Pump Down cycle with EPR

[herefishy]

Hi,

I've been servicing a couple of 4' open display merchandisers - each individually served by a dedicated condensing unit.

The system employs an evaporator pressure regulator and cycles on pump down via liquid solenoid valve.

I am concerned about proper pump-down control setting. The EPR is set at a minimum 18degF evaporator temperature (R-22). At times, when the EPR modulates, the suction pressure back to the compressor can drop to 15 to 20 psi so the cut-out is needed to be set lower than such occurance.

On pump-down (cut-out set at 5 to 10 psi) and cycle off, the suction pressure will rise to the 18degF SST pressure plus the superheat setting of the expansion valve (approx. 45 psi). Some short-cycling is evident during off-cycle, and obtaining satisfactory shut-down of the condensing unit under the cool ambient conditions that now exist is somewhat tedious.

I slept on it last night, and considered perhaps more satisfactory results would be obtained with use of a suction stop solenoid valve, as opposed to the liquid line solenoid valve in conjunction with the application of the EPR.

Any thoughts or observations?

[stan1488]

how excessive is the short cycling?

[Argus]

To my reasoning shutting the suction in preference to the liquid line may allow refrigerant to migrate or condense in the evaporator on the off cycle. This may give you an additional problem with liquid return when the compressor starts up. Added to this is the oil carry over on short cycles ? you may end up with most of your oil stranded around the system until it gets enough legitimate run time to pull it back.

If the temperature gradient in the cabinet is stable during this off-cycle, why not consider a timer relay providing a minimum off-time after pump down. As a solution it is relatively inexpensive and does not involve costly surgery to the refrigeration circuit. You can get them with adjustable time periods so that you can determine the optimum time period yourself.

Hope this helps
________
CT70

[chemi-cool]

In all freezers that I build, a EPR is present and I never had a problem.
To my opinion, it never closes completely so pump down should work.

If you encounter a real problem with pump down, you can increase superheat and set the EPR to a lower temp.

Chemi

[frank]

I've been servicing a couple of 4' open display merchandisers - each individually served by a dedicated condensing unit.

Hi Mark

If the units are served by individual condensing units, why do you need EPR's?

Why not convert the cabinets to control the pump down on stats?

[chillyhamster]

If they are low temp cases, are they EPR's. Presume the units have electric defrost. Are the Tev's Mop or N. If the TEV's are not Mop then the units should have crankcase pressure regulators to stop compressor starting under high crankcase conditions.

[herefishy]

Hi. The sytem installation application is specified by Masterbilt to include the EPR's. I talked directly to the factory. The units are a commercial temperature application - open dairy case, and do employ electric defrost.

I am able to set the low pressure control to limit any short-cylcling to something NOT TOO disconcerting.

One other thing that has always bugged me on these systems, is that the EPR is mounted exactly at the suction inlet to the condensing unit, with upt 60' of lineset between the control and the evaporator (case). Typically, this results in Johnny the trunk-slammer to mistaken the control for a "C"PR (crankcase pressure regulator) and after a period that I have been absent from primary service provider on these units (1-1/2 years or so), I find myself running about resetting the controls. I know what goes through the technician's mind, when the valve is being adjusted, and it has no affect on crankcase pressure (I've been there), but I got down to actually reading the information on the valve on the first visit (8years ago).

Perhaps if the valve was not so close-coupled with the condensing unit, the affect (that I describe) would not be so pronounced.

[herefishy]

Hi Mark

If the units are served by individual condensing units, why do you need EPR's?

Why not convert the cabinets to control the pump down on stats?

I hear you. That was my comment to the factory, when I first called them. Perhaps you are right, and the plant misunderstood the application. Perhaps I should call again to verify. It has been many years since I had that conversation.

I beleive that when the ambient is high (90degF to 110degF) in my part of the world, the high temperatures/pressures eliminate the operating conditions that I am observing now, in 60degF or so ambients, performing major service on the cases.

Regards.

[chillin out]

A few years ago I was working on an old belt driven comp which was leaking back accross the head causing it to short cycle but the customer would not buy a new one. So I rewired it to stop it short cycling, if i remember how I done it ill post it but im sure it can be done.

[frank]

You could of course increase the differential on the LP to make the "cut in" greater than 45psig, say 55psig. As the units are on R22 the standing pressures should be in excess of this at switch on.

[terrygoodrich]

I think Frank hit the nail on the head. If the units are sized right, why are there EPR valves? They would only serve to reduce efficiency and cause pumpdown issues. I would simply remove them or cycle the unit on the temp control and eliminate the pumpdown entirely. Was this unit put together at the factory or in the field? Masterbilt may have made it for use with a rack.

[Gary]

Why not connect the low pressure control to the evaporator side of the EPR?

[chemi-cool]

Hi Gary,

Wouldn't it might be dangerous for the compressor?

If the EPR will close when the pressure drops, the compressor will work for too long without cooling and might burn.

Chemi

[chillin out]

can you alter the wiring this way?

live feed through a nc timer to lp switch then comp contactor.
live feed through defrost timer, stat to s/valve
from the other contact on the stat conect this to the coil on the timer

run the system and time how long it takes to pump down then add a few secs and set the timer to this

now hopefully when the stat shuts down the valve it then counts the timer down then cuts the feed to the lp switch
when stat calls for cooling it lets the timer close and feeds the lp ( and also feeds the s/valve from the stat)