COMPRESSOR

Compressors: Types, Working Principle & Efficiency Factors

1. Function of a Compressor

• The compressor is the heart of a refrigeration system.
• It compresses low-pressure refrigerant vapor from the evaporator into high-pressure, high-temperature vapor before sending it to the condenser.
• This allows the refrigerant to reject heat in the condenser and continue the cycle.

2. Types of Compressors & Their Working Principles

A. Reciprocating Compressor

• Working Principle: Uses a piston and cylinder mechanism similar to an internal combustion engine.
• The piston moves up and down inside the cylinder, creating suction and compression strokes.
• Common in: Refrigerators, deep freezers, air conditioners.
• Advantages: High efficiency, suitable for high-pressure applications.
• Disadvantages: More moving parts, higher maintenance.

B. Rotary Compressor

• Working Principle: Uses rotating elements (blades, vanes, or screws) to compress refrigerant gas.
• Types:
  1. Rolling Piston (Vane Type) – A roller rotates inside a stationary cylinder, compressing refrigerant.
  2. Screw Type – Uses helical rotors to compress gas.
• Common in: Air conditioners, commercial refrigeration.
• Advantages: Compact, less noise, smooth operation.
• Disadvantages: Less efficient than reciprocating for high-pressure applications.

C. Scroll Compressor

• Working Principle: Uses two spiral-shaped scrolls—one fixed and one orbiting—to compress refrigerant gas.
• Common in: High-efficiency air conditioners and heat pumps.
• Advantages: High efficiency, fewer moving parts, low noise.
• Disadvantages: Costly, difficult to repair.

D. Wobble Plate Compressor

• Working Principle: Uses a rotating wobble plate connected to multiple pistons to compress gas.
• Common in: Automotive air conditioning.
• Advantages: Compact design, efficient.
• Disadvantages: Complex mechanism, high maintenance.

E. Swash Plate Compressor

• Working Principle: Similar to wobble plate but uses a swash plate for driving multiple pistons.
• Common in: Automotive air conditioners.
• Advantages: Continuous operation, good efficiency.
• Disadvantages: Wear and tear due to multiple pistons.

3. Lubrication Methods for Compressors

• Lubrication is essential to reduce wear, heat, and friction in compressors.
• Methods include:
  1. Splash Lubrication – Oil splashes onto moving parts (used in reciprocating compressors).
  2. Pressure Lubrication – Uses a pump to force oil into moving parts (used in screw compressors).
  3. Mist Lubrication – Oil mist is sprayed onto moving parts.
  4. Oil-Free Lubrication – Used in scroll and some rotary compressors to prevent contamination.

4. Factors Affecting Compressor Efficiency

1. Compression Ratio – Higher ratios reduce efficiency due to more work done.
2. Clearance Volume – Excess space in the cylinder reduces efficiency.
3. Suction & Discharge Pressure – Incorrect pressures increase power consumption.
4. Refrigerant Type – Efficiency depends on refrigerant properties.
5. Lubrication – Poor lubrication increases wear and reduces lifespan.
6. Cooling Method – Water-cooled compressors are more efficient than air-cooled.
7. Superheating & Subcooling – Proper refrigerant conditions improve efficiency.

5. Wet Compression & Its Effects

• What is wet compression?
  → It occurs when liquid refrigerant enters the compressor instead of vapor.
• Effects of wet compression:
  • Damage to compressor valves and pistons.
  • Reduced efficiency and increased power consumption.
  • Excessive foaming in the lubrication system.

6. Compressor Oil & Its Properties

• Functions of Compressor Oil:

  1. Reduces friction between moving parts.
  2. Cools the compressor by absorbing heat.
  3. Prevents rust and corrosion.
  4. Ensures smooth sealing between cylinder and piston.

• Essential Properties of Compressor Oil:

  1. High Viscosity Index – Should maintain lubrication at various temperatures.
  2. Chemical Stability – Should not degrade with refrigerant.
  3. Low Wax Content – Prevents freezing in cold areas.
  4. Non-Hygroscopic – Should not absorb moisture.
  5. Good Thermal Conductivity – Helps in heat dissipation.

1. Basic Compressor Questions

1. What is the function of a compressor in a refrigeration system?
   → It compresses low-pressure vapor into high-pressure vapor.
2. What are the main types of compressors used in refrigeration?
   → Reciprocating, rotary, scroll, wobble plate, and swash plate.
3. Which component follows the compressor in a refrigeration cycle?
   → The condenser.
4. Which type of compressor is most commonly used in domestic refrigerators?
   → Reciprocating compressor.
5. What is the main function of the suction valve in a reciprocating compressor?
   → To allow refrigerant vapor into the cylinder during suction stroke.
6. What happens if a compressor runs without lubrication?
   → It leads to overheating and component wear, causing failure.
7. Which compressor type has the least number of moving parts?
   → Scroll compressor.
8. What is the effect of a high compression ratio on compressor efficiency?
   → It reduces efficiency and increases power consumption.
9. What happens when liquid refrigerant enters the compressor?
   → It causes wet compression, leading to damage and inefficiency.
10. Which factor mainly affects compressor performance in an A/C system?
    → Suction and discharge pressure.

2. Reciprocating Compressor Questions

11. How does a reciprocating compressor work?
    → It uses a piston and cylinder to compress gas in two strokes.
12. Which stroke in a reciprocating compressor compresses refrigerant?
    → The compression stroke.
13. What is clearance volume in a reciprocating compressor?
    → The space left between the piston and the cylinder head.
14. What is the effect of excessive clearance volume?
    → Reduces volumetric efficiency and increases power consumption.
15. What are the two types of reciprocating compressors?
    → Single-cylinder and multi-cylinder.
16. What is the purpose of the discharge valve in a reciprocating compressor?
    → To release high-pressure gas into the condenser.
17. What type of lubrication is used in reciprocating compressors?
    → Splash lubrication or pressure lubrication.
18. Why is multi-cylinder compression more efficient than single-cylinder?
    → It reduces load per cylinder and increases smooth operation.
19. Which type of compressor is suitable for high-pressure applications?
    → Reciprocating compressor.
20. What is the main disadvantage of reciprocating compressors?
    → High noise and maintenance requirements.

3. Rotary Compressor Questions

21. How does a rotary compressor differ from a reciprocating compressor?
    → It uses rotating motion instead of pistons for compression.
22. What are the types of rotary compressors?
    → Rolling piston and screw compressors.
23. Which rotary compressor is used in large industrial cooling systems?
    → Screw compressor.
24. What is the main advantage of rotary compressors over reciprocating ones?
    → They are quieter and require less maintenance.
25. What is a common application of rotary compressors?
    → Air conditioners and water coolers.
26. What type of lubrication method is used in rotary compressors?
    → Oil mist lubrication.
27. Why are rotary compressors preferred for continuous operation?
    → They provide smooth and efficient performance with fewer vibrations.
28. What is a disadvantage of rotary compressors?
    → Lower efficiency in high-pressure applications.
29. Which compressor type is compact and has fewer moving parts?
    → Rotary compressor.
30. Which rotary compressor type uses helical screws for compression?
    → Screw compressor.

4. Scroll, Wobble Plate & Swash Plate Compressor Questions

31. How does a scroll compressor work?
    → It uses two spiral scrolls—one fixed and one moving—to compress gas.
32. Why are scroll compressors used in air conditioners?
    → They offer high efficiency and quiet operation.
33. What is a key disadvantage of scroll compressors?
    → They are expensive and difficult to repair.
34. Which compressor type is commonly used in automotive air conditioning?
    → Wobble plate and swash plate compressors.
35. How does a wobble plate compressor work?
    → A wobble plate moves pistons back and forth to compress gas.
36. What is a swash plate compressor?
    → It uses an angled swash plate to drive pistons in a circular motion.
37. Which compressor type is used in car A/C systems for smooth performance?
    → Swash plate compressor.
38. Why are wobble plate compressors compact?
    → They use multiple pistons in a small housing.
39. Which compressor type provides continuous compression in vehicle A/Cs?
    → Swash plate compressor.
40. What is the advantage of scroll compressors over reciprocating ones?
    → Better efficiency and fewer moving parts.

5. Lubrication & Compressor Efficiency Questions

41. Why is lubrication important in compressors?
    → It reduces friction, heat, and wear of moving parts.
42. What are the methods of compressor lubrication?
    → Splash, pressure, and oil mist lubrication.
43. Which lubrication method is used in high-speed compressors?
    → Pressure lubrication.
44. What is wet compression in a refrigeration system?
    → When liquid refrigerant enters the compressor instead of vapor.
45. Why is wet compression harmful?
    → It can damage compressor components and reduce efficiency.
46. What are the key properties of compressor oil?
    → High viscosity index, thermal stability, and non-hygroscopic nature.
47. What type of oil is used in modern compressors with eco-friendly refrigerants?
    → Synthetic oil.
48. What is the role of an oil separator in refrigeration compressors?
    → It removes oil from refrigerant vapor and returns it to the compressor.
49. How does compressor efficiency affect overall system performance?
    → Higher efficiency leads to lower energy consumption and better cooling.
50. What happens if a compressor operates without proper lubrication?
    → Increased wear, overheating, and possible compressor failure.

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