Replyover charged a little
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At an industrial site today assisting a tech doing pm's. Seen similar problems, but this is the first with this combo.
Scroll compressors, two per circuit, microchannel condenser, tube/fin evap, ~100' line set, evap ~30' above cond. History of evap coil leaks due to corrosive environment.
Issue: connected gauges to suction and liquid lines outside of enclosed condenser.
All four compressor sump heaters were open.
Suction ~110-120psi, ssh 10-12*f, fairly stable
Liquid line ~350- 410 psi 5-10 second swings, subcooling ~ 35*f swinging a couple of *f
Took 3 steps and about 1 hour to find problem.
I'll go thru the steps giving about an hour for your input before listing each step and conclusion. If I had a prize I'd give it to the first correct response!
Professional Member*
over charged a little
Professional Member*
Correct on first step.Originally Posted by Snapperhead
Recovered refrigerant to ~ 15*f subcooling. SSH unchanged, Liquid line pressure still swinging ~ 50#.
Step #2?
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those micro channels dont take much to get dirty correct ? I'd wash them next
This unit is used for cooling ... I assume ...
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Cooling production area where they are making covid mask. Coils only moderately dirty.
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Low ambient Cond Fan monitor sensing bulb correctly mounted ?
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about 70*f ambient, 2 of 3 fans steady operation
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Step #2
did test for non-condensables, saturated and ambient < 2*f difference, liquid line pressure still swinging
Step #3?
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Removed coil panels, put gauge and temp probe on compressor discharge.
compressor discharge ~ 100# higher than liquid line, swinging > 50#, pegged gauges a couple of times, discharge sh 2-4*f
temp probe on liquid line fairly steady, but I could feel spurts of spikes in liquid line temp for 1-2 seconds
noticed oil in compressor sight glass agitated
Problem solved. Answer?
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I was thinking from the beginning maybe someone topped off unit with wrong refrigerant , but doubtful
Is something going on with drier ?
Professional Member*
Extreme oil overcharge.
Liquid flooding compressor causing swings in pressure. Microchannel ports restricted by by liquid oil entering causing high pressure and high pressure drop.
evidence:
1. normal ssh
2. usually high evaporator approach & low suction pressure on chillers, only slightly out in this case with ahu
3. low discharge sh, oil not compressing cools discharge gas
4. microchannel high pressure drop & high approach
Corrective action: connect recovery cylinder to liquid connection, bleed liquid into cylinder very slowly while operation, heater band on cylinder, cylinder vapor connection full open to suction, distill oil out.
Unit has had a history of leaks and repairs.
Have seen excess oil in all types of systems, but this is the first time I've seen the extreme pressure swings and extreme microchannel pressure differential. Usually very little effect on tube/fin condenser pd or approach.
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Why would anyone add extra oil to it ?
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Oil heater burnt out, may not have seen oil level after starting with saturated oil
Split unit factory recommends adding oil on start, may have calculated wrong
Original compressors so not due to changeouts
Recovery cylinders may have been contaminated
Regular Guest
I have run into this before myself. So were your sight glasses on the sump completely full?
Professional Member*
NO. Low dsh sh with normal ssh is main indication.
Used to see it frequently on McQuay ALS and other screw chillers, but without the pressure swings with tube/fin or water cooled condensers. Sight glasses on oil seperators would always be low.
Professional Member
I'm intrigued by your makeshift oil separator. It makes perfect sense, but I would have never thought of doing that.
I'm not familiar with that particular unit, was there no way to drain the oil from the sump? Most units that I've worked on that have oil sight glasses will also have some sort of port there where one could remove oil. Is this method preferable in some way? I guess you don't lose any refrigerant at all this way, as opposed to blowing refrigerant saturated oil into a bucket.
But for units without a SG, I'll remember this topic if I ever see something like this. Now that I think about it, I have a 30GXN that's been acting up, maybe I can play around with that.
The great professors of the art are not immune from the malignancy of matter and the eternal cussedness of inanimate objects.
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Epa does not allow oil at pressures over 5psi to drain to open containers. Most scrolls do not have oil drains. Flooded evaps take the longest to recover the oil...
Professional Member* / ARP Committee
Oil saturated refrigerant is a pretty common problem. I find it most after compressors have been changed on packaged units. Most dont take the time to measure the oil out of the old compressor. I usually do, and remove that amount from the new.
Ive used the method EHSX described a few times for running systems.
Had one jag off add 5 gallons of oil to the wrong circuit on a machine.
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I was training manager for Dunham Bush 90’s. Used to say there would never be an oil shortage if everyone had a DB screw, typically always had excess oil, removed 5 to as much as 45 gals from systems in the field. Most of the time no one claimed to know where it came from.
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