Door sealA strip round the door that closes tight against the cabinet. It keeps the cold in. Also called the gasket.
Evaporator coilsThe refrigerant evaporates inside them. That takes heat from the air in the freezer. This is where the cold comes from.
Condenser coilsThe hot refrigerant gives its heat to the room here. It turns back to liquid. The coils are on the outside. Keeping them dust-free helps efficiency.
CompressorSqueezes the refrigerant vapor to high pressure and heat. It also pumps the refrigerant round the loop.
The cabinet: 3 2
ThermostatStarts the cooling when the freezer gets too warm. It senses the temperature.
InsulationFills the walls and door. That way little heat gets in from the room.
Inside: 2 1
FanBlows the air inside across the evaporator coils and round the freezer.
At the back: 3 1
Expansion valveA tiny constriction for the high-pressure liquid refrigerant. There it drops to low pressure and starts to boil, turning very cold.
How this list was compiled: Read 2026-09-27. (1) Wikipedia, Refrigerator, https://en.wikipedia.org/wiki/Refrigerator : refrigerant enters the compressor as low-pressure vapour and leaves as high-pressure superheated vapour; the condenser coils, cooled by room air, liquefy it; an expansion valve, a pinhole-sized constriction, drops it to low pressure; it vaporises in the evaporator coils, drawing heat from the air a fan blows across them, then returns to the compressor; the thermostat starts the cooling when it is too warm; door seals; cleaning condenser coils improves efficiency. (2) OpenStax, College Physics 2e, section 15.5, https://openstax.org/books/college-physics-2e/pages/15-5-applications-of-thermodynamics-heat-pumps-and-refrigerators : the four basic components are the condenser, expansion valve, evaporator and compressor, with a working fluid (refrigerant) moving heat. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
These are the parts of a freezer on an upright freezer cut through from the side, showing the parts that keep the cold in and the loop that makes it. Start with the 4 main parts.
Read the full guide
Keeping the cold in
Insulation fills the walls and door, and the door seal closes tight against the cabinet. The thermostat senses the temperature and starts the cooling when the freezer gets too warm.
Making the cold
The compressor pumps refrigerant to the condenser coils on the back, where it gives off heat and turns liquid. It passes the expansion valve, boils in the evaporator coils inside, taking heat from the air the fan blows over them, and returns to the compressor.
The coils on the back of a running freezer are warm: that is where the heat taken from inside goes.
Questions and answers
How does a freezer get cold?
Refrigerant evaporates in the evaporator coils inside, taking heat from the air. The compressor then pumps it to the condenser coils outside, where the heat goes into the room.
What does the compressor do in a freezer?
It squeezes the refrigerant vapour to a high pressure and temperature and pumps it round the loop, from the evaporator to the condenser.
Why are the coils on the back of a freezer warm?
They are the condenser coils. The heat taken out of the inside is given off there, into the room. Keeping them free of dust helps the freezer work efficiently.