VSD on screw machines

[Mike Hopkins]

I'm not endorsing anything here just looking for the general consensus on application of a frequency drive to a screw. I think this was argued out on another forum sometime back. A customer made the statement the other day that such and such has a screw machine out that is using a frequency drive. Now I thought why would you need a freq. drive on a screw, unloading capability is already there, maybe the benefits of lower inrush on start but that is a waste of a good drive if just for that. Maybe a fixed speed or limited unloading screw that uses plug valves instead of slide valve. Working on finding who, what, when. You fellas running any screws on freq. drives? A centrifugal I could see, they can make an enormous efficiency gain if controlled right and coupled with lower condenser entering temps. But a screw I don't see it, maybe the fellow was thinking vsd but in reality was a solid state starter? Solid state starters are available on most of the major players screws already though so that tends to cancel out that theory.

Mike Hopkins

[Andy]

Hi, Mike people will try anything. We have a sales guy who wanted to quote inverters on one of my jobs just to run at a higher speed to get a capacity match. We run at 50Hz most compressors are design for other markets that run at 60Hz that means we can run at higher speeds. A friend of mine told me about a job he was putting in where the normal loading gear was fitted, but at peak plant loading ( Mulitple Blast Freezers) the speedwas doubled on the compressors to add capacity. Anyway all looked good, the first compressor on the job run up alright, the second blew up the inverter within 3 Sec of starting. I would prefer Star Deta or delander wound two speed motors as they would be more rebust.
Regards. Andy.

[Frosty]

Hi Andy

I know your a fellow Star engineer but, Darlander two speed motors!!!!!!You obviously did not have any involvement with these demons when Star went through a phase of using them in the 80's......Arrggghhhhh....nightmare!!.....Don't even think about it.

If you are into system clean ups and continued replacement, then ok. Maybe I'm being a little unfair, after all, compressor design has come on a long way since those heady days!

VSD and screws? Yes, go for it. After all, a screw is most efficient when its kept within 85%(dependant on model/manufacturer) of its capacity (see the Howden charts)

No screw should be allowed to run at part load...it should be made a criminal offence!!

See you around

Frosty

:D

[Andy]

Hi, Frosty unfortunately I know all about dalander wound bitzers, having a site on which originally there was eight of these beast fitted. The dalander aplication I was thinking of was a NH3 blast freezing plant with large Sabroe booster screws fitted with these type of motors. This was to reduce the inrush of current on starting and to provide reduced capacity when only running one off the three blast freezers. Basically half speed half amps, it was like two motors wound into the same casing.
I am looking for a set of windings for one of the bitzers, as I have a Bitzer screw which is OK mechanically, but the windings have been incorrectly wound. Fancy helping us out?
Regards. Andy.

[JSherrill]

Frosty is correct. There are several companies here in the states that are running VFD for capacity control, maintaining 100% slide valve loading as long as possible to improve the screws electrical efficiency. I have had several class participants from Ben and Jerry's Ice Cream that swear by this approach. We are planning a series of experiments in our hands on lab to allow us to offer an unbiased opinion on this. Lab is just being completed and the experiment protocols are being written. should have a report in a few months.

[Frosty]

HI Andy

Give me a shout at Derby....ask for Pat Stacey...don't just ask for pat! there are three of us...the other two haven't got a clue..LOL Only jokin!

Bye Andy

Frosty

[Frosty]

Hi Jsherrill

Love to have a copy - keep me in mind will you?

Thanks...Frosty

[Mike Hopkins]

Finding a little time to spend and wondering...
On these machines fitted with the VSD, they must also have full time oil pumps? Micro would have to be programmed to signal VSD speed based on some parameters and also certain conditions, to coordinate with slide valve and also maybe variable VI. What would be the parameters / conditions considered? The VSD would come in to play when operating with slide valve 85- 100% and backing motor speed down for capacity reduction. With loads where slide valve is wide open VSD would ramp up speed as needed to maintain the load. My speculative pondering here only. This makes some sense of the matter, why run a motor
wide open and vary the slide valve when one could leave the slide valve open and reduce motor current. This would be much the same principle on a centrif. application of VSD. My thoughts on the full time oil pump come in here as screws tend to be oil hungry for bearing lube and rotor sealing matters.
Mike Hopkins

[Andy]

Hi, Mike on the jobs I am aware of the screw is loaded to 100% slide pos, then the speed is usually increased to the desired max capacity speed. Below 100% the screw runs as normal. As for oil pumps the screws we are using for VSD are Grasso's with a mechanical oil pump built into the screw driven of the input shaft.
Regards. Andy.