Superheat is a measured value. It is the difference between two temperatures. Superheat is measured as the difference between the actual temperature of the refrigerant vapor and the saturation temperature of the refrigerant at that same point. Superheat on the system's low side can be divided into two types: evaporator superheat and total (or compressor) superheat.
The evaporator superheat calculation would be as follows: The evaporator outlet temperature (30 degrees) minus the saturation temperature at the evaporator (23 degrees) equals the evaporator superheat (7 degrees).
This higher, fictitious superheat reading may lead the technician to adjust the TXV stem clockwise (open) to compensate for the erroneously high superheat reading. This could cause compressor damage from liquid flooding or slugging from too low of a superheat setting.
In this case, the evaporator superheat calculation would be: Evaporator outlet temperature (30 degrees) minus saturation temperature at compressor inlet (15 degrees) equals degrees superheat (15 degrees).
The superheat changed from 7 degrees to 15 degrees simply by reading the pressure at the compressor inlet instead of the evaporator outlet. This correct evaporator superheat would be 7 degrees. It is best to measure the pressure at the same location as you measured the temperature to exclude any system pressure drops.
However, suction line accumulators are often employed on these systems for added protection. This will help ensure the entire refrigerant entering the compressor is free of liquid. This will also help keep a fully active evaporator. Lower temperature applications generally utilize lower evaporator superheat. Please consult the case manufacturer if in doubt. In the absence of manufacturer's data, a chart such as the one shown in Table 2 shows guidelines for superheat settings.
There will always be a time when the evaporator sees a light load and the TXV may lose control of its evaporator superheat due to limitations of the valve and to system instability. This is where total superheat comes into play.
Sometimes referred to as compressor superheat, total superheat consists of evaporator superheat plus suction line superheat. A technician can measure total superheat by placing a thermistor or thermocouple at the compressor's inlet and taking the temperature. A pressure reading will also be needed at this same location.
The total superheat calculation is as follows: Degrees compressor in temperature (50 degrees) minus saturation temperature (23 degrees) equals total superheat (27 degrees).
In the above example, the total superheat was calculated to be 27 degrees. 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 floodback should be found and corrected. It is recommended that all TXV-controlled refrigeration systems have at least 20 degrees of compressor superheat to ensure that the compressor will not see liquid refrigerant (flood or slug) at low evaporator loads. Total superheats from 20 degrees to 30 degrees are recommended to ensure adequate compressor cooling and preventive liquid control to the compressor. The TXV, however, should be set to maintain proper superheat for the evaporator.
Air-cooled compressors are more vulnerable to slugging and valve damage because the suction gases are not heated by the motor windings. The gases enter the sidewall of the compressor and go directly to the valves. The 20 degrees of compressor superheat will be a buffer in case the TXV loses control of superheat at these low loads.
However, the evaporator superheat must still be maintained by the guidelines in the chart shown. A buffer of 20 degrees to 30 degrees of compressor superheat will also make sure that the refrigerant vapor entering the compressor is not too dense. Vapors at too high of a density entering the compressor will cause the compressor to have a higher-than-normal amp 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), can result in insufficient cooling of the stator and open the internal protectors.
It is not recommended to take the insulation off of the suction line to increase total superheat. This will cause the suction line to sweat from water vapor in the air reaching its dew point on the suction line. Freezing at this condensation may also occur if suction line temperatures are below 32 degrees. Water damage can occur.
TXVs often lose control of evaporator superheat at evaporator loads. Low evaporator loads can be caused by many different situations.
Causes for low load conditions on evaporator coils include:
Hunting occurs during periods of system unbalance (e.g., low 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. Too low evaporator superheat setting also causes the TXV to hunt.
In conclusion, a total superheat of at least 20 degrees can prevent the compressor from seeing any liquid refrigerant.
Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at email@example.com.
Publication date: 06/07/2004