How does a refrigerator work?

A fridge doesn't make cold. It pumps heat out of the box and dumps it into your kitchen. Touch the sides of a running fridge: they're warm. That's the heat from your food leaving.

The compressor, a small black can at the bottom back, is the pump. It pushes a fluid called the refrigerant through hot pipes, the condenser, hidden inside the side walls (older fridges have a black grille on the back). A filter-drier catches moisture, then a hair-thin capillary tube lets the liquid through slowly, so its pressure falls.

In the evaporator, a finned coil hidden behind the freezer's back panel, the refrigerant boils at about -25 °C. A small fan blows air over it into the freezer, and a damper lets some down a duct to the fridge below. A defrost heater melts frost off the coil now and then, and the water drains to a tray on the warm compressor, where it dries up.

All around sits thick foam insulation, and magnetic gaskets seal the doors. A thermostat switches the compressor on and off.

Heat always flows from warm to cold. A fridge can't stop it, only slow it down, and every watt that sneaks in has to be pumped out again by the compressor.

Heat soaks through the walls by conduction. How fast is set by a simple rule: Q = k × A × ΔT ÷ d. That's the material's conductivity k, times the wall's area, times the temperature difference, divided by the thickness. Double the thickness and half as much heat gets in.

Fridge walls are about 5 cm of polyurethane foam, full of tiny gas bubbles, with k ≈ 0.022 W/m·K: over 30 times better than brick. Some top models add vacuum panels, about five times better again. The catch: thicker walls leave less room for food.

Every time you open the door, cold heavy air spills out at the bottom and warm, humid kitchen air rolls in. Its heat, and the water it carries, which turns to frost, must be pumped out too.

A fridge runs the same loop as an air conditioner (see ACClear for every step in detail). The compressor squeezes the gas so it gets hot, the condenser gives that heat to your kitchen and the gas turns liquid, and the liquid then boils cold in the evaporator, soaking up heat from inside.

What's special about a fridge? Its refrigerant is usually isobutane (R600a), a cousin of lighter gas. It boils at −12 °C in open air and barely warms the planet. To boil at −25 °C, colder than the freezer, it runs at about 0.6 bar: less than the air's pressure.

Instead of a valve, a fridge uses a capillary tube, metres of copper as thin as a pin, soldered to the cold suction line. The warm liquid inside cools as it goes, and the returning gas warms up before it reaches the compressor.

The compressor doesn't run all the time. A thermostat switches it on when the freezer warms to its cut-in temperature and off at its cut-out. The hotter your kitchen, the more heat leaks in and the longer it runs: its duty cycle goes up.

Kitchen air carries invisible water vapour. When you open the door, warm, humid air rolls in. The coldest thing inside is the evaporator, at about −25 °C, so the vapour freezes straight onto it as frost, like frost on a winter window.

In a direct-cool fridge the freezer box itself is the evaporator. Frost builds up on its walls day after day, and every few weeks you switch off and defrost by hand.

A frost-free fridge hides a finned evaporator behind a panel and blows air over it with a fan. Every 8 to 12 hours, or when sensors say it's needed, a defrost heater switches on for a few minutes. The frost melts, runs down a drain to a tray on the warm compressor, and evaporates into your kitchen.

Why bother? Frost is full of air, so it's an insulator. A thick layer blankets the coil, which then has to run colder to pull the same heat through, and that costs electricity.

Food goes off mostly because of microbes: bacteria, yeasts and moulds. A few are harmless, but some cause food poisoning. Given warmth, water and food, a bacterium splits in two, then those two split, and so on. At body temperature Salmonella can double every 20 to 30 minutes: one cell becomes millions in a few hours.

Cold slows the chemistry of life. Most food-poisoning bacteria barely grow below about 5 °C, which is why a fridge runs at 3 to 4 °C. Food safety agencies call 5 to 60 °C the danger zone (FSSAI; the US says 40 to 140 °F). Leave cooked food out for no more than 2 hours, or 1 hour on a very hot day.

A few, like Listeria, keep growing slowly even in the fridge, which is why leftovers still have a use-by date. At −18 °C growth stops, but freezing doesn't kill most bacteria: they wake up when food thaws.

Where to put things: the door is the warmest spot, so keep sauces and drinks there, not milk. Ready-to-eat food goes up top, raw meat on the bottom shelf where it's cold and can't drip on anything, and vegetables in the humid crisper.

A fridge runs day and night for 10 years or more, so it's one of the biggest users of electricity in many homes. In India, the BEE star label tells you how many kWh a year it uses in a standard test. More stars, less energy. The bands tighten every few years: from January 2026 a 250 L frost-free fridge needs under about 163 kWh a year for 5 stars, while 1 star allows up to about 282.

Many new fridges have an inverter compressor. Instead of switching on at full power and off again, it runs slowly and nearly all the time, matching the heat leaking in. That keeps the coils closer to the temperatures they need, with no start-up losses, so it uses less energy and stays quieter.

You can help too. Leave a gap behind and beside it so the condenser can shed its heat. Keep the door gaskets clean and tight: a worn one lets warm, damp air leak in all day. Don't pack it so full that cold air can't flow from the vents, and let hot food cool a little first.