View Full Version : Hermetic strange sound.

02-02-2007, 10:34 PM
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)

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.

03-02-2007, 09:03 AM
This unit has a cpr fitted , and adjusted so it is not liquid , charge weighed in from nameplate.

Hi Fridg

Not sure what the noise is but one thing to remember is that a CPR valve only works on pressure and cannot distinguish between liquid or vapour. Liquid return should be handled by a suction accumalator if it is excessive.


03-02-2007, 10:36 AM
What aplication is this comp on. You mention comps in plural sujesting more than one. Have you changed compressors because of this noise. if it is more than one comp then I would look for the common link. The sound could be refigerant passing through the cpr. The fluctuation in pressure and running amps indicates to me the load balancing out and settleing down through the valve.

Is it causing a problem or is it just a funny noise?
I would be tempted to let it run and see what happens.

Cheers taz.

03-02-2007, 10:41 AM
Just thinking aloud, why have a cpr anyway?
Is it on a cappilary or with a TEV?
if its with TEV then is it on pump down?
Cappilary tube alows the pressurs to equilise anyway and should not need a cpr.


03-02-2007, 07:48 PM
leaking LP reeds inside the compressor?

03-02-2007, 11:47 PM
Sounds to me it could be the cpr throttling the suction to allow the comp to get away clean.


04-02-2007, 04:54 AM
The system is a commercial cabinet freezer , single compressor , cpr , and suction accumulator on a cap tube system.

Have changed compressors and the same thing happens.
The system runs fine after 10 seconds , it is just initial
start up. The sound is definitely coming from inside the
comp. (sssh - sssh - sssh) and is happening with every stroke of the piston i imagine.

There is something happening with these compressors as it is not the first.

The system should run fine , as i am close with the design team , and have worked on the same system with them.

I know what the system should run like , but this is just unexplainable.

04-02-2007, 12:34 PM
The system is a commercial cabinet freezer , single compressor , cpr , and suction accumulator on a cap tube system.

G'day fridg.
I bet it's also got hot gas defrost as well.Might be possible that it might be the cause of your noise.

monkey spanners
04-02-2007, 04:40 PM
Some small hermetic compressors have a dischagre muffler with a pressure release valve built in. If the compressor is pumping liquid or the head pressure is too high it would not be able to get rid of it quick enough through the wiggly (not feeling technical today) pipe from the head to the body, so the valve opens and dumps the gas back into the compressor. Have heard one "go off" when Rob overcharged something. Normal compressor noise with periodic PA-CHEEEWW noise:eek:
Where are you measuring the discharge pressure? I wonder if it has a restriction in the discharge somewhere. On start up it would be moving a greater volume of gas to lower the standing suction pressure to its normal running pressure. Maybe a restricted discharge would flow enough for normal running but cause the pressure release valve to open on starting. May acount for fluctuating suction at start up.

Cheers Jon

old gas bottle
04-02-2007, 07:07 PM
as above ,or have they got a built in discharge pressure relise valve,if there all doing the same they may be going of too soon,have a play with one that you have taken off,

06-02-2007, 02:24 PM
comp. (sssh - sssh - sssh) and is happening with every stroke of the piston i imagine.


How far from the comp Is the cpr?
My money would be on the cpr allowing the gas through.
Listern to the cpr with a screwdriver handle to your ear and the tip touching the valve.

I still wonder why it has a cpr on it any way? On a comercial fridge with a capilary will the load fluctuate enough to require a cpr?


09-02-2007, 06:53 AM
Personally, it may be the problem of oil return for the liquid migration, because when the comp. is shortage of oil, there will be a big noise, after some cycles, the oil will be return to comp, and everything become normal. If you fitted the comp. with the crankcase heater? This phenomenon often happen in the cold season.

09-02-2007, 11:41 PM
Temprite , how's things down there ? , and yes it does have hot gas.

Discharge is measured at condenser outlet
Cpr is for the off cycle to stop slugging , it is 1 foot from compressor

Still got no answers from the manufactures , they took this comp to the workshop and put it under it's paces with a 3000kpa head pressure , and still they did not get the noise we are getting.

Their next move is to dismantle the comp to have a look at the internals.

old gas bottle
10-02-2007, 08:23 AM
bet there using nitro to test it, wonder if it is liquid thats lifting the valve[if it has one] they wont find it using that.

10-03-2007, 11:54 PM
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 measurements 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.

02-03-2008, 06:19 PM
Just a guess man.........are they liquid fed into the compressor? I'm guessing oil is in the the evap accumulator and when it's off and standing the oil is making it's way back to the coldest point causing a hissing noise at the compressor and the refrigerant hits the hot dry windings. Dude.... don't judge me if i'm way off. i ain't fd with them for a while. The same thing happened to me with that bittzer comp at tafe and the fn oil just wouldn't return.

02-03-2008, 07:08 PM

I had a similar situation about 4 years ago with a company that manufactured medical laser machines. We had sold them literally hundreds of 1/4, 1/3 and 1/2 hp Copeland condensing units with no problems.

Then one day they we got the call. The compressors were making a strange noise, similar to what you have, but this noise was consistant. It was annoying for their users because these units were used in a medical enviroment......big problem.

We replaced the compressors but had the same problem. We sent the compressors back to the factory for inspection but no noise was heard. It was quite the dilemma.

I met with the Copeland reps and the laser manufactures engineers on the job. You could hear the noise, especially with a stethoscope and of course the good ol hi tech long screwdriver listening device. It was more predominant on the suction line.

It was quite the machine they had designed. The Copeland unit had all sorts of devices installed, CPR, EPR some type of HGB valve and other electronic controls. They were packed in good and tight and wrapped with insulation.

We finally sent the complete condensing unit, with all added components, back to the factory for testing. They found the problem!

It seems the laser manufacturer was rotating the compressor 180 degrees from its original mounting footprint. The dynamics of the compressor piping change and their added components in such tight quarters created the noise.:eek:

End of story. We never got the complaint again, that I know of, from the laser mfg.