bitzer 6f compressors blowing head gaskets

[chemuss]

Hi there, can anyone help me with this tricky fault i am facing. I have two bitzer 6f compressors and they are giving me grief in regards to blowing head gaskets on the discharge side of the cylinder heads. i have set my txvs to operate with a 4k superheat. The suction super heat 500 mm away from compressor suction service valve is 15k and the system is on r22. Condensor subcooling is set to5 - 10 k and the high side temp at both compressor discharges at service valves is around 90 degrees celsuis. I am trying to maintain the subcooling as i have been told it will help get the best performance out of the system. It seems like a possible flood back slightly some times on start up causing the problem. I heard it flood back once when i was next to machine and it only lasted for litteraly one secound. The system pumps down to 17 psi on its off cycle but suction header looks to still be a bit icy when compressors are off. I have checked llsv for diaphram by pass and have had it apart and it looks fine. I am wondering if it. i should have the system pump down to 10 psi to recover any left over excess refrig in suction header. The oil is not holding refrigerant in the sump and by this i am meaning the compressors are not overfill with oil as they are controlled by trax oil level controllers which are in good condition. The head presure maintaining is 220 - 240psi to maintain my subcooling. This one has got me stumped and any help would be appreciated. Thanks.

[monkey spanners]

Only really had head gasket problems twice, once when we changed from 3gs oil to poe and it brought a load more oil back and blew the gasket about three times till the oil level stabalised.

Second site customer had run compressor with leaky valve plate (would whistle when shut down) changed valve plate but kept blowing gaskets so decided head or block had warped from the heat as the paint had changed colour!

[al]

Are these single or dual stage compressors? If dual then air would be the most likely culprit, check your discharge temperature is peaking, this makes the gaskets brittle.

al

[Grizzly]

Hi Chemuss.
You are to be congratulated for a informative post with all the right things checked.
I will question your comment about maintaining the Head pressure so high.
What sort of condensers do you have.
I struggle with the high discharge pressures you are operating with it operating at 40+c and pumping down to
-22c.

What is the Amperage being drawn by the comps like? High?
What is the condition of the condensers like?
I tend to agree with Al, Air in the system would be the most likely culprit.
Good luck.
Grizzly

[RANGER1]

Agree with others dischage pressure to high, but need to know anbient temp first.
If ambient is in the lower region then it could also be overcharged amongst other things.

4 deg C sounds a bit low as well, maybe should be 6 deg C

What is this system, a freezer?

What type of defrost,& does it have a suction slop pot?

Does oil ever foam up in sump?

[joe-ice]

I have also come across this on plant that had faulty oil cicuit at times flooding compressor with too much oil, might be a good idea to connect high pressure switch directly onto the head of the compressor if not already there.oil in the head can give an hydraulic effect that wont show up at the discharge flange

[Grizzly]

Hi Chemuss.
I had a thought at work earlier, does your setup incorporate a check valve in the discharge line.
Which is not uncommon on these type of comps.
If so what is the state of it?
If that were passing. therefore allowing say.
Condensed discharge gasses (IE Liquid!) to sit on the discharge valves etc.
That would do some damage.
Especially on start-up!
Grizzly

[chemuss]

Thanks i have. had to change valve plate as it is faulty after it blew the gasket out on the high side. also replaced two pistons asthey where badly scored but managed to save compressor bores with a light hone. I have installed new head and valve plate gaskets with copper coat this time to get a tighter seal once torqued up. I lowered pump down setting to 10psi to try and draw as much liquid and oil back to condensor and reciever in off cycle. Ambient temperature out here in brisbane inland is +30 - +40 degrees. The system is a hydrocooler so we have a large stainless steel evaporator which has water pumped over it. water gets chilled to + 2 and then gets pumped over + 25-+30 degree corn and warm water gets recirculated back to stainless steel evap. Condensor is at same level as compressors and there is non return check valves on discharge lines to prevent oil migrating back to compressor cylinder heads in off cycle. Compressor cylinder heads where not overly dry or wet when i had compressors apart.

[chemuss]

The compressors are single stage aswell. Thx

[chemuss]

Thanks for the reply. The condensor is buffalo trident in good nick and has six fans at 500 mm each. staging of fans is as follows fan one and two on at 220 anb off at 210. Fans three and four on at 240 off at 230. Fans five and six on at 250 off at 240. I can change this on plc if you recommend better staging but i have worked pressures around having my subcooling sit between 5 and 10 k. Ambients are around + 30 - +40 degrees celsius. Compressors draw 33amps each leg and is single stage system. I pump system down to 10 psi when system cycles off to try and draw any excess pockets of oil or refrigerant back to reciever in off cycle to prevent any slugs on start up.

[chemuss]

Thanks for reply. There is only the suction header on this system that acts as accumulator. Ambient is around 30 - 40 degrees celsius. The machine is a hydrocooler. Chilled water flows over stainless evap and is then pumped over 20 -30 degrees celsius corn and then warm water is recirculated back to stainless evap. Water leaves evap at +2 degrees celsius to cool corn down to +6 degrees celsius. No defrost on this system as warm product keeps entering water over evap always warmer. Coil only ever ices up if clowns operating machine over fill it with water and clowns here are in large supply with no tent big enough to hold them.

[chemuss]

Thanks for reply. I think oil is ok as there are trax oil level controls that are working ok and decent size oil seperator that is only filled to middle sight glass. Oil does not foam in trax oil sight glass.

[chemuss]

Yep i have one for each compressor but have not taken them apart as i checked the sealing of them when i pumped system down and had suction service valves closed fully i let discharge pressure out of compressors and discharge pressure reduced to nil which proved the check valves seal while off. The comps also have hot gas bypass valves that operate on start up to relieve discharge pressure from heads for a breif secound but i have isolated them from system as once a comp blew up due to bypass diaphram failing.

[RANGER1]

I think you need a suction slop pot with liquid line coil going through it to boil off excess liquid refrigerant.

They don't cost that much, call it insurance.

The suctin line should be dry all the time without one.