Question about setting pressure switch in medium temp automatic pump down application

[doubleehvac]

Is there a rule of thumb as far as the pressure switch settings for R-22 medium temp equipment in automatic pump down systems? If not, what factors are involved in determining appropriate settings?

I am an HVAC guy that has been dabbling in refrigeration as a favor to some of my boss' friends. I have a call to go on tomorrow where the complaint is that the unit on satisfying is rapidly short-cycling several times and then shutting off. It is still holding box temperature, they just saw the remote condenser doing this yeaterday and called in. Our refrigeration tech (before he quit and I inherited his jobs) was here before and the limited info I have is - ancient copeland semi-herm that you can't even read the plate on, single evaporator mini-mart beverage cooler - automatic pump down. Condenser fan motor replaced by us about 6 months ago, long story short these guys like to mess with their stuff before they call. I figure the cycling at pump down could be an improperly set pressure switch. I of course will still check everything, but I figure I would go here first, and I need to learn more about them...

I also know I don't quite have enough info for most, and I have already applied for pro membership, I just thought as I cruise google for my answers, somebody may be able to reach out and give me a hand. I have to go here tomorrow... any help would be appreciated.

[doubleehvac]

Thank you google books... Tomczyk is the man...

http://books.google.com/books?id=tH6...tcover#PPP1,M1

[cretan]

Most pressure/temp charts have pressure control settings for different equipment.Can be downloaded off the web.If pressure control is adjusted properly and it still short cycles,check how fast the low side pressure climbs when compressor shuts off.

[local 832s]

Valve could be leaking by, LL that is causing pressure to rise.

[icemeister]

The low pressure control cut out setting should be approximately 5 psig.

The cut in should be approximately 10 psi below the suction pressure for that system when it's at design operating temperature...ie; A 35ºF walk-in cooler running on R22 with a normal operating suction pressure of 50 psig should cut in at about 40 psig.

The idea being you want the cut out low enough to clear the evaporator of refrigerant but have the differential wide enough to prevent the compressor from cycling back on during pumpdown.

As was mentioned, a leaking liquid line solenoid valve would cause short cycling on a pumpdown system. A leaking or broken compressor discharge valve would cause short cycling as well.

[doubleehvac]

Thanks for all you help, unit was short cycling on pressure switch because it was low on refrigerant - leak from the ancient receiver.