Understanding Service Compressor Replacements
This is part 2 of a 3 part series on understanding compressor replacements.
What are compression ratios and how do they affect compressors?
As discussed in the first part of this series, an air conditioning compressor’s re-expansion gas directly affects its volumetric efficiency at different system operating conditions. The volumetric efficiency of a reciprocating (piston) compressor can vary over a wide range, depending on the compressor design and the compression ratio.
The compression ratio is the ratio of the absolute discharge pressure (psia) to absolute suction pressure (psia), found using the formula Discharge Pressure Absolute ÷ Suction Pressure Absolute.
In that chart that accompanies part one of this series, the left side (Y-axis) represents compression ratios. As the compression ratio increases, the volumetric efficiency decreases in reciprocating compressors.
To convert any gauge pressure to absolute, add 14.7 (or 15 to make it easy) to the pressure reading from a manifold gauge set. 14.7 represents the atmospheric pressure that the manifold gauge already takes into consideration. (At 0 psig, the actual reading is 14.7 psia.)
Consider a few examples:
Discharge = 185 psig + 15 = 200 psia
Suction = 5 psig + 15 = 20 psia
This example uses a low temperature application range, and rounds the 14.7 (atmospheric) to 15. Using these psia pressures from the examples above, a 10:1 compression ratio is calculated
200 psia = 10:1
10:1 is a compression ratio commonly found in refrigeration applications. If you are thinking about air conditioning, it is often around 3:1 or 4:1
200 psia = 20:1
In this example, the suction pressure drops by 10 psia, which doubles the compression ratio. At this compression ratio, a compressor designed for a 10:1 ratio would have a tough time surviving. Do you think the Service Tech would notice this drop in absolute suction pressure? Probably not.
400 psia = 20:1
In this calculation, absolute discharge pressure is raised to 400 psia, essentially doubling it, to give the same 20:1 compression ratio. Do you think the Service Tech would notice if his discharge pressure doubled? Hopefully. In either case, the compressor would have a difficult time operating at double its rated compression ratio. After understanding how compression ratio affects volumetric efficiency, at what efficiency level do you think this system is currently operating at, versus its design envelope?
It is important to pay attention to those system pressures, and in the next article we will tie compression ratio and volumetric efficiency together for a better understanding of service compressor replacements.