When I first started talking about contributing to this magazine, I was concerned about just what to write. After all, the supply-house business owners don't usually require the same level of technical expertise as their industry customers, heating/cooling contractors. That makes it a bit more difficult to come up with ideas on what to cover in my column. But the editor back then assured me that any technical articles I wrote would be appreciated, because everyone recognizes the need to better understand the needs and concerns of supply-house customers. And that's what this article is about - the fundamental air-conditioning (refrigeration) cycle.
I'm sure that there are many common misconceptions about what is really going on inside the system, since we're dealing with a gas that is circulating inside sealed copper tubes. So, to demystify air conditioning for those who are interested, here's a view of what's happening inside the pipes.
The heart of an air conditioner and, in fact, the only moving part (other than the refrigerant itself) inside the refrigerant circulating system, is the compressor. Its function is two-fold:
- It moves the refrigerant through the system;
- It creates a pressure difference between its incoming (suction) line and its discharge (hot gas) line.
These two things are what make the cooling cycle possible.
Something else that makes the cooling cycle possible is the properties of refrigerants. For example, we refer to refrigerants as gas. However, we can tell by the weight, that inside the refrigerant drums the refrigerant is a liquid. What keeps the gas a liquid inside the drum? The pressure. And when this refrigerant in the drum gets a little warmer (sitting in your warm warehouse), a little of the refrigerant inside the drum evaporates, raising the internal pressure. It's this higher pressure that keeps the rest of the refrigerant liquid.
So, you can see that there is a direct relationship between the temperature of the refrigerant inside the drum and its pressure. Actually, if you connect a refrigerant pressure gauge to the port of a refrigerant drum, then follow the needle down to the temperature lines for that refrigerant, you can determine its exact temperature inside the drum. Look at a refrigerant pressure gauge and you'll understand what I mean. So the refrigerant inside a full or partly-full drum is referred to as saturated. That is, any slight temperature difference will cause some to either evaporate and raise its pressure, or to condense and lower its pressure.
Now, if you were to open the valve on a refrigerant drum, drastically lowering the internal pressure, large amounts of refrigerant would evaporate and this evaporating gas becomes very cold. In fact, if you were to turn a drum of R-22 upside down and allow some of the liquid refrigerant to pour out on the ground (don't let the EPA catch you), the refrigerant which may have been 85 degrees F inside the drum instantly evaporates at about -42 degrees F outside the drum.
What does this teach us? That changing the pressure of liquid refrigerant causes some to evaporate, and that makes it colder. This is how the cooling cycle works:
- by changing pressures,
- by adding and removing heat,
- by causing the refrigerant to change states - to evaporate and condense.
Now look at the illustration of typical temperatures, pressures, and states inside an air conditioner. The pressure of the gas coming out of the compressor is very high, about 220-psi (on a new high-efficiency R-22 system). It is also very hot, about 180F, and it is all gas (no liquid). We say it is superheated - that is, the gas temperature is well above what is needed to turn the refrigerant into a liquid.
However, within the first couple of bends in the condenser coil, this superheated refrigerant is cooled some 70 degrees and reaches the point where removing any more heat will cause it to start turning into a liquid (the point where it is saturated). And of course, that's the function of the rest of the condenser coil, to remove the heat and to cause all the gas to condense into a liquid.
Since the pressure throughout the coil remains about the same (maybe 2-psi drop across the coil), the refrigerant remains at about the same temperature throughout the coil as it condenses. Then in the last couple of bends when it is all liquid, the liquid is cooled below its saturation point. This is what we call subcooling.
Next, the high-pressure refrigerant (218-psi at 98 degrees F) is sprayed through a little hole we call the metering device, into a low-pressure situation (74-psi). Like pouring liquid refrigerant on the ground, this causes some refrigerant to quickly evaporate and lower its temperature. However, the positive pressure at this point keeps it from getting much colder than 40 degrees F. Of course, this part-liquid part-gas state means it's saturated again.
At this point the saturated refrigerant enters the evaporator (cooling) coil. And since there isn't much pressure difference across the coil, the refrigerant evaporates as it moves through the coil at about the same temperature (40 degrees F).
Finally, at the last bend or two of the evaporator, all the refrigerant is evaporated and the gas starts to warm above its saturation temperature (to about 55 degrees F). And what do we call a gas that is warmed above its saturation temperature? That's right, it's superheated again as it returns to the compressor.
In a coming issue we will look at the same diagram and cover some of the finer points of the air-conditioning cycle.