EPR valves

[LRAC]

Hi all

we have 4 large systems approx 15 hp each which we have inherited on a new contract. The problem we are having is that the EPR's are operating correctly in keeping the suction high in the evaps to stop them freezing, but the capacity of the compressors suction is causing the compressors to short cycle on the LP switches until the pressure comes back up.

The customer has been plagued with failures, the previous company fitted time delays between re starts but my view is that this just hides the problem rather than solving it.

The systems have CPR's as well as head pressure controls + unloading but still they short cycle, i'm begining to think they are oversized or i'm loosing my sanity.

What do you think guys and dolls.

Kind regards
Lrac

[Brian_UK]

Oversizing sounds possible.

Would a hot gas bypass, with liquid injection cooling, help the situation at all?

[US Iceman]

DO I understand you have an EPR and a CPR in the same suction line?

[LRAC]

yep your correct US iceman, EPR and CPR in the same suction lines, it seams the job was not thought out correctly.

The customer recalls that the CPR valves were an after thought following failures after defrost when the EPR's were fully open and the compressors could not take the high suction pressures.

This is going to be a nightmare and i appreciate any suggestions i fore see a complete rip out and downsizing the refrigeration compressors.

Kind regards
Lrac

[Peter_1]

Obviously some one who took the Danfoss schematic or a Sporlan one where all the possible regulators they fabricate were put in a real cycle.
Very rare you need these all together.

Compressors are indeed over sized. But I doubt that the compressors can't handle the short additional loads after a defrost.
If that was the case, MOP TEV's was also an option.

Try to find the type and brand of both evaporators and compressors and the gas in the system.
If the compressors are in capacity too high compared with the evaporators - or the evaporators are too small so that they evaporate too low - then you can do only a few things.
First, place a VFD on the compressors and reduce there speed to a fix speed or even better, one which is controlled by a LP sensor. Very easy to install this.
In Belgium, VFD's are heavy subsidized, is that also the case in the UK?

Second cheaper solution if there's R404a or R22 in the system, change it to R134a or a refrigerant with a lower specific enthalpy so that the compressor capacity matches better the evaporator capacity. We've done this already several times in bot directions (R134a tot R404a and vice-versa)

[licencetochill]

Hi LRAC,
Id get rid of those cpr's for a start,and see how it runs..your compressers my be oversized,but id look at every other option before pulling them out.is your suction line under sized?

[US Iceman]

Having both valves in the same suction line is lunacy. They cannot control anything, as they will work against one another.

I think it's as Peter said. Someone saw those diagrams where all different valve types are shown in one system.

That's just crazy.

...the EPR's are operating correctly in keeping the suction high in the evaps to stop them freezing...

If indeed the compressors are oversized, then a VFD would provide better capacity regulation and suciton pressure control. I would recommend removing the EPR's (less pressure loss in the suctionn line) and let the VFD and LP control vary the compressor speed to match the suciton pressure required.

The customer recalls that the CPR valves were an after thought following failures after defrost when the EPR's were fully open and the compressors could not take the high suction pressures.

Electric or hot gas defrost?

Why does the defrost pressure increase too high? Another option with this would be to remove the CPR's and just let the pressure rise (it will only do this until the compressor pulls the pressure back down).

With the use of a VFD, just let the compressor only ramp up some (not maximum speed as the high suction pressure could overload the motor). If the compressor runs at high suction pressure for awhile at reduced speed, the motor should not overload.

You should also check the total evaporator capacity connected to each compressor and finf out how much over-sized the compressor really are. That should give you an indication of the compressor speed actually needed at full load.

If you only have several evaporators on each compressor, the idea of changing the TXV's to also use a MOP valve is a good idea. That takes the place of the EPR and does add pressure loss to the suction line. This would also reduce the effect of high suction pressures after defrost.

You end up with a much more efficient system and less stupid valves to work on or cause future trouble.

[LRAC]

Thanks for the input guys we're going back to site monday to get a full day on site to evaluate the situation and put our thoughts across to the customer, whether they accept it or not is another question.

May be the shortest contract we've ever had it they wont follow our recomendations.

Kind regards
Lrac

[Pooh]

LRAC
What sort of systems are they? freezers or chillers, as I understand it the fact that you have epr's controlling the suction pressure to the evaporator indicates that you have different temperature rooms of one compressor otherwise why not control the suction pressure with the compressor loading alone. Are these pack systems?

Ian

[LRAC]

Hi Pooh

Its worse than that Pooh ive been speaking to a friend from the ex service company, none of us spotted that at each end of the coldstores is another panel, i didn't see it nor did the customer say anything.

Here goes its a dual temperature room in the meaning that it can be both chiller and freezer with a simple switch of a button, i didn't take too much notice on our preliminary visit but it looks like i missed that the EPR valves are bi passed by a solenoid valve when in freezing mode, could explain the CPR valves as well.

Looking forward to our investigation visit in the morning.

Kind regards
Lrac

[Brian_UK]

Have fun tomorrow Lrac, don't forget the Thermos of coffee it could be a long day.

Look on the bright side, a new customer who has an old problem resolved is normally a happy new customer.

[Pooh]

LRAC
unless they are worried about TD across the coil why are they not using straight forward pump down off a stat to control things, that way even after defrost it should not be possible to get high crankcase pressures as long as the LP switch is set correctly. I hate it when people complicate things for the hell of it.

Ian

[The MG Pony]

The optimum design has been reached not when there is nothing left to add, but when there is nothing to take away!

[Peter_1]

MG Pony, good one, will remember this.

[LRAC]

Sorry guys i wont be able to inform you on the outcome of todays visit, we have put the visit back due too some low lifes breaking in to our offices and workshops.

They did a b***dy good job, gained access by cutting a 2 metre hole in our buildings cladding and coming through the false ceilings, cut the alarm and phone lines.

As usual its the easy things that have gone, computer server, screens, tools, filing cabinetsm and not gergetting the copper tube. I wish they had been cleaner instead of putting all the paperwork over the office floors. One good thing they didn't take the microwave.

Thank god for laptops and taking them home.

Speak to you all soon when my headaches gone.

Kind regards
Lrac

[US Iceman]

Sorry to hear about the office problems Lrac. Hopefully, you will be able to find enough of the data on the laptops to restore your electronic files.

This is not a good way to start the week.

[Peter_1]

Backin up company files is so important.
We backup every evening to 2 different PC's and 1 external drive on distant location in our shop on an old PC which only serves for this purpose. There's even no screen connected on it.

[The MG Pony]

We seriusly need to do it like the bond films: WARNING This premmis protected by clamour! As* hats will be detonated!

[Pooh]

MG no good mate most of them do not know how to read

Ian

[LRAC]

Hi guys finally made second site visit, this is a good one.

Here we go 4 x 15 HP condensing units matched to individual evaps running a dual temp chill and freeze.
Each 1 on own controller and no interlocked defrost.

2 systems are used for holding the room at chill and all 4 are used on freeze cycle.Chillers have CPR & EPR and MOP TEV's. Each system has solenoid pump down working off eliwell ID974's.

Now the interesting bit 2 chill units when on freeze cycle bypass the EPR's with a solenoid valve arrangement but the bypass size is only 5/8 when the suction is 1" 3/8, we watched the 2 units continually bounce in and out on the LP's when on freeze cycle, reduced return flow. we have informed the customer of our findings and they have agreed to a system re-design of the pipework (suctions).

Here's what we think:
1, remove bypass system,
2, remove EPR,s
3, reset Eliwells to include fan stop temperature's
4, leave the CPR's in(just in case)
5, install anti short cycle timers independant of 974's

What do you think guys, looks like we've inherited a pup.
The controllers were completly set wrong!

Kind regards
Lrac

P.S here's a horrible word Loss adjuster which means we get jack s**t from the insurance company.

[Pooh]

LRAC
what are they storing in the room? slightly worried about td when on chill without the EPR's.

Ian

[LRAC]

LRAC
what are they storing in the room? slightly worried about td when on chill without the EPR's.

Ian

Hi Pooh
10 tons of vac packed olives delivered as frozen, some shipped frozen and the rest brought up to chilled dispatch depending on customers orders.

Kind regards
Lrac

[Pooh]

LRAC
should be no problem with removing the EPR's then.

Ian

[krgari]

LRAC
So what happend next. Did you fix the problem?

[LRAC]

LRAC
So what happend next. Did you fix the problem?

Hi Krgari

Job is scheduled in next week i'll let you know.

Regards
Lrac

[krgari]

I cross my fingers for you , guys!
Hope to heare good news from you next week

[LRAC]

Follow up visit report:

Removed EPR's, removed bypass systems and fully commissioned took us 3 days to complete.

Moved all panel probes for air temperatures so they all read the same and cycle correctly. We did leave the CPR's in just in case they are needed by the system following a defrost.

Installed timers to stop possible short cycling of the units and moved the liquid line solonoid valves upto the evaporators to stop liquid line scavenging on pump down.

Installed head pressure controls for low ambient conditions.

Hopefully that should be the end of it ?????

The flasks came in handy good tip.

KInd regards
Lrac

[old gas bottle]

i think your bang on the money there, i t maybe the epr,s were fitted before the elliwell 974,s and now theres no need, i agree on leaving the CPR,s in,i,am a big fan of those, set them low to start with and tweek from there on, i think glory is in sight as they say.