|MadSci Network: Engineering|
Air conditioners, refrigerators, and heat pumps all demonstrate Gay-Lussac's Law, which is a physical law that chemists use to predict how the temperature and pressure of a gas are related.
Shown as an algebraic equation, it is P/T = k, where k is a constant value (it doesn't change).
If you experiment with this equation, you find that if we compress a gas to a higher pressure, its temperature will rise. So let's say that we start with a gas (our refrigerant) that is in a pipe in our house, then we compress that gas (with a pump that we call a compressor). It will get very warm. If we pump that warm gas outside our house and run it through a coil with a fan blowing air over it (this is the condenser coil and is very similar to the radiator in a car) the warm gas cools off, and actually becomes cool enough that some of it turns into a liquid. This is just like steam condensing into water when it cools off, but the liquid in an air conditioner has a much lower boiling point than water does.
Now, an important thing to know here is that to make a gas turn into a liquid, you have to remove a lot of heat from it (this is known as the latent heat of vaporization). So, by the time our warm gas has cooled off enough to turn into a liquid in the outdoor cooling coil, a tremendous amount of heat has been removed from it. If you feel the air coming from the outdoor unit of an air conditioner, you can feel how warm it is. Although the basic idea of an air conditioner would still work even without turning the refrigerant into a liquid, it wouldn't work nearly as well, because you wouldn't be removing nearly as much heat from the refrigerant.
Once the refrigerant has cooled off enough to turn into a liquid, we pump it back into the house. Here is where things get a little tricky, because what we do next is reduce the pressure on the refrigerant. If you look at Gay-Lussac's law again, you see that if we reduce the pressure on the refrigerant, its temperature must also fall. So the refrigerant gets even colder than it was when it left the outdoor condenser coil. The cold refrigerant flows through the evaporator coil, where it absorbs heat from the air and warms back up. The evaporator coil is located inside the house, and it gets very cold because of the cold refrigerant inside it. The air blowing over the coil gets cold, and we have air conditioning.
I said the evaporator part was tricky: the tricky part is related to why it is called the "evaporator". Remember I said earlier that in the outdoor condenser coil we had to remove heat from the refrigerant to turn it to a liquid? Well, to turn it back to a gas (in other words, to make it evaporate), it has to absorb that much heat. And that's the tricky part. Because even though it absorbs a lot of heat, the temperature doesn't change until it turns into a gas. So we start with a cold liquid, add a bunch of heat to it (where does the heat come from? From the air in our house) and we still have a cold gas. That's why the change from gas to liquid makes the air conditioner work much better than if we tried to do it all with a gas.
By the time the refrigerant leaves the evaporator coil, it has completely turned to gas and has warmed up significantly. This is the gas that we started with at the beginning of this description, ready to be compressed to start the cycle all over again.
This basic cycle is how air conditioners (in both houses and cars) work, as well as refrigerators. The details of how it is implemented will be different for each application, of course. But the really neat thing is that if we put the evaporator coil outside, and the condenser coil inside the house, instead of cooling the house, we heat it. This is what a heat pump does. A heat pump is simply an air conditioner with valves that control which coil gets used as the evaporator and which gets used as the condenser. By simply flipping a switch, we can control whether it absorbs heat from the inside air and releases it outside (an air conditioner), or whether it absorbs heat from the outside air and releases it inside (a heater).
A WWW page called Pumping Heat Uphill explains this cycle from the point of view of a heat pump, and it includes a diagram that might make it easier to understand.
You probably realize by now that the entire process only works if the pressures inside the system and the boiling point of the refrigerant are all properly balanced. That's why air conditioner mechanics always have a bunch of pressure gauges with them!
An interesting device that is related to air conditioners and heat pumps is the Stirling Engine, which is sort of an air conditioner run backwards: instead of a compressor being used to change the pressure (and temperature) of a refrigerant, the changing temperature (and pressure) of a gas are used to move the compressor, turning it into an engine!
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