Walk-in box with outdoor compressor
I put a new outdoor compressor unit on my ancient walk-in box 5 years ago and it has been working fine until recently. It would randomly stop cooling for no apparant reason. Shutting it down for 15 minutes and restarting would cure the problem for a few days but it would re-occur. Thinking the old expansion valve was at fault, which had not been replaced when the compressor was, I replaced it with no results. My refrigeration man had recently died so I called another repairman who was available. At first he too was stumpted but then said it was the cold weather head pressure control valve but his reasoning doesn't make sense. The system runs on r22. When the compressor is first started the liquid line pressure goes up above 200 and the system works fine for a short while. Then the pressure drops to 180 and the evaporation coil stops cooling and the head pressure valve which is set at 185 cuts off the circulation to the condensing coil. The pressure does not increase at this point and to my thinking it should unless the valves in the compressor are bad. What is wrong with my thinking? Any help would be greatly appreciated. Thanks, Bob
Originally Posted by Bobby67
Does this system have a sightglass?
If so, what does the glass look like after 3-5 minutes of operation?
Is the small line hot or a bit warmer than outdoors?
Is the big line cold? Frost is good on this line for a freezer.
It is critical that units with these control valves be charged exactly right.
It does have a sightglass. The periods when it's working fine the sightglass shows a thick rush of liquid with bubbles. When it stops working, which used to be after some days but is now after only a few minutes, the sightglass shows almost nothing. When it's working the small liquid line is warm and the large line is very cold or frozen. When it's not working properly the liquid line is very hot and the large return line is nearly room temperature. This unit is a walk-in box for beer and it maintains 38 degrees with no problem when working properly. The charge may not be exactly correct but since it works quite well when it IS working properly, it's probably close enough. Thanks for your help.
Close enough isn't. If the charge isn't exactly correct, the unit will not work exactly correctly.
Originally Posted by Bobby67
A unit like this MUST have a clear sightglass at all times to operate correctly.
If your technician does not understand how to properly charge a system with a headmaster valve, then I'd suggest looking for a new tech.
I cannot educate you in the proper way to do it.
I would suggest that, at the VERY least, you have the leak repaired. Since the system is low on gas, there MUST be a leak.
There was a small leak on the expansion valve which I replaced before I called the professional. I told him it was probably running on an insufficient charge since I did not replace any gas before calling him. He seemed to think there was sufficient charge to maintain a head pressure of more 180 which it did do (about 240 I think) for a short time before dropping back to 180. His solution is to replace the cold weather head pressure control valve because it is not letting the charge pass though the condensing coil when the pressure drops below 185 which is its factory setting. But isn't that what it's suppose to do and shouldn't the pressure rise fairly quickly without the condenser to cool it? I will be calling a different professional unless someone tells me he's correct. Thank you all your help.
This cold weather valve of which you speak is a headmaster.
You're basically on track with regards to how the headmaster control operates during low head pressure situations - with a couple of exceptions.
The function of a headmaster is to 'stack' liquid refrigerant in the condenser - which in turn reduces the amount of space available to condense vapor into a liquid.
When a sufficient amount of liquid refrigerant is stacked in the condenser, the high side pressure builds and the headmaster starts to release the liquid it has been holding back into the receiver.
As you can see, a headmaster's function is not to simply dump discharge gas into the receiver as a way to increase system pressure, but rather to reduce the functional surface of the condenser by 'damming up' the flow of liquid to the receiver.
That said, hopefully it will allow you to better understand jpsmith's comments regarding adequate refrigerant charge.
There must be enough refrigerant in the system to allow the headmaster to do it's job.
Please keep in mind that based on your description of the problem, it appears the system is undercharged.....however, it is impossible to tell you with any certainty (without physically diagnosing the unit) just exactly what may be responsible for your particular refrigeration issue.
I am a touch concerned about your involvement in changing the TXV as it appears you are not overly knowledgeable in refrigeration service. If you did not properly pump the system down before changing the valve or sufficiently evacuate the refrigeration lines afterward....the problem you are experiencing may be self inflicted.
Bottom line here - if your refrigeration mechanic doesn't understand the operation of a condenser flooding valve, or the importance of adequate refrigerant charge......you may want to think about calling someone else to take a look at it.
"The problem is the average person isnt tuned in to lifelong learning, or going to seminars and so forth. If the information is not on television, and its not in the movies they watch, and its not in the few books that they buy, they dont get it" - Jack Canfield
Headmaster valves are very reliable. Very low failure rate.
This system is critically charged. This means that if it does not have exactly the correct charge, it wont work.
Call a real technician.
Thanks for the explanations, they are very helpful. I did pump the system down before replacing the txv and I pulled a vacuum afterwards. Since the problem occured before I performed the switch, I know I didn't cause it and only hope I didn't add to it. I will now be able to ask the next repairman more intelligent questions to be sure he knows how a headmaster works. The first guy admitted he did not even while saying it was not working properly.
Thanks again for all the help, it is much appreciated.
It turned out that the headmaster was perfectly fine and it WAS a low gas problem from the leak which I repaired. CRITICALLY CHARGED has been entered in my dictionary. Thanks.