View Full Version : Coalescing filter at low discharge pressure

11-05-2017, 11:47 PM
Some people believe that a single stage or high stage screw compressor can damage coalescing filter if operated at low discharge pressure(below 100 psig for ammonia). What do you think abou this statement?

12-05-2017, 12:31 AM
I will not say it will damage it, but it could. It will certainly cause it to pass oil due to the higher velocity.
Happens on air compressors as well. Not a function of the type of gas being compressed.

12-05-2017, 01:17 AM
It seems that some manufacturers design close to limit on coalescer sizing.
It obviously could keep pricing down to look more competitive.
I guess it should be asked for when ordering a compressor package.
Higher velocity, potentially more oil carry over.
In some cases special coalescer with more surface area can be used to help.

12-05-2017, 03:41 PM
Lower discharge pressure mean higher capacity and mass flow.
first stage of oil separator will increase its oil carryover added to the increase of refrigerant mass flow= possibly coalescing filter faillure.
oil separator has to be selected for:
nominal discharge pressure and also lowest discharge pressure
idem for highest suction pressure.

12-05-2017, 10:59 PM

This link sound fair


13-05-2017, 02:09 AM
I know one company and they replaced regular coalescer with super from Frick. Condensing pressure has been dropped by 5 psig not 40-50 psig how Frick claim.
Regarding linkedin article. It has many good points and I agree with them. However, I have couple concerns. About losing couple tablespoons of oil per year. I'm doubt that is possible. Typically, it is good if compressor loose 1 gal.(3.8 l) per month. Oil can be added once per year.
Compressor low differential pressure will not cause high velocity. Booster compressors have differential 20-30 psig and they don't loose oil. Yes, high velocity is the reason of oil carry over. Why it happens? Low condensing pressure will reduce density of the discharged gas. Mass flow is not changed. Lower density will lead to higher velocity of discharge gas and oil carry over can happen. Increase of suction pressure will lead to increase of mass low because of higher density of suction gas. This can lead to higher velocity of discharge gas and oil carry over can happen. High suction or/and high stage compressor more prone to oil carry over because of significant mass flow.

13-05-2017, 04:31 AM
What type of oil is this plant operating on?

If we manufacture oil separator for booster, it is much larger than high side, with more coalesces.
Frick just say standard oil separator on programme with varying conditions.
Do compressors lose oil when continually running, or do they cycle off much?


Assume they are following instructions when installing filters on page 26.
These coalesces have to be mounted with certain orientation, torque setting etc.
They recommend to change oil & filters to maximize filter life.
Be interesting if they ran oil levels a bit lower than normal, at least on one compressor to see if any difference.
On old ones removed, does it appear all end cover gaskets are seating 100%.
Some we do have to be carefully tightened , & centralised whole doing it.
In the past on Mycom ones we made like a pipe support cradle size of OD of filter, with pipe or similar welded to it, so one person hold weight of it, while other supports it installing end plate & tightening it.

Just suggestions!

16-05-2017, 02:56 PM
This plant uses Reflo-68A oil. Actually, it is good oil. One Frick compressor run 157,000 hours without overhaul. Do we have any criteria of normal or excessive oil carryover?

16-05-2017, 09:37 PM
What conditions is it running as well as oil & discharge temperatures?
Oil cooling method (don't tell me liquid injection!).

17-05-2017, 10:23 PM
This is typical cold storage. Pressures: suction 2-5 psig, discharge 100-150 psig. Temperatures: oil 125F, discharge 180F. Oil cooling is thermosiphon.

18-05-2017, 08:07 PM
This is typical cold storage. Pressures: suction 2-5 psig, discharge 100-150 psig. Temperatures: oil 125F, discharge 180F. Oil cooling is thermosiphon.

Segei, as you say all seems normal, so no easy answer.
Do Frick have any advice?
Most manufacturers are stating 15ppm or less.
I see that Frick will only guarantee new compressors if you use their oil only, so guess they would say same for coalesces.
Recently GEA informed us that one package using oil is because discharge pressure changed to quickly causing foaming!
Has compressor changed duty, or is it correct compared to today's selections?
Does oil analysis show oil viscosity as being correct, Suniso mineral oil for example can throw over lower viscosity part of it's blend & viscosity goes up when oil check carried out.

21-05-2017, 11:57 PM
I've done some math and let's look at the numbers.
During winter operation this plant run one compressor RWB -134 with displacement of 790 CFM. Suction pressure is 5 psig and ammonia density at this pressure is 0.073 lb/ft3. 790 x 0.073 = 57.67 lb/min.
57.67 x 453.6 g X 60 min X 24 hrs= 37,667,000 g/day Assume that oil carry over is 1 ppm. It will be 37.667 g/day or 37.667 x 30.5 = 1150 g/month. Oil density around 0.9. It means at 1 ppm of oil carry over, 1278 ml of oil will be drained every month. To drain 3.8 liter of oil every month, oil carry over will be 3 ppm. I think it is not bad for 20 years old plant. I'm not sure how people get 2 tablespoons of oil carryover every year, because it is 1500 times less than 3 ppm.