screwy sound

[milkyway]

hey guys., i am working with a screw machine and when the compressor shuts off for 1 second or so it sounds like it is running backwards., i am thinking that there is still pressure somewhere pushing on the rotors forcing them to turn for about 3 seconds., i know this is what the screw compressors does, just not sure what causes it.,

thanks fello thermal die nam mik engineers.,

[jsherhvac]

As you said, what you are hearing is probably gas flow noise, it takes a second or two for the disharge check valve to close and stop that gas flow.

[R123]

What kind of screw?

[BKS60]

It's may well be spinning backwards on shutdown. I have a YS thats close to 20 years old and when it shuts down it backspins almost as fast it runs forward. The tech that did the original startup is still in the area and I'm told he cussed the factory the day he started it.

But its still running everyday!!!

[ga1279]

mlkwal1, What brand is it, If it's a Trane you might just have check valve issues as when the motor stops pumping that check valve should slam shut, right now.-GEO

[milkyway]

sorry guys ., it is a rtaa 140

[ga1279]

Change out the check valves-now. -GEO

[heavymetaldad]

Depending on the volumn of the seperators and the pressure differentials............ yep, you can get a back peddle, even with functioning checks.

[Healey Nut]

The groan followed by the slapping sound on an RTAA is normal and yes it will be more obvious if the differentials are higher , dont be rushing to change the check valves coz it wont sound any different with new ones .

[R123]

The groan followed by the slapping sound on an RTAA is normal and yes it will be more obvious if the differentials are higher , dont be rushing to change the check valves coz it wont sound any different with new ones .

Agreed!

[ga1279]

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

[Healey Nut]

Not quite Guys

Ok so let me ask you this ga1279 ......If mlkwal1 goes ahead and changes the check valves and it doesnt make any difference to the sound at shutdown will you pick up his customers tab for the wasted time and money ???

[ga1279]

You are one of the most knowledgeable technicians on this site and I love like a brother, but we will have to disagree here as you and others are confusing semi-normal sounds at shut down with a problem. I have screws from all the mentioned manufactures at all kinds of location. I was involved with an original RTHA rebuild way before it was even thought of. I never mentioned anything about changing out a check valve because of strange noises, I did say to change out the check valve and / or investigate the cause of motor / rotor backspinning, reverse flow of discharge gas back to the suction side at the point of shut down. This is detrimental to the rotors in any screw machine. -GEO