WheelerHVACR: The Compressor's Role In The Air-Conditioning Cycle
The compressor is literally the heart of an air conditioner. It moves the refrigerant through the system and creates the pressure differences that cause the refrigerant to condense and evaporate. The compressor takes in superheated gas at about 40-psig and 55 degrees F, then raises its temperature to about 180 degrees F at about 220-psig by compressing the gas, thus reducing its volume. There are two reasons why the gas temperature rises during compression:
1. Since the gas volume is reduced, the heat is concentrated in a smaller area
2. Internal motor heat and friction losses are carried off in the discharge gas.
Since all hermetically-sealed compressors have their own internal electric motors, it's easy to understand why the returning gas must be cool. This is what keeps the motor windings from overheating. So anything that causes the returning gas to be too superheated (like a loss of refrigerant charge) will likely reduce the life of the motor. In fact, almost anything that goes wrong with the system adversely affects the compressor life.
For example, a dirty condenser coil or a failed fan motor causes higher pressure differences, which makes the compressor work harder. And a dirty evaporator or a failed fan motor causes liquid (unevaporated) refrigerant to return to the compressor. What's wrong with that? Well, the rule of physics goes: "You can't compress a liquid." So, something has to give.
Raising pressure differences across the compressor is the main cause of lost efficiency as air-conditioning systems age. A deteriorating condenser coil raises the discharge pressure while a dirty and deteriorating evaporator coil lowers the return pressure, which creates a higher compression ratio.
Compression ratio is the number of times the compressor must raise the return or suction pressure to reach a working discharge pressure. The average on a new system is about 2.7 times to 1. When that compression ratio reaches 4/1, the expected life of the compressor and the extra money it costs to operate it makes purchasing a new system the wisest choice. You can determine an air-conditioner's compression ratio by adding the atmospheric pressure (14.7-psi) to both the discharge and the suction pressures, then dividing the discharge by the suction. For example, a 350-psig discharge pressure and a 60-psig suction pressure shows a compression ratio of about 4.9/1 (350+14.7/60+14.7 = 4.88). Realize that this isn't caused by a compressor problem, it's the result of a system problem. So the whole system must be replaced.
Not all compressors are created equal. Compressors used on heat pumps and for high-temperature (desert) applications are usually more durable. However, that doesn't mean they are more efficient. Usually the reverse is true.
One of the nice things that I have seen in my lifetime is a proliferation of new compressor types. Years ago, about all that was available was reciprocating (piston type) compressors and centrifugals. Centrifugals are only used in very large commercial systems. However, both rotary and scroll compressors are now competing for the residential and smaller commercial-system market, while screw compressors challenge the large commercial-system market.
The nice thing about scroll and rotary compressors is that their compressor sections rotate in just one direction, unlike reciprocating pistons. As a result, they tend to just move refrigerant in gulps toward a higher pressure rather than slamming it there with a single upward stroke. This has three advantages:
1. There is no "clearance factor" (the re-expansion of gas that isn't vented through the valves on the compression stroke), which means that they naturally operate at a higher efficiency.
2. The lack of discharge valves and the movement of the gas allows for better tolerance of liquid in the return.
3. The lack of a compression stroke means that they are usually quieter operating.
Now, both scroll and reciprocating compressors are becoming available with capacity controls (50% and 100%) that allow them to more closely match the load and operate at higher efficiencies.
Rotary compressors are a great idea. The main moving parts are a hockey-puck-shaped piston that spins to move the refrigerant forward, and a spring-mounted vane that separates the high and low-pressure sides of the compressor. Unfortunately, these compressors are mainly limited to sizes under two tons so far, but they now virtually dominate the window air-conditioner and mini-split market.
Scroll compressors seem to be taking over the residential market, simply because they offer equipment manufacturers a way of increasing air-conditioning efficiency without adding condenser-coil size. However, their reliability has also made them very attractive to the commercial market. And although their individual size is limited to a maximum of 20 tons so far, air conditioners with multiple scrolls are now available to 100 tons!
Scrolls are a very simple design. A cup with a metal scroll fits inside a cup with another metal scroll. Then one scroll is rotated so the surfaces meet and move refrigerant along in little gulps.
I would be remiss if I didn't mention that all this competition of new compressor types has definitely resulted in improved and more efficient reciprocating compressor designs. So they haven't been counted out of the market. Screw compressors are really giving centrifugal compressors strong competition due to their ability to use almost any type of refrigerant. This is important in a time when refrigerant types are being limited by legislation.
So, things are changing for the better. Compressors are now quieter, more efficient, and more reliable than ever before in history. And if all the other system components were designed to last as long, the effective life of all air conditioners could be greatly extended.