Frascold screw vibration

[chilled kiwi]

Hi.
I have a Frascold R-TSH8-60 screw compressor that, when the economiser is operating, has a pulsation in the economiser line. This pulsation varies with speed and gets that bad it has snapped the brass fitting in the economiser port. The system is running at -5oC/30oC on R404a. The only way to stop the pulsation is to shut off the economiser, or overfeed the economiser so it floods back. I have tried a muffler, reducing the economiser suction line, and both. Has anyone else had these issues, or know of a way to reduce the pulsations?

[NoNickName]

Yeah, replace the liquid solenoid valve. It vibrates in resonance with the rotors. Is this unit in Blenheim?

[chilled kiwi]

Hi Nonickname.
Thank you for the advice.
I must admit to under stating the problem. I actually have 11 of these compressors with this issue. I think the solinoid valves are Parker Scems, so I will trial an Alco, Danfoss or Sporlan. They are in the Bay of Plenty.

[Grizzly]

Hi CK.
Nonickname has Your Frascold covered!
It can happen on other Makes as well.
I know at least 1 manufacturer that installs a non return valve as near to the economiser port as is practical.

When the lands of the screw rotor pass over the economiser port.
The result is a tiny resonance of the disrupted gas.
This then sends tiny shock waves up the port pipework.
Hence the resultant issues.
The non return dampens this!
Well that was how it was explained to me!
And when you compare the 2 systems. The result is obvious.
Grizzly
By the way how many solenoid valve stems have you replaced-so far?
Because thats what happens with some other makes.

[NoNickName]

I didn't actually replace any myself, but been on a number of systems where that appeared to be the problem.

Grizzly's advice is very sensible.

[oldesky]

I have seen this problem on medium temp screws where the resulting vibration has broken the capillary from the TX valve.I have replaced the TX valves with Honeywell tx valves and fitted a check valve. This has cured the problem. Also any reason why you are using the RTSH medium temp screw on a low temp application?

[NoNickName]

For -5°C evaporating, RTSH is fine. It's a medium temp application.

[chilled kiwi]

I have seen this problem on medium temp screws where the resulting vibration has broken the capillary from the TX valve.I have replaced the TX valves with Honeywell tx valves and fitted a check valve. This has cured the problem. Also any reason why you are using the RTSH medium temp screw on a low temp application?

Yep, broken a capillary, power element tails, access port tail, 1 1/8 copper elbow and a brass fitting. Luckily we have leak detectors which shut the system down and limits gas loss. As you can understand, the economisers are shut down until I find a cure. Planning to trial the Solinoid valve and then NRV reccomendations early next week.

[Magoo]

Economized at -5 'C SST, is that correct.
It would be punching huge amounts of vapour into the partial port of compressor. No wonder it is vibrating, would be putting huge loads on thrusts as well.
I stand to be corrected by "no nick name ".
Installed a similar machine with integral separator, book said no oil cooler required, died after two years operation, screw totally screwed. Current service company replaced with Bitzer and oil cooler.

[chilled kiwi]

[quote=Magoo;167615]Economized at -5 'C SST, is that correct.
It would be punching huge amounts of vapour into the partial port of compressor. No wonder it is vibrating, would be putting huge loads on thrusts as well.

According to the Frascold selection software, economising is fine.
It is all very interesting this. I was led to believe that these particular units were the only compressors in the world to have problems.:)

[NoNickName]

Magoo gave a very interesting view.
Frascold selection software does not recalculate intermediate pressure at eco port, therefore it always assumes refrigerant goes in, but in your case, the eco port pressure is lower than the intermediate pressure, so I assume Magoo may be right saying that refrigerant is coming out of it.

[Magoo]

Hi Chilled Kiwi.
Take your heart pills around about now.

[chilled kiwi]

Thanks for the help. I replaced the solinoid valve with a smaller one, and that fixed the problem on the fixed speed units. On the VSD controlled units, there was still some vibration at lower speeds, so I installed the NRV. Problem solved. Next units I install will have NRVs and Carel Electronic expansion valves.

[NoNickName]

Thanks for your feedback. I'm no longer at Frascold's, but the guys there may be interested.