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How a Fridge Works
Heat pumps and fridges explained in simple terms

You know that a fridge/freezer is an insulated box which is colder inside than outside, but if you've ever wondered how it uses energy to make this useful temperature difference, here's one of these interesting explanations. I can explain how a fridge works, but without getting too technical about it.

The first thing about fridges and freezers, which may be surprising, is that they don't make coldness. Although heaters and ovens make heat, fridges don't make cold, and instead they work by pumping heat out of the box. With less heat inside the box, you can see it's going to be colder. The heat that was inside the fridge is usually pumped out of those fins at the back, and you can feel they are warm. That's heat that was inside the fridge earlier on.

Next, looking at what it is about the machine that can absorb heat and transfer it, it's worth thinking about stuff that can absorb heat. Perfume always seems cold to the touch, and so does petrol/gasoline, and alcohol. These materials are all volatile; they evaporate easily. Butane lighter fluid must start off being the same temperature as the air, when it's in the can, but as soon as it's released, it's cold. That's another case of evaporation.

What happens is that it takes energy to evaporate the liquid, and that energy is absorbed from wherever the liquid is evaporated, leaving it colder than before.

Inside the pipework of a fridge, some volatile spirit is squirted through narrow gaps and evaporated, taking heat away. The stuff is then pumped round to the fins at the back of the fridge and turned back to liquid, releasing the heat, and then it goes round again, in a process of continual recycling.

You don't see the fluid and the cycle in which it gets evaporated and condensed, as it is in closed loop like blood going round inside a person.

Fridge designers have always had to make the difficult decision of what fluid to use. It has to be something that boils at a low temperature, but there are other factors that have to be considered. Perfume might seem a good choice, but it would be expensive. Butane lighter fluid would work, but if something went wrong it would be a serious fire risk. Ammonia was a favourite some time ago, although if it leaks out it can be poisonous. This is why CFC seemed to be such a good idea initially. It's not toxic to people, and it's not inflammable, and it's not expensive. The low boiling point of CFC and its continuous recyclability made it the favoured refrigerant fluid for a long time. It was only later found that if it escapes into the atmosphere it's got ecological problems. According to Iceland Appliances, the fluid they're using now in fridges and freezers is R600a, which is much better. They say "All our refrigeration appliances use R600a; a gas that is not only harmless to the ozone layer, but also climate friendly".

With a fridge being a heat pump, pumping heat from the inside to the outside of an insulated box, it's an interesting thought that it is possible to use the same principle of a heat pump to do other things. In a cold climate, how about pumping heat from outside of a house to inside? Well, air conditioning works, and that's doing something similar, except it's pumping heat from inside to outside. The amazing thing is that you get more heat than you pay for! That's because it's cheaper to move heat than to make it. A 1000 watt air conditioner can pump at least 3000 watts of heat out of a building. The reverse principle, using a heat pump to move heat into a building, has the same type of efficiency. That's with pumping heat from the air. Pump heat from the ground and you can get over 6000 watts for every 1000 watts of energy used.

If you're wondering why heat pumps aren't more fashionable and why electric fires are still in use anywhere, it's because the cost of a big heat pump is quite high, so it represents a major capital investment up front, whereas normal heaters are quite cheap to buy. Thinking about things longer term, buying a heat pump makes good sense, and can pay for itself in a few years.

Anyway, there it is. How a fridge works. And a few other useful things. Now a few points of note:

Mini Fridge* If you'd like to buy a fridge, there are a few electrical shops being promoted at this site, including the place that's got an obvious association with refrigeration, Iceland Appliances. Another good contact is Mini Fridge, where fridges that are small are available!

* Although it's true that in a heat pump you can get more heat than you pay for, this does not mean you can use it to produce "perpetual motion". You're using 1000 watts of prime grade top quality energy to get 6000 watts of low grade heat energy, so it's only a bargain if that's what you want, and it's a totally different matter trying to get any such energy back to being high grade again.

* If a fridge/freezer pumps heat out of an insulated box, then after a while, won't it just get colder and colder until there's no heat at all inside? No, because the box is not perfectly insulated. It's like having a leaky boat and a bilge pump to keep the level of water low enough in the boat to stop it from sinking.

* If you leave a fridge door open, will the room get colder? No. The heat will be pumped out of the back and then get round the front and go around and around like people in a comedy sketch going out of one door and in through another. If you want to cool a house down by using a fridge, you'd have to fit the fridge into the house wall. That's effectively the way an air-conditioner works, but the machine is more powerful than a domestic fridge as it has a room to cool, not just a few cubic ft.

* Air-conditioners are refrigeration engines and work on the same principle as domestic refrigerators. Dehumidifiers are very similar, but the cold part is used for causing condensation inside the unit so that moisture can be extracted from the atmosphere.

Other things: How a microwave oven works

Also see Freezer Fun for some practical ideas about doing silly things with a deep freezer.