We’ve talked about compression ratio a lot over the years. Compression ratio is the number you get when you divide the absolute head pressure (PSIG + 14.7) by the absolute suction pressure (PSIG + 14.7). It measures the efficiency of a compressor, and lower numbers indicate that the compressor is moving more refrigerant while consuming less power.

Newer equipment often has lower compression ratios than we’ve ever seen before. With financial incentives in play for high-efficiency systems, that isn’t a surprising development. We’re going to discuss a few things that can reduce the compression ratio of an HVAC system. 

Use Larger Coils

Heat transfer primarily happens in the coils. Sure, the suction line can pick up a little bit of heat between the evaporator outlet and compressor inlet (even if it’s insulated), but we really want a system to absorb and reject heat.

Larger coils have a larger surface area, allowing for better heat transfer. As the blower or fan pushes air over the coil and those molecules make contact with the fins, that’s an opportunity for heat exchange. Simply put, when there’s more opportunity for that contact, the more heat transfer you will have—both into and out of the refrigerant.

In general, larger, warmer evaporator coils are better for higher suction pressures, and larger condenser coils are better for reducing head pressure (and efficiency in general). However, keep in mind that while a larger and warmer evaporator coil raises the suction pressure and is good for efficiency, a warm coil isn’t as useful for dehumidification. We need a cold coil below the dew point to pull moisture out of the air.

Ensure Better Airflow Over BOTH Coils

We’ve already made a video that establishes the relationship between poor evaporator airflow and low suction pressure. Low suction pressure leads to a higher compression ratio, especially if the head pressure is also high. Poor condenser airflow is also one of the common causes of high head pressure.

We get higher compression ratios when we have a larger number at the front of the equation (head pressure) and divide it by a smaller number (suction pressure):

(340 PSIG + 14.7) / (120 PSIG + 14.7) = ~2.6

Vs.

(420 PSIG + 14.7) / (86 PSIG + 14.7) = ~4.3

When we have good airflow over both coils, the system can absorb and reject heat more effectively. On top of that, you mitigate two sources of high head pressure and low suction pressure.

Some common causes of poor indoor airflow in residential applications are dirty filters, dirty coils, collapsed ducts, and improper fan settings. We also encounter some situations where the cabinet insulation gets pulled into the blower, which will negatively affect airflow as well. Bert made a video about this issue a while back.

Poor condenser airflow is often caused by dirty, impacted coils or improper fan operation. 

Compressor Cooling Strategies (Enhanced Vapor Injection)

The compressor creates the pressure differential between the high side and the low side. When it works harder and runs hotter, the high-side pressure goes up. Modern HVAC systems have technology that we didn’t see several years ago.

The U.S. Department of Energy has raised its energy efficiency standards in recent years, especially for cold-climate heat pumps. Manufacturers have responded to these initiatives by finding ways to cool compressors.

One example of such technology is enhanced vapor injection. A cool liquid-vapor mixture is injected into the compression chamber to keep the compressor from getting too hot. As the compressor motor moves and heats up, it evaporates the saturated refrigerant until it’s fully vapor. From there, it goes back into the refrigeration circuit with the compressed refrigerant.

It’s important that we have a saturated vapor because it’s in the middle of changing its state, and some of the compressor heat gets absorbed as latent heat. On top of that, evaporation also has a cooling effect. Think about when you sweat and feel cooler as it evaporates or when you have rubbing alcohol in your hands and blow over it.

All that said, you’ll generally see compression ratios between 2.3:1 and 3.5:1 in most heat pumps. Medium-temp refrigeration is usually higher (up to 5.5:1), and low-temp refrigeration is even higher (up to 13:1). While a compression ratio of 4 would be fine for a medium-temp cooler, it’s not ideal for a heat pump and may indicate an issue with the system design or airflow.

Now, it IS possible to have too low of a compression ratio. When the system is off, the compression ratio is 1 because there is no pressure differential between the high and low sides; the compressor isn’t creating a differential. When we get compression ratios below 2, that means the compressor isn’t creating enough of a pressure differential to move refrigerant through the circuit effectively. However, most of the time, we’ll be trying to cut inefficiency due to high compression ratios and move more refrigerant with less energy wasted—and the three things we covered are three ways to do precisely that.