Wet Compression, Compressor Oil, Properties and Lubrication Methods

In refrigeration and air conditioning systems, proper compression and lubrication are essential for efficient and reliable operation of compressors. This chapter explains the concept of wet compression, the importance of compressor oil, its properties, and various lubrication methods used in different types of compressors.

Wet Compression

Wet compression refers to the condition in which the refrigerant entering the compressor contains liquid along with vapour. Ideally, only dry vapour should enter the compressor, but in some cases, small quantities of liquid refrigerant may be present.

Causes of Wet Compression

• Overfeeding of refrigerant
• Faulty expansion device
• Low evaporator temperature
• Poor superheating control

Effects of Wet Compression

• Damage to compressor valves and piston
• Reduction in lubrication efficiency
• Noise and vibration
• Reduced compressor life

Although a slight amount of wet vapour may help in cooling the compressor, excessive liquid can cause serious damage. Therefore, proper system control is necessary to avoid wet compression.

Compressor Oil

Compressor oil is used to lubricate moving parts, reduce friction, and ensure smooth operation. It also helps in sealing, cooling, and reducing wear and tear of components.

Functions of Compressor Oil

• Lubrication of moving parts
• Cooling of compressor components
• Sealing between piston and cylinder
• Reduction of noise and vibration

Properties of Compressor Oil

For proper functioning, compressor oil must have specific properties suitable for refrigeration systems.

• Viscosity: Proper thickness for effective lubrication.
• Low Pour Point: Ability to flow at low temperatures.
• Chemical Stability: Should not react with refrigerant.
• High Dielectric Strength: Suitable for electrical insulation.
• Low Volatility: Prevents oil loss due to evaporation.
• Non-corrosive Nature: Prevents damage to metal parts.

Types of Compressor Oils

• Mineral Oil
• Synthetic Oil (POE, PAG)

The selection of oil depends on the type of refrigerant and compressor design. For example, synthetic oils are commonly used with modern refrigerants like R-134a.

Lubrication Methods

Proper lubrication is essential for reducing friction and ensuring long life of the compressor. Different compressors use different lubrication methods.

1. Splash Lubrication

In this method, a part of the crankshaft dips into the oil sump and splashes oil onto moving parts during rotation.

• Simple and economical
• Used in small compressors

2. Pressure Lubrication

Oil is circulated under pressure using an oil pump. It ensures proper lubrication of all components.

• Used in large compressors
• Provides efficient lubrication

3. Centrifugal Lubrication

Oil is distributed using centrifugal force created by rotating components.

• Used in rotary compressors

4. Oil Injection Method

Oil is directly injected into the compression chamber for lubrication and cooling.

• Used in screw and rotary compressors

5. Mist Lubrication

Oil is mixed with refrigerant in the form of mist and circulated throughout the system.

• Ensures uniform lubrication

Importance of Proper Lubrication

• Reduces friction and wear
• Prevents overheating
• Improves efficiency
• Extends compressor life

Conclusion

Wet compression, compressor oil, and lubrication methods are critical aspects of refrigeration systems. Proper understanding of these concepts helps in maintaining efficient compressor operation and preventing system failures.

By selecting the right oil, ensuring proper lubrication, and avoiding wet compression, technicians can improve system performance and extend the life of refrigeration equipment. This knowledge is essential for ITI RAC students in practical and industrial applications.