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bernard
31-03-2005, 07:56 PM
Hi All

Could someone explain to me how the oil seperation process would be affected on a pack if

1.The pack was low on gas

2.The pack was greatly overcharged with oil

I am thinking that on both occasions the oil would flow to the evaporator.

Many thanks Bernard

Mark
31-03-2005, 09:29 PM
Hi All

Could someone explain to me how the oil seperation process would be affected on a pack if

1.The pack was low on gas

2.The pack was greatly overcharged with oil

I am thinking that on both occasions the oil would flow to the evaporator.

Many thanks Bernard

What type of separator?impingement or coalescent.Ie float operated or high pressure.

Gas shortage can reduce oil return.
Oil overcharge can cause oil carry over and or poor compressor oil return.

Kind regards.Mark :)

Mark
31-03-2005, 09:35 PM
I am thinking that on both occasions the oil would flow to the evaporator.

Amongst other places.yes :)

Mark
31-03-2005, 10:31 PM
Bernard.hi must get some sleep .I will continue this tommorow.

zzzzzzzzz

chillin out
01-04-2005, 09:30 PM
Hi Bernard

If the pack was low on gas I doubt the oil would be affected greatly . I mean yes the flow coming back from the evap would be slightly less but the oil should have been seporated before the evap anyway.
If the pack was really low on gas it should have shut down before any problems arrise.

Being overcharged with oil would only cause it to dumped straight into the suction pipe through the gas releaf valve. ( knacking the comp)

Mark
02-04-2005, 07:41 PM
A scroll pack using a pulsed oil managment system with a high pressure separator/resevoir,overcharge may cause carryover.

Another pack with resevoir overcharge of oil may bleed back through the suction header via pressure valve.

Low gas charge may cause case alarms.

Low gas charge may cause pack alarms.

Low gas may not cause either of the alarms.

Reduced gas charge may cause compressor cycling,hunting leading to a reduction in impingement through the separator.And consequential oil starvation may occur.

Low gas charge on a high pressure oil separator may disperse the oil around the system.

Coalescent separators are 99.9% efficient.

Float operated impingement separators are 99% efficient.

Sometimes they are not.

Coalescent separators are not velocity dependant.

Float operated separators are not velocity dependant.

Sometimes they are.

Floating the head pressure may effect compressor oil return.

It may not.

Best regards.Mark :)

Peter_1
03-04-2005, 08:41 AM
...the oil should have been seporated before the evap anyway.

I made and installed several packs in the past (all more than 14 years ago) - at least 8 as far as can remember - and none of them had an oil separator, and 5 of them even no oil float switch on the sump.
This was the time when packs were not so common to install and you had to make them yourselves.
The packs were even made in a special way according a 'mathematical' row, well... at least ... some sort of it.
A 0.5 HP, HP, 2HP, 4 HP, 7.5 HP (8 doesn't exist) and 2 pieces 15 HP.
The PLC then measured the LP. As long as LP was too high, the PLC switched to the next higher step (with delay times between it of course) We could regulate in steps of 0,5 HP.
If pressure became too low, then we switched too the next lower step, again delay times and in steps op 0.5 HP.
So there was no oil separator nor oil floats on the compressor.
All these packs are still running with the original compressors.
We even made a pack with Copeland machines and a 10 HP Bock machine between it. This machine was then relative new (+/- 5 years) and client asked us if it was possible to use this machine. Also this Bock (now 15 years old) is still running.

So, oil return is for me more proper tubing techniques, suction inclined to the compressors, speeds in the tubing calculated like it has to be done, a very well designed suction header,...

We also still use following technique to ensure oil return (just in case of...) switching all the SV's open twice a day during the night for 15 minutes so that all the tubes encounter a maximum gas flow. The main header was calculated at 75% of the full capacity so that speed was high enough.


If the pack was really low on gas it should have shut down before any problems arise

I disagree with this: the only thing that will happen with a pack is that it will switch to a lower capacity step. The controller 'thinks' that there is a reduced load on that moment, not a gas shortage.


Being overcharged with oil would only cause it to dumped straight into the suction pipe through the gas relief valve. ( knacking the comp)
Not of there is a suction accumulator.
And why should if come back sudden if lines are properly installed?
What do you mean with the gas relieve valve?

bernard
04-04-2005, 06:55 PM
Hi

If a pack was controlling say 6 cabinets and one was badly iced or had problems with the eev,then It would flood back lowering suction pressure therefore staging down the compressors causing all the other cases to go high temp.Is there away to stop this.


thanks bernard

Peter_1
04-04-2005, 07:38 PM
OK, 6 cabinets on a pack and one iced. It will flood back -perhaps- if the EEV didn't prevented it.
But.... the LP will not drop at all, the pack controller will maintain a stable suction pressure.
The only thing that will happen is that the pack perhaps will switch to a lower capacity step. If it's a frequency controlled compressor, speed will be slightly decreased. That's all.
So the LP on the other will remain the same.
Of course, mass flow will decrease in the main suction line and the suction header.
Same story if you lost gas: LP will remain the same but with fewer compressors.

Mark
04-04-2005, 07:41 PM
Hi

If a pack was controlling say 6 cabinets and one was badly iced or had problems with the eev,then It would flood back lowering suction pressure therefore staging down the compressors causing all the other cases to go high temp.Is there away to stop this.


thanks bernard



On a suitably designed system one case iced up out of ? should not effect the pack.
On a smaller scale an iced up coil may cause reduction in load .The valve would either momentarily shutdown completely or at a reduction in pulsed opening.Leading to HT in case/cr.
What do you mean by floodback Liquid?.

bernard
04-04-2005, 08:02 PM
I had a pack problem recently,4 scroll compressors,one would only run then pack would shut down,4 out of 5 cases high temp,suctions on pack badly frosted,switched off the good case frosting gone and all other cases returned to temp with compressors staging properly,case that was switched off-found set point set to low running with a/off at -7c on h/t case,adjust set point and all was well.On arrival hp was 150psi, lp would reach 40psi then pack shut down,r404a.Its on a Tescos site with one of they controllers thats controlled from America.I thought it was due to the frosting??

Bernard

Mark
04-04-2005, 08:15 PM
Do tescos use jtl??

bernard
04-04-2005, 08:19 PM
Hi mark

Micro cool on cabinets woodely front end but are changing to eigsten,they are changing all there pack controllers at mo

bernard

Peter_1
04-04-2005, 09:13 PM
....4 scroll compressors,one would only run then pack would shut down,4 out of 5 cases high temp,suctions on pack badly frosted....,...I thought it was due to the frosting??
What was LP when line was frosting and running on 1 compressor?
What was your SH?
Problem can't be caused by frosting of the line?
Why frosted the line? Iced up? Bad TEV?
There must be another explanation.
Set point 7F or 7C?

bernard
04-04-2005, 09:35 PM
Hi Peter

Maybe I,m missing something here but I,m always eager to learn.I took the problem to be that the cabinet was trying to achieve a temp that it was never going to reach therefore icing up causing floodback lowering the suction pressure,how low I don,t Know as the pack would shut down below 40 psi. I measured the a/off at -7 c.By adjusting the controller to the same setpoint as the rest of the cases and de-icing.The problem was gone which led me to ask the question,there is no problem now.However maybe my theory is wrong .


Bernard

Mark
04-04-2005, 10:04 PM
One controller set point deviation would not cause 4 or 5 cases to go H/T .

All cases EEV?