screwy sound

[Phisher]

gram and ga1279 I bet if the 2 of you were standing in front of a screw at shutdown you would both agree on what sounds normal and what sounds abnormal. It sounds like your interpreting what mlkwal1 is describing as a strange noise at shutdown differently. Just my unbiased opinion.

mlkwal1 you need to find some other screw machines and listen to them when the compressors cut off to get a better feel for what sounds normal. If in doubt I would rather replace the check than the compressor.

[milkyway]

very well said phisher.,actually when the other compressor shuts down , it just stops., and i was working on an rtaa 80 ton and those screws just shut down and turn off., no noise .,., but when a trane screw does turn backwards, obviously there is a problem, and most likely it is the check valve.,

[Phisher]

There should be a distinct sound when the check closes. Put the circuit into a manual pump down. If the check valve is holding then you should maintain a pressure difference between the condenser and evaporator. Have you checked the oil solenod on the oil supply line to make sure it closes when the compressor cuts off? I'm not sure if that can cause the compressor to back spin or not but it might be something to check. Any ideas on that?

[Healey Nut]

Ok let me try to expand on what is normal . Chiller running normal conditions , chiller goes into pumpdown compressor shuts off . for about 1 or 2 seconds at the most you will hear a groan or moaning sound followed by a slap as the check valve closes then silence . if this is what you are hearing , no problems . if you are hearing a continuous moan and no slap then yes you have check valve issues .

[ga1279]

No, I'm not talking about my golf game. The backspin of the screws is a pretty definite sound. I have replaced many bad checks out there, my wife says I'm just under deposited. Graham keep us posted on your "Green Machine"-GEO

[R123]

If the rotors on a GP compressor are allowed to backspin they will put an opposing thrusting force and eventually cause rotors to make contact. I don't think Trane change their guidelines from the mid-nineties. The development engineer for the smaller RTAA's 70-125 ton said there should no back-spinning of the rotors on shutdown. It could go bang, pop slap itself around a few times but never turn in reverse. There was a notice to this problem passed out during the early Pueblo classes, (memo not a service bulletin) -GEO

First of all, the RTAA140 doesn't have GP compressors, it has intermediate compressors.

It doesn't hurt the compressor to backspin for a short amount of time on shutdown as a result of pressures equalizing. The thrust is still against the thrust bearing when the presssures are equalizing. As a matter of fact, the RTHC and RTHD compressors don't have a 100% sealing check valve and they will roar for several seconds on shutdown. It is bad however, for a screw compressor to be powered running backwards as there is no thrust bearing on the opposite side of the rotors.

[milkyway]

Nope has chunk compressors .. check valve is most definitely bad. When I shut down circle 1 . Back spins for about 5 seconds. Sounds like a jet engine. And I don't hear the slap. When I shut down circle 2 .I hear the slap and the compressor shuts off..

[jsherhvac]

Check Valve it is.

[Dallas Duster]

Ok let me try to expand on what is normal . Chiller running normal conditions , chiller goes into pumpdown compressor shuts off . for about 1 or 2 seconds at the most you will hear a groan or moaning sound followed by a slap as the check valve closes then silence . if this is what you are hearing , no problems . if you are hearing a continuous moan and no slap then yes you have check valve issues .

Man we used to have a guy that would go around condeming the 2-3 second back spins and for some reason I would have to do the work. Always pissed me off when the customer was now smart enough to listen when the compressor shut off. I agree that if pressures equalize pretty quickly from back spin on a screw that the check valve should be replaced.

[Tech Rob]

There is a lot of information floating around out there about these mystical screw compressors. Who knows what to believe?

Is the discharge check valve there to isolate the refrigerant in the condenser when the machine is off, is it there to prevent reverse rotation, or both?

[milkyway]

There is a lot of information floating around out there about these mystical screw compressors. Who knows what to believe?

Is the discharge check valve there to isolate the refrigerant in the condenser when the machine is off, is it there to prevent reverse rotation, or both?

Going backwards

[Screwit]

Reading this post I think back to the first of the RTHB chillers we started out here in 94. In the early days of the screws the official company position was "no service work inside the compressor" replacement was the only option. From the first shutdown on this chiller you could here the slap of the check valve but then the screws would rotate backwards for the next 10-12 seconds while the compressor groaned and the chiller vibrated.

Obviously a leaking check valve but following much discussion with compressor engineering we were advised the decision was ours to replace the check valve, replace the compressor or leave as it was and run it until the compressor failed. Management made the call and the compressor was left as it was.

You know it is now 16 years later, this original untouched compressor has well over 60,000 hrs of service and yes still spins backwards for the same time every shutdown.

But I would still agree with the advice in this post, if the check valve is leaking then it should be changed out.

[rover]

RTAA I assume you have more than one compressor on this chiller, do nothing , let it run , if it blows then you know it was a screwy sound.