward, it draws refrigerant vapor from the evaporator into the cylinder through an intake valve. As the piston moves upward, it compresses the vapor. The piston displacement is the maximum theoretical volume of the compressor. The swept volume is the actual volume of refrigerant vapor entering the cylinder and is usually less than the piston displacement due to the re-expansion of some refrigerant gases that remain in the cylinder (in the dead volume) after discharge. The piston is designed to come as close as possible to the cylinder head, without touching it, in order to press as much of the vapor into the high-pressure side as possible.

Figures 37-2 and 37-3 are cutaway illustrations of reciprocating compressors. The intake and compression strokes of a reciprocating compressor are illustrated in Figures 37-4 and 37-5, respectively. In multiple-cylinder units, the cylinders may be arranged in pairs with one cylinder head covering two cylinders. One or more openings are provided for access to the crankcase for assembly and overhaul.

In larger reciprocating compressors, oil and refrigerant

Fig. 37-4 Illustration of Intake Stroke of Reciprocating Compressor. Source: The Trane Company
Fig. 37-5 Illustration of Compression Stroke of Reciprocating Compressor. Source: The Trane Company

mix continuously. In most compressors, the lubrication system is a positive displacement type pump that forces the oil to the components. Refrigeration oils are soluble in liquid refrigerant and generally mix completely at typical ambient temperatures. The term miscibility is used to describe a refrigerant's ability to mix with oil. Because oil is passed through the compressor cylinders to provide lubrication, a small amount of oil (about 1%) is constantly circulating with the refrigerant. In order to return oil and refrigerant gas, velocities must be high enough to sweep the oil along. If gas velocities are insufficient, oil will tend to lie on the bottom of refrigerant tubing. This potential becomes more pronounced under lower evaporating temperatures, since the viscosity of oil increases as temperature decreases.

In hermetic designs, the motor is directly connected to the compressor. Both are sealed inside a housing to lubricate the compressor. To lubricate a small hermetic compressor, the return suction gas is fed into a hollow disk mounted on the motor compressor shaft. Centrifugal force throws the oil and liquid refrigerant to the outer rim of the disk, after which it flows over the motor windings. Only the vapor refrigerant remains at the center and is drawn into the cylinders of the compressor. In larger hermetic compressors, the lubricating oil pump is driven off of the crankshaft. The oil flows through the bearing and the unloaders, and, simultaneously, through the crankshaft to the rod bearing, up the connecting rod, out of the piston, onto the cylinder walls, then back to the oil pump.

In external- (or open-) drive designs, the compressor is bolted together with the crankshaft extending through the crankcase. The crankshaft can be driven by a flywheel

Suction Pressure or Low Side Gage

Cylinder

Hand Expansion Valve c—

Suction Pressure or Low Side Gage

Cylinder

Head Pressure or High Side Gage

Compressor

Head Pressure or High Side Gage

Water

Compressor

Evaporator or

Cooling Coils

Liquid Line

Condenser

Receiver

Fig. 37-6 Illustration of Vapor Compression Cycle System with a Reciprocating Compressor. Source: The Trane Company

(pulley) and belt or can be driven directly by a coupling.

Small compressors usually have fins cast with the cylinders to provide better air cooling. Larger compressors may have water jackets surrounding the cylinders for cooling. Some compressors have cylinder liners (or sleeves) that can be replaced when worn.

Compressor pressure may range as high as 600 psi (41 bar), depending on the type of refrigerant used. Typically, currently applied reciprocating compressors operate on refrigerants such as HCFC-22 (R-22), HFC 134a (R-134a), and ammonia, which have high vapor densities and condensing pressures. Single-stage units are commonly applied to chilled water and other air conditioning applications. Two-stage compressors are used for low-temperature applications. Two stages of compression are achieved with booster and high-stage units, which can be two individual compressors or integral machines. Intercooling is generally used between stages.

Figure 37-6 illustrates a vapor compression cycle system with a reciprocating compressor. The two gauges on either side of the compressor are used to measure the suction, or low-side, pressure before the refrigerant reaches the compressor, and head, or high-side, pressure after the refrigerant is discharged from the compressor as a high-pressure vapor. An additional component, the receiver is added to the system in order to store liquid refrigerant and balance the system. Upon leaving the receiver, the liquid refrigerant passes through the expansion valve on the way to the evaporator. Whereas the compressor maintains a difference in pressure between the evaporator and the condenser, this difference can only be maintained with the use of the expansion valve, which separates the high-pressure part of the system from the low-pressure part. The valve is adjusted so that only just as much liquid can pass through it as can be vaporized in the evaporator, or cooling coil.

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