How does a refrigerator keep food cold?

A refrigerator moves heat rather than creating cold. A fluid called a refrigerant circulates in a loop, absorbing heat inside the cabinet as it evaporates and releasing that heat into the room as it condenses. This is why the back or underside of a working fridge feels warm. A compressor supplies the work that makes this possible, and a thermostat switches it on and off to hold the temperature. [1] [2] [3]

Heat flows on its own only from hot to cold. Moving it the other way requires work. According to HyperPhysics, a refrigerator is a heat engine in which work is done on a refrigerant to collect energy from a cold region and exhaust it into a warmer one. OpenStax describes refrigerators, air conditioners and heat pumps as heat engines run backward, with the heat delivered to the hot side equal to the heat taken from the cold side plus the work input. [3] [4]

NOVA summarizes the two ideas involved: compressing or expanding a gas raises or lowers its temperature, and heat only flows from hot to cold. [5]

The compressor squeezes the refrigerant vapor, making it hot and high in pressure. Danfoss compares this to the warmth you feel in a bicycle pump. The hot gas then flows through the condenser on the back of the fridge, where room air carries heat away and the refrigerant turns into a liquid. [1] [2]

The liquid then passes through a narrow capillary tube or valve, which drops its pressure. Its temperature and boiling point fall with it. Washington State University's Dr. Universe column says this liquid is the coldest thing in the fridge, colder even than the freezer compartment. Danfoss likens the pressure drop to the spray from an aerosol can. [2] [1]

In the evaporator inside the cabinet, the cold refrigerant absorbs heat from the food and air and boils into vapor. The vapor returns to the compressor and the cycle repeats. You can feel a similar effect when a drop of alcohol evaporates on your skin and leaves it feeling cool. [1] [6]

The Physics Hypertextbook gives typical figures for vapor-compression refrigeration: about 1.5 atmospheres and -10 to +15 °C on the low-pressure side inside, and about 7.5 atmospheres and +15 to +40 °C on the high-pressure side outside. It notes that the energy absorbed and released during boiling and condensing is the latent heat of vaporization. [6]

A thermostat monitors the inside temperature. It turns the compressor off when the cabinet is cold enough and back on when it warms. Danfoss also notes that keeping food cold slows the reproduction of harmful bacteria, which is the purpose of all this. [1]