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A cold cabinet, a pump, and coils letting heat out Three separated parts laid out like exhibits in a case. On the left a thick block with a room hollowed out of it holding two small packages: the cold cupboard where the food sits. In the middle a squat solid block: the pump that does the work. On the right a stack of four long bars with three arrows leaving them towards the edge: the coils, and the heat going out of them into the room. Nothing here makes cold. The heat that was in the cupboard has been carried across and let go on the right.

A Fridge Cannot Make Cold

About 10 minutes

Go and put your hand on the back of the fridge in your kitchen. Not the door. The back, or the two sides down near the floor.

It is warm.

That is a strange thing for a cold machine to be, and it is the whole story. There is no such thing as coldness that a machine can manufacture and pour into a cupboard. Cold is just what is left when heat has gone somewhere else — and a fridge is a machine for carrying heat out of one box and letting it go in the room.

Heat only ever runs downhill

Leave a hot drink on the table and it cools down. Leave a cold drink there and it warms up. Both are the same rule: heat slides from the warmer thing into the cooler thing, until they match.

It never goes the other way on its own. A cool room has never made a warm drink hotter.

So the inside of a fridge is a problem. The air in there is colder than the kitchen, which means heat is leaking in through the walls and door all day long. To keep the cupboard cold, something has to keep taking that heat back out and pushing it into a kitchen that is already warmer. That is heat going uphill, and nothing does that for free.

Which is why a fridge has a plug on it. The electricity is not making cold. It is paying for the uphill trip.

The loop that does the carrying

Sealed inside the walls and pipes is a small amount of a liquid that boils at a very low temperature. It never gets used up and it never comes out. It just goes round and round the same loop, picking heat up at one end and putting it down at the other.

The loop a fridge pushes its liquid around A closed loop of pipe drawn as a rounded rectangle, crossing a dashed line that marks the wall of the fridge. The left-hand side of the loop is inside the cold cupboard and arrows point into it, labelled heat in. The right-hand side is out in the room and arrows point away from it, labelled heat out. A pump sits on the bottom of the loop and a very narrow gap sits on the top. Small arrows around the pipe show the liquid always travelling the same way round, so heat picked up on the inside is always carried to the outside and let go there. inside the fridge out in the room heat in heat out the pump a tiny gap
  1. The pump squeezes the gas hard. Squeezing anything makes it hotter, and this gas comes out of the pump hotter than the kitchen.
  2. It travels through the coils at the back, out in the room. Because it is now hotter than the room, its heat leaks away into the kitchen — downhill, the ordinary direction. As it loses that heat it settles back into a liquid.
  3. It is pushed through one very narrow gap into a much wider pipe. On the far side of the gap there is far less pressure squashing it together, and the liquid boils.
  4. Boiling takes heat, and the only heat within reach is inside the cupboard. So the food and the air give theirs up, and the gas carries it back round to the pump.

Read the loop again and notice what is never happening. Nothing is being chilled. Every step is heat moving the way heat always moves — out of the warmer thing into the cooler one. The trick is that the machine keeps changing which one is warmer.

Why letting go makes something cold

Step three is the part worth slowing down for, because you already know it on your own skin.

Get the back of your hand wet and blow on it. It goes cold, and the water was not cold to start with.

When a liquid turns into a gas, it needs a push of energy to do it, and it takes that energy as heat from whatever is nearest. On your hand, the nearest thing is your hand. Inside a fridge, the nearest thing is the milk.

That is all evaporation is: boiling is a way of taking heat. A fridge simply arranges for something to boil in the right place, over and over, forever.

Somebody seals a kitchen shut and leaves the fridge door wide open all day. What happens to the kitchen?

That answer catches almost everybody, and the reason is worth having.

With the door open, the fridge takes heat out of the room and puts it straight back into the same room. That part cancels. But the pump is still running, and everything a motor does ends up as warmth in the end. So the kitchen gets back all its own heat plus everything the electricity supplied.

A fridge with its door open is a heater. A slow, expensive, badly designed heater, but a heater.

Two things you can check today

Feel the back of the fridge, near the bottom. Warm, and warmest just after you have heard the machine humming.

Then look at the shelf of your freezer, or the cold plate at the top of an older fridge, and find the frost. Frost forms where the pipe is coldest — that is the spot where the boiling is happening, and it is marked out for you in ice.

One end warm, the other end frosted, and the same small amount of liquid going round between them all day.