Function, Construction, Working and Application of Hermetic Compressor (Fixed Speed and Variable Speed)

In the ITI Refrigeration and Air Conditioning (RAC) trade, the compressor is the most important component of any refrigeration system. The book "Compressor" explains in detail the function, construction, working, and applications of hermetic compressors, including fixed speed and variable speed (inverter) types such as reciprocating, rotary, and scroll compressors.

Function of Compressor

The main function of a compressor in a refrigeration system is to compress low-pressure, low-temperature refrigerant vapour coming from the evaporator and convert it into high-pressure, high-temperature vapour. This process enables the refrigerant to circulate through the system and release heat in the condenser.

• Maintains refrigerant flow in the system
• Increases pressure and temperature of refrigerant
• Ensures continuous refrigeration cycle

Hermetic Compressor

A hermetic compressor is a sealed unit in which the compressor and motor are enclosed within a welded steel shell. These compressors are widely used in domestic refrigerators, air conditioners, and small commercial systems.

Advantages

• Compact and leak-proof design
• Low maintenance requirement
• Quiet operation
• High reliability

Construction of Hermetic Compressor

The hermetic compressor consists of the following main parts:

• Outer Shell: Sealed steel casing.
• Electric Motor: Drives the compressor mechanism.
• Compressor Unit: (Reciprocating/Rotary/Scroll type).
• Suction and Discharge Lines: For refrigerant flow.
• Lubricating Oil: Reduces friction and wear.
• Mounting Springs: Reduce vibration and noise.

Types of Hermetic Compressors

1. Reciprocating Compressor

This type works on the piston-cylinder mechanism similar to an internal combustion engine.

• Construction: Piston, cylinder, crankshaft, suction and discharge valves.
• Working: The piston moves up and down, drawing refrigerant during suction stroke and compressing it during compression stroke.
• Application: Domestic refrigerators, small AC units.

2. Rotary Compressor

Rotary compressors use a rotating mechanism to compress refrigerant.

• Construction: Rotor, vane, cylinder.
• Working: The rotating vane traps and compresses refrigerant gas.
• Application: Split air conditioners, medium cooling systems.

3. Scroll Compressor

Scroll compressors use two spiral-shaped scrolls for compression.

• Construction: Fixed scroll and orbiting scroll.
• Working: Refrigerant is trapped between scrolls and compressed as the orbiting scroll moves.
• Application: High-efficiency air conditioners, commercial refrigeration.

Fixed Speed Compressor

Fixed speed compressors operate at constant speed. They start and stop depending on temperature requirements controlled by a thermostat.

• Simple design
• Lower initial cost
• Higher energy consumption due to frequent ON/OFF cycling

Variable Speed (Inverter) Compressor

Variable speed compressors, also known as inverter compressors, adjust their speed according to cooling demand. They use electronic control systems to vary motor speed.

• Energy efficient operation
• Stable temperature control
• Reduced noise and vibration
• Longer compressor life

Working of Inverter Compressor

The inverter system converts AC power into DC and then back into variable frequency AC to control compressor speed. Sensors detect temperature changes and send signals to the control PCB, which adjusts the compressor speed accordingly.

Unlike fixed speed compressors, inverter compressors do not stop frequently but run continuously at varying speeds, ensuring efficient cooling and energy saving.

Applications of Hermetic Compressors

• Domestic refrigerators
• Split and window air conditioners
• Deep freezers
• Water coolers
• Commercial refrigeration systems

Comparison: Fixed Speed vs Variable Speed

Conclusion

The study of hermetic compressors is essential for understanding modern refrigeration systems. Different types such as reciprocating, rotary, and scroll compressors are used based on application requirements. The introduction of inverter (variable speed) technology has significantly improved efficiency, performance, and reliability.

This chapter provides RAC students with the necessary knowledge and practical understanding required for installation, servicing, and troubleshooting of compressors used in refrigeration and air conditioning systems.