[iac]

I have a 200-ton Millennium screw chiller. I was looking into installing a variable speed drive, but it appears that it is not recommended for a screw. I've seen VSDs installed on many screw air compressors, so I was wondering if anyone could tell me why it is not recommended for a screw chiller. The system already has VSDs on 6 of 8 AHUs. I was also considering VSD for the remaining 2 AHUs and the 7.5 hp cooling tower fan, but I don't know how the system will respond to the changes. Any comments would be appreciated. Thank you.

 

[txhvac]

On your air-cooled screws, the oil is pushed through the system by discharge gas. As you may know, screws are very sensitive & require lots of oil, for the bearings & rotors. You'd run a risk of not getting the minimum lift out of your compressor to return the proper amount @ part loads. I know McQuay has been putting Benshaw solid state starters on their screws. The technology is going in that direction, I believe Carrier has a VSD screw in the works or out now. Hope this can help.

 

[roadhazard]

Both York and Carrier have a version of screw using a vsd in place of a slide valve. However the system is designed for proper oil return and the compressors are of a different design. As far as taking a standard screw with the slide valve design and putting a vsd on it, It probably would not be pretty.

 

[clydemule]

Carlyle publishes information in their application guide on their screw compressors what is required when using a VFD.

 

[absrbrtek]

I dont see why it couldnt be done if you added an aux oil pump. Problem would be in the logic controlling the VFD and slide. Be reinventing the wheel basicaly.

 

[blue32]

Scott says it's a no-no, I think it's a matter of maintaining rpm for oil seal tween slide and rotors, just a thought.

 

[acjourneyman]

I think it could be done, I was going to do one for a customer of mine on a 70 ton CHSA using an oil pump, a liquid pump and doing motor cooling through an orifice or metering device of some kind.I knew I could get it to work but the job was 200 miles away and it was on a surgery chiller at a hospital. I have been around long enough to know that that enough was reason not to.If it was in my town in a comfort cooling application I would have done it.

 

[iac]

Thank you all for the comments. It would seem that VSD on the screw chiller would be a more technical retrofit than I had anticipated. The management probably wouldn't go for it, particularly since it's a relatively new chiller. It appears that I will be able to get enough energy savings out of other areas to satisfy them, however. I'm not sure that the energy savings would have been very extensive anyway for a constant torque device. Thank you once again for your help.

 

[chillerfreak]

I think it could be done, I was going to do one for a customer of mine on a 70 ton CHSA using an oil pump, a liquid pump and doing motor cooling through an orifice or metering device of some kind.I knew I could get it to work but the job was 200 miles away and it was on a surgery chiller at a hospital. I have been around long enough to know that that enough was reason not to.If it was in my town in a comfort cooling application I would have done it.

K...you have been wanting to do this for a long time.....sorry but York beat you to it...

 

[duke of earl]

I wouldn't want the ownership of this retrofit for sure. Field R&D can get real expensive.

 

[acjourneyman]

K...you have been wanting to do this for a long time.....sorry but York beat you to it...

I know they did but remember I was thinking about this in 01, I don't think it would be that difficult, drive the slide valve wide open, slow the motor down, external liquid pump to maintain DP across EXV and external oil pump inline to compressor. Can someone please do this to prove it will work, I know the factory didn't want any part of it but were sure interested in the outcome if we did it.

 

[chillerfreak]

The York YCAV actually uses a 4 pole 200 hz. motor....you could rewind the Trane..

 

[the007]

When I was at the York factory in 03 they had a screw they were testing on a VFD. We discussed it at length with Mr. Rose from Frick and they don't like the idea of it because of torques on the rotors and the other stuff discuss here such as oil and liquid flows. If it were me, I wouldn't try it, but then again if you can get a customer willing to take a chance............

 

[stanbyyourword]

i have put some thought and research on this subject, firstly it would be beneficial to research if your package has availablity to an auxillary oil pump available to install , then to unloading pactice , u have option of installing solid state starters such as rthd, or placing vfd for unloading vs slide valve, regards stan