Ice Breaker: The Hot Pull Down

December 5, 2011
KEYWORDS superheat
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While troubleshooting or installing refrigeration systems using a thermostatic expansion valve, it is a relatively common task to measure the superheat value of the refrigerant leaving the evaporator. This could be a very useful measurement to determine how a system is operating.

However, there is a major consideration when using this measurement to analyze the operation of a system. If the heat load on the evaporator is very high — in other words, if the air temperature entering the evaporator is very high — the refrigerant’s superheat value leaving the evaporator will be high. The thermostatic expansion valve will not be able to feed enough refrigerant into the evaporator.

Under this condition, the thermostatic expansion valve will be open to its maximum port size, and it will have reached its limit of how much refrigerant it can feed into the evaporator. This is assuming that the pressure drop and temperature of the liquid entering the valve remain constant. This is not an indication of a system problem, but simply a system working outside its design range. Do not interpret this as an issue and attempt to reduce the refrigerant’s superheat value. Trying to adjust the expansion valve, adding refrigerant to the system or some other change is not the answer. In fact, it can (and more than likely will) cause a problem.

For accurate superheat calculations, the case (return air) temperature should be within 5°F of its design conditions. If the case temperature is initially high, this will require running the system for a period of time and allowing it to drop before relying on this measurement as a system check.

So, as with many aspects of our trade, this measurement needs to be taken using some common sense and a little knowledge. If the refrigerant’s superheat leaving the evaporator is high and the case is hot, wait to make a decision as to whether this is a problem. If the superheat leaving the evaporator is high and the case is within 5° of its normal design temperature, there is an issue. If the refrigerant’s superheat value is lower than normal regardless of the return air temperature, there is a problem.

On larger cases or cases packed with product, if the case temperature is high, it will take time for it to drop. This may present a problem for you. Do you wait for the temperature to drop, or do you make a return visit, or do you simply not need to even take this reading? It’s normally best to take this measurement, but it is not always practical. So, again, common sense must prevail. If it is a new installation or if the repair you made requires taking this measurement, you need to wait. However if the repair you made does not really relate to taking this measurement, you don’t need to wait.

Understanding this relationship can help you avoid creating additional problems and wasting time on a job, which is definitely good for you and your customer.

Publication date: 12/05/2011

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