Oil recovery problem in water cooled centrifugal chillers

[Mechya]

Hello to all..
During the low cooling load demand period combined with low ambient conditions, the problem of oil recovery to the oil sumps becomes very serious and sometimes the oli level in the sight glass dispappears, this requires oil charging. Again as the demands increases and also the discharge pressure increases the oil reappears forcing to drain the oil. My question is, is there a way to control the oil levl instead of this charging and discharging at varying loads.
The chiller in discussion is a large capacity (1000 Tons) water cooled centrifugal chiller.
Comments and suggestions are highly appreciated

Thanks.

 

[tombmech]

I wish this was a help, but unfortunately, no. This is a problem inherent in the particular chiller design. Some chiller designs have been corrected with an oil pump that ensures pressure and circulation, regardless of load. Unfortunately, that's a problemmatic fix for a pre-existing chiller.

You may have better luck with a controls modification. You could artificially induce additional load through recirculation/bypass, or introduce a higher cutout to keep the chiller from running at loads that would cause oil loss.

I had a similar problem with a bunch of water-cooled screw machines. Luckily, they were all in the same building, and we were able to re-configure the loads on each chiller to minimize the effects. We routed year-round computer loads to problem chillers, for instance, and removed load from others.

You might try trending the oil return vs. loading, and make your own determination. As I recall, we found that it took very little additional load to put the chillers in a safe regime vs. lack of oil return.

It was a very irritating problem. As you suggest, the chillers would run lightly loaded with no apparent issue, then would shut down on low oil after a week or two at that condition.

 

[joeywpittman]

read up on this

http://www.redicontrols.com/HP_Products_pg_OAM_Purger_IN-H.htm

 

[tombmech]

I'm not sure a purge unit addresses the same issue. I believe the problem described was a failure of oil return - due to lower pressures with low loads. It's an oil level safety that trips in the sump, not a refrigerant contaminant issue. It occurs in a matter of a week or two, or even just a few days. The oil always returns under high loading, when higher pressures enable the complete circulation of the oil.

Purge units address very small quantities relative to a long-term degradation of the refrigerant, instead.

 

[Mechya]

Thanks very much Mr. tombmech for the advice, and as u have rightly pointed out that the OAM Purger deals with entirely a different subject than the question in the thread. However this has started a new discussion, can anybody comment on the real application of these equipments (OAM Purgers) in relation to the high claims made by the suppliers of improved performance or energy savings. Does anybody have used them to see the difference?. Suggestions or experiences are highly appreciated.
Thanks....

 

[tombmech]

A purge device is a proven - and actually necessary - piece of equipment on most low-pressure chillers (R-11, R-123). They are not really useful on high-pressure machines (R-134a). The refrigerant contamination most often occurs on the negative pressure side of a low-pressure centrifugal - contaminants are sucked in. That's what fouls the oil. Simple leaks are your biggest concern on a high-pressure machine.

You may want to elevate this to another thread, and solicit responses from others more expert than me.

 

[Trigout]

For the oil level going low you will need to add an artificial load. tombmech is correct that you may not need much more load to keep the oil level from going low.
We had purg units on all centrifugals and I suggest you try to get funding for them.

 

[Mechya]

Trigout
As u have pointed out, HOT GAS bypass is in ON mode most of the time for artificially loading of the chiller. If I have not misunderstood your statement, could you please explain, How does a Purge unit eliminate the problem of low oil level during the low load conditions? and do u mean the OAM Purger units?.. If they (OAM Purge) are used for any other purpose than the above discussed point, can u detail here as to what benefits do u have using the purge units.
I hope I did not ask many points.

and as always many thanks to tombmech for the valuable advices.