The capacity of a compressor is measured in Btu per hour (Btuh) or Btu per minute (Btu/min). Remember, there are 12,000 Btuh or 200 Btu/min in one ton of refrigeration. The capacity of a piston-type reciprocating compressor is governed by many factors, including the following:

• Number of cylinders in the compressor;
• Density of refrigerant vapors entering the compressor’s cylinder;
• High- and low-side system pressures;
• Compressor motor’s speed (RPM);
• Piston’s bore or diameter; and
• Piston’s stroke.

Let’s look at how the density of refrigerant entering the compressor’s cylinders affects the mass flow rate of refrigerant through the compressor.

Flow Rate of Refrigerant

The mass flow rate of refrigerant through the compressor depends on the piston displacement and how dense the gases are filling up the piston’s displacement volume. As mass flow rate goes up, so does system capacity. The units for mass flow rate are in pounds/minute, while the density of refrigerant vapors is measured in pounds per cubic feet.

The volume that the piston displaces depends on the piston diameter, the RPM of the compressor, and the length of the stroke of the piston. 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 that will fill this fixed volume depends on how dense the refrigerant gases are entering the piston’s cylinder.

The volume that the piston sweeps in one revolution is 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 entering or filling the cylinder. The units for mass flow rate are in pounds/minute. It is this mass flow rate of refrigerant that is so important for system capacity:

The service technician does have some control over the density of refrigerant coming into the compressor.

Density and Superheat

The density of the refrigerant vapor depends on its temperature and pressure. The lower the temperature of the superheated gases entering the compressor’s cylinder, the denser it will be. Also, the higher the pressure of these superheated gases, the denser they will be.

The refrigeration system’s pressures can dictate how much refrigerant flow rate will flow through the system. If the pressure of the suction line that delivers refrigerant gas to the compressor cylinders is high, the density of refrigerant vapors will be high and the mass flow rate of refrigerant will be high. On the other hand, if the pressure of the suction line is low, the density of the refrigerant vapors will be lower and the refrigerant flow rate will be lower.

Any time a fixed volume (compressor’s cylinder) is filled with a higher pressure, more refrigerant gas molecules will be present, causing a higher refrigerant density. 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.

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.

Service technicians need to make sure that the compressor does not have too much evaporator and compressor superheat. To determine compressor superheat, technicians should take the temperature of the suction line entering the compressor and the suction pressure at that point and convert it to a saturation temperature. The difference between the two will be the compressor superheat.

Many systems today are designed with very little superheat 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. By adjusting the evaporator superheat with the thermostatic expansion valve (TXV), compressor superheat will also change. However, before attempting this, always consult with the compressor manufacturer and find out what is the ideal compressor return gas temperature or what is the maximum return gas temperature allowed for that particular compressor application. Other factors that affect compressor superheat include:

• Length and insulation of the suction lines,
• The ambient or surrounding temperature the suction line is exposed to, and
• Liquid/suction line heat exchangers.

Actual Compressor Capacity

Both the high- and low-side system pressures can be expressed as a ratio called a compression ratio, which is the absolute discharge pressure divided by the absolute suction pressure:

Most service technicians realize that their service gauges read zero when not connected to a system, even though there is a pressure of approximately 15 psi on the gauges exerted from atmospheric pressure. These gauges are calibrated to read zero at atmospheric pressure. Therefore, in order to use the true or "absolute" discharge and suction pressure at zero gauge pressure or above, a technician must add 14.696 psi — or approximately 15 psi — to the gauge reading.

When referring to absolute pressure, psia is used to label the pressure's magnitude, and psig labels the pressure's magnitude when referring to gauge pressure. The following example shows a compression ratio calculation for pressures above or equal to zero gauge pressures:

A compression ratio of 8 to 1 is expressed as 8:1 and simply means that the discharge pressure is eight times the magnitude of the suction pressure.

Compression Ratio

Now that we know how to calculate a compression ratio, let's dig a bit deeper into what that physically means when applied to a cooling system.

In reciprocating compressors, there has to be some clearance space between the piston at top dead center and the valve plate, otherwise there would be a collision of the two. This intentionally designed clearance volume — or clearance pocket — traps a certain amount of refrigerant vapor after the discharge valve closes. Even though compressor manufacturers are reducing the amount of clearance volume found between the valve plate and piston head, some clearance will always remain.

The clearance volume gas pressure is assumed to be at the discharge pressure if we ignore valve weight and valve spring forces. The vapor left in the clearance volume has been compressed to the discharge pressure. Once the down stroke of the piston starts, this same clearance volume vapor has to be re-expanded to just below the suction pressure before the suction valve can open and let new vapors into the cylinder. The piston, however, will have already completed part of its suction stroke, and the cylinder will already have been filled with re-expanded clearance vapors from the clearance volume before new vapors enter.

These re-expanded clearance volume vapors take up valuable space that new suction vapors coming from the suction line cannot occupy. Hence, suction vapors from the suction line will fill only part of the cylinder volume that is not already filled with re-expanded discharge gases. Therefore, the total volume of the piston's cylinder is not completely utilized in taking in new refrigerant gases, and the system is said to have a volumetric efficiency.

The volumetric efficiency is expressed as a percentage from 0 to 100 percent, depending on the system in question. Volume efficiency is defined as the ratio of the actual volume of the refrigerant gas pumped by the compressor to the volume displaced by the compressor pistons. A high volumetric efficiency means that more of the piston's cylinder volume is being filled with new refrigerant from the suction line and not re-expanded clearance volume gases. The higher the volumetric efficiency, the greater the amount of new refrigerant that will be introduced into the cylinder each down stroke of the piston, and thus more refrigerant will be circulated each revolution of the crankshaft. The system will then have better capacity and a higher efficiency.

On the other hand, the lower the discharge pressure, the less re-expansion of discharge gases to suction pressure. Also, the higher the suction pressure, the less re-expansion of discharge gases, because of the discharge gases experiencing less re-expansion to the higher suction pressure and the suction valve will open sooner. How much of the piston displacement is filled by new refrigerant vapors depends on system pressures and valve design.

A service technician can control, to a certain extent, how high or low the discharge and suction pressure will go. If the discharge (condensing) pressures can be kept low and the suction (evaporating) pressure can be kept as high as possible without affecting the refrigerated product temperature, the compression ratio will be low and the volumetric efficiency will be high. This will cause a higher mass flow rate of refrigerant to flow through the compressor and increase the capacity of the system.

Here are some of the causes for high head (condensing) pressures:

• Recirculated air over the condenser;
• Undersized condensing unit;
• High ambient;
• Non-condensable (air) in the system;
• High case humidity or heat load;
• Dirty condenser;
• Overcharged system; and
• Condenser fan out.

Here are some of the causes for low suction (evaporating) pressures:

• Not enough programmed defrosts;
• Defrost heater faulty;
• Undercharge of refrigerant;
• Low case heat load;
• High humidity load on coil causing excessive frost;
• Evaporator fan out;
• Iced evaporator coil;
• Dirty evaporator coil; and.
• Defrost time clock faulty

In conclusion, keeping the refrigerant return gas to the compressor as dense as possible will increase system capacity. Also, keeping suction pressures as high as possible without sacrificing product temperature and keeping the head pressure as low as possible will increase system capacity.