ACHRNEWS

Compressor Capacity and You

January 8, 2007
FIGURE 1. Two-cylinder compressor with exposed pistons, cylinder walls, and suction valves.

The service technician cannot control many factors that affect a reciprocating compressor’s capacity. However, the tech can control a few factors that can help the system have more capacity and run more efficiently.

 

The following factors can affect compressor capacity:

• Compressor rpm.

• Compressor piston diameter (bore).

• Compressor piston stroke.

• Number of pistons.

• Density of refrigerant vapors coming to compressor.

• High- and low-side system pressures.

Of all these factors, the density of refrigerant coming to the compressor is the one factor that service technicians have control over. Let’s look at how the density of refrigerant to the compressor determines what the mass flow rate of refrigerant through the compressor will be.

Equation 1.  Click on the equation for a larger view.

MASS FLOW RATE

The mass flow rate of refrigerant through the compressor depends on the piston displacement and how dense the gases are that are filling up the piston’s displacement. Density is measured in pounds per cubic feet of refrigerant. The volume that the piston displaces depends on the piston’s diameter, the compressor rpm, and the length of the stroke of the compressor.

 

Piston displacement is usually expressed in cubic feet per minute (cfm). This volumetric displacement is a fixed volume per revolution of the crankshaft. How many molecules of refrigerant will fill this fixed volume? It depends on how dense the refrigerant gases are coming into the piston cylinder.

Figure 1 shows a two-cylinder compressor with exposed pistons, cylinder walls, and suction valves. The volume that the piston sweeps is called its displacement.

In fact, the mass flow rate of refrigerant through the compressor is a product of the piston displacement and the density of the refrigerant filling the cylinder. (In Equation 1, the units for mass flow rate are in pounds/minute.)



How much of the piston displacement is filled by new refrigerant vapors? It depends on system pressures and valve design. (This topic will be covered in my Feb. 5 column.)

This mass flow rate of refrigerant is very important for system capacity. The service technician does have some control over the density of refrigerant coming into the compressor. The density of the refrigerant depends on its temperature and pressure. The lower the temperature of the superheated gases coming into the compressor, the denser they will be. Also, the higher pressure the superheated gases are, the denser they will be.

(This column concentrates on the temperature of the refrigerant as it enters the compressor and how it affects density. The Feb. 5 column will cover how pressures affect density of gases coming into the compressor.)



SERVICE TECHS AND SUPERHEAT

The service technician can make sure the compressor doesn’t have too much compressor superheat. The higher the compressor superheat, the hotter the refrigerant gases will be coming into the compressor. This will cause a lower refrigerant density and a lower mass flow rate of refrigerant through the compressor.

It is this mass flow rate that seriously affects compressor capacity. By taking the temperature of the suction line entering the compressor and the suction pressure at that point, and converting it to a saturation temperature, the difference between the two is the compressor superheat.

Many systems are designed today with very little superheat both at the evaporator and coming into the compressor. Many manufacturers are using 3° to 4°F of evaporator superheat to maximize the evaporator’s coil efficiency. A previous article mentioned that the compressor superheat should be at least 20° to ensure that there would never be liquid coming back to the compressor. This only applies to systems that have had a history of liquid floodback.

By adjusting the evaporator superheat with the thermostatic expansion valve, compressor superheat can also be adjusted. However, before attempting this, always consult with the compressor manufacturer and find out the ideal return gas temperature, or the maximum return gas temperature allowed for your application.

Other factors that affect how much superheat the compressor sees are liquid/suction line heat exchangers, length of the suction lines, and the ambient temperature to which the suction line is exposed.

Publication date: 01/08/2006