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.
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.,
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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.
What kind of screw?
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!!!
I'm good at making things cold...You can ask my first two wives!
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
Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO
sorry guys ., it is a rtaa 140
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Change out the check valves-now. -GEO
Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO
Depending on the volumn of the seperators and the pressure differentials............ yep, you can get a back peddle, even with functioning checks.
\m/
original member of the racoon brotherhood
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 .
The toy chest is officially full ... I got a new toy..... 2007 Aston Martin V8 Vantage and yes it still gives me goosebumps
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I"m not a service tech, I’m retired ….I used to be a thermodynamic transfer analyst & strategic system sustainability specialist
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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
Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO
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 ???
The toy chest is officially full ... I got a new toy..... 2007 Aston Martin V8 Vantage and yes it still gives me goosebumps
You bend em" I"ll mend em" !!!!!!!
I"m not a service tech, I’m retired ….I used to be a thermodynamic transfer analyst & strategic system sustainability specialist
In the new big shop , greasin', oilin' . tweakin' n shinin' !!
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
Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-GEO
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.
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.,
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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?
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 .
The toy chest is officially full ... I got a new toy..... 2007 Aston Martin V8 Vantage and yes it still gives me goosebumps
You bend em" I"ll mend em" !!!!!!!
I"m not a service tech, I’m retired ….I used to be a thermodynamic transfer analyst & strategic system sustainability specialist
In the new big shop , greasin', oilin' . tweakin' n shinin' !!
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
Once in a while everything falls into place and I am able to move forward, most of the time it just falls all over the place and I can't go anywhere-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.
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..
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