A typical industrial reciprocating compressor will run at a compression ratio of approximately four. This allows industrial equipment to run for a very long time with only routine maintenance.The compression ratio of the compressor is a value that describes the ratio of the absolute discharge and absolute suction pressure; the equation for calculating the compression ratio is shown in the following equation. It is important to remember that you must use absolute units when calculating the compression ratio. The calculation is straightforward and quite easy to do. In and of itself, this ratio is not all that interesting directly, but it affects other compressor performance characteristics, like volumetric efficiency, that we are a lot more interested in.
For now, let’s consider only reciprocating compressors. In a reciprocating compressor, refrigerant is drawn into a cylinder as the piston moves down and is compressed as the piston moves up (Figure 1). Obviously, the piston cannot be designed to hit the top of the cylinder, so there is always a small volume of compressed refrigerant that re-expands as the piston begins its next suction stroke. The suction valve can’t open to allow in new refrigerant until the expanding gas reaches the suction pressure (Figure 2).
The volumetric efficiency of a compressor is the ratio of the volume of refrigerant entering the compressor divided by the displacement of the compressor. This re-expansion in reciprocating compressors reduces the volumetric efficiency of the compressors because it reduces the amount of refrigerant that can get in. One of the reasons we are interested in the compression ratio is that it is the most significant variable in determining volumetric efficiency. As you can see in Figure 3 and Figure 4, increasing the compression ratio causes a decrease in the volumetric efficiency.
Figure 1: A piston draws in refrigerant in a suction stroke and compresses the refrigerant in the discharge stroke.The other reason that understanding compression ratios is important is that manufacturers design products to function in specific compression ratio ranges. You may recall a previous article on low and ultra-low-temperature refrigeration that explained why we need to use two-stage systems to achieve low temperatures when using reciprocating compressors. It is common to hear that you should limit reciprocating compressors to compression ratios of 10 or less. This is not exactly true. Most compressors have application envelopes that allow operation at compression ratios higher than 10 but, there are also compressors whose allowable compression ratios are closer to eight.
Compressor at risk
Operating compressors at high compression ratios, even if they are allowed by the manufacturer, has risks, and can shorten the life of the compressor by causing additional wear on compressor components such as the piston pin (because the expanding vapour can push the piston down in the suction stroke and prevent lubrication). In addition, it can lead to overheating and excessive discharge temperatures.
Given this, care should be taken to make sure to prevent operating compressors outside their application envelopes, and when it comes to compression rations, low suction pressure is the most common place to go wrong. Figure 5 demonstrates that as you decrease the evaporating temperature, even small increases in condensing temperature can cause large increases in compression ratios.
Figure 2: The gas that’s trapped in the top of the cylinder at the end of a discharge stroke must re-expand to suction pressure before the suction valve can open and let in refrigerant.Take a system operating at 110 F (the blue line in Figure 5) and imagine that normally it is operating at 10 F evaporating temperature, which means a pretty low compression ratio of 8.5. However, this system has a pump down control, and the pressure control is set for cut out at five PSIG (which is about -29 F). During this time, the compressor would be operating at a compression ratio of 13.5. Is this okay? Probably, but not for every compressor. A medium-temperature compressor not designed for extended range may not be rated for a compression ratio this high. Now consider the condenser is dirty and the condensing temperature jumps to 130 F (the orange line in Figure 5). In this case, when we pump down, we have a compression ratio of almost 15.5.
Continuing with our example, what if a technician working on this system sees the compressor come on and off a few times every time it pumps down because of residual liquid in the evaporator and thinks the reason the system is doing this is because the low-pressure control is set too high, and they adjust it down to zero PSIG? The compression ratios in the above example jump to 17.5 and 23.5 for the 110 F and 130 F condensing temperatures, respectively. The same thing happens when running systems outside of their design range (i.e., turning a cooler into a freezer) or operating with a low refrigerant charge.
Figure 6 shows the application envelope of a real compressor. The refrigerant is R134A. What is the minimum suction pressure this compressor can handle? The lowest temperature in the envelope is -25 F, and since R134A has a pressure of 11.3 PSIA (3.4 psi in a vacuum) at -25 F, that’s the lowest pressure it can handle.
Figure 3: Sample volumetric efficiency of a six-cylinder compressor operating at -10 F evaporating temperature (dew point) using R449A.Max condensing temp
Note that the maximum allowable condensing temperature when this compressor is operating at -25 F is only 105 F. This maximum condensing temperature is a pretty low value and would limit the application of this compressor if you were trying to use it in a freezer.
What is the highest compression ratio at which this compressor is allowed to operate? Normally, this would be the corner with the highest condensing temperature and lowest evaporating temperature, but since this envelope has a substantially higher condensing temperature allowed at 20 F, I checked there too. Figure 6 has a red dot on the corner of the application envelope where the highest compression ratio will normally occur and a green dot at the location I decided to check just in case. At the red dot, the compression ratio is 13.2. At the green dot, the compression ratio is 8.8.
I purposely kept the topic of compression ratios and volumetric efficiency limited to reciprocating compressors in this issue, but since scroll compressors are much more popular in air conditioning and small refrigeration applications, we will take a closer look at what makes them different in an upcoming issue.