Service technicians will often experience frost on a suction line or on the compressor in some refrigeration applications. Thinking that liquid refrigerant must be present because there is frost is a fallacy in most cases, especially when dealing with medium- and low-temperature refrigeration. Frost accumulation simply means the moisture in the air has reached its dew point temperature and has frozen into a crystalline structure.
Frost forms because the surface temperature of the refrigeration equipment has reached 32°F. In many cases, the compressor, suction line, or a component in the suction line will be below freezing temperatures and will have frost accumulation while still operating normally.
Figure 1 shows frost coming back to a suction gas-cooled compressor. The suction filter, suction line, and end bell of the compressor are all frosted; however, in this system, the evaporator had more than 10° of evaporator superheat, and the compressor had more than 25° of superheat. Figure 2 shows a frosted suction line and compressor head of an air-cooled compressor. Again, this compressor had more than 20° of superheat coming to it, meaning liquid refrigerant cannot be present.
It is important for service technicians to differentiate between evaporator superheat and compressor superheat.
Evaporator superheat starts at the 100 percent saturated vapor point in the evaporator and ends at the outlet of the evaporator. The 100 percent saturated vapor point is the point where all the liquid has just turned to vapor and is the saturated evaporator temperature. The temperature at this point can be determined from a pressure/temperature chart once the system’s low-side pressure is known.
The evaporator outlet is where the remote bulb of the thermal expansion valve (TXV) is located. Technicians usually place an insulated thermistor or thermocouple at the evaporator outlet to get the evaporator outlet temperature. A pressure gauge at the same point as the temperature reading will give the technician the saturated vapor temperature. Most manufacturers of larger evaporators supply a Schrader fitting at the evaporator outlet close to the remote bulb of the TXV for measuring pressure.
Evaporator superheat is measured as the difference between the actual temperature of the refrigerant vapor at the evaporator outlet and the saturation temperature of the refrigerant at that same point as taken from the pressure/temperature chart (see Equation 1). Some suggested evaporator superheat amounts are shown in Chart 1.
Evaporator Outlet Temperature
- Saturated Evaporating Temperature
= Evaporator Superheat
If a technician were to measure the pressure at the compressor inlet instead of the evaporator outlet, a higher, fictitious evaporator superheat value will be read. As the refrigerant travels the length of the suction line, there will be an associated pressure drop from friction and/or restrictions caused by long runs of suction piping, oil accumulation, filters, or valves. This will cause the pressure at the compressor to be lower than the pressure at the evaporator outlet.
This higher, fictitious superheat reading may lead the technician to adjust the TXV stem clockwise (open) to compensate for the fictitious high superheat reading. This could cause compressor damage from liquid flooding or slugging from too low of a superheat setting. Therefore, it is always best to measure the pressure at the same location as you measured the temperature to exclude any system pressure drops.
There will always be a time when the evaporator sees a light heat 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.
Total or compressor superheat is all the superheat in the low side of the refrigeration system. It is all the superheat the compressor experiences. It starts at the 100 percent saturated vapor point in the evaporator and ends at the compressor inlet. Total or compressor superheat consists of evaporator superheat plus suction line superheat.
A technician can measure total superheat by placing an insulated thermistor or thermocouple about 6 inches from the compressor’s inlet on the suction line and taking the temperature. A pressure reading also will be needed at this same location. The pressure gauge reading at the same point as the temperature reading will give the technician the saturated vapor temperature once a pressure/temperature chart is used.
Compressor superheat is measured as the difference between the actual suction line temperature of the refrigerant vapor and the saturation temperature of the refrigerant arrived at from a pressure/temperature relationship.
Compressor In Temperature
- Saturated Evaporating Temperature
= Compressor Superheat
It is possible to have a TXV adjusted to control superheat at the coil (evaporator superheat) and still return liquid refrigerant to the compressor at certain evaporator low-heat loading conditions. It is recommended that all TXV-controlled refrigeration systems have some compressor superheat to ensure the compressor will not see liquid refrigerant (flooding) at low evaporator heat loadings. The TXV, however, should be set to maintain proper superheat for the evaporator, not for the compressor.
Compressor or total superheat assures there is no liquid refrigerant present at the compressor, and the saturated vapor in the evaporator has gained some sensible heat before reaching the compressor. Compressor or total superheat will be a buffer in case the TXV loses control of superheat at these low loads.
A buffer of compressor superheat will also ensure the refrigerant vapor entering the compressor is not too dense. Density vapors that are too high and enter the compressor will cause the compressor to have a higher than normal amperage draw. This will overload the compressor in many instances and could open thermal overloads. On the other hand, excessive superheat in the suction gasses and/or long periods of low mass flow rate (for example, 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. Because of this, always consult the compressor manufacturer for the maximum return gas temperature the compressor can handle to prevent overheating.
Many appliance and refrigerated case manufacturers are working 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 or compressor superheat, one may add a liquid/suction heat exchanger or even run a bit longer suction line to allow heat gains from the surrounding ambient temperature to heat the suction line. 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 of this condensation (sweat) may also occur if suction line temperatures are below 32°, and water damage can occur once the ice or frost melts. 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.
In conclusion, the only way a service technician can tell if liquid refrigerant is coming back to the compressor (floodback) is to measure the system’s total or compressor superheat at the compressor. If there ever is a time where the compressor superheat drops to zero, there now could be liquid refrigerant coming back to the compressor. Don’t be fooled by frost.
Publication date: 2/5/2018