HVAC Compressor Motor Compression Ratios

Normal and Abnormal HVACR Equipment Compression Ratios

Typical HVAC Compressor Motor Compression Ratios

The compression ratio for an HVACR compressor refers to the ratio of the compressor's high side output pressure to the pressure at its input or suction line. The compression ratio CR is a useful diagnostic because lower OR higher compression ratios than normal tell us that the compressor isn't working as it should.

Measured as the low side or suction pressure and the high side or compressor output pressure we obtain two pressure readings and from them compute the compression ratio.

We simply divide HVACR compressor motor's output pressure (high side) by input pressure (low side or suction side) to get the compression ratio.

CR = (Output Pressure psi + 14.7 psi) / (Input Pressure psi + 14.7 psi)

Why we add 14.7 psi to gauge pressure when calculating CR

HVACR refrigerant gauge sets are calibrated to show zero pressure when not connected to a source. Actually the gauge is being subjected to the normal atmospheric pressure which, at sea level, is about 14.7 psi.

Therefore to calculate accurate compressor compression ratio correctly we must convert gauge pressure to absolute pressure by adding the standard atmospheric pressure at sea level (1 ATM = 14.7 psi) to both the high side output or discharge and to the low side suction or input to the compressor.

That accounts for the effect on the equipment of the surrounding atmospheric pressure.

Why we use absolute pressure numbers rather than negative (suction) pressure

Since we're looking for the pressure difference (ratio) that the compressor motor is producing between its input and its output sides, even though the low side pressure is "negative pressure" or "suction" we treat it as a positive number in computing the actual or effective compression ratio of the compressor motor.

"Input pressure" as used here is also called "suction pressure" but always used in absolute form (that is, not as a negative number).

What's the difference between psig and psia?

psig = gauge pressure or psi(gauge)

In the pressure chart above on this page, if you look carefully you'll see that the manufacturer specifies that their suction line data is in psig.

psia = atmospheric pressure or psi(atmospheric)

To be technically correct we note that 0 psig is, at sea level, actually 14.7psia so if we use an HVACR gauge set and measure 300 psig, the absolute pressure would be expressed as 300 psig + 14.7 = 314.7psia. Or in sum:

psia = psig + 14.7 psi

Notes to the table above

1. Watch out: be sure to use "absolute" pressures when calculating compression ratios for HVACR equipment. The calculation is easy, as explained in the CR formula given above.
2. Hundy, Guy F., Albert Runcorn Trott, and T. C. Welch. Refrigeration and air-conditioning. Butterworth-Heinemann, 2008.

Why are HVAC Compression Ratios Given as a Range

While we gave above the easy calculation of the compression ratio CR for an HVACR compressor motor, that just produces a single number.

The proper way to express a compression ratio in HVACR equipment is as a compression ratio range that varies depending on the cooling or heat pump system's evaporator unit load at the air handler.

The load on the HVAC compressor varies depending on the outdoor temperature and also on the indoor temperature and thermostat setting.

That' s why when you look at an HVAC compression ratio chart or data you'll see that the expected compression ratio will usually be given as a range of numbers such as 2.3:1 to 3.5:1 as "standard" as we gave just above.

Abnormal HVACR Compression Ratios

Notes to the table above

1. Compression ratios outside the typical compression ratio ranges given in the earlier table indicate that diagnosis and repair are needed.
   
   But as you see above, an abnormally-low compression ratio, for example less than 2.3:1, sometimes has a different interpretation from an abnormally-high compression ratio, such as above 3.5:1 in a conventional air conditioning system.
2. Note on abnormally high compression ratios:: the appearance of the same cause in both high and low compression ratios can be confusing. In fact abnormally high compression ratios can be caused by either an abnormally low suction pressure or an abnormally high head pressures (or both).
   
   That's why you'll see similar causes listed at both abnormally low A/C compression ratios and abnormally high A/C or compressor compression ratios.
3. Li, Haorong, and James E. Braun. "A methodology for diagnosing multiple simultaneous faults in vapor-compression air conditioners." HVAC&R Research 13, no. 2 (2007): 369-395.
4. Mtibaa, Amal, Valentina Sessa, Gilles Guerassimoff, and Stéphane Alajarin. "Refrigerant leak detection in industrial vapor compression refrigeration systems using machine learning." International Journal of Refrigeration 161 (2024): 51-61.
5. Scrivener, Greg, Compressors and compression ratios [Web article], Plumbing & HVAC Magazine, 286 King St W Unit 203, Oshawa, ON L1J 2J9, Canada - retrieved 2024/11/20
6. Stevens, S. J., U. S. L. Nayak, J. F. Preston, P. J. Robinsons, and C. T. J. Scrivener. "Influence of compressor exit conditions on diffuser performance." Journal of Aircraft 15, no. 8 (1978): 482-488.

Causes of abnormally low compression ratios

An abnormally low compression ratio may tell us that the system has lost all of its refrigerant or that there is a mechanical failure in the compressor motor itself so that it can no longer produce higher pressure on its output side.

If the current draw of the motor is abnormally high the motor is failing and seizing. If the current draw is low we suspect lost refrigerant or a failed valve or other problem in the compressor motor.

If the AC compressor isn't compressing, the problem is typically internal, a worn compressor piston or a failed valve.

As your compressor is typically a sealed unit, repair isn't feasible. The unit gets replaced.

Watch out: before replacing the compressor,be sure the cause isn't something else easier and less costly to repair.

• Check the TEV - Thermostatic Expansion Valve operation - stuck "open".
  
  An expansion valve that sticks wide open will also give low compression ratios, lower current draw.
• Check the refrigerant charge Level
  
  Abnormal compression ratios can be also an effect of refrigerant overcharge (which also risks liquid slugging and compressor motor damage).
• Check the current draw of the compressor: a failing compressor motor may draw low current and run without head pressure if an internal valve in the compressor has failed (stuck open)
  
  Alternatively, if the compressor motor or a compressor motor bearing is failing the compressor may be unable to develop pressure.
• Other causes of low compression ratios are in the table above.

Causes of abnormally high compression ratios

• Check the TEV - An abnormally high compression ratio may point to a thermostatic expansion valve that is stuck closed.
  
  If the TEV is stuck closed then the high side pressures, at the output side of the compressor will be abnormally high (and may be unsafe).
• Refrigerant overcharge
• Clogged refrigerant piping
• Other causes of high compression ratios are in the table above.

High CRs at the compressor not only increase the system's operating cost but also a high CR leads to overheating that in turn damages the equipment.

More About Abnormal Residential Air Conditioner Compressor Pressures

Put another way, high temperature or a low compression ratio (high pressure on the air conditioner low side or low pressure on the high side) is a sign of a problem.

If pressure on the high side is very low the cause may be one of the problems we listed just above.

If pressure on the high side is abnormally high, we may have a TEV that's stuck closed (the low side pressure remains low or abnormally low) or if the TEV opens, low side pressure might increase as well, and we may exceed the operating temperature and pressure of the system.

The Low side temperature must be low enough to get transfer of heat from the indoor air into the evaporator coil. The High Side temperature must be high enough to get transfer of heat into the outdoor air.

• See SEER RATINGS & OTHER DEFINITIONS for additional definitions of the high side and low side of an air conditioning system.
• See COMPRESSOR CONDENSER for an explanation of how and why high side and low side pressures in the cooling system enable an air conditioner to move heat from indoors to outdoors.
• See OPERATING TEMPERATURES HVAC for a discussion of the typical temperatures at which various types of cooling systems operate.

HVACR Compression Ratio Research & Source Citations

• Kraft, Frank F., and Tommy L. Jamison. "Mechanical behavior of internally pressurized copper tube for new HVACR applications." Journal of pressure vessel technology 134, no. 6 (2012): 061213.
• McQuiston, Faye C., Jerald D. Parker, Jeffrey D. Spitler, and Hessam Taherian. Heating, ventilating, and air conditioning: analysis and design. John Wiley & Sons, 2023.
• Nandagopal, Nuggenhalli S. HVACR Principles and Applications. Springer, 2024.
• Nunes, T. K., J. V. C. Vargas, J. C. Ordonez, D. Shah, and L. C. S. Martinho. "Modeling, simulation and optimization of a vapor compression refrigeration system dynamic and steady state response." Applied Energy 158 (2015): 540-555.
  
  Abstract excerpt:
  
  This paper introduces a dimensionless simplified mathematical model of a vapor compression refrigeration (VCR) system, in order to optimize the system dynamic response.
  
  The model combines principles of thermodynamics, heat and mass transfer applied to the system components with empirical correlations, assigning thermodynamic control volumes to each component, which yield a system of ordinary differential equations with respect to time that is integrated explicitly and accurately with low computational time.
  
  Appropriate dimensionless groups are identified, and the results are presented in the form of normalized charts for general application to similar systems.
  
• Perevozchikov, Michael M., and Hung M. Pham. "Scroll compressor for mobile HVAC/R application." (2004).
• Yang, S., J. C. Ordonez, and J. V. C. Vargas. "Constructal vapor compression refrigeration (VCR) systems design." International Journal of Heat and Mass Transfer 115 (2017): 754-768.
  
  Abstract excerpt:
  
  This paper introduces a mathematical model and a structured procedure to optimize the internal structure (heat exchanger areas) and pressure ratio of a vapor compression refrigeration system so that the refrigeration rate, the coefficient of performance (COP), and second law efficiency are maximized.