Vapor Compression: Compressed Version

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Vapor compression: Compressed version

By Michael Bradley

The vapor compression cycle is a means of transferring heat from one location to another, by changing the pressure of a substance at various points within the system. Essentially, we are utilizing a low pressure to induce vaporization in a refrigerant, and then a high pressure to push it back together and condense it. As the refrigerant changes state, the laws of thermodynamics state that there must also be a heat gain or loss in the substance (although not necessarily a temperature change). This heat gain or loss is what we utilize in the vapor compression cycle to transfer heat from one location to another.

Picture this. Our refrigerant is like a “heat sponge” that we have to keep using again and again to cool our house on a hot day. We send it through the evaporator in our AC unit at a low pressure, and allow it to “suck up” all the heat it can hold from our house. We then send that heat soaked sponge outside to the condenser where we squeeze it within an inch of its life to get all the heat out. After doing this, we send our dried out sponge back into the house to grab more heat for us. It will do this again and again until it reaches whatever point we set for it with our thermostat. Shooting in, sucking up heat, running outside, dumping it off.

This, is EXACTLY what we are doing in a vapor compression system. The only difference being, that our “dry sponge” is refrigerant in the liquid state, and our “wet sponge” is refrigerant in the gaseous/vapor state. We are using a bit of science (thermodynamics) which tells us that heat is transferred when we change the state of our refrigerant, and using that to build a system which knocks it back and forth between states as efficiently as possible. When heat causes a change in our refrigerant’s state, it’s called “latent heat” (versus “specific heat”, which causes a change in temperature).

So REALLY, here’s what you need to know. Lowering the pressure of our refrigerant allows it to vaporize and absorb heat at lower temperatures (this is how we soak up heat in the evaporator), and raising pressure allows it to condense at a higher temperature than what’s outside (this is how we reject heat in the condenser). By changing the state of the refrigerant (as opposed to its temperature), we achieve a much faster and more efficient method of heat transfer (For example. Changing 211 degree water into 212 degree water takes only one BTU, while changing 212 degree water into 212 degree steam takes 970.4 BTU’s.). This is why we compress the refrigerant, as opposed to simply circulating it.

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