Maneurop pump down

[G Cook]

Went to a freezer room today which was iced up, turned out heaters were not being energised as the compressor was not reaching lp set point when pumping down - heaters wired through compressor contactor auxilary.
Condensing unit is a maneurop MGZ18C12G with oil seperator, refrigerant r404a. Pressures were suction 3.2 bar (with evap iced up and frost up to the compressor), high side 18 bar, ambient around 25c, currant draw was around 9.3 amps.
Tried to pump down by shutting reciever valve to eliminate sol valve, suction pulled down to around 10 psi and would not get any lower, closed oil return line using hand valve, this made no differance, then shut in suction valve still the compressor would not pull any lower, then i switched off the compressor, it held the pressure.

Where am i going wrong? If compressor was not pumping then surely I would not have 18 bar high side and suction pressure would rise when i turned compressor off? Shutting the hand valve on oil return should rule the seperator out?
When i arrived at the site the compressor was tripped off on internal overload no doubt this was due to it being over heated while attempting to pump down.

Any ideas? Thanks in advance!

[install monkey]

internal relief valve passing or not pumping efficiently-hence overheating from constant running

[G Cook]

Thanks Install.

The conclusion I came to was internal relief valve, although was not sure as suction seemed to be fairly constant, I suppose the valve could just be constantly bleeding through rather than blowing at a certain pressure? If it was not pumping would it achieve the 18 bar?

[install monkey]

i had a walk in freezer only doin -10- tested the compressor and would only pump down to a 6 inch vac- decided the comp was poor pumping- changed it and all ok- if you cant pump down at the liq sol valve ,reciever outlet or suction valve then something is causing the sump pressure to stay above the lp cut out range.- relief valve usually opens at 350psi- more likely to be valves on cylinder head passing- with all valves shut stop the pot and see how ur suction pressure increases.

[G Cook]

Thats the strange thing, it doesnt move

[install monkey]

is the running current higher or lower than normal- is the compressor 5yr old?- try increasing the lp switch to make sure it cuts out- just reread ur original post- set the lp to cut out at 15psi to see if it reaches setpoint- if its chugging its nuts off then it will overheat again- so compressor worn- take an oil sample and acid test- i suspect its dying

Thats the strange thing, it doesnt move

[monkey spanners]

Think bad suction valve would make it pump poorly, but if the dischage valves are still good they will stop any pressure leaking back.

I'd change the comp and then cut the old one open and see whats fubared!

[Tayters]

Oo, nice job for a Saturday. Hope it wasn't in the small hours.

All the poor pumpers I've come across have had pressure pretty much the same both sides, but hey, I've only come across a few!

Evidentely there is a pumping fault as it would only get to 10psi. My guess is that it has only just gone belly up hence the pressures you were getting.

SST is only -11*C - a bit high. Surely if you had an iced up coil suction would be lower because less heat would be absorbed therefore highlighting the compressor fault.
Was the condensing pressure measured with a clear indoor (and outdoor!) coil?

If it was only the suction reeds leaking then if the unit is pumped down it would hold the 10psi because the passage to the discharge side is sealed. When running, some of the gas on the 'exhaust' stroke would be pumped back into the suction thereby raising the pressure.

Cheers,
Andy.

[G Cook]

LRA 12 amps, this thing was running around 9. I'm sure i seen 2003 stamped on it.

Don't mind replacing compressor as long as its the compressor which is at fault. Don't suppose theres much else it could be other than the seperator, there was no rushing through the oil glass and shutting the hand valve should close that line anyway I suppose.

[monkey spanners]

If the seperator was letting by, the oil line would be hot as it would have discharge temp gas going down it, and the pressures would equalise too.

Think those maneurop comps have the suction valves in the pistons, not that it would make any difference :o

http://www.youtube.com/watch?v=ZjjHybec9LE

After watch vid see they have a disc valve but its in the head, was thinking of those copeland discus comps i think...

[install monkey]

defrost it, increase the lp switch to cut out higher -so it doesnt struggle to pump down-make sure the crankcase heater is working-if fitted-see if it gets to temp otherwise pot change

[G Cook]

As you said install, got rid of ice, increased lp and left it to get on with it. Was back the other day and its not doing to badly, suction pressure had dropped considerably. No doubt the comp is on its way out, but you know how it goes-if theres still a bit of life......

Thanks all

[Len Fiddler]

Good morning The Maneurop internal PRV will blow at 30 bar differential and should reset below 6 bar
As the compressor is suction gas cooled on a freezer application the suction valve would have a hard frost cover and frost over the top of the compressor. If you have frost under the compressor Panic as this is liquid. Len