Basic Compressor Functions

The compressor's job is to circulate refrigerant through the system by means of vapor compression, similar to the way your heart moves blood through your circulatory system.

Refrigerant circulation is measured in lbs/min or lbs/hour; this is called mass flow rate—the mass flow rate changes depending on the density of the refrigerant and the compression ratio.

The denser (higher the pressure) the refrigerant is coming back from the evaporator, the greater the mass flow rate. The lower the suction pressure, the lower the mass flow rate.

The ability of the compressor to move refrigerant efficiently is often measured in volumetric efficiency. This is a measure of how much refrigerant enters the suction line vs. how much leaves the outlet of the compressor in the discharge line. The difference between the two is the loss or waste to re-expansion of the gas in the compressor cylinder (in a reciprocating compressor).

The greater the compression ratio (absolute head pressure divided by absolute suction), the lower the mass flow rate will also be, and the lower the volumetric efficiency will be. In other words, low suction with high head pressure is the worst-case scenario for mass flow rate and volumetric efficiency when the compressor is working as it should.

Proper lbs/min or lbs/hour of refrigerant circulation is vital to the capacity of the evaporator, condenser, and metering device—as well as the cooling of the compressor if it is refrigerant cooled.

The compressor size (pumping ability) controls the system's lbs/min or lbs/hour mass flow rate.

Compressor pumping action also performs two other functions.

1. It maintains the evaporator pressure: when the compressor runs, it lowers evaporator pressure. This sets evaporator pressure, operating TD, and BTUH capacity.
2. It increases condenser pressure: when a compressor runs, it pumps heat into the condenser; this causes condensing temp and TD to go up until heat can flow out of the condenser as fast as it enters.

As evaporator heat load and temp increase, compressor heat output increases and drives condenser TD even higher to increase condenser heat rejection.

Compressor response to changing evaporator heat loads

Here is a way of thinking about load and how it impacts mass flow rate, compression ratio, and volumetric efficiency:

Higher heat loads produce vapor faster than the compressor can remove it from the evaporator. When this occurs, the evaporator pressure and temperature go up with the increased heat load.

The compressor's flow in lbs/min or lbs/hr increases as the suction pressure increases and the compressor draws more amps due to pumping more refrigerant.

Lower evaporator heat loads produce vapor slower than the compressor is removing it from the evaporator. Evaporator pressure and temperature go down with the reduced heat load. The compressor's flow in lbs/min or lbs/hour goes down. The compressor draws fewer amps due to pumping less refrigerant.

Compressor's Volumetric Efficiency

The goal is to keep the volumetric efficiency as high as possible. With a higher VE, a compressor produces more lbs/min or lbs/hour of refrigerant flow.

System operating conditions, evaporating, and condensing pressures directly affect compressor pumping ability. The VE ratio of condenser pressure to evaporator pressure is called compression ratio. To calculate compression ratio, convert pressures to absolute values (add 14.7 to existing pressure), then divide condenser pressure by evaporator pressure.

Volumetric Efficiency Charts

VE (volumetric efficiency) charts show the effect of compression ratio on volumetric efficiency: As CR goes up, VE goes down. As CR goes down, VE goes up. Our goal is to keep the volumetric efficiency of the compressor as high as possible for capacity, energy usage, and compressor longevity.

Factors that determine system CR

System compression ratio is based on a few factors, primarily desired space temp and temperature of the cooling medium. Corresponding evaporator and condenser pressure establish the compression ratio the compressor must work against. Refer to the compression ratio chart for each compressor as a guide.

Keeping Volumetric Efficiency Up

To improve VE, you must keep the compression ratio low. You can do this by keeping condenser pressure low, maintaining a clean condenser, and supplying it with a cool condensing medium (proper temperature and flow of air or water across the condenser coil or condenser HX). You must also keep the evaporator pressure up; don't run the evaporator pressure any lower than needed to do the job. A lower compression ratio allows the compressor to pump more lbs/min or lbs/hour through the system. Higher compression ratios reduce the compressor's ability to maintain the desired mass flow rate.

Compressor Approved Application Range (operating range) 

Hermetic and semi-hermetic compressors are designed for specific evaporator temperature ranges. The range of evaporating temps varies by manufacturer and model, and you will need to do some reading to be sure you have it right. Evaporator temperatures above maximum approved temperature result in motor overload: drawing excess amps and overheating. An evaporator temperature below the minimum approved application temperature will result in poor motor cooling due to a low lbs/hour flow rate.

Compressor Datasheets

Datasheets show compressor performance in its approved application range. Data may be shown in a table or as performance curves. These tables or curves will show the following: capacity, mass flow rate, power, and current. These can be used for design, proper commissioning, and system diagnosis. Just keep in mind that the compressor, when working properly, is still at the mercy of system conditions; it is up to us to set it up for success.

Compressor Amp Ratings

Compressor amps change as the evaporator and condenser temperatures change. Under load conditions, the compressor could draw more than rated load amps and not necessarily be in any danger of motor overload—as long as the motor amperage drawn is well below trip amperage. Most compressors will run at less than rated load amps during normal conditions but may run high under heavy evaporator load. All of this can be found by looking carefully at the compressor charts or curves.

—Louie Molenda