oil separator addition

[friocold2008]

Dear fellows:

Recently I got a maintenance contract with a medium sized supermarket where most of the medium temperature cases are individually paired with small remote condensing units, some of them located quite far. I learned from the log that there had been a few cases of compressor replacements, mostly related to insufficient oil supply returning to the compressor (at least that is what the diagnosting tech wrote ). Suction lines seem to be correctly inclined, and suction risers have suction traps, so I don´t see much trouble with the piping. I´m planning to install oil separators for the compressors in those cases where the piping is very long (25 to 30 meters, sometimes longer). Since this is the first time I´m dealing with these devices, I have one question for the experts: oil separators have three connections- refrigerant inlet, refrigerant outlet, and oil return. From the catalogs, I can see that the oil return connection is to be taken to the crankcase. Most of the condensing units have hermetic compressors, therefore, should I route the oil return back to the suction line? One more doubt: I guess that the pressure of the returning oil out of the separator must be quite high (discharge pressure maybe?), do I need to reduce this pressure somehow (regulating valve) before it reaches the low side of the system ?
Thank you for any comments.

[Brian_UK]

I would think that the oil return line could be taken back to the suction line without problem. A reasonable distance from the compressor shell itself might be wise.

The pressure in the oil separator will be at compressor discharge pressure but the oil is returned via a needle valve so there shouldn't be any high pressure problems created when the valve opens to return some oil.

One manufacturers' view on their products here...
http://www.castel.it/pdf/catalogo_en/08_Separators.pdf

[taz24]

Dear fellows:

Recently I got a maintenance contract with a medium sized supermarket where most of the medium temperature cases are individually paired with small remote condensing units, some of them located quite far. I learned from the log that there had been a few cases of compressor replacements, mostly related to insufficient oil supply returning to the compressor (at least that is what the diagnosting tech wrote ). Suction lines seem to be correctly inclined, and suction risers have suction traps, so I don´t see much trouble with the piping. I´m planning to install oil separators for the compressors in those cases where the piping is very long (25 to 30 meters, sometimes longer). Since this is the first time I´m dealing with these devices, I have one question for the experts: oil separators have three connections- refrigerant inlet, refrigerant outlet, and oil return. From the catalogs, I can see that the oil return connection is to be taken to the crankcase. Most of the condensing units have hermetic compressors, therefore, should I route the oil return back to the suction line? One more doubt: I guess that the pressure of the returning oil out of the separator must be quite high (discharge pressure maybe?), do I need to reduce this pressure somehow (regulating valve) before it reaches the low side of the system ?
Thank you for any comments.


I agree with what you have said and would add that it might be wise to add a solenoid valve in the oil return line so the line can be shut when the comp is not running. This will stop any oil passing when the comp shuts down.
If you are fitting to the suction I would fit ball valves each side of the oil seperator and in the oil return line to the comp. That way you will be able to isolate the thing if you ever need to maintain it.

taz

.

[friocold2008]

Thank you very much for your interest and nice suggestions. I´m going to take a look to the link supplied by Brian.

[Tesla]

I've only worked on much bigger sysems with oil return using an oil resivour. They generally use something like a Sporlin CV20 pressure reducing valve, and isolation, solonoid valves, and sightglass go without saying

[NoNickName]

I second brian and taz.

[smilies]

Also, if oil return is suspect, where is it logging? With the replacement of each compressor, that much more oil is being added to the system. I would check that the suction piping size is adequate for the load. Are the risers the same size as the connecting pipe or smaller?

[friocold2008]

Following your suggestions, I went back and checked the suction lines sizes for the correct velocity. They seem to be all in the safe range. Additionally, I can tell you that the suction line sizes aren´t of the same size as the service valves for each system: they are one size smaller (1-1/8 vs 7/8, since this is a US made equipment). One more question: from the information I found in the suggested link, I realized that the oil separator is not 100% efficient, so that there is still some very little fraction of the oil passing through it. Would the suction line traps be still needed even after the installation of the separators, or could they be eliminated in order to reduce line total pressure drop ?

[seraaco]

Hi I don't think removing P traps is a good idea, and by the way are you sure there is an oil return problem? I would make sure first, you know how things are down here in Latin America, prior tech could'n be sayin' ALL CORRECTLY and just trying to keep out of his responsability.
Have you checked refrigerant charge? could be low
or liquid flooding back to the compressor
when did it start to happen? since start up or a few years later? 25 meters of piping should not be a problem for oil return, by the way could be easier for me if you speak spanish, thanks.

[wilsoncheung]

What kind of AC?
I don't think medium temperature cases would need oil separator.

[friocold2008]

Thank you all for your interest in my problem. By the way, how many oil traps are recommended for a vertical riser which is abou 10 meters high? Two of the systems which exhibit problems have only one trap located at the bottom of the riser. Is that ok?

To Seraaco: I sent you a message in Spanish to your inbox describing the situation. Maybe I will be able to explain me better in my native language. Please read it.

To Wilson: I also thought that medium temperature refrigeration systems working with R-404a did not require oil separators. But this is a special situation that maybe need them in order to assure quick oil return.

[seraaco]

Hi friocold i haven't received your mail yet you can send it to my yahoo.
gracias

[Scramjetman]

Are you sure it's definitely an oil problem. You my find it is something else smashing the compressors. A classic is when condensers are mounted above the compressor ( ie on the roof) and no discharge line traps to stop oil and liquid refrigerant running back down the discharge line from the full condenser during the compressor off cycle.

I've got 2 x water cooled chillers that have been retrofitted to air-cooled ( by others) with 3 out of the 4 compressors smashed because there are no discharge traps and the condensers are 10m higher than the compressors.

There should be a loop in the discharge line at the condenser where the discharge line rises higher than the manifold then drops back down into the top of it. At the compressor there should be a deep trap ( reaching to the floor) to intercept any liquid or oil that may drain back down the liquid line before it can get to the heads.

Oil can be a problem if you are having to top up the oil level constantly and there are no oil leaks. The systems are closed so the oil is either being pumped out into the discharge line or is stuck in the evaporator. Either way it is somewhere in the system and it must be returned to the compressor somehow. If this is not the case, and there is no requirement to keep adjusting the oil level, I'd be looking closely at the discharge lines and/ or TX valve superheats.

[friocold2008]

Thank you very much scramjetman for offering a fresh point of view on this matter. The systems I`m trying to troubleshoot use remotely located condensing units which have both the compressor and the air cooled condenser in one single package. I haven´t checked if they include an oil/liquid refrigerant line trap in the discharge line, but I´ll check it as soon as I get to the jobsite on Monday.