Measuring Superheat
Measuring the amount of superheat allows the technician to determine if all refrigerant has boiled off in the evaporator and to measure approximately the efficiency of coil operation. It can also be a measurement of the margin of safety against flooding the compressor with liquid refrigerant.
The four steps in calculating superheat are as follows:
1. Measure the suction pressure with gauges.
2. From the pressure, determine evaporator temperature using a P/T chart.
3. Measure the temperature of the suction line at the outlet of the evaporator.
4. Subtract the evaporator temperature from the suction line temperature.
Because evaporator temperature is determined from suction pressure, accuracy depends on how physically close to the evaporator the pressure reading is taken. An excellent reading can be taken by installing a tee in the TEV external equalizer line or at the outlet of the evaporator.
Example of superheat calculation on a 35°F walk-in refrigerator:
35°F (at the coil outlet or TEV bulb) 2 25°F (evaporator temperature) 5 10°F superheat.
The amount of evaporator superheat designed into a system varies from as much as 15°F for some AC systems to as low as 3°F for most ice machines. As a rule of thumb, 10°F is usually adequate for commercial refrigeration systems. However, for optimum effi ciency, the technician should check with the equipment manufacturer.
The technician should know which type of superheat measurement is recommended by the factory. An evaporator manufacturer might specify that most of its refrigeration evaporators are designed with a 10°F superheat and that it is calculated by using the suction line temperature at the evaporator outlet. However, a compressor manufacturer may recommend a 20°F or 30°F compressor superheat. This is the total of the superheat in the evaporator plus the sensible heat picked up in the suction line. The compressor superheat, or total superheat, calculation is determined by taking the suction line temperature 6 inches from the compressor inlet, not at the outlet of the evaporator.
Compressor manufacturers want to make sure there is enough superheat at the compressor inlets to ensure that the compressors will not be damaged by liquid fl ooding from the evaporator.
An example of pressures and temperatures on the low side of a refrigeration system. Courtesy of Refrigeration
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