Wierd Low superheat to compressor on LPR

HI Folks, can any one help? i have a cold store on a lpr pack, 12cyl grasso compound, and i am getting a low superheat at the compressor when i can see the level in the drum at half the sight glass , being the correct charge to submerge the liquid line being subcooled. i am evaporating at -33 c and "dry return" is at 31-32 c, there is a hp float that controls the main llsv and a bleed v/v always energised both with a 3mm orifice plate. any suggestions why i should be getting such a low superheat at the m/c

Thanks

Maybe the subcooling you have is too much for the thermal load at this point.

Sounds like it is normal as when compressor is drawing gas, or in your case saturated gas you would expect maybe 1-2 deg c s/heat.
Its not like a tx valve which is controlling s/heat in an evaporator,so suction gas is gauranteed dry.
If you have correct level in suction accumulator, oil in crankcase is clear of foaming or liquid , discharge temp is to hot to touch and crankcase is warm i would say no problem.

That sounds about right for a star spec LPR system.

Are you backing up liquid at the condensor and making the float level?

Also what is the wet return temp and liquid line temp into and out of the LPR subcooler?

As stated by others, you are not looking for super heat here, instead you need to be looking for the main liquid valve operating regularly, a decent (-15K or better) subcool and a wet return from the evaporator

Thanks for the reply guys, i understand the fundamentals you are describing and initially is what i thought and have come across but the machine controller is tripping on low S.H ( 2 c) my thoughts are that the discharge is running hot and my suction pressure being slightly high that the comp suction vlv`s are passing slightly, making it impossible to pull the suction down enough? What you think?

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Originally Posted by mpwp

Thanks for the reply guys, i understand the fundamentals you are describing and initially is what i thought and have come across but the machine controller is tripping on low S.H ( 2 c) my thoughts are that the discharge is running hot and my suction pressure being slightly high that the comp suction vlv`s are passing slightly, making it impossible to pull the suction down enough? What you think?

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Hello mpwp:)

I have never seem a low superheat cut-out on an LPR system, but it could be do on a Telestar.

High discharge temps indicate you are not carrying over liquid. Is the oil foaming in the sump or the oil temps low this would indicate liquid carryover.

The low superheat cut out could be low super heat on the oil return from the LPR, ie the oil rec solinoide is stuck open/passing, you could isolate the oil rec line to see if this stops the fault.

What discharge temp have you, the suction press seems about normal.

Happy hunting:cool:

Kind Regards Andy :)

Thanks again for the reply,
The plant is not a star lpr, it has a monitron control that cuts out at 2 c S.H (comp being a Grasso Rc 1293e) Ye the discharge is around 150 c being far too hot and i dont think it is actually liquid just that the compressor is struggling to pull down the suction and the return temp is becoming colder the longer the plant runs eventually meeting the saturation temp. There is no foaming of the oil and its temp is fine. The only doubt in my mind is that are the suction v/vs passing due to the high HT and suction pressure or is there something passing from high to low ie equalistion v/v or 4 way, im thinking the compressor suction v/vs.?!? Cheers.

When was comp looked at last?
Is it due for inspection?
Did problem suddenly occur,or has it gradually occured?
Maybe you could check discharge valves with shutting m/c down,closing discharge stop and see how valves hold.
As for 4 way valve goes have no experience on how to check,but if in correct position for ports it shouldnt be to bad.Does it have a particular ice pattern on it thats not there anymore?

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Originally Posted by mpwp

Thanks again for the reply,
The plant is not a star lpr, it has a monitron control that cuts out at 2 c S.H (comp being a Grasso Rc 1293e) Ye the discharge is around 150 c being far too hot and i dont think it is actually liquid just that the compressor is struggling to pull down the suction and the return temp is becoming colder the longer the plant runs eventually meeting the saturation temp. There is no foaming of the oil and its temp is fine. The only doubt in my mind is that are the suction v/vs passing due to the high HT and suction pressure or is there something passing from high to low ie equalistion v/v or 4 way, im thinking the compressor suction v/vs.?!? Cheers.

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Hello mpwp:D

Sorry I forgot there are a lot of LPR's built by people other than Star.

Monitron suction tranducers wander, if this is out the suction supheat warning will come up.
This transducer also does your oil cutout, I have seen a compressor destroyed by this.

As with all electronic controls, best fit mechanical safetys which will protect the compressor when they fail.

Discharge temp is too high, I would pull it down and rebuild, plates, rings, big ends, small ends and check the end float on the mains.

Good luck with this one;)

Kind Regards Andy D.

Thanks andy, ye the transducers and temp probes was the first thing i checked as they often drift as you stated. The monitron has a suction transducer for the lp and sh measurment and also there is a crankcase transducer that is used to calculate the d.o.p. I have orded the gear to perform a topend o/haul and while m/c is down i`ll check for small/big end play as there is a threaded hole in the top of the piston, i`ll make a tool to fit in and clock the play @ t.d.c (we all know how the conrods get a battering on a compound). also i`ll check the valves with some paraffin to see if they pass.
Hope this will sort it as the spares for grasso are a horiffic price, should have stayed working for a manufacturer!lol!

Thanks every 1

interchange discharge valve and sonde suction ;)

A refrigeration system without any super heat only happens in heaven.
You must live right.

have you checked for non condencables in system?

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Originally Posted by RANGER1

Sounds like it is normal as when compressor is drawing gas, or in your case saturated gas you would expect maybe 1-2 deg c s/heat.
Its not like a tx valve which is controlling s/heat in an evaporator,so suction gas is gauranteed dry.
If you have correct level in suction accumulator, oil in crankcase is clear of foaming or liquid , discharge temp is to hot to touch and crankcase is warm i would say no problem.

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I also agree with you.

Quote:

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Originally Posted by mpwp

Thanks for the reply guys, i understand the fundamentals you are describing and initially is what i thought and have come across but the machine controller is tripping on low S.H ( 2 c) my thoughts are that the discharge is running hot and my suction pressure being slightly high that the comp suction vlv`s are passing slightly, making it impossible to pull the suction down enough? What you think?

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I it is tripping change parameter superheat alarm and put it to 0,5 K. I think you can easy change it (with UNISAB II/III SABROE you can do it easy...with Grasso I don't know). For a flooded system it's normal the SH be nearly 0 K because inside the surge drum we are talking about saturated vapor.

About the discharge temperature you have said on another post 150ºC and in fact is really hot and it should be on the limit of oil temperature. What's intermediate pressure of the compound compressor?

Also what's the discharge pressure?

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Originally Posted by Ponca Dave

A refrigeration system without any super heat only happens in heaven.
You must live right.

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Yes that's it: Althought we are dealing with saturated vapor on the dry suction line it happens that the line is not 100% free of heating inlet by thermal conduction by the insulation and the pressure drop since the drum surge to the compressor (pressure drop on the connection, curves, pipe and valves). The pressure drop bring the state of the saturated vapor to a region of superheated (see p-h diagram).

When you overall the compressor it is a good idea to get the manuals from the Grasso web site and print them off they are very helpful and pretty good compared to the Sabroe ones.