Bryan does an in-depth review of how to adjust a thermostatic expansion valve, also known as a TXV, TEV, or TX Valve. He explains why one would adjust a TXV and how to tell if a TXV is working.
You almost never need to adjust a TXV unless you're trying to prevent floodback or fixing a TXV that was adjusted by someone else. Adjustments should be very slow, and it's unwise to do an adjustment without a valid reason to do so.
The TXV has a few different sources of force. The bulb exerts an opening force according to the temperature of the suction line, as the TXV tries to maintain a constant superheat. So, the bulb needs to be mounted to the suction line sufficiently, and the powerhead needs to maintain enough refrigerant charge. (Some powerheads are replaceable, and others are not.) The internal or external equalizer also applies a closing force to counter the opening force. Spring force also exerts a closing force to assist the equalizer's closing force.
For a TXV to work properly, it needs to have a full line of liquid going into it. Therefore, you need to make sure that you have sufficient subcooling or a full sight glass. Before condemning or adjusting a "bad TXV," be sure to check your superheat with probes or clamps to ensure that you don't have a different problem. (It's also worth mentioning that evaporator and compressor superheat can vary quite wildly, especially if the suction line is very long.) TXVs also don't set superheat at a fixed temperature; there is a range based on load conditions. You also don't want to adjust the valve so that it's too far open, as that can cause floodback.
We have to adjust TXVs based on evaporator superheat, and the bulb should be insulated and well strapped to the top or side of the suction line. When you're ready to adjust the TXV, remove the stem cap on the bottom to expose the spindle.
It's easy to make the adjustment if you make a mark at the edge of the spindle. That way, you can keep track of how many turns you make. Many manufacturers recommend turning the spindle only a quarter-turn at a time. Turning the spindle clockwise compresses the spring, which allows less refrigerant to enter the evaporator coil and increases the superheat. Turning the spindle counterclockwise has the opposite effect (less closing force).
Balanced port TXVs help balance the inlet pressure; refrigerant flowing in through the inlet of the valve also affects the valve, and the balanced port tries to prevent it from having as much of an impact. Balanced port TXVs make the operation more stable over a wide range of load conditions.
Hard shutoff or non-bleed TXVs shut when the system turns off. These TXVs are a bit more likely to require a hard start kit in single-phase residential applications.
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9 ноя 2019
