oil not returning on 15 hp med temp refrigeration

[circa1]

i have a problem with existing unit 15 hp 3 evap coil 3 tx valves 4ton external equillized suction from condensing unit on roof 1 3\8 liquid 5\8 reduced to 1 1\8 to one coil 7\8 to second coil oil not returning since we changed compressor copeland semi
10 super heat but noticed suction pressure 55 psig at each coil but at compressor 45 psig there is a 10 psig drop and oil not returning to compressor
changed oil in new compressor to mineral using r22 ***** 35 years in the trade my first defeat this system is a flooded system head master set at 200 psig sight glass clear no hunting or liquid flooding

[LRAC]

Hi Circa 1

Have you changed it too R22 please clarify what refrigerant it was on before.

Regards
LRAC

[circa1]

r22 before and after no change

[LRAC]

Hi Circa 1
On a system this size is there an oil seperator fitted.

[circa1]

no oil separator 5 c box med temp

not needed

[refteach]

Was there a viscosity change?

[circa1]

no modifications made compressor only changed tx valves no difference suppeheat 12 f

[Samarjit Sen]

As per your initial statement you said the condensing is located on the roof . Where are the evaporators located and what are the vertical heights of the suction and the liquid lines. Have you provided any loops in the liquid line. What are the sizes of the liquid lines and the capacities of the individual evaporators.

Although at 5 Deg C Te Oil Separators are not required but in such installations where the condensing units are located higher than the evaporators, using oil seperators will be better. Moreover the piping has to be laid properly designed.

[US Iceman]

A couple of questions...

Is the oil level slowly decreasing in the new compressor crankcase site glass?

Where are you measuring the superheat? At the evaporator outlets or at the compressor suction?

[Dan]

i have a problem with existing unit 15 hp 3 evap coil 3 tx valves 4ton external equillized suction from condensing unit on roof 1 3\8 liquid 5\8 reduced to 1 1\8 to one coil 7\8 to second coil oil not returning since we changed compressor copeland semi

I'm sorry. 1-3/8 liquid. You mean suction with a 5/8 liquid, correct? So you are feeding a 7/8 suction from the last coil to feed liquid, 1-1/8 after passing the second coil to feed liquid and 1-3/8 suction after the last coil to feed liquid.. and this 1-3/8 suction is trapped before rising to the compressor unit? Original piping?

45 psig there is a 10 psig drop and oil not returning to compressor

A lot of good questions being asked in this discussion. Here's my best guess not knowing those details. You could have a horizontal suction pipe that is either sagging or pitched incorrectly that is oil laden, causing the pressure drop and lack of oil return.

[monkey spanners]

Are all the tev's working? Ive seen low oil problems on ice builders when one of the tev's was faulty. They can flow enough to let refrigerant/oil in but theres not enough velocity to carry the oil back out of the evaporator. Ive also seen pipes squashed flat due to ice causing the same problems. Have you changed the oil from ester to mineral?

When i did a lot of CFC conversion work for my old boss, the guys from castrol said the best refrigerant/oil combination was CFC/HCFC and ester oil. Ester oil lubricates better than mineral, and if you get any metal to metal contact the chlorine reacts with the hot metal to sort of case harden it.

cheers Jon

[US Iceman]

oil not returning since we changed compressor copeland semi

This was the original statement.

If indeed there was no oil return problem before the compressor was changed, why would it start after the compressor is changed? If this holds true, I'd say the replacement compressor is a smaller displacement which establishes lower suction line velocity.

On the other hand, perhaps this condition existed before the compressor was changed, and this was the cause of the initial failure.

Dan was going with the same line of thought I was considering... oil logging in the system = no oil return & higher suction line pressure loss.

[Frostycold]

Hi If the CU is on the roof then the evaporators are obviously lower than the CU. From the terminology used I am assuming that the system is cooling only. If so you should put some oil traps in the suction line or not only will you burn out compressors but eventualy you'll reduce the efficiency of the system as the evaporators will fill up with oil
Regards Frostycold

[old gas bottle]

hi mate, was the compressor a new unit or rebuild?,if the later it might be just a bad one causing the problem,i would ditch the mineral oil out of the system and use a good semi synthetic that will mix with the residual mineral left trapped,also i would fit a seperator for good measure,as you know sometimes a change of compressor can bring more problems.

[Electrocoolman]

Hi,
All large R22 systems I've worked on have had oil separator fitted, even where theres very little difference in height between evaps and condensing unit. R22 is not very miscible with mineral oil. How about using Alkyl Benzyl oil as its recommended for use with a lot of R22 based 'drop ins'.

Cheers Adrian