Screen over turbo?

For posts specifically relating to fusor design, construction, and operation.
Post Reply
David Kunkle
Posts: 284
Joined: Thu Apr 17, 2014 12:43 pm
Real name: David Kunkle

Screen over turbo?

Post by David Kunkle »

Hi all,

Wondering if I really need a screen over the opening to my turbo. It's getting mounted very close to the top of my 20" cylindrical chamber on a 90 degree elbow. I already have a piece of #30 mesh SS screen that I was going to weld over the 4" opening to the elbow inside the chamber. I could see it being necessary if the turbo was hanging from the bottom of the chamber. I figure, worst case, any debris should fall straight down. Wondering if it's worth the loss of pumping speed vs. the odds of some object winding up in the turbo being mounted so high?
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
User avatar
Dennis P Brown
Posts: 3159
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Screen over turbo?

Post by Dennis P Brown »

A screen has a lot of advantages and really no dis-advantage so why not? As someone who discovered the hard way (a glass tube scattered under vacuum putting small pieces of glass into my turbo (luckily not running) - got most out and it is running again), you never know what might fall in. The cost to replace a turbo if the blades hit junk while spinning is very high and the screen is trivial to install ahead of time - why not?
User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 7:05 am
Real name:

Re: Screen over turbo?

Post by Chris Bradley »

Assuming you are implying you have one with the turbo, leave it on. As well as being protection from particulates, it is a last defence against any plasma discharges that might enter the vacuum stream and find their way to any electronics down-stream.
prestonbarrows
Posts: 211
Joined: Sun Jun 24, 2012 1:27 am
Real name:

Re: Screen over turbo?

Post by prestonbarrows »

Pumping speed is not critical for a fusor since most of the time it is running under zero or almost zero input gas load at pressures well above the ultimate pressure of a turbo. In fact, you will probably want a choke valve between your turbo and chamber so you can have high enough chamber pressures without needing to flow 100's of SCCM through the system.

A screen will only lower the pumping speed by a few percent. Use one if at all possible.
Last edited by prestonbarrows on Mon Oct 20, 2014 10:35 pm, edited 2 times in total.
User avatar
Bob Reite
Posts: 576
Joined: Sun Aug 25, 2013 9:03 pm
Real name: Bob Reite
Location: Wilkes Barre/Scranton area

Re: Screen over turbo?

Post by Bob Reite »

I have a screen over my turbo. Unless I'm experimenting at a really high flow rate, I have the turbo running in "Standby mode" at 2/3 of rated speed after I get the chamber pumped down.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.
John Futter
Posts: 1848
Joined: Wed Apr 21, 2004 10:29 pm
Real name: John Futter
Contact:

Re: Screen over turbo?

Post by John Futter »

Leave it on
unless you have a limitless budget and plenty of turbos in stock
David Kunkle
Posts: 284
Joined: Thu Apr 17, 2014 12:43 pm
Real name: David Kunkle

Re: Screen over turbo?

Post by David Kunkle »

Looks pretty well unanimous. I'll have the screen welded in over the entrance to the elbow. Didn't realize it would only cut pumping by a few percent, so no reason not to put the screen in. Thanks guys.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
prestonbarrows
Posts: 211
Joined: Sun Jun 24, 2012 1:27 am
Real name:

Re: Screen over turbo?

Post by prestonbarrows »

David Kunkle wrote:... Didn't realize it would only cut pumping by a few percent...
In molecular flow, the pumping speed is ideally proportional to the pump inlet area. If you block half the inlet, you get about half the pumping speed.

So if your screen has, say, 50 square centimeters of open area for every 100 square centimeters of mesh, you will loose about 50% of your pumping speed. Most screens are something around 90% open so you will loose in the ball park of 10% maximum pumping speed. But again, pumping speed is not that important for a hobby fusor.

It is usually more important to make sure you have a high conductance path to the chamber (wide short pipes). Especially for smaller turbos, it is easy to choke their max pumping speed by trying to pump through a long bellows etc.
David Kunkle
Posts: 284
Joined: Thu Apr 17, 2014 12:43 pm
Real name: David Kunkle

Re: Screen over turbo?

Post by David Kunkle »

Looks like my #30 mesh screen has about 45% open area. There goes 1/2 my pumping speed which I will need down the road. Looking at other screen available on ebay, 8 mesh is 60% open area. Preston said most screens are around 90% open. Am I looking at the wrong types? Anyone know where I can get the right type? Thanks.

My turbo didn't come with a factory screen. When I looked at some of the usual pump suppliers, the prices seemed pretty ridiculous. They seem to use 28 mesh on the larger sizes and 74 mesh on the smaller fittings. The 28 mesh looks like it uses similar wire sizes to my 30 mesh.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
Jerry Biehler
Posts: 975
Joined: Tue Nov 24, 2009 3:08 am
Real name:
Location: Beaverton, OR

Re: Screen over turbo?

Post by Jerry Biehler »

According to the manual for my Varian 301 it says the screen will drop throughput by 10% and that is a pretty fine screen.

Leave the screen in, if something were to get into it some how it would be bad. The other day a friend had a leybold maglev just suddenly grenade, it ripped itself of the mount to the chamber. Same thing would happen if something got sucked in it.
Post Reply

Return to “Fusor Construction & Operation (& FAQs)”