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I am having a bit of bother with a plant with a PHTQ expansion valve.
Could anyone help and explain how these valves operate.
I believe there is a 24vac supply to the actuator and another 2 wires which I presume control the modulation. What do these 2 wires do and how do they open/close the valve.
There is some info on the Danfoss site but I'm not sure if it is enough for you, have a look anyway....
http://uk.refrignet.danfoss.com/ra/p...selectedTab=IN
Thanks Brian.
I had a look at the Danfoss site earlier and while there is some info it is not really that detailed.
One of the main problems with the job that this valve is on is that it is not controlled by a Danfoss controller but by a Star PCB. So there is no user interface to see what is happening.
Hi Dallan:)Quote:
Originally Posted by dallan
The valve is a heat motor. Heat is applied to the heater part, that warms a bimetal strip/element, which causesthe valve to open. The other set of wires (small ones) are a temperature feed back to the board;)
The control board is a Star built MEV board. Level input is by a thermistor (usually a white one with 13.5 ohms reading when out of refrigerant) a voltage circuit is complete through the thermistor, when liquid covers the thermistor the voltage drops as the current in the circuit increases,the board senses this and reduces or increasees the voltage to the PHTQ accordingly.
The board will usualy have other functions, load up relays ect, these receive an input for every stage of loading on the compressor, they actually reduce the amount the valve can open at high loading values to stop liquid carry over into the compressors.
In short the following problems are usual.
1/thermistor burnt out or about to burn out (check ohms when out of refrigerant)
2/white resistors on board burnt out (you can see this and measure for open circuit)
3/Oil in thermistor chamber (check oil rectification system)
4/weak element in PHTQ, replace (needs replaced every 5 to 10 years usually)
5/sticking main valve the TQ element controls this main valve, but in R22 the amounts of moisture it carrys aroungd can rust the valve and it will stick
Good luck:)
let me know if I can help any.
Kind Regards Andy:)
Andy,
Thanks for the input, it actually helped to clarify things a lot.
With regards to the LPR systems can you explain how the oil rectification works, what is the point of the bypass round the expansion valve how can you be sure that the refrigerant charge is ok.
Thanks
David
Hi dallan:)Quote:
Originally Posted by dallan
LPR's:D now that is something not a lot of people work on.
Oil Rec is normally on ***** systems, the ammonia system is different.
A feed on cold liquid and oil mix is taken from the liquid level, this feed is then heated by hot gas or liquid until the liquid is gone and the oil is returned to the compressor suction:)
Oil rec is sometimes internal the liquid in the liquid line passes into the LPR into a 4" shell, this liquid fills up and then flows out the top connection to the valve station. Inside the shell has a coil which takes it's feed from the liquid level and then passes up thru the shell and out the top, out of the LPR and to a point higher than the liquid level in the LPR.
Bypass valve is for trim control (balancing liquid feeds to serveral evaporators) or for emergency use in the liquid control has a problem.
Refrigerant charge is usually gauged by the one or two sightglasses on the LPR (older systems have two, but only charge to the bottom one half glass).
Better results can be obtained by checking liquid subcooling, wet return temps (main and each evaporator if there is more than one).
Star also make a HF chiller, looks a bit like an LPR, but is a flooded liquid chillers (these are charged to a half glass or just above the tubes, same thing). Mostly the HF chiller has TQ valves, only seen one maybe two sites with TQ valves on LPR's.
Kind Regards Andy:)