Support
changes in suction line pressure can be observed as the valve
opens and closes.
Hunting thermostatic expansion valves can damage the
compressor.
A cause of hunting can be improper adjustment of a properly
sized TEV such as superheat set too low. A low superheat setting
can allow the superheat to fall to zero. This condition will cause
the valve to close and the superheat will then quickly rise to well
above its setting.
This in turn can cause the valve to reopen wider than
necessary and overshoot the superheat setting. This condition
is called hunting. On one of the valve's swings to the wide-open
position, liquid refrigerant can flood the suction line and allow
liquid to enter the compressor.
Yet another cause of hunting is an oversized refrigerant
metering device. An over-sized valve dispenses liquid refrigerant
into the low side too rapidly and then tries to correct by closing.
This is then followed by the valve opening rapidly and again
dispensing too much refrigerant. Again, the problem detected
will be a valve that is hunting. It is also possible that dirt, debris or
even water in the refrigerant piping system enters the TEV which
could cause this undesirable situation.
It should be noted that variations in the level of superheat of
around 0.5°C or less are not considered hunting while variations
greater than that amount should be addressed.
The superheat must be allowed to change for the sensing
bulb on the TEV to make corresponding adjustments to the valve
opening while it maintains the set superheat.
An erratically operating or failed thermostatic expansion valve:
Liquid slugging or poor system performance will also occur if a
refrigerant metering device is not properly regulating refrigerant
flow from the high side to the low side of the refrigerant system
due to sticking or failure.
One can remove the expansion valve and perform a bench test
which will allow you to accurately set the expansion valve and test
the operation.
Sarel, when it comes to expansion valves all that you need
is understanding and some attention to detail. I hope that this
helps you with your expansion valve issues, a poorly installed
or incorrectly adjusted valve can most certainly cost you a
compressor. RACA
References:
1. ACRA
2. Danfoss
Thank you for all your questions. Send your problems (and
sometimes your creative solutions) to [email protected]
with ‘Solutions Page’ in the subject line. You may include
pictures.
www.hvacronline.co.za
RACA Journal I April 2020
61