Question: What is the minimum pressure drop across a TXV valve that still allows it to function properly?

-John West Jr., Richmond, Va.

Answer: In a theoretical world, a thermostatic expansion valve will operate with only one-pound pressure drop across it. The thing is refrigeration systems do not operate in the theoretical world, but the real world.

One of the most misunderstood issues in the refrigeration industry is how to correctly size an expansion valve. Expansion valve nominal capacities are listed per an ARI standard that states HCFC-22 valves will be rated at 40°F for the evaporator with 100°F liquid entering the valve and 100 pound pressure drop across the valve.

A typical air-cooled air conditioner installed today will operate with closer to a 150-pound pressure drop across the expansion valve. The 50-pound increase in pressure drop will increase the capacity of an Alco ANE 3-ton expansion valve from3 tons to 3.7 tons. The 0.7 ton could be enough capacity increase to cause a hunting or liquid flooding problem.

Another and even more serious expansion valve sizing problem is caused by matching the valve to compressor horsepower or capacity. A couple of examples are a walk-in cooler with two evaporator coils. Should you size your expansion valve by compressor capacity it would be two times too large. Another example is a 3 horsepower compressor operating at -30°F evaporator. At this condition, this compressor is only rated at approximately 13, 000 Btu. If you installed a 3-ton expansion valve, it would be greatly oversized.

The only way to correctly size a thermostatic expansion valve is match its capacity to the evaporator capacity using the following information: evaporator capacity in Btuh/ton, refrigerant type, pressure drop across valve, liquid refrigerant temperature entering valve, and evaporator temperature S.S.T.

-Hugh Redmon, application engineer, Alco Controls