A refrigerator removes thermal energy from a relatively cold compartment and transfers that energy to a warmer surrounding environment by using external work. Most household refrigerators use a vapour compression refrigeration cycle involving a refrigerant, compressor, condenser, expansion device and evaporator. The refrigerant circulates through these components and undergoes changes in pressure, temperature and phase. In the evaporator inside the refrigerated compartment, low pressure refrigerant absorbs heat and typically changes from liquid to vapour.
The compressor then raises the refrigerant pressure and temperature by doing mechanical work on it. In the condenser outside the cold compartment, the refrigerant rejects heat to the room and condenses into a liquid. The expansion device then reduces the refrigerant pressure, causing cooling and preparing the refrigerant to enter the evaporator again. The cycle continuously repeats while the compressor operates.
The evaporator is the component where useful cooling occurs. Low pressure refrigerant enters the evaporator at a temperature below that of the refrigerated space. Heat naturally flows from the warmer food, air and internal surfaces toward the colder refrigerant. The absorbed heat causes the refrigerant to evaporate or boil at relatively low pressure. Phase change allows the refrigerant to absorb substantial latent heat without requiring an equally large increase in temperature. The resulting vapour travels toward the compressor.
Efficient evaporator design maximises contact with the refrigerated air or surfaces while maintaining appropriate pressure, temperature and refrigerant flow. Fans may circulate air through the evaporator in larger refrigerators and freezers. Frost and ice accumulation can reduce heat transfer and airflow, which is why automatic defrost systems are used in many modern appliances.
The compressor is the part of the refrigeration system that supplies most of the mechanical work required by the cycle. It takes relatively low pressure refrigerant vapour from the evaporator and compresses it to a higher pressure. Compression raises the refrigerant's temperature, allowing it to reject heat to the room when it reaches the condenser. Hermetically sealed compressors used in domestic appliances generally contain an electric motor and compressor mechanism within a sealed housing.
Different compressor technologies include reciprocating, rotary, scroll and variable speed designs. Variable speed compressors can adjust their output according to cooling demand and can improve efficiency under suitable operating conditions. The compressor must be designed for the refrigerant, pressure range, lubrication requirements and expected operating conditions.
The condenser releases heat from the refrigerant into the surrounding environment. The refrigerant enters the condenser as a high pressure, high temperature vapour and transfers energy to the surrounding air. As the refrigerant loses enough energy, it condenses into a high pressure liquid. The heat rejected by the condenser is greater than the heat extracted from the refrigerator compartment because it includes the electrical work supplied to the compressor.
This is why the back or sides of a refrigerator can become warm. The condenser must have sufficient surface area and airflow to remove the required heat. Dust accumulation, poor ventilation or high room temperature can reduce condenser performance and increase energy consumption. In modern appliances, condenser arrangements vary and may be integrated into cabinet walls rather than exposed as traditional coils.
The expansion device creates the pressure difference needed for the refrigeration cycle. A high pressure liquid refrigerant passes through a capillary tube, thermostatic expansion valve or electronic expansion valve, depending on system design. The pressure drops as the refrigerant passes through the restriction, and some of the liquid may flash into vapour. The resulting mixture enters the evaporator at a much lower pressure and corresponding saturation temperature.
The expansion process is approximately throttling, meaning that enthalpy remains approximately constant across the restriction even though pressure and temperature change. The expansion device also regulates refrigerant flow in systems where variable demand requires active control. Correct expansion control is essential because too much or too little refrigerant flow can reduce evaporator performance and potentially damage the compressor.
Refrigerators work because the refrigerant has thermodynamic properties that allow it to evaporate and condense at useful temperatures and pressures. Modern refrigerants are selected according to pressure characteristics, chemical stability, heat transfer properties, toxicity, flammability and environmental impact. Historically used chlorofluorocarbons and hydrochlorofluorocarbons contributed to ozone depletion, leading to international controls and replacement by alternative refrigerants.
Some hydrofluorocarbons have high global warming potential, prompting further transitions toward lower global warming potential alternatives such as hydrocarbons, carbon dioxide and other refrigerant classes where suitable. Refrigerant selection is therefore an engineering and environmental issue. The exact refrigerant and system architecture vary among appliances and regions, and servicing refrigeration systems requires appropriate technical procedures because refrigerants can be pressurised, flammable or environmentally regulated.
The refrigerator demonstrates the second law of thermodynamics because heat is being transferred from a colder region to a warmer environment through the expenditure of work. Without external work, the natural direction of heat transfer would be from the warmer room into the colder compartment. The coefficient of performance of a refrigerator is the heat removed from the cold space divided by the work supplied to the refrigeration system.
A higher coefficient of performance indicates that more heat is removed per unit of supplied work under the defined operating conditions. Real refrigerators perform below reversible theoretical limits because of compressor inefficiency, pressure drops, imperfect heat exchangers, heat leakage through insulation, motor losses and other irreversibilities.
Refrigerator performance depends on insulation, door opening frequency, food loading, ambient temperature, condenser ventilation, thermostat settings and component condition. Thick insulation reduces unwanted heat leakage into the cold compartment. Opening the door allows warm humid air to enter and can increase the cooling load. Placing hot food inside increases the amount of heat that must be removed before the food reaches storage temperature. Poor airflow around the condenser can make heat rejection more difficult and increase compressor operating time.
The thermostat or electronic controller starts and stops or modulates the refrigeration system to maintain the desired temperature. The refrigerator therefore does not create cold as a substance. It continuously removes thermal energy from its interior and transfers that energy, plus the electrical energy consumed by the compressor and other components, into the surrounding environment.
