Thermostatic Expansion Valve TXV TEV

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The Job of the Thermostatic Expansion valve is to open and close to maintain a steady amount of superheat in the evaporator.
#1 By maintaining the superheat we have the correct amount of refrigerant changing state in the evaporator to absorb heat for the best operation even with changing conditions.
#2 This also helps protect the compressor from refrigerant floodback but it is limited in how much it can close.

#3 Protect the compressor for high superheat and overheat.

The TXV can only do this if 

A: We have the proper airflow across the evaporator. Often airflow problems lead to misdiagnosed TXV’s. We really need a measured airflow reading. The hand in front of the grill saying “its good” is not. Again CFM is a number that needs to be measured.

B: We provide the TXV with a full column of liquid. In air conditioning this is where subcooling becomes so very important. It ensures we are feeding it enough refrigerant to do its job. Even if we have enough subcooling at the outdoor unit, the liquid line in a hot attic can heat the refrigerant and loose subcoolings. Also restrictions such as a clogged liquid line drier, kink or even the screen before the TXV can become clogged. This restriction often leads to misdiagnosed TXV’s. In a perfect world we would have a service port before the TXV or at least a sight glass before every TXV to ensure we have a full column of liquid. This is called a clear sight glass meaning no vapor bubbles.
If a system has a liquid receiver, It will generally not have subcooling relying on the sight glass and never overfilling the recovery more than 80% during pump down.
I have seen many TXV misdiagnosed because they were simply low on charge, had a restriction or liquid line ran through a hot attic. 

C: It is protected from heat

Often the TXV is damaged during installation by overheat during brazing. This can be from the sensitive sensing bulb overheating and damaging the power heat of overheating the valve body and damaging the moving parts and spring inside. This does not immediately kill it but causes long term damage that may show up a few weeks to a few years later. Protect the TXV from heat!

D: Its protected from vibrations. Often the transmission tube (capillary tube) is left to rub against other components. Once the isolated charge is compromised from the power head to the sensing bulb, the TXV will no longer operate.

E: Its properly installed.
The location of the sensing bulb is critical and it varies by manufacturer. The sensing bulb must be mounted metal to metal and the proper position on the suction line. The equalization tube must also be in the correct location and secure.

F: Kept clean from contaminated
This includes

1 Flowing nitrogen while brazing to prevent oxidation that can buildup and block the operation

2 A proper deep vacuum (dehydration) and decay test remove microscopic moisture. thi s moisture alone can cause issues inside the TXV but it also turns to acid that eats copper form the system and the copper flakes can cause that can clog the TXV

3 Clean lines. Copper shavings, foreign objects and debris can clog the moving parts. Using a line set cleaner such as a “pig” can push these out of the system before charging. 

4 Flux and soft solder both often clog TXV operation. Flux is an acid and the overfeed of solder creates balls and slag that stop the TXV up.
5 additives. Some “leak fix” additives can cause issues inside the TXV

6  Additives form manufactures. One compressor manufacturer used a rust inhibitor that did not react well to POE oil that caused TXV to stick. This gaveTXV’s a very bad reputation.

1st, you must ensure proper airflow across the evaporator coil

2nd provide the TXV with enough subcooled liquid.
3rd Check superheat to see if the TXV is doing its job. Some can be adjusted, while others will have to be opened to inspect the screen or replaced. 

Multiple links below in videos and articles with much more details of TXV operation.