How a Refrigerator Works

A refrigerator works by using the refrigeration cycle to remove heat from the interior of the appliance and transfer it to the surrounding environment. By effectively moving heat energy to the outside, the transfer cools the space inside. This process relies on the principles of thermodynamics and the properties of refrigerants to efficiently transfer heat in the system.

Refrigerants are chosen because they possess certain properties that allow the refrigerator to "hack" the ideal gas law, pV = nRT, which relates pressure (p), volume (V), the amount of substance (n), and temperature (T). Properties like a low boiling point allow the refrigerator to mechanically manipulate the pressure and volume of the refrigerant to prompt heat absorption in relatively cool environments and heat dissipation in relatively warmer ones. (More on Refrigerants here)

The refrigeration cycle involves four main components: the compressor, the condenser, the expansion valve, and the evaporator, all of which manipulate the pressure of the refrigerant to absorb heat inside the compartment and release it outside the compartment.

The cycle begins with the compressor, which is essentially a pump driven by an electric motor. The compressor's primary function is to compress the refrigerant gas, increasing its temperature and pressure.

The hot, high-pressure refrigerant gas exits the compressor and enters the condenser, a series of coils or tubes located at the back or underneath the refrigerator. The gas releases heat to the surrounding environment, primarily through conduction and convection. As the gas cools in the condenser, it condenses into a high-pressure liquid, releasing its latent heat of vaporization.

The high-pressure warm liquid refrigerant flows through an expansion valve, a small orifice or throttling device that restricts the flow of liquid refrigerant. This device allows expansion on the evaporator side, causing a significant pressure drop. This process is adiabatic, meaning there is no heat transfer. As the pressure drops, the refrigerant expands and some of it undergoes a phase change from liquid to vapor, becoming a low-temperature, low-pressure gas and liquid mixture.

The cold, low-pressure refrigerant liquid and gas mixture enters the evaporator, a set of coils or tubes located inside the refrigerator compartment. The mixture absorbs heat from the air inside the refrigerator, primarily through conduction and convection, causing the air to cool. The refrigerant liquid in the mixture vaporizes and gains the latent heat of vaporization as it absorbs heat from the interior of the refrigerator. This process continues until the desired temperature is reached. When the compressor is shut off, the flow of refrigerant stops, and the cooling stops.

The refrigerant now warmed and transformed back into a low-pressure gas, returns to the compressor, where the cycle begins anew.

The refrigerator's temperature is maintained through a feedback control system involving temperature sensors and thermostats. When the temperature inside the compartments rises above a set point, the thermostat signals the compressor to start the refrigeration cycle. Once the desired temperature is reached, the thermostat signals the compressor to stop.

You can think of the refrigerant as a sponge for heat. A tightly gripped (i.e. compressed) sponge is plunged into a water-filled basin (the inside of the refrigerator). Releasing the sponge (reducing the pressure/expansion) allows it to absorb water (heat) from the basin. The soaked sponge is removed from the basin and squeezed (compression), causing the water to pour out into the kitchen. Repeat this process many times and you can remove much of the water from the original basin.

Another comparison is the human body's cooling mechanism—sweat. Your body uses a phase change (evaporating water) to cool you off. Your body supplies the heat to make that happen which cools you off. When the air is dry evaporation takes place rapidly. and the cooling effect is high. When the air is humid, the sweat doesn't evaporate as well and you feel hot and sweaty.

Good refrigerants have an extremely low boiling point to be able to absorb heat even in chilly conditions like the inside of a fridge. They’ve also got a high critical point, which determines the highest temperature they can exist as a liquid and a gas, and a high latent heat, which means they can absorb more heat during phase changes (like from a liquid to a gas). Finally, they rely on low viscosity (to be able to flow throughout the system with minimal pressure changes, so the pressure can be manipulated to facilitate heat transfer) and high density (to maximize the amount of energy stored in the compound, to further increase its effectiveness). Refrigerants are chosen for their chemical structures that possess these properties, among others like low toxicity and cost effectiveness.

A refrigerator works based on the principles of thermodynamics, pumping a refrigerant through a cycle of tubes which absorbs heat inside the fridge and dissipates it outside.