Refrigeration compressors use compressed air to provide refrigeration and air conditioning. It works by converting low pressure gas to high pressure gas. Refrigeration compressors come in three different types: piston/reciprocating, screw, and scroll. Each type uses a different process to create the refrigeration cycle.
Piston compressors, also called reciprocating compressors, provide compressive force using a piston and cylinder. They have the lowest cost and an easy, compact design compared to other types. It works well in high pressure applications due to their large power output range. Disadvantages of this system may include high maintenance costs and vibrational issues. Their design typically prevents them from running at full capacity.
A screw compressor uses a pair of high-speed helical rotors to compress the refrigerant. They come in two configurations: single-screw and twin-screw compressors. They give excellent output and product life. They do require lubricant to ensure sealing between moving parts, reduce noise, and cool down the refrigerant. Screw compressors tend to cost more and take up more space compared to piston compressors.
Scroll compressors, also called spiral compressors, consist of two spiral-shaped rollers. Scroll compressors use less parts, so they have a better output and produce less sound than piston compressors. With less torque variations on the engine, spiral compressors have increased reliability and less sensitivity to the presence of liquid at the inlet. They have a power limit and must combine with other compressors to reach a higher level of power.
Refrigeration systems offer a choice between open, hermetic, and semi-hermetic.
An open refrigeration system features a separate motor and compressor. A coupling sleeve or a belt and pulley system connects the compressor drive shaft to the motor. This allows use with different types of motors, including electric, diesel, and gas. Open refrigeration systems mostly see use in high power applications and have several ways of adjusting the power. It allows easy access to internal parts for repairs.
A hermetic, or airtight refrigeration compressor keeps the electric motor and compressor enclosed in a hermetically welded envelope. Because of this, users cannot access the parts for repair, and must replace the system if any part breaks. Luckily, these systems remain fairly affordable. The main advantage lies in the fact that the sealing system does not depend on the state of the joints. Disadvantages include the fact that the cooling capacity can only be adjusted by varying the frequency of the supply current. They work in low and medium power requirements.
A semi-hermetic compressor holds the motor and compressor in a non-welded hermetic envelope, with parts still accessible for repair. The refrigerant or liquid cooling systems cool the refrigerant. Unlike an open-refrigeration system, a semi-hermetic system has no rotating seal on the transmission shaft. The removable parts still use static seals, which means hermetic refrigeration compressors have more complete sealing. Semi-hermetic compressors typically see use in medium power requirements. While they offer easy access to parts for repair, this type of compressor also costs more than hermetic compressors.
A fixed speed compressor operates at one constant speed, switching on and off to meet the needs of the equipment. They work best in applications that require a constant air demand. A variable speed compressor adapts to meet the needs of the unit. It recovers quickly from door openings, making them ideal for food service applications.