Help with Theory of Operation - Cooler

[David40]

I do mostly A/C work and I'm not that familiar with complex refrigeration system. I've come across an old system I have been asked to repair but I'm not sure about the theory of operation for this system.
I would appreciate it if anyone could describe to me how this system is supposed to operate correctly in theory so I can figure out what is wrong.

The system: A reach-in cooler running on R-22 consisting of two 2-Ton condensing units supplying four evaporators (2 each) with a TXV in each unit. Operating temp is controlled by a low pressure switch on each condenser. Defrost is accomplished by condenser off time (no heaters) controlled by a defrost timer on each condenser, which are set at 40 minutes every 3 hours. Also, each condenser has a solenoid valve on the high side that appears to energize during defrost and is also controlled by an adjustable thermostat who's sensing bulb runs through the wall into the cooler.
The uncertainty I have is the proper reason/function of the solenoid valve and the thermostat that is tied into that.
Thanks for any help.

PS. I should mention that it looks like the system was modified at some point because there are disconnected temperature switches on the wall near each of the thermostats for the solenoid valves, that looks like the box temp used to be controlled by two thermostatic switches rather than the currently used pressure switches.

[Brian_UK]

What are solenoid valve pipes connected to?

[David40]

At the condenser in the liquid line. Since posting this I have done some research and tests, and discovered that the problem is likely a bad discharge valve in the compressor. Now that I have isolated the likely problem I still have the issue of how the controls should be properly configured. I had thought this system's temperature was controlled solely by the pressure switch, but it looks like this is a "pump down" type of system so it uses a bulb thermostat to control that liquid line valve. I'm not sure how to set them both to work together properly.

[icecube51]

40 minutes of defrosting evrey 3 hrs? I only hopes that the products stays good and there is no foodinvestigator passing at the time. if you give it 8 or 10 minutes every 2 hrs, you will get better results,and the products will not be harmd.

Ice

[David40]

That's the way it was set when I got there and like I said I don't have much experience with what would be considered typical or expected settings for this system. Since this box has two plants I suppose if the defrost cycles are staggered then one pair of evaporators is still cooling while the other pair takes it's turn defrosting. I'm still trying to learn the balancing act it takes to maintain the proper temp while preventing icing. Is there any training of information available that you know of that runs through the proper steps to configuring the control settings on one of these pump down systems?

[icecube51]

i think the best way to help you out here, is if you can make a small design or picture of the existing plant so we can define what is in a surten place.
i think the designer was using the hotgas return to defrost the other cycle. and when one thermostat closes a magnetvalve the system go on pumpdown and stops. so when the thermostats opens the magnetvalve the pressure equilise and the cycle restart from the beginning. is a verry rude and basic system, it works, but thats it.

Ice

[David40]

Yes, I'd like to do that, both a diagram and a schematic. I'm waiting to hear if the customer wants to let it limp along the way it is or wants me to repair it and tune it to perfection. Either way I really want to understand how this system is supposed to perform.