Oil carry over to the system

[lana]

Hi,
I am working on a 75HP Trane water chillers. One of them is working with glycol te=-20°C and the other at te=+5°C.
The first one is working fine but the other one empties the oil from the crankcase in 15 mins?!!!
The piping is very short and everything is in one compact unit. The compressors are old and they have been repaired.
Any idea?
Thanks.

[Dan]

I would guess that the tolerances in the oil pathways are too large, thus the oil pump is pumping against a low resistance and the oil is literally being pumped into the discharge at a rate much greater than the return. Another possibility might be bad discharge valves.

I did a search on the web and found this comment:

"In older compressors, oil failures can occur because of the pressurization of the crankcase due to blow-by from the pistons or piston rings."

That''s one of those statements that I too often nod my head to without thinking of the dynamics that this would put into play. Are we literally pushing the oil out of the crankcase and into the motor housing? How would we test for that? Perhaps turn the compressor off and see the level rise?

[US Iceman]

...the other one empties the oil from the crankcase in 15 mins?!!!

Several questions first of all....

What is the suction superheat?

Does this occur on start-up of the compressor?

What is the operating superheat on the discharge line during start-up?

Is the compressor fitted with a crankcase heater? If so, is it working?

One thing I have seen where the crankcase looses oil this fast is due to liquid in the crankcase. When the compressor starts, the crankcase pressure is suddenly reduced and the liquid flashes off.

When the liquid flashes off, it carries the oil right out of the crankcase very quickly as you mentioned. This is the only condition I know of where this can happen as fast as you stated.

...thus the oil pump is pumping against a low resistance and the oil is literally being pumped into the discharge at a rate much greater than the return.

If the rings were this bad, some additional oil carryover would probably occur. However, since the compressor discharge pressure is higher than the oil pump discharge pressure, the oil pump cannot pump oil into the compressor discharge. The high pressure gas would probably pressurize the crankcase and could result in a additional carryover also. I do agree with the comment you found on the net.

Usually the result of sustained high superheated discharge temperatures from excessive suction superheats and/or compression ratios or even non-de-superheated post-evap hotgas injection etc.

I agree with this, as these are all contributing factors for an increased carryover rate. I do doubt though that these would cause the loss rate to be as fast as lana said.

[TXiceman]

A few thoughts...I don't remember Trane building a compresor rated to run at -20dc (-4 dF) SST. Seems like the lower limit for their recips is -12 dC (10 dF) SST.

As suggested above, what is the suction superheat and what is your discharge condition?

Also, do you have the right oil in the machine for your refrigerant and conditions?

Ken

[Dan]

However, since the compressor discharge pressure is higher than the oil pump discharge pressure, the oil pump cannot pump oil into the compressor discharge.

Interesting point, Iceman But oil does indeed travel to the discharge, does it not? And wouldn't a pump moving more oil as a result of less resistance put more oil into the areas such as cylinder walls to cause it to travel to the discharge at a greater rate?

[US Iceman]

...oil does indeed travel to the discharge, does it not?

Dan, I agree with this.

The gas flow is through the compressor. Oil will flow with the gas as it leaves the compressor. I don't think the pump is pumping more oil. If anything, the oil pump is probably as worn as the rest of the compressor, so oil flow rates or pump discharge pressures could be lower.

If the tolerances are "loose" on the oil ring (on the piston), less oil is scraped off of the cylinder walls. The oil then blows past the oil ring and into the gas stream leaving the compressor.

The only time I've come across a compressor trip on oil so soon was when with excessive all round wear as the compressor oil warmed so the oil pressure would drop from 60psi to 20psi on account of reduced viscosities.

I have seen this also. The oil safety switch trips within a short period of time, but this is different than the oil safety tripping for loss of oil in 15 minutes.

...do you have the right oil in the machine for your refrigerant and conditions?

A very good question. Oil miscibility can be a large contributing factor for this.

Where's lana???

[Dan]

If the tolerances are "loose" on the oil ring (on the piston), less oil is scraped off of the cylinder walls. The oil then blows past the oil ring and into the gas stream leaving the compressor.

Blows past the oil ring and into the gas stream leaving the compressor? From a lower pressure to a higher pressure? You started it, Iceman. Let's sort this out.

How do things move from a lower pressure to a higher pressure?

[US Iceman]

Hi Dan,

Looks like an interesting predicament, heh???

My thoughts on this...

Blows past the oil ring was probaby the wrong term to use. If the oil ring does not scrape the oil off of the cylinder, and if the compression rings are loose, I feel the oil is left on the cylinder wall.

When the piston travels back up to top dead center of the stroke, the oil is "stripped" off of the cylinder into the discharge gas stream.

This is what I was thinking when I wrote the "blows past the oil ring" comment. In other words, the tolerances are bad enough to promote "oil pumping" out of the compressor.

As Marc correctly pointed out, increased suction superheat and other problems can contribute to this effect.

How do things move from a lower pressure to a higher pressure?

You use a compressor! OK, I'm kidding.