does the txv closed when the system is off

[comfortable air]

when I do the vacuume with a txv meter system, a idea come into my mind. does the txv always keep close when the compressor is off? so the system pressure can not balance by itself? do I must hook my gage to the high side and low side at the same time to do right vacuume?

[rclaoac]

The TXV opens and closes based on refrigerant superheat of the outlet of the evaporator. Why would you want to vacuum with only one hose hooked up to the system? It would take longer and you'd have to hook up your other hose later to check pressures when you charge up the system.

[filterbuoy]

I believe txv is closed by evap pressure plus spring force,opened by tx bulb pressure.So if system has no evap pressure (i.e.-vacuum) tx should be open.Probably are some exceptions.

[karl k]

Evap pressure and bulb pressure fight against each other. The evap pressure closes the valve and the bulb pressure opens the valve. With a straight liquid charge, this force should be roughly the same at all times. The spring acts to close the valve. That is how superheat is maintained. The bulb pressure, and therefore the suction line temp needs to be a little higher than the evap pressure to act against the spring and feed the evap.

Therefore, the TXV is closed when the system is off. It should be assumed that it is closed when under a vaccuum to avoid any trouble even though theoretically, if the bulb temp was high enough, it could act against the spring pressure and cause the TXV to open.

That should cover good practice as well as what could actually happen.

[hvacpope]

the right answer is "it depends on the kind TXV" some of them will be open others will be almost closed.

[jamaladwani]

In most cases the txv closed ,and will not open in the next running until the suction went down to the designed MOP [maximum operation pressure] lest the compressor be overloaded specialy in low temp.That is in work,but if the coldstore in off case that will depend on the ambient temp that the suction line related to .

[Edited by jamaladwani on 10-10-2006 at 11:55 AM]

[newbeetech]

but in theary, if one were to pull a vacuum on both sides, hi and low , and if the txv ia open are we fighting aginst hi side and low side never accomplishing nothing, just a thoutht

[hvacpope]

Originally posted by newbeetech
but in theary, if one were to pull a vacuum on both sides, hi and low , and if the txv ia open are we fighting aginst hi side and low side never accomplishing nothing, just a thoutht

that is so true,thats why I always run the compressor on high speed while pulling a vacuum, it takes care of the fighting right away.

[hands]

Originally posted by newbeetech
but in theary, if one were to pull a vacuum on both sides, hi and low , and if the txv ia open are we fighting aginst hi side and low side never accomplishing nothing, just a thoutht

This would only be true if you were using two vacuum pumps, which you never want to do. You should connect a hose to the high and low side access valves and then the other end of the hoses to a Tee on the vacuum pump. You should never run the compressor while the system is under a vacuum unless you want to replace the compressor.

[hands]

Originally posted by comfortable air
when I do the vacuume with a txv meter system, a idea come into my mind. does the txv always keep close when the compressor is off? so the system pressure can not balance by itself? do I must hook my gage to the high side and low side at the same time to do right vacuume?

TXV's open and close from the pressure inside the sensing bulb which is connected to the suction line right after the evaporator. If the bulb is warn, the TXV will be open, if it's cold, it will be closed. You should connect your gauges to both the high and low sides and the center (yellow) hose to the vacuum pump. You can also use vacuum hoses connected to the high side, low side and a Tee on the vacuum pump. You should also a micron gauge to check your vacuum level.

[comfortable air]

thank you. I got. when the system is off, there is no superheat in the sensor, the valve is closed. when it is in the vacumme presure. the valve is open.

[kewlguy1]

I believe there are different types of txv's.
Bleed and non-bleed meaning some equalize pressure after compressor shut down. A non-bleeding valve would close positively.

[comfortable air]

I just can not understand one processure when I read the text . the system has a solenoid vavle before the txv vlave. they asked people power the solenoid valve to make it open when do the vacuume.they said the valve may trap air in the liquid line between the expansion valve and the solenoid vavle. I don't think it is necceccery, because if the suction go down to vacumme, the txv valve will open, and the air tripped between the solenoid valve and txv valve will be vacuummed out. why do they asked people use the extra power to open the solenoid valve?

[Edited by comfortable air on 10-09-2006 at 10:29 PM]

[chiller mekanik]

I'm not sure if can even remember all of the reasons for doing this but its how I was taught & I've done it this way for so long now I do it without even thinking about it.

When you start the vacuum pump, open the high side valve on the manifold first, watch the suction gauge, you should see it starting to pull down.
The mechanic I trained under could have written a book on "checks & balances",(God rest his soul)his reasoning for this was to prove that the schrader on the high side was feeding, I think.
But, we always did the same procedure even when the high side was a valve you could mid seat & there was no schrader.
Whenever we had a solenoid valve, he would power it long enough to see the suction gauge go down to about 10", then we de-energized the valve, opened the low side gauge & let it rip.
So having done this procedure possibly hundreds of times, when the equipment is idle & void of refrigerant, the valve is open.(no pressure to push on springs)

When the equipment is off & full of refrigerant, thats different, for one thing the TEV doesn't know if the compressor is on or not, it will simply try to do what it is designed to do, maintain a constant superheat.
If the sat. temp. of the refrigerant in the evaporator is 70* & the suction line temp @ the sensing bulb is also 70*, the valve will be closed or pretty close to it, as it is trying to get the suction line temp @ the sensing bulb upto 80*,(if 10* superheat is the target)thats why alot of TEV systems seldom equalize, which is why they should have start assist components.(on single phase equip.)

[comfortable air]

I see,mr. mekanik. when we hooked the gage at the low side and high side. both side will pull the system presure down. since the txv valve is closed, evaporator pressure will going to vacuume state, the presure can not hold the txv valve close, the txv valve will open and the air between txv and solenoid valve will leased out. so I think it is unnessessary to power the solenoid valve when we do the vacuume. am I right?

[jayguy]

powering the solenoid is always a good idea, since it will allow for faster evacuation. you can also pull the coil and install a magnet to pull the coil up. it may not make much difference but it does not hurt. one thing to remember, these are good practices for TXVs. Electronic expansion valves (EXV) will close in most cases...some are used as the expansion device AND the solenoid valve.

[rob10]

Originally posted by hvacpope

Originally posted by newbeetech
but in theary, if one were to pull a vacuum on both sides, hi and low , and if the txv ia open are we fighting aginst hi side and low side never accomplishing nothing, just a thoutht

that is so true,thats why I always run the compressor on high speed while pulling a vacuum, it takes care of the fighting right away. BAwaaaaaaaaaaaaaaaaaa