About compressor oil loop in supermarket racks

[coldkeeper1]

It´s my understanding that the refrigeration oil flows from the oil reservoir tank to each compressor by means of the pressure differential existing between the outlet of this tank and the crankcase, being this first pressure somewhat higher than the second. My question: is this pressure differential kept at all times, or it´s created only when the float valve at the oil separator opens, allowing oil and high pressure gaseous refrigerant to rush into the reservoir through its inlet ? If so, does it mean that the oil reservoir pressure and crankcase pressure become the same whenever the float valve is shut ?

[750 Valve]

yes the pressure is bled into the oil reservoir with the oil as the high side float(s) open and shut.... but.... there is a check valve or nrv or pressure differential valve fitted to the oil reservoir, usually rated at 20psi, all additional pressure is bled into the suction header until this valve seats, leaving the reservoir 20psi above crankcase pressure. I guess if there was no oil feeding from the sep thru the hi side float(s) then you could have a scenario where the res and crankcases are at equal pressures, until the hi side float(s) open up again and bleed some more oil through. This is why we use 2 floats in parallel, if one hangs up and doesn't open then the other one will soon enough.

[pw93]

My rack has a second solenoid and coiled cap tube to supply pressure when the oil separator float has closed.

[coldkeeper1]

Sounds very practical. Might be a new system I have not seen before. Could you tell me what year /brand is the rack you´re working on?

[smilies]

My rack has a second solenoid and coiled cap tube to supply pressure when the oil separator float has closed.

Where does this line branch off from and what controls the solenoid valve?
Are you sure somebody just didn't remove the float pin and install this 'metering/cooling' device to replace it? It's rather common. The solenoid is is there in case of power outage to prevent liquid migration into the compressor crankcases.

[pw93]

It is a Hussman rack from the late 80's or early 90's.

[pw93]

The pressure line is tapped into the common discharge header before the oil separator. The solenoid appears to be energized at all times that there is power to the rack control circuit; so your surmise about the purpose of the solenoid is probably correct.