Faulty TXV

[marc5180]

If i have a faulty TXV what pressures would i expect to see because im reading 0bar on my suction line and everything seems to point at the TXV, i dont know weather it is set up right because i have to bridge out the LP switch to get the unit to run and i dont like leaving it running for long like this.

[monkey spanners]

Hi Mark

Check out the sporlan site for tev fault diagnostics.
You could try warming the bulb by hand or with some warm water to see if this improves the suction pressure (temporarily) if so check to see if someone has been beggering about with the superheat screw (how many turns in/out compared to factory spec, look up their number in the rac yearbook) If its lost the charge from the bulb, the valve will shut down, more charge is lost the more it shuts. have seen them shut tight (in the past i have cut the capiliary off the element on the tev and used a gauge line with ofn held against the cut/element to put some pressure on the diaphragm so i could pump the system down)

Have had a faulty solenoid do what you describe, opened enough to equalize the system when not running but could not flow enough with the comp running, with a raised head pressure (cardboard on condenser:p) you could see a temperature drop across the solenoid with a infra red thermometer.

Where are you measuring the hot gas bypass temperature?
Is there pleanty of load on the evaporator?

Cheers Jon

[marc5180]

Im measuring the hot gas bypass as it leaves the condensor before it goes into the evapoator.. The Hot gas bypass looks like a TEV but bigger but when i take the temp from inside the condensor i get 31deg on the hot gas bypass but when it gets to the evaporator its -25 which i dont get??? Can i alter the hot gas bypass cos this doesnt seem right to me...........When you say "If its lost the charge from the bulb, the valve will shut down".... Do you mean lost its refrigerant? I dont think it had lost any gas. I think someone has been messing with the TEV because the nut was loose on the end of it?

[monkey spanners]

Hi Marc
I don't have too much experience with hot gas bypass. From what you have said i'd be looking at the tev, i'd want to see a rise in suction pressure on warming the bulb (system running), maybe tip some warm water over the valve body incase it has some ice in it.
As to "lost its charge from the bulb" the bulb and the element/bellows on the valve body is a sealed system, as the temperature in the bulb rises so does the pressure this pushes on the bellows which pushes on a pin to open the valve and let more refrigerant through. bulb colder, pressure drops,less pressure on bellows/pin valve shuts down a bit to let less refrigerant through. If it looses some or all of its charge the valve will be more shut than you would expect for the given pressures and temperatures.
Check out the sporlan valves site for cut away pictures of valves and explanation of thier operation.

Cheers Jon

[The MG Pony]

ya you can never read them enough IMO more you read and understand the valve easier it is to deal with them.

[marc5180]

Hi Marc
I don't have too much experience with hot gas bypass. From what you have said i'd be looking at the tev, i'd want to see a rise in suction pressure on warming the bulb (system running), maybe tip some warm water over the valve body incase it has some ice in it.
As to "lost its charge from the bulb" the bulb and the element/bellows on the valve body is a sealed system, as the temperature in the bulb rises so does the pressure this pushes on the bellows which pushes on a pin to open the valve and let more refrigerant through. bulb colder, pressure drops,less pressure on bellows/pin valve shuts down a bit to let less refrigerant through. If it looses some or all of its charge the valve will be more shut than you would expect for the given pressures and temperatures.
Check out the sporlan valves site for cut away pictures of valves and explanation of thier operation.

Cheers Jon

held the bulb in my hand this afternoon for about 5 mins with no rise insuction pressure at all... but the TEV was fully frosted over.

[monkey spanners]

I'd want to check the inlet filter/strainer in the tev (if fitted) for rubbish (bits of gasket, or drier etc).
Some valves (small danfoss, teddington) you can check to see if the bulb has lost charge, with the liquid line disconnected, the orifice/inlet filter will be about 1.5mm out from the body, you can push it in with your finger against the pressure of the bulb/bellows.
You could try adjusting the superheat screw to lower the superheat/open the valve to see if it raises the suction pressure (make a note of how many turns you moved it just in case) If it improves, and no one has adjusted it before then something must have gone wrong with the tev.
This is assuming you have got liquid at the tev inlet.
It does sound like the tev is faulty. With some makes(sporlan) you can just change the bulb and bellows (element)
Something else to check is the temperature range of the tev (usually marked on the element/bellows part), e.g. -60 /-10 , -40/+10 etc, if its the wrong valve for the evaporating temperature the system runs at (too low a range) the valve won't flow enough for the system to work.

Cheers Jon