Lower-temperature refrigeration applications often experience frost on their suction lines, compressor end bell, and/or part of the compressor head. When the air’s dew point temperature is reached from coming in contact with the cold suction line, compressor end bell, or compressor head, water vapor in the air is cooled below its dew point temperature and will condense into liquid. When this condensed liquid reaches 32°F, it will freeze into frost. Frost is simply condensed water vapor or dew that has reached 32°F and has frozen.
A fallacy that many service technicians believe is that if there is frost on the system’s suction line, compressor end bell, or the compressor head, there is liquid refrigerant reaching the compressor. All frost means is that the suction line or compressor is below freezing and that the moisture in the air has reached its dew point temperature, condensed, and then froze. If technicians experience this, there is no need for alarm, as long as there is proper compressor superheat protecting the compressor from liquid flooding or slugging.
Flooding and Slugging
Flooding is liquid refrigerant coming back to the compressor’s crankcase during a run cycle. Slugging is liquid refrigerant or oil actually entering the compressor’s cylinders and/or valve arrangement and being pumped by the compressor. In order for flooding or slugging to occur, the compressor has to be experiencing no superheat. In other words, the temperature coming into the compressor would be the same as the evaporator temperature. This would indicate that there was no compressor superheat and that liquid refrigerant was entering the compressor. In either scenario, whether the system does or does not have compressor superheat, the suction line, end bell and/or compressor head would still be frosted. This is why it is of utmost important for service technicians to measure superheat at both the evaporator and compressor to make sure the compressor is protected from flooding and/or slugging.
One of the worst enemies of a compressor is liquid refrigerant, because liquids cannot be compressed. Refrigeration and air conditioning compressors are vapor compressors, meaning they are designed to compress refrigerant vapor, not liquid refrigerant. The compressor is often referred to as the heart of the refrigeration system. Without the compressor as the refrigerant vapor pump, refrigerant could not reach other system components to perform its heat transfer functions.
A reciprocating compressor, 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.
It is important for service technicians to understand the difference between suction gas-cooled and air-cooled compressors. In an air-cooled compressor, the suction return gas does not pass over the windings of the compressor; the return gas simply enters the compressor through the suction service valve on the side of the compressor. This gas enters the suction valve and cylinders right away without seeing any other heat source. If there is any liquid (refrigerant or oil) entrained in this suction gas, the valves and/or pistons/rods themselves can be seriously damaged.
This is not the case for refrigerant gas-cooled compressors, where liquid refrigerant coming back to the compressor must first pass around or through the motor windings. There is a good chance that the windings will be producing enough heat to vaporize any liquid refrigerant before it is sucked up through the suction cavities to the valve structures. Refrigerant must travel in close proximity to the motor windings before it flows uphill and enters the compressor’s valve structures and cylinders.
The only way a service technician can tell if liquid refrigerant is coming back to the compressor is to measure the compressor superheat. This can be accomplished by taking the evaporating pressure with a gauge set and converting it to a temperature with a pressure/temperature chart. Next, with a thermometer or thermistor, measure the compressor inlet temperature on the suction line about 6 inches from the compressor inlet. The compressor inlet temperature should always be warmer than the evaporating temperature; if it is at the same temperature or colder, liquid refrigerant is present at the compressor.
To figure out compressor superheat, subtract the evaporating temperature from the compressor inlet temperature:
Compressor inlet temperature - Evaporating temperature = Compressor superheat
For example, an R-134a system has low-side pressure at the compressor of 20 psig or 23°F (refer to an R-134a pressure/temperature relationship chart). The compressor inlet measured temperature is 50°F.
Total superheat calculation:
50°F (measured compressor in temperature) - 23°F (saturation temperature) = 27°F (total superheat)
In this example, the total superheat is 27°F. 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 (flooding or slugging) at low evaporator loads. The TXV, however, should be set to maintain proper superheat for the evaporator, not the compressor. And, 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, because in this circumstance, evaporator superheat readings are sure to be high. Again, 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.
Another enemy of a compressor is very dense refrigerant vapors coming into the compressor. Sometimes these vapors are so dense that they require tremendous amounts of energy and work to be compressed. The more dense 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. Often, if the situation is severe, the compressor’s electrical motor circuit will be electrically opened by one of its internal and/or external overloads.
To ensure that neither liquid refrigerant nor too dense refrigerant vapor enters the compressor, the proper amount of superheat has to be set at the 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, the evaporator must be kept active 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, these 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; and
- 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.
For example, in an R-134a system the pressure reading at evaporator outlet is 25 psig or 29°F (refer to a R-134a pressure/temperature relationship chart). The 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 the following:
- Dirty filter before evaporator coil;
- End of the refrigeration cycle;
- Low on refrigerant charge;
- Defrost circuit malfunction causing evaporator coil icing;
- Low air flow across evaporator coil;
- Evaporator fan motor not operating; or
- Iced up or dirty evaporator coil.
Anytime the evaporator coil sees a heat load less than what it is designed to see, a TXV can lose control and hunt. Hunting is nothing but the valve overfeeding and then underfeeding in 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. This is where total superheat comes into play.