Vapour compression refrigeration cycle

Vapour compression refrigeration is the cycle used in refrigerators, water coolers, cold stores and air conditioners. A refrigerant (R-134a, R-410A, R-600a, ammonia in industry) absorbs heat when it evaporates at low pressure inside the cold space and rejects that heat when it condenses at high pressure outside. An electric motor-driven compressor supplies the work needed to move heat from cold to hot.

Sketch of the cycle

        Heat out (to room/air)
             ↑  ↑  ↑
      +----[ CONDENSER ]----+
      |   high P, high T    |
   (2)|                     |(3) liquid
      |                     ↓
 [COMPRESSOR]        [EXPANSION VALVE]
   ↑  driven by             |
   |  motor M               | (4) low P,
(1)|  low P vapour          |  cold mix
      +----[ EVAPORATOR ]---+
             ↓  ↓  ↓
        Heat in (from food / room)

Working (four processes)

1. Compression (1→2): the compressor draws low-pressure, low-temperature vapour from the evaporator and compresses it. Its pressure and temperature rise well above room temperature (superheated vapour).
2. Condensation (2→3): hot vapour passes through the condenser coils (fins at the back of a fridge, or outdoor unit of an AC). Heat flows to the surrounding air, and the refrigerant condenses into high-pressure liquid.
3. Expansion (3→4): the liquid passes through the expansion valve or capillary tube. Its pressure drops suddenly; part of it flashes to vapour and its temperature falls far below the space temperature.
4. Evaporation (4→1): the cold liquid–vapour mixture flows through the evaporator inside the cabinet/room. It takes latent heat from the food or air and boils into vapour. This heat absorption is the cooling (refrigerating) effect. The vapour returns to the compressor and the cycle repeats.

Electrical circuit of a domestic refrigerator

 L ──[Thermostat]──[OLP]──┬──[Relay coil]── Run winding ──┐
                          │                               │
                          └─[Relay contact]─ Start winding┤
 N ───────────────────────────────────────────────────────┘
 Door switch ── Lamp (in parallel across L–N)

• Thermostat switches the compressor motor on when the cabinet warms and off when the set temperature is reached.
• Starting relay (current relay or PTC) connects the start winding for a moment at starting; overload protector (OLP) trips the motor on overcurrent or overheating.
• The compressor motor is a hermetically sealed single-phase induction motor.

Coefficient of performance: COP = refrigerating effect / work input = (h₁ − h₄)/(h₂ − h₁), typically 2–4, i.e. 1 kW of electric input removes 2–4 kW of heat.