8 Basic Refrigeration and Refrigerator

1. Fundamental Concepts

• Refrigeration: The process of removing heat from a low-temperature reservoir and transferring it to a high-temperature reservoir.

• Unit of Refrigeration (TR): 1 Ton of Refrigeration is the amount of heat required to melt 1 ton (1000 kg) of ice at 0°C in 24 hours.

  • $1 \text{ TR} = 3.5 \text{ kW}$

  • $1 \text{ TR} = 50.4 \text{ kcal/min}$

  • $1 \text{ TR} = 210 \text{ kJ/min}$

• COP (Coefficient of Performance): Ratio of heat extracted (Refrigeration Effect) to the work supplied.

  • $\text{COP} = \frac{\text{Desired Effect}}{\text{Work Input}}$

  • Note: COP is always greater than 1 for practical refrigerators.

2. Vapor Compression Refrigeration System (VCRS)

This is the most common cycle used in domestic refrigerators.

• Compressor: Increases pressure and temperature of the refrigerant vapor (Heart of the system).

• Condenser: Rejects heat to the surroundings; refrigerant changes from high-pressure vapor to high-pressure liquid.

• Expansion Valve/Capillary Tube: Drops the pressure and temperature; refrigerant becomes a low-pressure liquid-vapor mixture.

• Evaporator: Absorbs heat from the storage space; refrigerant turns into low-pressure vapor.

3. Domestic Refrigerator Components

• Compressor Type: Usually Hermetically Sealed Reciprocating or Rotary compressor.

• Condenser Type: Air-cooled (Natural convection) wire-mesh or finned type located at the back.

• Expansion Device: Capillary Tube (diameter 0.5 to 2.28 mm) is used due to its simplicity and lack of moving parts.

• Evaporator: Usually “Plate Type” or “Finned Type” located in the freezer compartment.

• Refrigerants Used: * Old: R-12 (CFC)

  • Modern: R-134a (HFC) or R-600a (Isobutane – highly flammable but eco-friendly).

4. Key Parts & Controls

• Thermostat: A temperature-controlled switch that starts/stops the compressor to maintain the set temperature.

• Relay (PTC or Electromagnetic): Used to start the compressor motor by engaging the starting winding.

• OLP (Overload Protector): Protects the compressor motor from overheating or high current.

• Accumulator: Placed at the evaporator exit to prevent liquid refrigerant from entering the compressor (prevents “Slugged” compressor).

• Filter Drier: Removes moisture and debris from the refrigerant before it enters the capillary tube.

5. Important Cycle States (Exam Points)

6. Common Troubleshooting for RAC Mechanics

• Short Cycling: Frequent start/stop of compressor; usually caused by a faulty thermostat or low refrigerant.

• Sweating on Cabinet: High humidity or door gasket leak.

• Frost Accumulation: Excessive moisture entry or failure of the defrost heater (in frost-free models).

• Choked System: Moisture freezing inside the capillary tube; requires vacuuming and recharging.