Aircon, Refrigeration and Heat Pumps
Firstly let me say that we are specialist in only one of these disciplines which is air conditioning but even then we specialise only in the airconditioning of vehicles.
All three of these disciplines are related in that they all use the same techniques to move heat or the lack of heat, i.e. coldness, from one place to another. They may appear to actually create heat or cold but in reality they just shift heat around and so obtain the desired effect whether cooling or heating.
In the USA the working components of a frig can look very similar to the components of an AC system of a vehicle.but in Europe the components tend to be more different in appearance but they do much the same job. The components of a Heat Pump are also closely related even though it seems to be achieving quite the opposite effect.
How do they all work? Let me explain just one system, say, aircon. The similarities should be obvious.
The compressor on a car aircon system is sucking refrigerant gas from the component in the area where we can detect the cold is coming from - called the Evaporator. The compressor is powered by the engine via a rubber belt and after sucking this gas in it compresses the gas to quite a high pressure. If you have ever pumped up a cycle tyre by hand you will be aware that the hand holding the front of the pump can get quite hot. This is the result of compressing all gasses - they get very hot as the pressure increases. This hot, high pressure refrigerant gas from the compressor is directed into a radiator at the front of the vehicle where the much cooler outside air is able to take some of this heat away. This has the effect of condensing the gas back to a liquid in exactly the same way as steam that is cooled will condense back to water.
So coming from the bottom of this radiator we have refrigerant in a liquid form although it went into the top as a gas (a vapour). Although now a liquid, this refrigerant is still at high pressure but is now just warm, not actually hot. We direct the liquid through a narrow pipe back towards the Evaporator where it is squirted through a tiny hole into the evaporator (remember this is just where we started - where the compressor is sucking from). Because the gas is being sucked away by the compressor the pressure in this evaporator is quite low and the high pressure droplets of refrigerant being squirted through the tiny hole have room to expand and try to turn back into a gas, BUT and this is the crucial bit - to return to a gas the liquid droplet needs to absorb heat in order to change state and evaporate. This it does by stealing some heat from the car's interior. You feel the aircon is creating cold but in reality all that is happening is that some of your heat is being taken away. Once the refrigerant has all evaporated into in a gas, the compressor sucks it back in again and it goes around the circuit again.
That's it. Don't knock it - it works fine, simple though it is.
Now you know about the Evaporator. I'm sure that you must have realised that the radiator which arranged the condensing of the refrigerant vapour into a liquid is called "The Condenser". This is placed right at the front of the engine bay immediately in front of the water coolant radiator. The tiny hole that squirts the refrigerant through is called "The Expansion Valve".
That's the system on a vehicle. On a refrigerator the condenser is the rack of black pipes and finning usually on the back of the cabinet. On a freezer it is in the form of a small radiator with a cooling fan usually on the side. On an aircon unit in a building it is usually in a box with cooling fans outside the main structure. This would be a box of perhaps just a couple of feet square on the side of a house but on a large building it could be a massive block on the roof.
Now for something quite clever. Suppose that you reverse the direction of flow of refrigerant and were to cool this outside box to very low temperatures, say, minus 6° Celsius and then turn off the compressor. What would happen to this box in a typical winter temperature of say plus 4° Celsius. It does not take a genius to work out that the box will gradually come up to the outside temperature of plus 4° Celsius. In other words the box has absorbed some heat. Not a lot, just 10 degrees Celsius but if we restart the compressor and move this little heat to where we can add that to the inner room temperature of perhaps only 15 degrees C we can make a total of 25° C. Clever eh. This is what is a heat pump. It can pick up heat from where there is a little and moves it to where we need it. It does not create heat (which is expensive), it simply moves heat from one place where it is free to where we can use it. This movement is relatively inexpensive - just one compressor and two fans.
With a refrigerator, a freezer or a heat pump the compressor is usually powered by the mains supply so it is normal to have an electric motor to drive it. Except in very large installations this electric motor can be fixed directly to a compressor and both units can be installed inside one sealed steel box so ensuring that no refrigerant can possibly leak away. As a result most units of these types have a very long service life needing virtually no maintenance. In the case of a car however the vast majority of compressors are powered by the engine via the main rubber drive belt that runs the alternator and the power steering etc. As a result it is not possible to completely seal the compressor so there is a small natural annual leakage that gradually reduces the refrigerant charge. This means that it needs regular maintenance perhaps every three years or the efficiency will rapidly degrade and problems or failures will result.
The refrigerant chosen will depend on the temperatures required and the legislative constraints, usually environmental, of the region. Many refrigerants have been found to be non-eco, others are dangerous, others are not very effective. A balance needs to be found. There is a feeling that "natural" refrigerants must be better however that has not been borne out in practice, some are horribly dangerous (everything natural is not benign), some work so inefficiently that it takes far too much energy to achieve reasonable cooling - hardly not what we are seeking. The most efficient refrigerants tend to be either dangerous (very flammable or otherwise very nasty to humans if they leak out) or have some environmental concerns. There is a current search for better refrigerants for both refrigeration and airconditioning. Some are available but at high cost and with some concessions to flammability. The latest refrigerant which will be used in all cars built after the end of 2016 is called R1234yf and is very specialised (and very expensive).