Excessive oil foaming

[mohamed khamis]

Hi guys

I came cross a problem related to a reciprocating 40 HP compressor Carlye (semi-hermetic) works in Dx system. This system with R-407C the operating design conditions should be 62 PSI (corresponds to 5 C) and 255 PSI (48 C at ambient air 35C). The system consists of two units sharing the evaporator (interlacing connections) The main problem is whenever, the system is on one unit works very well in terms of the pressure range but the second one has problem of oil foaming and almost evacuate the crack case from the oil. The two units are identical and similar in every thing except the operating pressure 70 PSI and 290 PSI. Nevertheless, I made a pump-down system to be activated the oil foaming in the oil sight glass still there. the crankcase is not heated well but u can say slightly cool even though the crankcase heater is on. I know this problem of liquid migration to the system but why one unit is working well and the other does not although the both are identical?

Now I m going to suggest other solution and I need ur advice to tell me which of them is the most effective to prevent or at least reduce this foaming and oil migration from the compressor:

1- reverse the motor rotation to reverse the oil pump rotation.

2- adjust the TXV to more tightly.

3- relieve some gas from the system, I guess it is overcharged than the other, but the technician told me he charge by almost the same number of gas cylinder.

4- give a small current to the motor to assist the heater during the off time.

Thank you in advance

Best regards

Mohamed

[Grizzly]

Mohamed.
Do you have FOAMING, created by the introduction into the crankcase
of liquid refrigerant.
Or do you have CHURNING, created by the oil level in the crankcase being to high. Old time Engineers used to refer to this as a "wet crank".
where the white metal bearings ran partially submerged in a bath of oil.
In short
The first thing I would check is the amperage being draw by the crankcase heater. Compare it with the 2nd one.
Is the errant machine the "lead compressor." Do they both share a suction header (which one has preference on the suction return?) That would explain a lot !
Or is the foam there all the time?
In which case is the crankcase normal temp or cold?
Please tell us more.
Grizzly

[mohamed khamis]

Many thanks Grizzly for your input

Actually, I don not know the difference between the two expressions (foaming and churning) as far as I know both are the same. Anyhow, that's I noticed yesterday in the site, the efficient system has very warm crankcase and cylinder walls but that for second one is cool or slightly cool. And I put my hand on the area nearest to the heater is very hot, that's mean it works.

The two units/systems share the evaporator and each one has separate compressor, distributor,header collector typically like the attached file.

Best regards

[NoNickName]

If one compressor is warmer, then it probably has got a broken discharge reed. That would also explain the foam (gas return into the crankcase)

[mohamed khamis]

If one compressor is warmer, then it probably has got a broken discharge reed. That would also explain the foam (gas return into the crankcase)

Ok I forgot to tell that I observed the oil return pipe from the oil separator was sweat for this compressor this means the discharge reed has been broken, is not it?

[NoNickName]

Well I'm not sure.... how can cold gas or vapour end up in the oil separator?

[nike123]

If one compressor is warmer, then it probably has got a broken discharge reed. That would also explain the foam (gas return into the crankcase)

As I understood his posts, colder of compressors has foaming and not warmer one!

[nike123]

Cold crankcase usually mean liquid in crankcase and therefore foaming. Check your superheat at compressor and compare with hot one! Also check subcooling!

[NoNickName]

Cold crankcase usually mean liquid in crankcase and therefore foaming. Check your superheat at compressor and compare with hot one! Also check subcooling!

It can also be. Sure.

[mohamed khamis]

Cold crankcase usually mean liquid in crankcase and therefore foaming. Check your superheat at compressor and compare with hot one! Also check subcooling!

Well I know but how can I solve this problem?

[nike123]

Well I know but how can I solve this problem?

First we need to know what is the problem to be able to solve it. Make asked measurements!
If superheat is low, then you have to check sufficient air flow, proper functioning of TEV and if necessary adjust superheat at TEV after you are sure that all other thing are OK!

[Grizzly]

Mohamed
Many thanks for the excellent sketch.
If I understand what you have shown. It means that each system is separate despite being within the same evaporator.
With the foaming Compressor having a cold or colder crankcase etc.
Basically what the guys are trying to politely say is!
It would appear that you have all the classic symptoms of liquid refrigerant being pulled back to the compressor.
Which basically means that instead of the refrigerant boiling off into a vapour within the top section of the evaporator tubes.
Liquid is being drawn along the suction line and into the compressor.
If you measure the superheat of both compressors you should find that the good one will have a higher s/heat than the faulty one.
The nearer to negative s/heat you get the nearer to liquid.
Simply put if temperature of the refrigerant pipe out of the evaporator.
Is at a lower temperature than the temperature for it's given pressure (shown on a pressure temperature chart aka comparator!) there is liquid within the pipe!
So if this is the case winding in the expansion valve will lower the point within the evaporator that the refrigerant is boiling off.
Giving a higher s/heat and eliminating the liquid refrigerant being carried into the compressor!!
Not the best worded explanation but hopefully you understand?
Grizzly

[mohamed khamis]

Mohamed
Many thanks for the excellent sketch.
If I understand what you have shown. It means that each system is separate despite being within the same evaporator.
With the foaming Compressor having a cold or colder crankcase etc.
Basically what the guys are trying to politely say is!
It would appear that you have all the classic symptoms of liquid refrigerant being pulled back to the compressor.
Which basically means that instead of the refrigerant boiling off into a vapour within the top section of the evaporator tubes.
Liquid is being drawn along the suction line and into the compressor.
If you measure the superheat of both compressors you should find that the good one will have a higher s/heat than the faulty one.
The nearer to negative s/heat you get the nearer to liquid.
Simply put if temperature of the refrigerant pipe out of the evaporator.
Is at a lower temperature than the temperature for it's given pressure (shown on a pressure temperature chart aka comparator!) there is liquid within the pipe!
So if this is the case winding in the expansion valve will lower the point within the evaporator that the refrigerant is boiling off.
Giving a higher s/heat and eliminating the liquid refrigerant being carried into the compressor!!
Not the best worded explanation but hopefully you understand?
Grizzly

Really thank you very much for excellent explanation. However, I expected all of this before going to the site but why the unit is right and the other is faulty although the both are perfectly identical.

Now I am thinking about it is excessive oil in the compressor, may be causes this foaming. Because I observed the oil level in the faulty compressor is full in the sight glass in the shut down mode but 3/4 the sight glass for the right one. And I think it is not recommended form compressor supplier that. Anyhow, I will go next week to the site and remove the oil little bit until make it the same with other and adjust the TXV after that and see what is happened. Once again, my appreciation to your kindness.

Best regards

Mohamed

[Grizzly]

Mohamed
I think you have now worked out the solution yourself.
A good oil level to maintain is a maximum of 3/4 sight glass when off, (usually this will reduce to approx 1/2 a sight glass when running) anymore and the spinning crank is "churning" up the oil.
BUT! A good engineer will find out what the manufacturers recommended oil level is!!
As excessive oil level will turn a recip compressor into a very efficient oil pump!
To much oil is worse for a system than a slightly low level.
let us all know how you get on?
Grizzly

[nike123]

Really thank you very much for excellent explanation. However, I expected all of this before going to the site but why the unit is right and the other is faulty although the both are perfectly identical.

Now I am thinking about it is excessive oil in the compressor, may be causes this foaming. Because I observed the oil level in the faulty compressor is full in the sight glass in the shut down mode but 3/4 the sight glass for the right one. And I think it is not recommended form compressor supplier that. Anyhow, I will go next week to the site and remove the oil little bit until make it the same with other and adjust the TXV after that and see what is happened. Once again, my appreciation to your kindness.

Best regards

Mohamed


High level of oil in sight glass doesn't necessary mean that you have excess oil in system. If you have liquid refrigerant diluted in oil in crankcase, you also have high level in sight glass. You must be sure that crankcase heater is working and that sufficient time is passed to allow that all liquid refrigerant in crankcase has evaporated before removing excess oil.

Also, before adjusting TXV read all posts in this thread:
http://www.refrigeration-engineer.com/forums/showthread.php?t=11903&highlight=adjust

[Grizzly]

High level of oil in sight glass doesn't necessary mean that you have excess oil in system. If you have liquid refrigerant diluted in oil in crankcase, you also have high level in sight glass. You must be sure that crankcase heater is working and that sufficient time is passed to allow that all liquid refrigerant in crankcase has evaporated before removing excess oil.

Also, before adjusting TXV read all posts in this thread:
http://www.refrigeration-engineer.com/forums/showthread.php?t=11903&highlight=adjust

Good point Nike, as they say well presented!!!
Grizzly.

[mohamed khamis]

Hi there

Thank you all for inputs. Thanks God, I have solved the problem yet and the system is working very well now and the oil is stayed in the sight glass with a little bit foaming on the surface as recommended by the compressor manufacturer.:D

However, I got another call from the contractor informing me that there is shaking/fluctuation in LP Gage between 50 PSI and 60 PSI in another system. Where the plant consists of 4 systems each has 2 units. So I will go to check it, any body could expect what is this problem?.

I am afraid this is an erratic performance of TXV (hunting) due to some liquid inside suction line side. He told me the HP Gage is maintained at 250 PSI while there is a shivering in LP readings. :(

Any participation is highly appreciated in advance

Best regards

[Grizzly]

Hi there

Thank you all for inputs. Thanks God, I have solved the problem yet and the system is working very well now and the oil is stayed in the sight glass with a little bit foaming on the surface as recommended by the compressor manufacturer.:D

However, I got another call from the contractor informing me that there is shaking/fluctuation in LP Gage between 50 PSI and 60 PSI in another system. Where the plant consists of 4 systems each has 2 units. So I will go to check it, any body could expect what is this problem?.

I am afraid this is an erratic performance of TXV (hunting) due to some liquid inside suction line side. He told me the HP Gage is maintained at 250 PSI while there is a shivering in LP readings. :(

Any participation is highly appreciated in advance

Best regards

Mohamed
Thank you for updating us, all to often people do not bother!
But please could you advise as to which problem it was that "you have solved the problem"
Grizzly

[mohamed khamis]

Mohamed
Thank you for updating us, all to often people do not bother!
But please could you advise as to which problem it was that "you have solved the problem"
Grizzly

Me too.....The system which has excessive foaming now it is working well...the problem was related to excessive oil amount than the required by manufacture.

Now there is another system has another fault which is fluctuations in LPS Gage....so any advice or suggestion will be much appreciated.

Best regards

Mohamed

[hendry]

Hi guys

I came cross a problem related to a reciprocating 40 HP compressor Carlye (semi-hermetic) works in Dx system. This system with R-407C the operating design conditions should be 62 PSI (corresponds to 5 C) and 255 PSI (48 C at ambient air 35C). The system consists of two units sharing the evaporator (interlacing connections) The main problem is whenever, the system is on one unit works very well in terms of the pressure range but the second one has problem of oil foaming and almost evacuate the crack case from the oil. The two units are identical and similar in every thing except the operating pressure 70 PSI and 290 PSI. Nevertheless, I made a pump-down system to be activated the oil foaming in the oil sight glass still there. the crankcase is not heated well but u can say slightly cool even though the crankcase heater is on. I know this problem of liquid migration to the system but why one unit is working well and the other does not although the both are identical?

Now I m going to suggest other solution and I need ur advice to tell me which of them is the most effective to prevent or at least reduce this foaming and oil migration from the compressor:

1- reverse the motor rotation to reverse the oil pump rotation.

2- adjust the TXV to more tightly.

3- relieve some gas from the system, I guess it is overcharged than the other, but the technician told me he charge by almost the same number of gas cylinder.

4- give a small current to the motor to assist the heater during the off time.

Thank you in advance

Best regards

Mohamed

hi Mohamed,

i feel it has to do with the design.
to me, oil foaming is due to insufficient suction pressure to keep up with the system.

you might have undersized piping & valves somewhere.

oil migration is quite normal for centralised system.
you need to look into common areas where you could salvage & equalise the lube oil system of both compressor.

"have a safe & productive day"
:)

[mohamed khamis]

hi Mohamed,

i feel it has to do with the design.
to me, oil foaming is due to insufficient suction pressure to keep up with the system.

you might have undersized piping & valves somewhere.

oil migration is quite normal for centralised system.
you need to look into common areas where you could salvage & equalise the lube oil system of both compressor.

"have a safe & productive day"
:)

Hi hendry

Thanks for your participation the problem was solved allahdullah

Best Regards

Mohamed