Hermetic strange sound.

[fridg]

Had a few small hermetic compressors on freezer cabinets that make a strange pumping sound.

(like on a fly spray can when you press the nozzle down then let go then press again)



It only makes this sound for the first 10 seconds after an off cycle , and even when the cabinet has been turned off for 24 hrs.

Monitored suction pressure and it fluctuates up and down 100 kpa , amp draw fluctuates up and down with the suction pressure , and this is just for the first 10 seconds while the comp makes this noise. Head pressure is normal , cabinet pull down rate is normal.


After this time the unit runs fine , amps good , suction good , no noise.

This unit has a cpr fitted , and adjusted so it is not liquid , charge weighed in from nameplate.


The engineers and reps have come in to take it away , for an explanation on why this occurring.

[hillbilly tech]

I have heard what your talking about before and i figured the compressor was just getting weak or something,anyway i just letem do there thing to they die any more no matter what they sound like.My thought was high pressure leaking threw to the low pressure a little ,valves or something.Gotta be something:D

[rocket]

sounds like you have a "Flooded Start", migration of vapor during the off cylce. It will kill the compressor. Check your crankcase heater also check superheat at compressor. It won't last long and the replacement compressor may suffer the same fate.

[frozensolid]

Could be a flooded start, but I think a weak discharge valve is bleeding until high side pressure comes up.

[aprophet]

is the compressor a scroll type compreesor

[fridg]

I am leaning towards a valve problem also. it is a reciprocating comp.


The manufacturer has tested the unit with a 3000 kpa head and they did not get the elusive noise.

Their next plan of attack is to dismantle the unit to have a look at the internals.

It has a cpr set so the flooded start is very unlikely , i have worked on a fair few of these systems and they all have the same specifications , but lately the comps seem to have changed internally for some reason.

[rocket]

flooded start is "OFF CYCLE", when compressor is not running.
refrigerant migrates to compressor during Off Cylce (CPR not involved)

[fridg]

Well got the answer back , and it was good and bad.

the reps and engineers pulled it apart and found no indescrepencies with the comp. They got the engineering measurement from the company that makes it and everything is on the right scale and measurements.

So it is unexplained why this was happening , and i have worked on the same system again and there was no problem.


Cross our fingers now is all we can do.