Last week I wrote about heat transfer into fridges in the context of operation in vacation mode. It is tempting to think that if energy is moving into the fridge as a result of heat transfer from the warm room to the cold food compartment in the fridge, then why can’t we use the energy to power the fridge. A fridge that operated on this basis would be categorised as a perpetual motion machine of the second type because it would contravene the second law of thermodynamics and so it can’t exist. One of the great pioneers of thermodynamics, Rudolf Clausius expressed the second law as ‘heat does not pass from a body at a low temperature to one at high temperature without an accompanying change elsewhere’. In other words, something has to be done, generally in the form of work, to move energy from a cold to hot place, e.g. from the food compartment of the fridge to the warmer room.
In a domestic fridge, the work is supplied in the form of electricity to drive a compressor – that’s the thing making most of the noise coming from your fridge. It is compressing a refrigerant gas (typically from atmospheric pressure to about 8 times atmospheric pressure) and in the process raising its temperature (perhaps by 80°C) as it pushes the gas into a condenser. In the condenser, the hot refrigerant transfers heat to the colder room and in the process condenses from a gas to liquid dropping its temperature, perhaps by 30°C. Then, the liquid refrigerant flows into an expansion valve where its rapid expansion to a gas lowers both its temperature (perhaps to -20°C) and pressure (typically from 8 times atmospheric pressure back to atmospheric) before it is sucked into the heat exchanger inside the food compartment where its very low temperature causes heat transfer from the compartment to the refrigerant, i.e. it removes the unwanted energy. The compressor sucks the gas out of the heat exchanger and the whole cycle starts again with the unwanted energy being dumped into the room by the condenser, which is the warm panel on the back of your fridge.
If you understood all of that then well done, if not then try again following the steps on the schematic diagram.
The temperatures and pressures are expressed rather vaguely because they depend on the design of the fridge and the settings you select on the control panel.