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"Why are you adjusting that Thermal Expansion Valve?"
To answer this question we first need to look at the main functions of the TXV. The TXV is designed to keep the evaporator as full of saturated refrigerant as possible while at the same time protecting the compressor from slugging or overheating. This is accomplished by maintaining a set superheat. The typical TXV comes factory set for 8-12 degrees of evaporator superheat, which in most cases should be fine. What is the correct setting for evaporator superheat? While it is true lower superheat will make the evaporator more efficient, we need to be more concerned with the compressor or total superheat. If we check the superheat on the suction line 6” from the compressor we should normally have at least 20 degrees of total superheat. This will ensure the compressor is not slugging with liquid even when frost is present. The suction line temperature at the compressor should not exceed 65 degrees to ensure proper compressor cooling. The last but most important reading is the discharge line temperature taken 6” from the compressor. This temperature should never exceed 220 degrees, as higher discharge temperatures will lead to compressor failures. Reducing superheat will lower both the compressor suction line and discharge line temperatures.
The TXV cannot be adjusted open or closed, it is a modulating valve. Turning the adjustment stem clockwise will only increase spring pressure causing a higher superheat. Turning the adjustment stem counterclockwise will decrease spring pressure reducing superheat. When an operating refrigeration system stops working, the TXV may have lost its power head charge or it may have become restricted, but it’s not likely that it came out of adjustment. Clean or replace the malfunctioning valve, don’t try to readjust it. When replacing a TXV, protect it from overheating while brazing as this will damage the new valve. The sensing bulb should be mounted snuggly to the suction line on the evaporator outlet at the 9:00 or 3:00 position for smaller suction lines 5/8" or less. For suction lines 7/8" and larger the bulb should be mounted at the 4:00 or 8:00 position. Adjustments may be needed on a new system at startup, especially when you have long line runs.