Refrigeration and air conditioning compressors are vapor compressors, meaning they are designed to compress refrigerant vapor, not liquid refrigerant. Liquids cannot be compressed, which is why liquid refrigerant is one of the worst enemies of a compressor. The compressor is often referred to as the heart of the refrigeration system. Without the compressor, refrigerant could not reach other system components to perform its heat transfer functions.
Reciprocating compressors, along with many other types of compressors, cannot handle liquid refrigerant coming into them. Serious mechanical damage will occur to the compressor’s valve structure and drive train when liquid refrigerant enters the compressor’s cylinders or end bell. If the liquid refrigerant doesn’t do direct damage to the valve structures, it will do indirect damage to the internal drive components of the compressor when it dilutes the crankcase’s oil and degrades its lubricity.
Vapor Density
Another enemy of the compressor is very dense refrigerant vapors coming into the compressor to be compressed. Sometimes these vapors are so dense that they require tremendous amounts of energy and work to be compressed. The denser the vapors, the more mass they contain. Too much mass flow rate can often overload and stress the compressor’s motor, causing high amperage draws and overheating conditions. If the situation is severe, the compressor’s electrical motor circuit will often be electrically opened by one of its internal and/or external overloads.
To ensure that neither liquid refrigerant nor super dense refrigerant vapor enters the compressor, the proper amount of superheat has to be set at the thermostatic expansion valve (TXV). This superheat is referred to as evaporator superheat. The proper amount of evaporator superheat ensures that the compressor is not going to experience any liquid refrigerant or very dense refrigerant vapors entering it. The proper amount of evaporator superheat also keeps the evaporator active with phase-changing refrigerant.
The amount of evaporator superheat that is required for a certain application will vary. Lower temperature applications generally utilize lower evaporator superheats than medium- and high-temperature applications. The reason being is that in low-temperature applications, it is of utmost importance to keep the evaporator as active as possible throughout as much of the evaporator coil as possible. This ensures a high net refrigeration effect by filling out the evaporator as much as possible with phase-changing refrigerant.
Always follow manufacturer’s guidelines for setting evaporator superheat; however, in the absence of manufacturer's data, the below guidelines for evaporator superheat
settings can be followed:
For commercial refrigeration applications with evaporator design temperatures of 0° to 40°F, the evaporator superheat should be set between 6° and 8° F.
For low-temperature refrigeration applications with evaporator design conditions of -40° to 0°F, the evaporator superheat should be set between 4° and 5° F.
For air conditioning and heat pump applications with evaporator design conditions of 40° to 50°F, the evaporator superheat should be set between 8° and 12° F.
Superheat
Superheat is defined as any heat added to completely saturated vapor that results in a rise in temperature (sensible heat change) of the vapor. Superheat on the system’s low side can be divided into two types: evaporator superheat and total (compressor) superheat.
Evaporator superheat starts at the 100 percent saturated vapor point in the evaporator and ends at the outlet of the evaporator (see Figure 1). It is this superheat that the technician must set at the TXV to protect the compressor. A service technician can put a temperature measuring device at the evaporator outlet to obtain the evaporator outlet temperature. Using a pressure/temperature relationship, a pressure gauge at the same point as the temperature reading will give the technician the saturated vapor temperature. The difference between these two temperatures is the evaporator superheat.
FIGURE 1: Superheat is defined as any heat added to completely saturated vapor that results in a rise in temperature (sensible heat change) of the vapor. Courtesy, ESCO Press
Example:
R-134a system
Pressure reading at evaporator outlet is 25 psig or 29°F (refer to a R-134a pressure/temperature relationship chart)
Evaporator outlet measured temperature is 35°F
Evaporator superheat calculation:
35°F (evaporator outlet temperature) – 29°F (saturation temperature) = 6°F (evaporator superheat)
There will always be times when the evaporator sees a lightened heat load and the TXV may lose control of its evaporator superheat due to limitations of the valve and to system instability or system problems. TXVs often lose control of evaporator superheat at low evaporator heat loads, which can be caused by many different
situations, including:
Defrost circuit malfunction causing evaporator coil icing;
Low air flow across evaporator coil;
Evaporator fan motor not operating;
Iced up or dirty evaporator coil;
Dirty filter before evaporator coil;
End of the refrigeration cycle; or
Low on refrigerant charge.
Any time the evaporator coil sees a heat load lower than what it is designed to see, a TXV can lose control and hunt. Hunting is nothing but the valve overfeeding and then underfeeding, trying to find itself. Hunting occurs during periods of system unbalance (low heat loads) when temperatures and pressures become unstable. The TXV tends to overfeed and underfeed in response to these rapidly changing values until the system conditions settle out and the TXV can stabilize. It is this overfeeding condition that hurts compressors. An evaporator superheat setting that is too low also causes the TXV to hunt, which is where total superheat comes into play.
Total superheat is all the superheat in the low side of the refrigeration system. It starts at the 100 percent saturated vapor point in the evaporator and ends at the compressor inlet, see Figure 1. It is sometimes referred to as compressor superheat. Total or compressor superheat consists of evaporator superheat plus suction line superheat. A technician can measure total superheat by placing a temperature measuring device on the suction line about 6 inches from the compressor inlet and taking the temperature. A pressure reading will also be needed at this same location for a pressure/temperature relationship in getting the saturation temperature.
Example:
R-134a system
Low-side pressure at compressor is 20 psig or 23°F (refer to a R-134a pressure/temperature relationship chart)
Compressor inlet measured temperature is 50°F
Total superheat calculation:
50°F (inlet compressor temperature) – 23°F (saturation temperature) = 27°F (total superheat)
In the above example, it is possible to have a TXV that is adjusted to control superheat at the coil (evaporator superheat) and still return liquid refrigerant to the compressor at certain low load conditions. If so, the conditions causing the refrigerant floodback should be found and corrected. It is recommended that all TXV-controlled refrigeration systems have some compressor superheat to ensure that the compressor will not see liquid refrigerant (flood or slug) at low evaporator loads. The TXV, however, should be set to maintain proper superheat for the evaporator, not the compressor.
When setting evaporator superheat at the TXV, make sure the system has stabilized to its designed refrigerated space temperature; otherwise, meaningless superheats will be read. Don’t expect a TXV to hold proper superheat under high evaporator heat loadings. Under these high heat loadings, evaporator superheat readings are sure to be high. As mentioned before, it is of utmost importance to always wait for the system to pull down to the design refrigerated space temperature before taking an evaporator superheat reading.
Compressor Types
Air-cooled compressors are more vulnerable to slugging and valve damage because the returning suction line gases are not heated by the motor windings. The gases enter the sidewall of the compressor and go directly to the valves. Compressor or total superheat will be a buffer in case the TXV loses control of superheat at these low evaporator heat loads. However, the evaporator superheat must still be maintained by the above guidelines. A buffer of compressor superheat will also make sure that the refrigerant vapor entering the compressor is not too dense.
As mentioned earlier, very dense vapors entering the compressor will cause the compressor to have a higher-than-normal ampere draw. This will overload the compressor in many instances and open thermal overloads.
On the other hand, excess suction gas superheat and/or long periods of low mass flow rate (e.g., an unloaded compressor or variable capacity compressor at lower capacities) can result in insufficient cooling of the rotor, motor, and stator and open the internal protectors.
Always consult the compressor manufacturer for the maximum return gas temperature the compressor can handle to prevent compressor overheating. Many appliance and refrigerated case manufactures work with compressor manufacturers to determine the optimum amount of compressor superheat so as not to overheat compressors. The proper amount of compressor superheat will ensure a cool running compressor and at the same time, ensure the proper density suction gasses for good capacities.
Remember, the TXV controls evaporator superheat. To obtain more total superheat, add a liquid/suction heat exchanger, or even run a bit longer suction line to allow heat gains from the surrounding temperature to heat the suction line. It is not recommended to take the insulation off of the suction line to increase total superheat, as this will cause the suction line to sweat from water vapor in the air reaching its dew point on the suction line. Freezing of this condensation may also occur if suction line temperatures are below 32°F, then water damage can occur.
If at all possible, do not sacrifice (raise) evaporator superheat to get the amount of total superheat needed. This will not maintain an active evaporator, and system capacity will suffer. Suction line accumulators are often employed on systems for added protection. This will help ensure that all of the refrigerant entering the compressor is liquid free, and this will also help keep a fully active evaporator.
