Put a hot cup of chai on the table and it cools down. Put an ice cube in your hand and it melts. Heat always flows the same way on its own, from hotter things to colder things, never the reverse. This is so reliable that physicists made it one of the fundamental laws of nature, the second law of thermodynamics. And yet, sitting in most kitchens, there’s a machine that spends every minute of every day making heat go the wrong way. It takes heat out of a cold box and pushes it into a warmer room.
The first thing to understand is that a fridge does not make cold. There’s no such thing as adding coldness. Cold is just the absence of heat. What a fridge does is pump heat out, the way a water pump moves water uphill. And like a water pump, it needs energy to do it, which is why it’s plugged in.
The trick at the centre of it is a liquid called a refrigerant. Many household fridges today use one called isobutane, a close relative of the gas in cooking cylinders. What matters about it is that it boils at a very low temperature, around minus 12 °C at normal pressure, and that boiling soaks up heat. You know this from your own skin. When sweat evaporates, it takes heat with it, which is why you feel cool in a breeze. Turning a liquid into a gas always absorbs energy.
So the refrigerant flows through thin pipes inside the walls of the fridge, called the evaporator, at low pressure. There it boils, and as it boils it draws heat out of everything around it: the air, the milk, yesterday’s sabzi. The inside of the fridge gets colder.
Now the refrigerant is a gas full of heat, and it has to get rid of it somewhere. That’s the job of the compressor, the part that hums. It squeezes the gas into a much smaller space, which raises its pressure and its temperature, often to well above room temperature. The hot gas then flows through the coils on the back or underneath the fridge, called the condenser. Because the gas is now hotter than the kitchen, heat flows out of it into the room, naturally, the normal downhill way. As it loses heat, the gas condenses back into a liquid. If you’ve ever put your hand near the back of a fridge and felt it warm, that’s the heat from your food leaving the building.
Notice what the pressure is doing in all this. A liquid’s boiling point isn’t fixed: it depends on the pressure around it, which is exactly why water boils at only about 88 degrees high in the mountains. The fridge uses that fact twice. In the evaporator, it keeps the refrigerant at low pressure so that it boils at a temperature colder than your food, and soaks up heat. In the condenser, it keeps it at high pressure, so that the same substance condenses at a temperature hotter than your kitchen, and gives the heat away. The compressor’s real job is to move the refrigerant between those two pressures. That is the price of pushing heat uphill: you have to do work, and the electricity bill is the work.
Last, the liquid passes through a narrow tube or valve that lets its pressure drop suddenly, which cools it right down, and it flows back into the evaporator to boil again. The cycle repeats, over and over, all day.
The choice of refrigerant has a history of its own. Early fridges used gases like ammonia and sulphur dioxide, which worked well but were dangerous if they leaked. In the 1930s these were replaced by chlorofluorocarbons, or CFCs, which seemed perfectly safe, until scientists discovered in the 1970s and 1980s that they were destroying the ozone layer high in the atmosphere. The Montreal Protocol of 1987 phased them out, and the gases in modern fridges, such as isobutane, don’t damage the ozone layer.
Here’s the impressive part. A good fridge moves two or three times more heat out of its box than the electrical energy it uses to do it. It isn’t creating energy; it’s just using a little energy to move a lot of heat, the same way a small pump can lift a lot of water. The same idea, run in reverse, is how air conditioners cool rooms and how heat pumps warm houses in cold countries.
And it explains an old trick question. If you open the fridge door on a hot day in a closed room, will the room get cooler? No. The fridge just takes heat from the front and dumps it out the back, into the same room, and its motor adds a little more heat as it works. The room ends up slightly warmer. The only way to cool a room is to put the hot end somewhere else, which is exactly why an air conditioner has a box outside the window.