Semi - Hermetic Compressor

[dannyk]

Friend of mine has a walk in freezer and condensor has a mini semi-hermetic compressor on it (3 phase), i have mainly worked on smaller systems and never really worked on a semi-hermetic, the compressor runs (quite loudly) and pulls the freezer down to around -5,by the time it reaches this temp there is a massive knocking noise coming from the compressor and it shakes like hell, then it knocks the contactor out, after 10 seconds the contactor pulls back in then out etc, the pressures are fine and system pumps down ok etc, is this a common fault on these? can anybody tell me what kind of overload device these compressors have on them? It would appear that the overload is knocking it out but like i say i have never really worked on these, thanks in advance!!

[nike123]

How is set low pressure switch? How much superheat you have?
What is compressor model#? Is it pump down system?
How is overload relay set? -5°F or °C?

[dannyk]

Compressor is a Frascold, Type - B210Y, 3 phase, R22 gas. System is pumpdown system, solenoid remains open, i also have continuity through pressure switch, so it must be a compressor problem. When it runs the compressor head gets really hot and compressor gets louder and louder. With regards to overload relay i have never worked on this type of compressor, only overloads i know are klixons, will the overload relay be found in the control panel?

[cadwaladr]

they have a thermister and relay located in the top/connection/terminal box on the compressor,sounds like it may be tripping has it got a tjermal o/load ,normally mounted on the contactor could be set for auto/reset,sounds like it is bol!!!!,i am led to believe its hard to get them repaired?

[cadwaladr]

by the number you quote its a 2 h/power

[monkey spanners]

Could be running on two phases, put your amp probe on each of the three lives in turn, amps should all be similar. If not could be duff contactor, blown fuse, faulty windings.

Or if its not electical, could be a snapped con rod, try a pump test and see what happens, you could take the head off and see also, you will need some new gaskets if you do though.

[dannyk]

Thanks for the promt reply guys, will check phases first, it does have an overload on the contactor, system is on r22 so if it is compressor will probly just get it replaced with r404 instead of repair, going to job tomorrow so will keep you posted, thanks again!!

[cadwaladr]

if you do have to replace it change the contactor and o/load,the times i have seen repeat failures because this has not been done,and give the rating of the contactor some thought,10/16 amp sounds right.clean condensor etc

[nike123]

If electrical parts are OK (I would change contactor and check control circuit every time I see such behavior) than you should measure superheat and subcooling.
By Frascold software, if you have -15°C evaporation (-5°C ambient) and total superheat of 15°C (suction pipe at 0°C) you than probably have discharge temperature around 120°C-130°C, witch is high and temperature protection should react.
If it is badly executed you could have chattering and then overload protection trip after few chatters.

Also that compressor have INT69 motor protector which are known to me as, sometimes, source of problems like this. So, make bypass of it, but closely monitor current and discharge temperature.

I would suggest to change gas to R404A since R22 is not for low temperature application with these compressors without additional measures for overheating compressor.
Simple change of gas (oil, TXV etc....) will bring down discharge temperature to acceptable range of 80-90°C.

[chemi-cool]

All The symptoms you describe are of a broken valve in the compressor head.

have it fixed and change the TXV to a new one. These things can loose some of the charge or get stuck from dirt.

[cadwaladr]

All The symptoms you describe are of a broken valve in the compressor head.

have it fixed and change the TXV to a new one. These things can loose some of the charge or get stuck from dirt.

good shout chemi,taking the head off is a good idea i think the gasket could survive ,strange how it pumps down ok?

[chemi-cool]

The gasket should be changed too.
It will pump down OK with a broken valve but will not hold it.

I would take the valves plate for grinding as well, makes it like a new one. [If the rods and pistons are in good shape]