Why Compression Ratio Matters

In HVAC/R, we are in the business of moving BTUs of heat, and we move BTUs via pounds of refrigerant. The more pounds we move, the more BTUs we move.

In a single-stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. The thing that changes is the number of pounds of refrigerant being moved with every stroke (reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly, a higher compression ratio results in fewer pounds of refrigerant being moved. The lower the compression ratio, the more pounds are moved.

In A/C and refrigeration, the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 PSI) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like this:

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change. However, in general, under near-design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system:

  • In air conditioning applications, compression ratios of 2.3:1 to 3.5:1 are common, with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency air conditioning equipment.
  • In a 404a medium-temp refrigeration (cooler) application, 3.0:1 – 5.5:1  is a common ratio range.
  • In a typical 404a 0°F to -10°F freezer application, 6.0:1 – 13.0:1 is a common ratio range.

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter? 

When the compressor is functioning properly, the lower the compression ratio will be, and the more efficient and cool the compressor will operate. Therefore, the goal of the manufacturer's engineer, system designer, service technician, and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity indicate mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity 

Compression ratio lower than designed = Possible indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don't be tempted to skip past this; it is a vital concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale; high pressure is higher on the chart. Horizontal (left to right) is the heat content scale; the further a value is right, the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. That is where the suction gas enters the compressor. As it is compressed, it goes to point #3, which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself). There is also heat added when the refrigerant cools the compressor motor windings.

Once the refrigerant enters the discharge line at point #3, it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil, it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air to which the condenser is rejecting its heat. In other words, in a typical straight cool, air-cooled air conditioning system, the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high, or if the condenser coils are dirty, blades are improperly set, or the condenser coils are undersized, point #2 (condensing temperature) will be higher on the chart. Therefore, it will put more heat strain on the compressor and reduce compressor efficiency and capacity.

As the refrigerant is changed from a liquid-vapor mix to fully liquid in the condenser, it travels from right back to the left between points #4 and #5 as heat is removed from the refrigerant and into the outside air (on an air-cooled system). Once it gets to #5, it is fully liquid, and at point #6, it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled and the load on the coil, but the LOWER the pressure and temperature of the evaporator, the less dense the vapor will be at point #2 when it re-enters the compressor, and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

The greater the vertical distance between points #2 and #4, the higher the compression ratio, which means that low suction pressure or high head pressure may result in higher compression ratios, poor compressor cooling, lower efficiency, and lower capacity.

In some cases, we can't do very much about high compression ratios. When a customer sets their A/C down to 69°F (20.55°C) on a 100°(37.77°C) day, they will simply have high compression ratios. When a low temp freezer is functioning on a very hot day, it will run high compression ratios.

But in many cases, you can reduce compression ratios by:

  • Keeping set temperatures at or above design temperatures for the equipment. Don't be tempted to set that -10°F freezer to -20°F or use that cooler as a freezer.
  • Keep condenser coils clean and unrestricted.
  • Maintain proper evaporator airflow.
  • Install condensers in shaded and well-ventilated areas.

Keep an eye on your compression ratios, and you may be able to save a compressor from an untimely death.

—Bryan

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3 responses to “Why Compression Ratio Matters”

  1. I’m reading an RTS PPT describing how low loads and suction pressure cause high compression ratios. It seems like compression ratio would be determined simply by comparing the maximum (uncompressed) and minimum (compressed) cylinder volumes.

  2. I don’t see how low loads would cause high compression ratios if fact if it cycles less the condenser would actually run cooler lowering the ratio. Please link this RTS article.

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