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

[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.

[ertan]

helfen sie mir micro tech 2 parameter und plan adresse

[HallsEngineer]

A screw machine is at peak efficency when the slides are at 100% as there is no gas passing back to the suction round the unloading valves. This means that economiser fuction works better as does suction efficiency meaning less oil cooling is required as the suction is taking peak gas flow constantly ans as long as the superheat isnt astonomical you sometimes dont need oil cooling at all. All the VSD is doing is changing the motor speed to match the load. This enables us to over speed the compressor to 3600rpm and down to around 800rpm. If this isn't enough turn down. you need two compressors and a better system designer.

[gary cato]

the 23 xrv from carrier is a tri-rotor screw with no slid valve only vfd. very goog unit, quite,and very reliable. up to 500 tons.

[2400martin]

We have the experience that Fr drive make the compressorpack more flexible. You recieve more flexible cooling capacity on a m2 . and that means investment in 1 time - if you need later more capacity your compressor packs are prepared on it. however you need design teh pack on this. only than this can be a succes. regardless the used compressor. we buils and re-build new and used compressors sets with fr driven. and we give two years warranty. of cours it can be tricky- basic you create a onter variable in the amount delivered to the suction of teh compressor. but after making 7 years mistakes , we can final we have a workable product. Bur guys it true we had sometimes pain in the stomage. But we learned a lot. blistering, lack on lubrication, hammering- bended rotors. basic I would not miss it. I helps me each dayby my job.

[antonio nunes]

Hi everybody...i´ve following this thread with most interest, since i´m specifically working on (trying) to optimize an older screw Stal S71(marine app.)to chilling purposes. It has a fixed Vi and offcourse slide-valve..the issue is that is running most of its yearly working hours in current limitation mode( In = 720 A but is limited to 300 A)..meaning that is often running on/off---------I assume that as soon as it starts over-compressing(not enough load), the control unit shuts the el.-motor down.
My question now is: wich info/data(from manufactor) do I need to get hold of, in order to know if a variablefrequency driver would do the job?
The unit has an external oil pump..thereby is oil (injection perhaps) not a problem...
All indput, links, docs are heartly welcome.......cheers.....Antonio(marine engineer trainee)

[glenn1340]

I know these are VSD aircompressors but I wonder if the motor would be suitable for refrigeration purposes.
www.ingersoll-rand.gr/downloads/rotary_nirvana.pdf