 When we talk about refrigeration in general, what we're describing is a process by which we are consuming work to push heat in a direction that it does not naturally want to go. If you think about a temperature difference driving heat transfer, maybe from some high temperature to some low temperature, heat is naturally driven by that temperature difference. When we tap into that natural transfer of heat, we can produce work. This process is a heat engine. We are producing a net work as a result of heat transfer. That heat transfer is naturally driven from high temperature to low temperature. That heat engine or power cycle is one of two common thermodynamic cycles that we consider. The other is the exact opposite. We have the same temperature distribution, but this time we are driving heat in a direction that it would not naturally go. We are consuming work in order to push that heat back up the waterfall. That process is performed by a refrigeration cycle, and it's referred to as a refrigeration process. Note here, though, that we don't necessarily imply cooling with that process. If we were to stick this device into the side of a room, for example, in such a way that the key out end of the device were pushing heat into the room, we are heating the room. Conversely, if we were to stick the device into the side of a room in such a way that the cooling side of it was pointed into the room, we would be pulling heat out of the room, pushing the heat outside of the room, and therefore cooling the room. A refrigeration cycle can operate in a heating mode or a cooling mode entirely dependent on which end of the device we care about. I like to think of this like a shop vac. A shop vac has a suction side and a blower side, and you can use it as both a vacuum and a blower by attaching the hose to different ends. One cycle that can perform refrigeration is the vapor compression refrigeration cycle. In the vapor compression refrigeration cycle, we are using the process of going from liquid to vapor and vapor to liquid to move heat around. I think the easiest way to think of this is to imagine a tube, maybe a hose, and in that hose we are first evacuating everything out of the hose. Once we've evacuated it, we fill it partially with a fluid, maybe water. As a result of being filled with only that fluid, some of the fluid will be illiquid and some of the fluid will be a vapor. It will be in saturation. If we were to add a pump or compressor to one side, maybe we could move the fluid around. And if we consider an ideal case here, that movement of the fluid would happen for free. It would just rotate around and we'd have an approximately equal distribution of liquid and vapor. Now if we were to restrict the fluid by adding a valve to one end, we aren't allowing as much fluid to pass through the valve as wants to pass through the valve. As a result of that, we build up a high pressure side and a low pressure side. On the high pressure side, we are going to have more liquid. And on the low pressure side, we will end up with more vapor. The latent energy associated with going from vapor to liquid will result in a lot of energy being emitted from that transition. If we were to tap into that, say by adding a heat exchanger or a radiator or even just a sufficient amount of airflow, we can allow that heat to work in our favor. We can push heat out and if we were to add a heat exchanger to the other side, we can pull heat in. On the high pressure side, that vapor going to liquid occurs in a condenser. On the low pressure side, the liquid becoming a vapor occurs in an evaporator. These four devices make up this simple vapor compression refrigeration cycle. If we establish some state points here and model this ideally, we have a compression process from one to two. We have a heat exchange process from two to three. We have a restriction or expansion process from three to four. And we have a heat exchange process from four to one. Remember that when we say simple here, we're referring to the fact that we have the least complicated setup that we will consider. We can add devices to this to produce certain desirable effects in the same way that we can add devices to a Brayton cycle or a Rankine cycle. So one to two, we consider as isentropic compression. In two to three, we have approximately isobaric heat rejection. In three to four, we have isenthalpic expansion. And in four to one, we have isobaric heat absorption. Those four processes allow us to take in work and push heat in a favorable direction that it would not naturally want to go. And again, depending on which end of the device we care about, what we accomplish is either heating or cooling. The cycle itself is the same. Let's try an example.