Air Conditioning - What does the Thermal Expansion Valve really do?

If an A/C system is properly charged and, while running and cooling, a qualified person attempts to add refrigerant, does the TXV act in a normal way to prevent more refrigerant from being added to the system since the system is properly charged and working fine? Or, if refrigerant can't be added in this scenario, does it sound reasonable that something such as the TXV is preventing refrigerent from being added.

I posted this question in a much different format earlier with no success. I thought I would try again in a to the point question. Just trying to resolve two conflicting recommendations.

Thanks

A TXV controls or "meters" the amount of liquid refrigerant that flows into the evaporator based on the temperature of a sensing bulb that is usually (but not always) located on the suction line where it leaves the evap coil. A txv that is stuck open will allow too much freon into the coil resulting in a somewhat higher suction line pressure and little or no superheat.

In response to your questions, if I understand them correctly:

A system can still be overcharged if the txv is working properly. It is not a device that prevents someone from adding too much freon.

If a txv is not working correctly and is "wide open", it may in some cases (due to higher suction line pressure) hinder the addition of more freon. However, a good technician would know if the system charge is incorrect or if the txv is stuck open by looking at the subcooling and superheat numbers. Charging of this type system is done by checking subcooling and adjustment of the txv (if adjustable) is verified by the degrees of superheat.

If on the other hand the txv is stuck closed, the suction line pressure will be too low, the high side pressure will also be low, but the superheat will be too high. To an untrained eye, the system will "look" undercharged. At this point an inexperienced person will add a whole bunch of freon in hope of bringing up the suction pressure. This will result in an overcharged system with low suction pressure and high side pressure that is now much too high.

In a nutshell, what jim-connor said.

Many different types of TXVs. The temperature differential of the evaporator inlet and outlet typically determine the opening and closing of the TXV seat to either add (flood) refrigerant flow or constrict (starve) refrigerant flow to the evaporator. Refrigerant levels will affect the TXV if considerably out of range as the TXV is limited in opening size but it is much more tolerant of a charge level off a small amount. Keep in mind that the refrigeration process is incredibly simple. When the refrigerant changes state from liquid to gas its ability to absorb heat (latent heat)without increasing its sensible (measurable) heat increases exponentially. The TXV cannot detect latent heat but only measurable heat so what it is looking for is when the refrigerant enters the evap in its liquid/vapor mix form and absorbs all the latent heat it can, as it turns (boils) into a pure vapor. It then begins to absorb measurable heat (superheat) so the TXV is always trying to make sure just the right amount of refrigerant based upon sensible heat is realeased into the evap to completely absorb all the heat it can and have just a touch of superheat to prove that all latent heat has been absorbed plus this makes the vapor less likely to turn back into a liquid before reaching the compressor and damaging the compressor valves. If the TXV starves the evap the liquid boils completely off before reaching the end of the evap tubes resulting in high superheat and poor cooling since very little of the coil is being utilized. If the TXV floods the evap the liquid never boils off and absorbs the latent heat as it should, resulting in no superheat and of course poor cooling. As you can see a properly operating TXV controlled system is far superior to the fixed orfice types. You will find most of the fixed orfice systems are 10 or less SEER
Former Member apologizes for the book but man this subject really interests him LOL

I am redoing my sister's basement and I found a thermal expansion valve on the cold water line that tee's off the exterior hose line. What would be the purpose of this? Pressure build up from the outdoor heat? I can't imagine there would be that much pressure to require this. Anyone know? Thanks.

Sure it is not a pressure reducing valve?, where full city water pressure (say 80-100 psi) is allowed to go to the spigot, but reduces for everything in the house?

I agree with Jim also.

This article maybe a bit technical but might be educational.
http://www.emersonclimate.com/flowcontrols/pdf/2006FC-174R1TXV%20re-article.pdf