TXV hunting

[dcsac1]

Hello, I have a Dunham bush vertical screw compressor, with a sub cooler, that feeds a air handler with a VFD. The air handler has a DX coil with six circuits, and three liquid line solenoids for capacity control. The problem I have is that when the building gets close to full load conditions the txv hunts so bad that we loss capacity and then the building. The suction pressure runs about 80 to 85 psig, and the superheat fluctuates between 2 and 35 degrees. The equipment is 22 years old and as they stuff more people and equipment in the building the capacity issue just gets worse. If i could figure a way to keep the superheat from letting the coil warm up to 70 -75 and then back down to 45 -48 i think we would eliminate the lack of capacity. Does any body have a good idea, that could help me. PS the txv valve is a alco valve. Some circuits work better than others but most hut really bad, and yes I have tried to adjust the superheat setting on the TXV. Thanks for your help.

[Gary]

Generally, a TXV hunts because the orifice is too large for the load. This could mean that the orifice is too large or it could mean that the load is too small (evaporator airflow problems), but...

It could also mean that the high side pressure is too high. The TXV opens for a little refrigerant, but because the pressure pushing it through is too high it gets a large surge of refrigerant instead.

I would check for airflow problems in both evaps and condenser or high head pressure caused by non-condensables or overcharge.

[US Iceman]

Variable air volume and DX valves/coils do not work well together. Expansion valves like to operate steady state. Sudden or frequent load changes can cause the valves to hunt like crazy. Having a subcooler on the liquid feed probably hurts the valve operation as the colder liquid increases the valve capacity too.

If the subcooling is just enough to overcome the static pressures of vertical lifts in liquid lines that helps the valve capacity by ensuring 100% liquid is available to the TXV inlet. However, the liquid is still cold so the valve capacity also increases at the same time.



The equipment is 22 years old and as they stuff more people and equipment in the building the capacity issue just gets worse.


That does not help. Increased building loads can't be fixed by tweaking TXV's. That might explain where some of the higher suction pressures and superheat readings come from. Because... that is exactly how the system would respond to higher loads.



PS the txv valve is a alco valve.


I have had better luck with Sporlan balanced port TXV's because they will adjust better to changing conditions.

If you consider changing these, be warned. The distributor tubes have a nasty habit of coming out of the distributor if you get the connection too warm. Have seen this happen with others. Then you have a real mess on your hands.:eek:

Yes.... I have seen a few of these.;)

[Segei]

Try to throttle hand valve before TXV. Sometimes it helps.

[Magoo]

Check TEV external equaliser lines, if sweating or freezing the vav is internally bypassing and confusing the vav reaction, making them hunt wildly.
magoo

[dcsac1]

Thanks for the good ideas, The high super heat, and hunting occur when there is a full load and also at a part load. What I have gathered is these TXV's are not a balanced port TXV and that really hinders the operation? Would it be best to replace with a balanced port TXV or go with the new electronic TXV? I have no experience with the electronic TXV's so I don't know how they are suppose to work. Has anybody out there changed out an old TXV and replaced it with the electronic models, and how did it work, is it worth the extra cost?

[US Iceman]

I can't speak from experience about the electronic TXV's. They may better work for this application than even the balanced port valves.

The problem with these types of system is the changing loads imposed on the evaporator by the VAV fans. This will drive a normal TXV quite batty. They hunt like crazy.

I have had extremely good luck with the balanced port TXV's under varying conditions, however, I have not used them on a VAV system like yours. The systems I have seen were before the balanced port valves were available to the general market. At that time they were only available to OEM's.

Please remember, my comments are based on my past experiences. I can't in good conscience say exactly what your issue relates to without either seeing it or having a lot of good data to review.

[Gary]

If the TXV is externally equalized, the sensing bulb must be mounted upstream from the equalizer line. If mounted downstream it will sense seal leakage in the TXV, causing it to hunt.

Seal materials have improved over the years, but leakage was common in older TXV's. Still, even with newer TXV's it is good practice to mount the bulb upstream.

[Gary]

Thanks for the good ideas, The high super heat, and hunting occur when there is a full load and also at a part load. What I have gathered is these TXV's are not a balanced port TXV and that really hinders the operation? Would it be best to replace with a balanced port TXV or go with the new electronic TXV? I have no experience with the electronic TXV's so I don't know how they are suppose to work. Has anybody out there changed out an old TXV and replaced it with the electronic models, and how did it work, is it worth the extra cost?

Balanced port is not the answer, nor is EEV. You need to find the cause.

You have told us nothing about the high side (high side pressure, liquid line temp, condenser air in and air out temps,etc.). Are the condenser fans cycling on and off?

[Magoo]

I agree with Gary, there is something inherently wrong in system, you should establish that fact and correct it., try replacing existing TEV's with current replacement vavs. You need to establish a base line and work around issues from there.
magoo