9 Compressor, Condenser, Evaporator

1. Compressor (The Heart of the System)

• • Primary Function: To suck low-pressure, low-temperature refrigerant vapor from the evaporator and compress it into high-pressure, high-temperature vapor.

  • Process: Isentropic Compression (constant entropy).

  • Types Used in RAC:

    • Reciprocating: High pressure, low volume (Domestic fridges).

    • Rotary: Low noise, high efficiency (Window/Split ACs).

    • Centrifugal: Large capacity (Central AC plants).

    • Scroll: Quiet, efficient, fewer moving parts.

  • Key Fact: The compressor handles vapor only. Liquid refrigerant entering the compressor (slugging) can cause mechanical failure.

  • Lubrication: Uses specialized refrigeration oil (POE or Mineral oil) mixed with the refrigerant.

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2. Condenser (The Heat Rejecter)

• Primary Function: To reject heat absorbed from the refrigerated space and the heat of compression to the outside medium (air or water).

• Process: Constant Pressure Heat Rejection (Isobaric).

• Phase Change: Refrigerant changes from High-Pressure Vapor to High-Pressure Liquid.

• Sub-cooling: Cooling the liquid refrigerant below its saturation temperature to increase the refrigeration effect and reduce flash gas.

• Types:

  • Air Cooled: Natural or forced convection (Domestic units).

  • Water Cooled: Shell and tube, shell and coil (Industrial units).

  • Evaporative: Uses both air and water for cooling.

3. Evaporator (The Cooling Coil)

• Primary Function: To absorb heat from the space or substance to be cooled.

• Process: Constant Pressure Heat Absorption (Isobaric).

• Phase Change: Refrigerant changes from Low-Pressure Liquid to Low-Pressure Vapor.

• Superheating: Ensuring the vapor leaving the evaporator is $5\text{°C}$ to $10\text{°C}$ above its boiling point to protect the compressor from liquid.

• Cooling Load: The evaporator capacity is measured in Tons of Refrigeration (TR).

  • $1 \text{ TR} = 3.5 \text{ kW} = 12,000 \text{ BTU/hr} = 50.4 \text{ kcal/min}$.

• Types:

  • DX (Direct Expansion): Refrigerant cools air directly.

  • Flooded: Evaporator is kept filled with liquid refrigerant (High efficiency).

  • Fin and Tube: Used in ACs to increase the heat transfer surface area.

Summary Table for Quick Revision

(Note: LP = Low Pressure, HP = High Pressure)