Vacuum Ejector Drawing / Guidance

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Steve Goodbody
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Joined: Thu May 25, 2006 7:16 am

Vacuum Ejector Drawing / Guidance

Post by Steve Goodbody »

Hi there,

Does anyone have a drawing or sketch of a vacuum ejector that they've built and found to be successful? I'm aiming for 20" Hg at 60psi.
Testing mine on air this weekend, built to a Keith Wilson drawing from a 1980's Model Engineer, it pulls to 17" Hg at 80psi but only 8" at 60psi.
The ejector was bench-tested off the loco with no leak source.

The cone jet diameter is No.73 (0.024") and the venturi minimum diameter is 1/16". I've tried varying the distance of the cone to the venturi mouth but that had little impact. If no drawings are available then if anyone has experience/guidance on likely successful jet and venturi diameters then that would be much appreciated.

Many thanks
Steve
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Fred_V
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Re: Vacuum Ejector Drawing / Guidance

Post by Fred_V »

I've built a couple of them. You may not get 20" at 60#. If you need more power then use a larger diaphragm. I have 2 drawings in Autocad if you are interested. one is from Dcokstader.
Fred V
Fred V
Pensacola, Fl.
Steve Goodbody
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Re: Vacuum Ejector Drawing / Guidance

Post by Steve Goodbody »

Thanks Fred, the drawings would be much appreciated. I'll PM you with my Email if that's okay.
Do you recall what vacuum you've achieved with yours?
Best regards
Steve
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Fred_V
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Re: Vacuum Ejector Drawing / Guidance

Post by Fred_V »

I can't remember what I got on the small one but i think 17 or 18. you won't get a total vacuum with these like you would with a pump. With air brakes you only get what the air pressure is so at some point you will loose brakes. On my Hunslet i have just been pulling Mr. Johnson into reverse and that slows a whole train really well going down grade. Add some throttle and it will stop.
Drawings are on the way. i have not built the Dockstader unit yet.
Fred
Fred V
Pensacola, Fl.
Steve Goodbody
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Re: Vacuum Ejector Drawing / Guidance

Post by Steve Goodbody »

Drawings received and printed - many thanks Fred. I'll make up one of each and see how they compare on air on the bench.

I'd hoped for a bit more suck than the 17" I'm currently getting. Keith Wilson's original article suggested that the ejector would pull 20" at 40psi but it's quite possible that that was an exaggeration. However PNP in the UK also markets an ejector which they say will pull 21" above 'about 35psi' and so I'll remain naively optimistic until I've exhausted the options (pardon the pun!).

Best regards
Steve
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dnevil
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Re: Vacuum Ejector Drawing / Guidance

Post by dnevil »

The book "So You Want To Build A Live Steam Locomotive" provides a cross section diagram of a steam driven vacuum ejector on page 79.

Regards,
Daris
Daris Nevil
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NP317
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Re: Vacuum Ejector Drawing / Guidance

Post by NP317 »

It is also possible that the injectors will develop more vacuum with steam, that with equivalent air pressure.
Steam has higher mass/density that compressed air.
~RN
jma1009
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Re: Vacuum Ejector Drawing / Guidance

Post by jma1009 »

Hi Steve,

Apologies for the delay, I don't log in too often.

Your dimensions are as per the Brian Hughes / Reeves design that Keith Wilson copied. There is a 1/32" gap between the 2 cones.

I used No.53 and No.74 for both which is quite close to the Hughes/Reeves design and got very good results - much better than your test results. As I muck about with injectors making a few from time to time etc I have a full set of injector reamers I made the entry into the steam cone as you would on an injector, and the exit from the ejector cone tapered to match the ID of the ejector exhaust pipe and reducing the parallel portion of the exhaust cone accordingly.

Concentricity is very important and very good machining etc.

I did meet Keith Wilson once.

Cheers,

Julian
Steve Goodbody
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Re: Vacuum Ejector Drawing / Guidance

Post by Steve Goodbody »

Many thanks Julian,
I'll try the ejector cone modifications as you suggest - it's great to hear that you have had good success after modifying the design.
Like you, I'd used an injector steam cone inlet reamer to make the ejector steam cone entry.
Best regards
Steve
Steve Goodbody
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Re: Vacuum Ejector Drawing / Guidance

Post by Steve Goodbody »

As an update, I ultimately used the cone configuration described by DAG Brown at the back of his injector book and this seems to work well. On air the ejector pulls to 18 inches Hg from 40psi upwards which is what I was aiming for.

Fred - the cone configuration in your drawings are similar but not identical to the Brown design and were much appreciated. They are in my "things to experiment with down the road" folder!

Thanks again to all,
Steve
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Fred_V
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Re: Vacuum Ejector Drawing / Guidance

Post by Fred_V »

Thanks Steve; I'll look that up.
Fred V
Pensacola, Fl.
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