Control scenario

I came across a system the other day that looks as though a mistake was made in the control scenario. Water cooled R12 recip. compressor. 3 ton TXV, external equalized. Liquid line solenoid valve. Hot gas solenoid valve feeds hot gas regulator to side inlet of outlet of TXV. Temperature control is done by pressure switch on suction of compressor. When command to cool is issued ALL solenoids are energized. Now pressure
switch has control of the compressor start/stop.
Pressure switch set to maintain 5-8*C space temp.
As space temp. drops suction pressure will drop, but hot gas kicks in and raises suction pressure
so unit will never satisfy low pressure switch. There is a hand valve feeding the hot gas solenoid that if closed allows system to pull down and cycle on pressure switch. I think what happened here is that unit was controlled by temp.
at one time and someone decided to use pressure type control but never took the hot gas into consideration. Current control scenario doesn't make sense. How can you have pressure control and hot gas bypass at the same time? I have some ideas
floating around. One scenario that surfaced was that maybe they were regulating that hand valve to feed a little hot gas in to keep the evap. active and promote some oil return? Note that when the pressure switch satisfies the solenoids remain energized. Only thing that kills power to them and starter is computer command to stop cooling, also stops evap. blower. Soliciting
thoughts, comments, concerns from fellow fridgeheads.
Mike Hopkins

Hi Mike,
there are two main lines I think you have to follow to solve such a trouble.

1) Why is there this hot gas bypass? Is that system designed for a high pressure work? What kind of service the system does?

2)Obviously the wiring is incorrect; when the temperature is reached, the liquid solenoid and even the hot gas solenoid must be de-energized, if the scenario has to be a pump down-like.

Try investigate these paths.
Have a nice day

I believe you should put temp control in the system to cycle liquid and hot gas solenoid to pump down and turn off the compressor. The hot gas line must have been installed to false load the system perhaps the compressor was desiged a little big for the system. However keeping the soleniods open with the compressor off sounds a little risky to me causing flood back on start up.

Two statements need elucidation: "Hot gas solenoid valve feeds hot gas regulator to side inlet of outlet of TXV." What kind of regulator? Is it maintaining minimum evaporator pressure? What kind of system is this cooling or intended to cool?

2nd: "Only thing that kills power to them and starter is computer command to stop cooling, also stops evap. blower." Does this computer monitor the controlled medium/space temperature or is it simply periodic start-up/shut-down?

First thought is that pressure control is not intended to control temperature, just to shut compressor off after pump-down, but some tech has misunderstood system and set press control higher in an attempt to control space temp. Depends on answers to above.

Rog

These systems should be sized close to the load of the box and load added within the box. The hot gas solenoid opens when the space temp is reached and raises the temp of box slightly and shuts back off. This allows for a flat line on box temp. and keeps these compressors operating. They don't cycle compressors except to defrost. This gives longer compressor life. You will see these types of boxes on scientific boxes. We have installed and serviced many.

Quote:

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Originally Posted by Timsutton2

These systems should be sized close to the load of the box and load added within the box. The hot gas solenoid opens when the space temp is reached and raises the temp of box slightly and shuts back off. This allows for a flat line on box temp. and keeps these compressors operating. They don't cycle compressors except to defrost. This gives longer compressor life. You will see these types of boxes on scientific boxes. We have installed and serviced many.

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Bingo! Except something has to cycle the hot gas bypass on and off or the regulator must be set to keep evap temp at design - eg open and false load the comp on a lowering of evap pressure.
Sounds like maybe the regulator is not doing what it should and allowing the suction pressure to fall lower thereby allowing the room temp to fall - hence somebody has set the LP to cycle and keep temp, a bit of a compromise on the original intention.
What sort of regulator is it? Is it adjustable?