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Refrigeration Compression Ratio

Refrigeration Compression Ratio

Heat, pressure, and reactive chemicals are the tools of the chemist to create reactions.  He/she uses equipment to search for better ways to make a reaction proceed swiftly and more completely.  Often, the chemist must supply great quantities of heat to accomplish the reaction.

The refrigeration service and installation person has a great chemical reactor at his/her disposal.  He or she has reactive materials in abundance — refrigerant, oil, cellulose, copper, oxygen, moisture, acid, etc.  There is plenty of pressure and heat, and at times, there is more than he or she knows what to do with.    The last thing we want a refrigeration system to do is create chemical reactions.  We want a chemically stable and trouble-free system.

The competent service technician can do much to minimize these reactions.  Dehydration through deep vacuum, acid removing dryers, and filters can help any system, but high discharge temperature, caused by an improperly designed system, is virtually impossible to correct.  Too many systems are designed to “meet a competitive situation”.  It is one thing to design a job as economically as possible and yet correctly.  It is another thing to deliberately undersize and cut every corner on installation in order to be “competitive”.

Many individuals involved in design or service of refrigeration systems are not aware of the dangers involved in a system with back pressure or suction pressure that is too low.  Studies have shown that fewer than 10 percent of service people know how to calculate compression ratios, let alone know what the ratio means.  High compression ratios mean high discharge temperatures.  The rate of chemical reaction doubles for each 18°F rise in discharge temperature!

It is obvious that systems running abnormally high head temperatures will develop problems quicker and more frequently than systems running at normal temperatures.  The compression ratio affects discharge temperature more than anything else.

Compressor manufacturers can tell you what the maximum compression ratio is for a specific compressor, but an accepted rule is no more than a 10:1 compression ratio.

Absolute Pressures

In order to determine a system’s compression ratio, we must deal in absolute pressures, not gauge pressure readings, since a refrigeration system is a closed system, not open to atmospheric pressure.  Service technicians have ordinary gauge sets where gauges do not register atmospheric pressure, but read zero when not connected to a pressurized system, P.S.I.G.

It is easy to obtain absolute pressures, P.S.I.A., at zero or above gauge readings, P.S.I.G.  Simply add 15 Lbs. to the gauge reading.  This makes P.S.I.A. head pressure readings easy to determine.  It is computing the P.S.I.A. suction pressure when the system is running in a vacuum on the low side that causes the most confusion.  An easy formula to work with is to subtract the reading in inches from 30 inches and divide the answer by two.  We can now solve the formula for finding the compression ratio:

 

Example: 

A system is running at a head pressure of 160 Lbs., as shown on a standard manifold gauge set.  The suction pressure is shown as 10 inches of vacuum.

Converting to P.S.I.A.: P.S.I.A. Head Pressure     =   160  +  15   =  175 Lbs.

                                  P.S.I.A. Suction Pressure   =    30 - 10   =   20   =   10

                                                                                         2              2

        Compression Ratio   =  175    =  17.5 : 1

                                                                                    10

This example system is in a lot of trouble!  Unless the cause of the high compression ratio can be found and corrected, there will be many compressor failures.  The high compression ratio will cause high discharge temperatures, leading to many burnouts.

Let’s examine this system when the suction pressure is 10 P.S.I.G.

            P.S.I.A. Head Pressure = 160 + 15 = 175                  Compression Ratio = 175  = 7:1

            P.S.I.A. Suction Pressure = 10 + 15  = 25                                                      25

This is well within our guideline of 10:1.  This system should last a long time.

The examples also demonstrate what a great influence suction pressure has on compression ratio.  A change in head pressure does not influence the compression ratio as much as suction pressure.  If the head pressure in both our examples were 185 Lbs. instead of 160 Lbs., the first example would have a compression ratio of 20:1 and the second example 8:1.

High compression ratios are a major reason systems run hot.  There are other reasons a system can exhibit high discharge temperatures, but knowing how to find the compression ratio can greatly aid a service technician in discovering what is wrong with a troublesome system.