Is this a pump-down system?
I've had a problem with a walk-in cooler that is intermittantly dropping below freezing (usually around 18 degrees). The customer originally called with frozen product and by the time I got there it was cycling normally. Pressures were good. I checked the bulb on the TXV and there was no corrosion and it was in the correct position. I concluded that it had to be a thermostat sticking and changed the thermostat. A week passed with no problems and then the customer called back with the same complaint. I checked the system again with no visable problems. I marked the thermostat just to make sure no one was adjusting it (it is behind the coil and out of sight) 3 weeks went by and the same problem occured. I did adjust the position of the thermostat bulb on the evap coil, hoping that this will make a difference. I have never been able to catch it not cycling on and off. The customer is getting frustrated and so am I.
Anyone have any suggestions?
SAM
Is this a pump-down system?
Experience is what you have an hour after you need it.
Once had a cooler doing that. As I recall on that particular unit, tstat was wired to break one leg of the 208v contactor coil (vice using a liquid line solenoid valve & low pressure control). Other side of the contactor coil remained hot (not a good setup by whoever installed unit).
The tstat would open at proper temp, but contactor remained pulled in. Tested voltage @ contactor coil & still had 120v present (not 208).
Therefore, that 120v kept the coil magnetized enough that it kept the contacts closed.
Discovered the line routed through the tstat was grounding through moisture inside a junction box mounted to the exterior of that outdoor-installed walk-in cooler. The wet grounding wasn't a good enough "short" to trip any circuit protection. However, it was just enough to keep that coil alive.
So...one thing you might look for as a possibility...now & later.
Op, if it is on a pump down. Pump the system down and take the LLSV apart and clean it.
Now that you mentioned that, I realize now they usually are. I work on a large variety of equipment other than refrigeration which generally breaks both lines. My bad on that.
Unfortunately, with the problem being so intermittent - and if it's a pumpdown system - and no possibility of an electrical issue I'd recollected and shared - then the solenoid valve would be the suspect.
If only you could catch it in the act.
Change the entire solenoid assembly, if this is a high risk situation concerning product loss.
Suspect the LL solenoid valve, if a pump down system.
If the temp control is only controlling a solenoid valve then its a pump down system. Temp control closes solenoid valve, when set point reached, system pumps down and low pressure switch shuts comp off. Then opens solenoid valve, on a call for cool, pressure rises on low side and low pressure switch closes and brings comp on. If solenoid valve is failing open system will not pump down so it will not shut off.
I agree with everyone else change the solenoid valve
Yep. LLSV.
If its a serviceable valve take it apart and rebuild it, you may have a piece of copper shaving sitting on the seat. If its serviceable I don't see having to braze in a new valve..
Could be improperly solenoid sized also, oversized valve will cause similar situation