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maxmejia
29-10-2008, 12:21 AM
I have 3 ref. systems with two 20 Hp. recip. tamden compressors they where ship from Spain. ref. 404a for three big walk in freezers. Two of them are working properly but one of them have one compressor that is loosing the oil and the other compressor is gaining all. The oil separator is OK (if we run each compressor separately every thing is fine). The internal equalizer holes are ok. The working pressures are fine. They are complety level. So every thing is ok, and those d.... compressor sould be working fine, but one of them is keeping losing oil and no getting back from the oil separator (have a common line that go in the suction tamden). I need help, I am running out of solution.
Today I will try to put one external equalizer in the bottoms of the cranckase's compressor. Remember the other two are fine.
Max

Grizzly
29-10-2008, 11:31 AM
High Max
Not sure what you have!
But is it possible that the errant compressor is wearing out.
Causing it's crankcase pressure to be higher than the other.
Therefore allowing the other to take the bulk of the returned oil.
Whereas the suction which is shared
would be at the lower pressure or average of the 2.
Just a thought?
Do you know what make these are
Dorin maybe?
Grizzlyhttp://www.refrigeration-engineer.com/forums/images/icons/icon5.gif

BESC5240
29-10-2008, 01:59 PM
You should mesure the crankcase pressure of each compressor.
If they are different, it could mean :
- (exessive) wear on one of the compressors
- (small) internal bypass (valve plate, gasket, ...)
- there is something wrong with the internal pressure regulating valve; I don't know what brand of compressors you use, but Copeland compressors have this valve. It connects the crankcase and the suction side cylinder head. It slowly reduces the pressure difference by means of the “venturi” effect. The oil foams less and only limited oil/refrigerant foam is transferred to the oil pump, thus slowing down pressure decrease in the crankcase over a certain period of time. Now, if in one of the compressors this valve is broken you can get differences in crankcase pressure , and consequently in oil level.

US Iceman
29-10-2008, 02:33 PM
Are you describing a tandem compressor to be:

A) two compressors with a common suction service valve and two discharge service valves?

or

B) two compressors in parallel, each with a suction and discharge service valve?

Grizzly
29-10-2008, 03:14 PM
Are you describing a tandem compressor to be:

A) two compressors with a common suction service valve and two discharge service valves?

or

B) two compressors in parallel, each with a suction and discharge service valve?

Good point well presented Iceman.
I assumed the first option.
I suppose I should never assume!
Grizzly

US Iceman
29-10-2008, 11:40 PM
I think it is option A also, and if so, I think the comments BESC5240 mentioned are worth a look. It seems to me that the old information I saw on tandem compressors had a check valve of sorts to equalize the oil levels without all of the oil float regulators, etc.

taz24
29-10-2008, 11:45 PM
Are you describing a tandem compressor to be:

A) two compressors with a common suction service valve and two discharge service valves?

or

B) two compressors in parallel, each with a suction and discharge service valve?


Tandem compressors used to look like this

http://nationalcompressor.thomasnet.com/ImgMedium/copeland-tandem.jpg

Not sure if this is what you mean.

taz.

US Iceman
30-10-2008, 02:06 AM
That would be the one Taz. Thanks for the picture.

BESC5240
30-10-2008, 09:37 AM
In the picture attached you will see a single semi- hermetic compressor.
The valve I mentioned earlier is marked with an arrow.
It is mounted between the crankcase and the suction chamber in the cylinder head. It serves to gradually reduce the crankcase pressure at start up.
If it is broke, the crankcase pressure will reduce rapidly at start up, causing the liquid refrigerant in the oil to boil off... causing foam ... reducing lubrification 'quality' ... cutting out the oil pressure switch.
In systems with large superheat and low liquid fload back it could be the broken valve isn't noticed.

But when you have a tandem where one of the compressors has a broken equalisation valve ... you get different crankcase pressures ... different oil levels ...

And small different in pressure can result in a large difference in oil level. (0,1 bar results in 10 cm level difference, approx.)

boy
30-10-2008, 01:58 PM
maxmejia,

Is this a compressor rack?do you have a common discharge and suction header?any information oil pump pressure of those 3 comp?please check crankcase filter and change oil the comp.

regards,

maxmejia
31-10-2008, 11:29 PM
Hei my dear college, they are tamden one suction and two discharge. Tell you what. I decided to open the compressor today to check all mechanic parts, so on monday a let you know what I found because every thing point to valve, compression rings, etc. problems.
Thanks for your help, is good to count with you.
Max