EVAPORATOR

Evaporators: Working Principle & Function

Working Principle:
An evaporator is a heat exchanger that absorbs heat from the surrounding environment (air, water, or any medium to be cooled) by allowing a refrigerant to evaporate. The refrigerant enters the evaporator as a low-pressure liquid and absorbs heat, changing into a vapor, thus cooling the surrounding area.

Function of Evaporators:

1. Heat Absorption: Transfers heat from the environment to the refrigerant.
2. Cooling Effect: Reduces the temperature of air, water, or any substance in contact.
3. Superheating: Ensures complete evaporation of refrigerant to prevent liquid refrigerant from entering the compressor.

Types of Evaporators Used in Different Applications

1. Refrigerators:

   • Plate Type Evaporator
   • Finned Tube Evaporator

2. Water Coolers:

   • Shell and Coil Evaporator
   • Immersion Coil Evaporator

3. Bottle Coolers:

   • Finned Tube Evaporator
   • Bare Tube Evaporator

4. Window & Split A.C.:

   • Finned Tube Evaporator
   • Cross-flow Evaporator

Common Evaporator Types:

1. Bare Tube Evaporator: Simple construction, used in water coolers and domestic refrigerators.
2. Finned Tube Evaporator: Common in air conditioners for better heat transfer.
3. Plate Type Evaporator: Used in freezers and refrigerators for direct contact cooling.
4. Shell and Tube Evaporator: Used in large cooling applications like industrial refrigeration.

Superheating in Evaporators

• Definition: Superheating occurs when the refrigerant, after complete evaporation, absorbs additional heat, raising its temperature above the saturation point.
• Purpose:
  • Prevents liquid refrigerant from entering the compressor (avoids damage).
  • Ensures maximum efficiency of the refrigeration cycle.
  • Maintains proper pressure and temperature balance in the system.

Function of Accumulator & Types

Function:

1. Stores excess refrigerant and prevents liquid slugging into the compressor.
2. Ensures only vapor refrigerant enters the compressor.
3. Acts as a reservoir to supply refrigerant during low load conditions.

Types of Accumulators:

1. Vertical Accumulator: Used in large refrigeration systems with high refrigerant flow.
2. Horizontal Accumulator: Used in compact refrigeration units like window ACs.

Methods of Defrosting

1. Manual Defrosting: Requires turning off the system to remove ice buildup manually.
2. Automatic Defrosting:
   • Electric Defrost: Uses heating elements to melt ice.
   • Hot Gas Defrost: Redirects hot refrigerant gas to the evaporator coil.
   • Water Defrost: Sprays warm water over the evaporator.
   • Reverse Cycle Defrost: Uses heat from the condenser by reversing the refrigeration cycle.

Heat Exchanger in Refrigeration Systems

• Function: Transfers heat between two fluids to improve efficiency.
• Types Used:
  • Air-cooled heat exchangers
  • Water-cooled heat exchangers
  • Plate heat exchangers
• Applications:
  • Pre-cooling the refrigerant before expansion.
  • Recovering heat from condenser to improve efficiency.

Evaporators & Working Principle

1. What is the function of an evaporator in a refrigeration system?
   → Absorbs heat and allows refrigerant to evaporate for cooling.

2. Which type of heat exchanger is an evaporator?
   → A low-temperature heat exchanger.

3. What is the state of the refrigerant entering the evaporator?
   → Low-pressure liquid (partially vaporized).

4. What is the state of the refrigerant leaving the evaporator?
   → Low-pressure superheated vapor.

5. What causes the refrigerant to evaporate in the evaporator?
   → Absorption of heat from the surroundings.

Types of Evaporators & Their Applications

6. Which evaporator type is commonly used in household refrigerators?
   → Plate type evaporator.

7. Which evaporator is used in window and split ACs?
   → Finned tube evaporator.

8. Which type of evaporator is submerged in a liquid for cooling?
   → Immersion coil evaporator.

9. What type of evaporator is used in industrial refrigeration?
   → Shell and tube evaporator.

10. Which evaporator type is best for bottle coolers?
    → Bare tube evaporator.

11. Which evaporator has the highest heat transfer efficiency?
    → Finned tube evaporator.

12. Which evaporator type is widely used in deep freezers?
    → Plate type evaporator.

13. What is the main advantage of a shell and tube evaporator?
    → Handles large cooling loads efficiently.

14. Which evaporator type is commonly used in water coolers?
    → Shell and coil evaporator.

15. Which evaporator type provides direct cooling in refrigerators?
    → Plate type evaporator.

Superheating in Evaporators

16. What is superheating in an evaporator?
    → Heating the refrigerant above its boiling point after complete evaporation.

17. Why is superheating necessary in an evaporator?
    → To prevent liquid refrigerant from entering the compressor.

18. How does superheating affect refrigeration system efficiency?
    → Increases efficiency by ensuring complete evaporation of refrigerant.

19. What happens if there is excessive superheating?
    → Reduces cooling efficiency and increases compressor workload.

20. How is superheating controlled in an evaporator?
    → By adjusting the expansion valve or refrigerant flow.

Accumulators & Their Function

21. What is the purpose of an accumulator in a refrigeration system?
    → Prevents liquid refrigerant from entering the compressor.

22. Where is the accumulator placed in the refrigeration cycle?
    → Between the evaporator and the compressor.

23. Which type of accumulator is used in compact refrigeration units?
    → Horizontal accumulator.

24. What happens if an accumulator is not used in a refrigeration system?
    → Liquid slugging may damage the compressor.

25. How does an accumulator store excess refrigerant?
    → By temporarily holding it in a separate chamber.

26. What type of accumulator is used in large refrigeration systems?
    → Vertical accumulator.

27. What is a key benefit of using an accumulator?
    → Ensures only vapor enters the compressor.

28. What component is used along with an accumulator to control refrigerant flow?
    → A heat exchanger or expansion valve.

Methods of Defrosting

29. What is defrosting in refrigeration systems?
    → Removing ice buildup from the evaporator coils.

30. What is the simplest method of defrosting?
    → Manual defrosting.

31. Which defrosting method uses electric heating elements?
    → Electric defrosting.

32. Which defrosting method circulates hot refrigerant gas?
    → Hot gas defrosting.

33. Which defrosting method sprays warm water over the evaporator?
    → Water defrosting.

34. What is the main advantage of automatic defrosting?
    → Prevents ice buildup without manual intervention.

35. Which defrosting method reverses the refrigeration cycle?
    → Reverse cycle defrosting.

36. Which refrigeration systems require frequent defrosting?
    → Freezers and cold storage units.

Heat Exchangers in Refrigeration Systems

37. What is the function of a heat exchanger in refrigeration?
    → Transfers heat between two fluids for improved efficiency.

38. Which type of heat exchanger is commonly used in air conditioning?
    → Finned tube heat exchanger.

39. Which heat exchanger is used in large industrial cooling systems?
    → Shell and tube heat exchanger.

40. What type of heat exchanger is used to cool refrigerant before expansion?
    → Subcooling heat exchanger.

41. Which heat exchanger uses direct contact between refrigerant and air?
    → Air-cooled heat exchanger.

42. Which heat exchanger type improves energy efficiency by reclaiming heat?
    → Heat recovery exchanger.

43. Which component in an air conditioner acts as a heat exchanger?
    → The evaporator and condenser coils.

44. What is a counterflow heat exchanger?
    → A heat exchanger where fluids flow in opposite directions.

45. Why is a heat exchanger important in an evaporator?
    → Enhances heat absorption and cooling performance.

46. Which heat exchanger type is most compact and efficient?
    → Plate heat exchanger.

47. What is the main purpose of using a heat exchanger in a split AC?
    → To improve refrigerant efficiency and cooling performance.

48. What happens if a heat exchanger in an evaporator is clogged?
    → Reduced cooling efficiency and system failure.

49. What is the role of a secondary heat exchanger in some refrigeration systems?
    → Improves efficiency by utilizing excess heat from the primary cycle.

50. Which heat exchanger is commonly used in water coolers?
    → Shell and coil heat exchanger.