View Full Version : Parallel rack

14-05-2014, 08:56 PM
Looking for help from anyone out there, I have a parallel rack system in a grocery store 5 compressors in total 3 medium temp and 2 low temp compressors. I am having an issue with the low temp side which has two 10 door freezers, a 7 door freezer and a 12x14 walk in freezer. My temp in the two 10 door freezers is rising slowly, I have a high suction pressure aprox. 32 psig (refrigerant is r-507) and my discharge is pretty good at 240 psig. Epr valves are adjusted as cold as they can go, checked the tx valves and they are wide open giving a superheat of approx. 6' F. Any one have any ideas on how I can get my temperatures down? Does this sound like it could be a compressor issue with a leaking valve? any advice would be greatly appreciated .


Rob White
15-05-2014, 08:09 AM

You don't say what type of compressors?

Are the scroll or recip?

Two thing stand out from your explanation above.

One is your losing your suction pressure and the other is you say
the valves are wide open?

The valve wide open bit worries me. Have you manually adjusted
the valve so they are fully open or is it just an expression, stating
that the valves are working and seem to be operating correctly?

With something like this you need to go back to a point when it was OK
and then find out what has changed. All the valves failing at the same time
is unlikely but it is possible for a compressor to be playing up.

If the valves are OK I would look to the comps and do a pump test on
them both.




15-05-2014, 02:12 PM
34 psi is only like a -2F SST. Almost all freezercases and walk in require at least a -10 SST. I would start checking comps.

15-05-2014, 08:40 PM
Sorry the compressors are recip, the expansion valve are adjust open to get 6'f superheat but cannot be adjusted any more. And I do understand 32 psig is high, just need to figure out why. Thanks for the help, it is greatly appreciated

16-05-2014, 06:25 AM
Hi jsw
Are your two freezer compressors running and fully loaded?
If so as stated before there could be a problem with the valves in a compressor.
What type of condenser do you have?
What is the ambient temp?
I presume the other freezers are playing up as well.
The more info you can provide the easier it will be for someone to point you in the right direction.

Rob White
16-05-2014, 09:42 AM
Sorry the compressors are recip, the expansion valve are adjust open to get 6'f superheat but cannot be adjusted any more. And I do understand 32 psig is high, just need to figure out why. Thanks for the help, it is greatly appreciated

I'm a bit simplistic when it comes to fault finding.

You need to know when it was alright and what has changed.

I still don't quite understand your response with the valves???
I know it's me being dense but you say

"expansion valve are adjust open to get 6'f superheat but cannot be adjusted any more"

Are you saying the valves are fully open and can't be opened any more and the superheat
is 6degs or are you saying the valves are open enough to deliver 6degs superheat and don't
need to be adjusted any more??

I know I might be pedantically fixating on the valves but if they are fully open and you only
have 6degs of superheat, something is very wrong.
If you are saying the valves are set correctly and delivering 6degs superheat, then that's different.

Go back to basics. Was it working? When was it working? has anything changed physically?
How old are they? how long have they ran? and again find out what has changed since they were ok.

Has the refrigerant level changed, you say it's on 507? How long has it been on 507?

Go back to basics, fit the gauges to the system and run it, then fit the gauges to each
compressor and test them. Do a full pump down test to see if the compressor valves are passing.
Measure the temperature of the cylinder heads and see if there is a difference.



16-05-2014, 12:35 PM
The expansion valves are adjusted fully open and I cannot turn the stem anymore. The refrigerant has always been 507, and the system is about 12years old. The other freezers are all little high in temp running at the same temp about 8'f. It wouldn't be so bad but the ice cream is a little soft. There has been an evaporator changed in a bunker style cooler that did lose a lot of refrigerant. Receiver is approx 20 percent and liquid line Site glass is full. May fluctuate a bit

16-05-2014, 02:05 PM
Ok, from your latest post you may well suspect a refrigerant shortage.
First, however & coming in cold, I'd be thinking:
How do you know that you still have pure liquid at the expansion valves?
Why were the TEV's adjusted? Frankly, I believe the only time they should ever need adjustment is at commissioning.
What's the suction superheat at the compressors? Or failing that, what's the ΔΡ between the evap. outlet & the SSV? Ie, could you be picking up excessive heat in the suction pipe run?
Is the system a gas or electric defrost? We used to run our packs with barely enough charge to keep everything at temp. Which meant that in cold weather, refrigerant backed up in the condensers & caused what looked like a refrigerant shortage. It was worst, iirc, on hot gas defrost systems with A8 valves. We fixed it by adding enough refrigerant to force the pressure up & get the liquid migrating to the receivers.
Another weird issue I once found on a pack fitted with EPR's - one run appeared to be short of gas, but there was plenty of liquid to serve it. Eventually the issue was discovered to be a leaking EPR stem. On opening the cap, the pressure escaped and allowed the EPR to operate correctly.

I agree with Rob. Put the TEV's back as you found them. Check the system for blockages by looking for temperature drops where they shouldn't be. An old indicator of a blocked TEV is that it becomes encased in a block of ice all over, as the blockage causes too much duty to be done at the inlet strainer.
Since you have recip. comp's, do an efficiency test on each.
Assuming all ok on those, make sure the stub / run isn't stuck in defrost. Or partially stuck if it's a gas defrost.
Make sure the doors are sealing correctly & not being left open for extended periods.
Are all the fans running correctly, and the air flows are all good - overstocking, poor cleaning regimes & blocking the return air grilles were all common issues when I was doing supermarkets.
Then if this stub is the last on the system, you can start thinking about refrigerant shortages.

16-05-2014, 09:54 PM
Rob and Freezer I agree with alot of what you say. The only thing is that if the system were low on refrigerant or restricted flow you shouldn't have have high suction pressure it should be low due to the lack of mass flow.

The superheat reading at a higher SST doesn't mean much as to keeping the case cool. With the too high of SST it doesn't matter what your superheat is.

There has to be some type of inefficiency in the system high head pressure, dirty condensers, false load. With the info given though I would start checking compressor efficiency's and work on getting the suction pressure lower.

Rob, The explanation of how approach the service is spot on. The should teach it like that in trade school.

16-05-2014, 09:55 PM
Thanks everyone for your help, turned out to be a bad compressor

Rob White
17-05-2014, 12:37 AM

Thanks for letting us know.



17-05-2014, 11:32 AM
Good to know you found it. As Rob said, thanks for the update. :)