Compression Ratio – Capacity, Efficiency & SHR Class

Compression ratio is the ratio of head pressure to suction pressure in an HVAC/R unit. Generally, compression ratios approach 2.5:1 on high-efficiency, 16+ SEER A/C systems. The highest compression ratio in A/C compressors is typically 11:1. Refrigeration compression ratios tend to be quite a bit higher (up to 26:1).

To calculate a system’s compression ratio, you take your suction and discharge pressure right by the compressor. Then, you turn them into absolute readings. To do that, you add atmospheric pressure (14.7 PSI) to your gauge pressure reading. You divide your head pressure (discharge pressure) by your suction pressure to yield a ratio.

When you pump down a scroll compressor, the floating seal prevents a system from running with too high of a compression ratio. High head pressure AND low suction pressure both contribute to high compression ratios.

Lately, air conditioning manufacturers have attempted to drop compression ratios by increasing the sizes of their condenser coils. Increasing the sizes of condenser coils helps expand the surface area; when that happens, you can get the refrigerant temperature closer to the outdoor ambient temperature. (As you could probably assume, outdoor ambient also affects compression ratio.) While those reduce head pressure, they don’t do anything to address issues about low suction pressures.

Mass flow also plays a role in compression ratio. It correlates with the suction and discharge pressures. Higher discharge pressure indicates lower mass flow, and higher suction pressure indicates higher mass flow.

Compressor ratio and efficiency go hand-in-hand. Scroll compressors tend to have lower compression ratios than reciprocating compressors and are more efficient. Reciprocating compressors retain some gas in their pistons; the retained gas expands after most of the discharge gas leaves, leading to inefficiency.

Sizing is also a vital component of the compression ratio. Oversizing, in general, is NOT a best practice in installation. (You can see the benefits of large evaporator coils in dehumidification and large condenser coils in reducing compression ratio, but they are not perfect fixes and can lead to other complications.) Suction filter/driers are often improperly sized and end up being too restrictive. Improperly sized suction filter/driers have the same effect as a kinked suction line, which horribly impacts the compression ratio.

Sensible heat ratio (SHR) indicates the amount of humidity removed from an airstream across the evaporator coil compared to the sensible heat. Essentially, it compares latent heat to sensible heat and is expressed as a percentage of sensible heat. It typically does NOT impact the compression ratio, but conditions that affect the SHR may also affect the compression ratio.

When it comes to capacity, we often encounter limiting factors in each system. There may be difficulties rejecting or absorbing heat. Additionally, not all latent capacity can be converted to sensible. So, increasing airflow won’t directly increase your capacity because of that inconvertible latent heat.

We also answer viewers’ questions and talk about:

Vapor injection
Theoretical (and real) ways to reduce compression ratios with water
Dirty evaporator effects on volumetric efficiency
Dew point and its impacts on SHR and suction pressure
Manual J and latent heat load
MeasureQuick testing and functioning
Noisy compressors

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