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1 2010 Emerson Climate Technologies

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AE17-1268

Application Engineering

B U L L E T I N

The compression ration at which a compressor operatesis critical to compressor life. Field failure experiencewould indicate a growing need for better understandingon the part of the operating and service engineer as tothe causes and effects of excessive compression ratios.

Compression ratio is a term describing the compressioncycle in a compressor cylinder, and while suction anddischarge pressures are used to calculate it, the ratioitself is independent of pressures.

By definition, compression ratio is the ratio of theabsolute discharge pressure (psia) to the absolutesuction pressure (psia). Absolute pressure is definedas the pressure existing above a perfect vacuum.Therefore in the air around us, absolute pressure andatmospheric pressure are the same. A pressure gaugeis calibrated to read 0 psig when not connected to apressure producing source, so the absolute pressureof a closed system will always be gauge pressureplus atmospheric pressure. Pressure below 0 psigare actually negative readings on the gauge andare referred to as inches of vacuum. A refrigerationcompound gauge is calibrated in the equivalent of

inches of mercury for negative readings. Atmosphericpressure at sea level is approximately 14.7 psi, or 29.92inches of mercury, so two inches of mercury on thegauge are approximately equal to 1 psi.

Atmospheric pressure varies with altitude and weatherconditions, but for purposes of this discussion we willconsider only conditions at sea level.

To illustrate the calculation of compression ratio,consider two different examples.

Example 1

Suction pressure = 30 psig = 44.7 psia

Discharge pressure = 433 psig = 447.7 psia

Compression ratio = 447.7 = 10 to 1

44.7

Example 2

Suction pressure = 10 inches of vacuum = 9.7 psia

Discharge pressure = 83 psig = 97.7 psia

Compression ratio = 97.7 = 10 to 1

9.7

It is important to distinguish between excessivepressures and excessive compression ratios, becauseeach condition causes totally different wear patterns.

Excessive pressures, either discharge or suction, cancreate bearing loading beyond the bearing design limits,and eventually will cause connecting rod, crankshaft,and bearing damage.

With excessive compression ratios, the wear occurs at

the pin connecting the piston to the connecting rod. Asthe gas is compressed on the piston discharge stroke,it becomes much more dense than the entering suctiongas. Since the gas remaining in the cylinder clearancearea does not leave the cylinder on the discharge stroke,it re-expands on the suction stroke. As the compressionratio increases, the difference between the two densitiesbecomes greater, and eventually a point is reachedwhere the residual high pressure gas actually pressesthe piston down for most of the down stroke, not onlypreventing suction gas from entering the cylinder, butmore critically keeping the piston pin, starving the pin oflubrication and creating a wear pattern. Eventually theround pin hole in the connecting rod will be worn egg

shaped, and from that point on the play between rodand pin will quickly accelerate additional wear.

Enlarging and strengthening the rod and pin bearingsurfaces will increase the compressors ability towithstand high compression ratios, but the only wayto prevent pin damage is to maintain the compressorsoperation within allowable limits. Compressorsare capable of operating within the limits of theirrecommended operating parameters without piston pinstress, but operation beyond those limits can cause pindamage.

Field failures of this type can result from operation

after losing a condensing fan motor, operating underloss of charge conditions, or more commonly, fromoperation at excessively low suction pressures. Theservice engineer who sets the low pressure control fora lower cut-out point beyond the compressor designlimits to obtain a lower evaporating temperature maybe unknowingly starting a failure pattern. The numberof low pressure controls that are found bottomed out infield investigations indicates this is not an uncommonoccurrence.

Compression Ratio as It AffectsCompressor Reliability

AE17-1268 December 1982Reformatted November 2010

Application Engineering

B U L L E T I N

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2 2010 Emerson Climate Technologies

Printed in the U.S.A.

AE17-1268

Application Engineering

B U L L E T I N

Changes in suction pressure at low evaporatingtemperatures have a much greater affect oncompression ratio than the same pressure difference at

higher pressures, so thefi

eld setting of the low pressurecontrol is obviously the most critical area, as shown inTable 1.

It has been a commonly accepted industry rule of thumbthat compression ratios above 10 to 1 were the limitsof a compressors capability, but the figures in Table1 clearly demonstrate that compression ratios of thatmagnitude are routine in every supermarket, and mustbe exceeded for air-cooled operation of low temperaturesystems in desert areas.

Both compressor construction and time of exposure arecritical factors in determining a compressors capabilityto withstand high compression ratios. Lighter duty

welded compressors such as the Copelaweld normallyshould not be applied beyond compression ratios or 7.5to 1. The heavier duty Copelametic models can operate

at compression ratios up to 15 to 1 and higher forreasonable periods of time, but for long life, compressionratios of 12 to 1 or less are desirable.

While there has not been any laboratory testing toevaluate the effects of operating in a vacuum withsome of the non-standard refrigerants such as R-13,field experience indicates wear patterns of a similarnature may be aggravated at lower compression ratioswith R-13 than with R-502, which could mean thesolvent qualities of some of the ultra low temperaturerefrigerants may be a complicating factor.

In addition to the piston pin wear problem, excessivecompression ratios are directly related to excessivedischarge temperatures. It is possible that excessivedischarge temperatures can be controlled by liquiddesuperheating valves or other means, but the basic

mechanical wear problem can only be prevented byoperating within acceptable limits.

Table 1Effects of Changes in Pressure on Compression Ratio with R-502

Condensing

TemperaturePSIG PSIA

Evaporating

TemperaturePSIG PSIA

Compression

Ratio

110F 247.9 262.6 0F 31.08 45.78 5.7

-10F 22.56 37.26 7.1

-20F 15.31 30.01 8.8

-30F 9.20 23.90 11.0

-40F 4.11 18.80 14.0

-50F .2 14.60 18.0

-60F 7.15 11.18 23.5

100F 216.2 230.9 -30F 9.20 23.90 9.7

110F 247.9 262.6 11.0

120F 282.7 297.4 12.4

130F 320.8 335.5 14.0