Compression Ratio: A Valuable Technician Tool

Compression Ratio: A Valuable Technician Tool

Compression Ratio is a significant tool for a technician. It will tell you how the refrigeration

cycle is running / pumping and an excellent check of how well it is doing. The rule of thumb

indicated in industry reading material is not to exceed an 12:1 ratio. This ratio is specified by

compressor manufacturers for their individual compressors. We will use the 12:1 ratio as a

maximum rule of thumb.

12:1 (spoken Twelve to One) is a comparison of the "absolute" high side pressure to the

"absolute" low side pressure. The term "absolute" means that the pressures have been adjusted

to reflect pressures measured in psia (pounds per square inch absolute). This is necessary to

allow a true ratio regardless of whether the pressures are above or below atmospheric pressure.

An example:

Using R-22 in an air conditioning application, gauge suction pressure is at 70 PSI and gauge

discharge pressure is at 250 PSI. First you need to convert these gauge pressures to PSIA .

That means that 15 pounds is added to each gauge pressure. You might recall that our

atmospheric pressure is equal to 14.7 pounds pressure at sea level but our gauges typically

represent atmospheric pressure at 0 PSIG .

(250psi + 15psi= 265 psia) / (70psi + 15psi= 85 psia) = Compression Ratio

265psia Do the math and you will find an answer of 3.1 to 1 or 3:1 (close enough). You are

85psia comparing how many times higher the discharge pressure is as compared to the

suction pressure which is "Compression Ratio". This is well below the suggested

maximum of 12:1.

Exercise: Calculate the ratios below

Good

Bad

#1. R-22 High side @ 290 psi / Low side @ 20 psi Compression Ratio @______________________

#2. R-404A High side @ 215 psi / Low side @ 46 psi Compression Ratio @______________________

#3. R-401A High side @ 191 psi / Low side @ 1 psi Compression Ratio @______________________

#4. R-134A High side @ 260 psi / Low side @ 2 in Hg Vac Comp. Ratio @______________________

#5. R-502 High side @ 290 psi / Low side @ .2 psi Compression Ratio @______________________

#6. R-410A High side @ 417psi/ Low side @ 132psiCompression Ratio @______________________

Take a look at the gauges to the

left and calculate the compression

ratio from the readings shown. Is

it a good ratio or one that will

cause trouble?

Good?

or

Trouble?

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