Inside the refrigeration evaporator, refrigerant is changing state from a liquid to a vapor (boiling), at a pressure and temperature that's about 15° to 20° below the desired final temperature of the space or product being cooled.
The system is designed so that the refrigerant will be almost totally vaporized by the combination of the pressure drop through the metering device and the heat load being absorbed from whatever the system is cooling.
In a medium temperature reach-in or walk-in refrigerator, evaporating temperature at or near design conditions should be about 20° to 25°.
In a reach-in or walk-in freezer; evaporating temperature at or near design conditions should be about -10° to -20°.
With both medium temperature and low temperature fan coil evaporator units, my experience has been that once the box has cooled to within about 10° of desired temperature, a temperature difference of 4° to 6° between entering air and leaving air is normal.
If the temperature difference is less than 3° or more than 10°, check the refrigeration evaporator coil and fan, and check all of the system's operating characteristics.
System Evaluation Manual
has a cycle diagram and guidelines for evaluating pressures and temperatures of air conditioning and refrigeration systems.
If you need to replace a deteriorated refrigeration evaporator, the easiest way to determine what size you need will be to use the model number off of the old one.
If there is no model number on the evaporator, see if there's a model number on the expansion valve (if it uses an expansion valve).
This will tell you the capacity of the coil.
If there is no legible information anywhere on the coil; measure the dimensions of the coil, measure the outside diameter of the coil piping, measure the spacing between the fins, and count the piping passes through the coil.
These measurements, and some other miscellaneous information, will help the distributor or manufacturer determine what size your replacement refrigeration evaporator should be.
I hope this page has helped, and please, feel free to contact us with any specific HVAC questions you might have, including questions about refrigeration on Guam, and air conditioning on Guam.
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