2 Refrigeration Systems and Components

The refrigeration cycle

A vapor-compression refrigeration system transfers heat from a cooler space to warmer surroundings by circulating refrigerant through a sealed loop. The compressor supplies energy to keep the cycle moving, the evaporator absorbs heat, and the condenser releases it. The heat released at the condenser equals the heat removed from the cooled space plus the energy added by the compressor.

The refrigerant passes through four main states:

1. At the evaporator outlet, it is low-pressure vapor.

2. At the compressor outlet, it is high-pressure, hot vapor.

3. At the condenser outlet, it is high-pressure liquid.

4. At the metering-device outlet, it is a low-pressure, cold liquid-vapor mixture.

The refrigerant then returns to the evaporator, completing the loop. Lower pressure allows refrigerant to boil at a lower temperature, while higher pressure allows it to condense at a higher temperature. This pressure-temperature relationship enables the refrigerant to absorb heat inside the appliance and reject heat to the surrounding air.

The four main components

The compressor draws low-pressure vapor from the evaporator and compresses it to a higher pressure. Compression raises the refrigerant’s temperature so that it can release heat in the warmer surroundings at the condenser. The compressor also circulates refrigerant through the system. In many household refrigerators, it is a sealed unit near the bottom rear, started and stopped by a thermostat or electronic control in response to cooling demand.

The condenser releases heat from refrigerant to surrounding air or water. As hot, high-pressure vapor loses heat, it condenses into high-pressure liquid. In a household refrigerator, the condenser may be a coil at the back or a heat exchanger built into the cabinet walls; some designs use a fan to move air across it. Because the condenser rejects both heat absorbed in the evaporator and energy added by the compressor, it is normal for it to feel warm during operation.

The metering device restricts refrigerant flow from the high-pressure side to the low-pressure side. The sudden pressure drop causes some liquid refrigerant to flash into vapor, producing a cold liquid-vapor mixture that enters the evaporator.

A capillary tube is a narrow, fixed-length tube commonly used in small appliances. It provides a fixed restriction and does not actively adjust its opening as operating conditions change. Larger or different systems may use a thermostatic expansion valve (TXV), which varies refrigerant flow to help maintain appropriate superheat—the temperature of vapor above its boiling temperature—at the evaporator outlet.

The evaporator is the heat exchanger in the space to be cooled. Low-pressure refrigerant absorbs heat from the appliance interior and boils, changing from a cold liquid-vapor mixture into vapor. Fans may move air across the evaporator, or air may circulate naturally; the resulting vapor returns to the compressor. The evaporator does not create cold: it absorbs heat from the cooled space and carries that heat away in the refrigerant.

Controls and operation

Controls coordinate system components and help maintain the desired temperature. A thermostat or temperature sensor detects the temperature in the refrigerated space and calls for cooling when needed; it commonly starts and stops the compressor. In appliances that accumulate frost on the evaporator, a defrost control periodically manages a defrost cycle so frost does not block airflow or reduce heat transfer.

Some systems use high- or low-pressure switches as pressure controls to protect equipment or regulate operation when pressures fall outside intended limits. Electrical overload protection helps protect the compressor motor from excessive current or temperature. A TXV or electronic expansion valve adjusts refrigerant feed to the evaporator, while a capillary tube is a fixed restriction.

When the thermostat calls for cooling, the compressor circulates refrigerant. The evaporator absorbs heat from inside the appliance, and the condenser releases that heat to the surroundings. When the set temperature is reached, the control typically stops the compressor. The exact control sequence depends on the appliance design.

Following refrigerant through a refrigerator

Suppose the refrigerator interior warms above its set temperature. The thermostat signals for cooling, and the compressor starts. It draws vapor from the evaporator and sends hot, high-pressure vapor to the condenser, where heat is released to the room as the refrigerant becomes liquid.

The liquid passes through the capillary tube, where its pressure drops. In the evaporator, it absorbs heat from the refrigerator and boils into vapor. That vapor returns to the compressor, and the cycle continues until the interior reaches the control’s target temperature.