There are generally four types of compressors in use today: reciprocating, rotary, centrifugal, and the screw type.

Some of these compressors, used commercially, could be belt-driven or hermetic. The hermetic type has its motor and compressor coupled directly together in a sealed (airtight) metal housing, whereas the others are powered with a separately mounted motor and driven with a V belt(s) and pulleys.

All residential and many light commercial refrigeration units are operated with hermetically sealed motors and compressors because they are compact and require little space. In heavy commercial and industrial use, where large capacity units are required and space is not a problem, the separately mounted motors with v-belts and pulleys to drive the compressors work well.

A reciprocating compressor is similar to an automotive engine with a piston moving up and down in a cylinder. Instead of relying on exploding and expanding gas to drive the piston, the piston is powered by an electric motor. As the piston moves downward (or backward), the inlet valve opens and vapor is drawn in from the evaporator. When the piston starts up toward the top (or forward), the inlet valve closes and the gas is compressed (thereby raising its temperature). Before the piston reaches the top (or its most forward position), the discharge valve opens and allows the gas to be propelled into the condenser (see fig. 28). They are used in commercial and residential units, light and heavy applications. These compressors can be hermetic or externally driven.

A rotary compressor also operates on a principle similar to the automotive rotary engine. It can be hermetically sealed or rely on an external conventional electric motor. They are used in commercial and residential units. As the rotor revolves inside the cylinder on an eccentric cam, the spring-loaded vanes pass the intake and discharge ports. As one vane passes the intake port, suction begins and cold vapor is drawn into the cylinder. As the rotor continues its turn, the gas is compressed. When the other vane clears the discharge port, the gas is propelled through the high-pressure line into the condenser. (See fig. 29.)

A centrifugal compressor might be thought of as a “squirrel cage”-like blower motor. As the impeller turns, a vacuum is created at its center, causing cold vapor to be drawn in. It is then compressed and expelled into the discharge port through the sides (caused by high rpm). These compressors can be hermetic or externally driven and are used commercially. (See fig. 30.)

A screw-type compressor is similar in operation to a turbocharger. It has two cylindrical vanes with deep, spiraling flutes that mesh together like gear teeth. The extremely high rpm of the vanes meshing into each other creates vacuum on one side and high pressures on the other, causing refrigerant vapor to be drawn in (from the intake port), compressed, and forced out through the compressor discharge port. Screw-type compressors are used in heavy commercial applications.

A liquid receiver is a liquid storage tank used on larger commercial systems and on systems equipped with expansion valve or a low-side float-type refrigerant control explained on pages 140 through 170. (See figs. 33 and 118.)

The high-pressure-side service valve is usually located on the receiver, and the low-pressure-side service valve is installed on the compressor suction inlet.

Bolted-type serviceable semi hermetic motor compressors are equipped with high- and low-pressure-side service valves attached to the compressor housing. If the compressor has to be replaced, it can be isolated and removed from the rest of the sealed system without having to discharge the refrigerant from the system (see figs. 119, 32.) The service valves are bolted to the compressor housing with two bolts from one side and connected to the tubing by flared connections from the other side.

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