Steam Turbine Question

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Phil Tucker
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Steam Turbine Question

Post by Phil Tucker » Sun Apr 14, 2019 11:44 am

I bought an inexpensive Chinese "steam turbine" to play around with, in the hopes of using it to power a turbo generator. It works just fine on air but not on steam. When I apply steam pressure it turns very slowly, eventually grinding to a halt. Since I have no knowledge of steam turbine design I was hoping that someone here might have an idea why this is so. I'd like to know if there's any reason why this particular design would not work on steam. Perhaps it needs a drain to blow out the condensate? See picture below -- steam or air enters from an orifice in the upper right, and exits through another port in the lower left and out the exhaust.
turbogen1.jpg

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ALCOSTEAM
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Re: Steam Turbine Question

Post by ALCOSTEAM » Sun Apr 14, 2019 1:11 pm

My first guess would be that the expansion due to the heat from the steam is causing it to bind up somewhere. Was it technically made for steam or just a model of a steam turbine? Expanding steam can get a turbine spinning real fast, this being brass makes me wonder if it was ever intended to be powered by steam.
What is the brand or where did you come by this. I got a friend that is looking for a decent sized steam turbine to gear down to power a generator.

Phil Tucker
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Re: Steam Turbine Question

Post by Phil Tucker » Sun Apr 14, 2019 2:11 pm

I didnt think it was binding due to expansion because I was able to spin it by hand. Maybe there is a slight increase in dimensions somewhere and it doesn't take much to foul things up? No its not meant for steam just air. But for the price I thought it was worth taking a chance. Got it from Ebay years ago and had it sitting on the shelf for a while.

Asteamhead
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Re: Steam Turbine Question

Post by Asteamhead » Sun Apr 14, 2019 3:47 pm

Hello Phil,
Maybe the air inlet should be much smaller in diameter for using steam. A diameter of about 1 mm (1/32") should be fine!
In addition the outlet might be enlarged to reduce back preasure.
For use with steam the simple ball bearing to be seen won't work for a long time, anyway. Working steam turbines need a safe separation between the steam chamber and the bearing.
But your turbine should work for trials yet (any construction will do so, as long there is enough steam :P ).
Asteamhead

RET
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Re: Steam Turbine Question

Post by RET » Sun Apr 14, 2019 11:09 pm

Hi Phil,

You might get it to work, but the design is all wrong to have it work well at all on either air or steam.

First, the impeller (turbine wheel) should never fit the housing tightly anywhere (this is true for both air and steam).

Second, The inlet (steam or air) jet must be tangential to the turbine wheel and it needs to be delivered through a converging/diverging nozzle. If the nozzle design is at all close to being right, the discharge jet will be supersonic.

Third, You get your efficiency by having the turbine blades turn the air or steam jet through as big an angle as possible, at least between 135 and 165 degrees or even 180 degrees if you can make it work and that is achievable in some designs. One of the best designs is a balanced Pelton wheel. Look it up if you haven't heard of it.

Fourth, There has to be enough space in the casing so the discharge is not restricted and the housing exhaust piping should create as little back pressure as is practical.

If you are going to use steam, what has been said before is also true. The design needs to separate the bearings from the steam, otherwise the bearings will rust quickly. The smaller units will easily hit 40,000 rpm. when used as a generator so balancing is important.

Several years ago, Bill Huxhold of the Toronto Society of Model Engineers made a number of working turbogenerators in all three scales, 1/16th, 1/12th and 1/8th scales. They worked very well on about 40 to 50 psi. air or steam pressure and they didn't use a lot of air or steam to do it.

This topic has been discussed before, and if you do a search on this website you will find several threads that deal with it. I have contributed to some of them.

Best of luck.

Richard Trounce.

RET
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Re: Steam Turbine Question

Post by RET » Mon Apr 15, 2019 7:11 am

Hi,

One thing in my post isn't clear. When I used the word "balanced," I was referring to the flow of the jet against the turbine buckets, not the balancing of the wheel itself although that is also quite necessary.

The bucket design should be symmetrical and the bucket should split the jet flow into two equal parts with each part turning its portion of the flow in the opposite direction to the other. Draw yourself a picture and you will see the concept. It is possible to bring the jet flow in from the side, but it is hard to make that design as efficient as the symmetrical type.

Also, remember that if you do the math, even at 40,000 rpm, the jet speed is going to be at least 4 or 5 times the peripheral speed of the turbine since in a properly designed nozzle, the jet will be supersonic. If you look it up, you will find that Pelton wheels are used for both water and steam (or air).

Richard Trounce.

tetramachine
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Re: Steam Turbine Question

Post by tetramachine » Mon Apr 15, 2019 1:13 pm

A couple things to also consider. How far from the throttle valve is the turbine , air has less viscosity than steam, possible excessive distance allows the steam to expand to much before entering turbine. Have you put a pressure gauge just before the turbine entry, compare pressure air vs steam.
I thing you are not getting enough heated steam to the turbine.
My wheels don't slow me down

Phil Tucker
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Re: Steam Turbine Question

Post by Phil Tucker » Tue Apr 16, 2019 6:06 pm

Thanks, everyone, for that information. Now that I've gotten a couple of good suggestions on where the trouble could be I'll see if I can't get her running. Will update you with my findings...

RET
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Re: Steam Turbine Question

Post by RET » Tue Apr 16, 2019 9:48 pm

Hi Phil,

This is what I'm talking about.
Turbine Wheela.jpg
Balanced flow Pelton Wheel.
The jet hits the center of the wheel tangentially under the little "roof" that you see over each bucket pair. The roof guides the flow so it can't escape radially away from the wheel and the flow is split in two directions towards each side of the wheel. Each bucket pair turns the flow through about 135 degrees towards each side of the wheel and that is where the turbine's efficiency comes from. If you can turn that flow through a greater angle, the efficiency will be even higher. If you think about it, turning the flow through 90 degrees is equivalent to letting the jet go "splat" against a radial vane or gear tooth. Turning the flow through more than 90 gives you even greater efficiency.

The inside of the casing is wide enough that the discharge from the buckets can freely flow away from the wheel. If you compare this design with what you have in your picture, you can see why yours can't work very well. The vanes are the wrong shape. This is the same design that Bill Huxhold used very successfully in his turbogenerators.

The nozzle is made as if it was a combination of the steam inlet nozzle for the inlet portion of an injector combined with the pressure recovery portion of an injector for the output. To get true supersonic flow, the transition between the inlet and discharge portions of the nozzle has to have a definite angle, it cannot be smooth.

The discharge portion of the nozzle is cut away on one side to fit into the periphery of the turbine wheel. The wheel clears the cut away portion of the nozzle by between 5 & 10 thousandths of an inch.

With this information you can make your own turbogenerators, but its still a lot of work. Remember, these things turn at 40,000 rpm. or higher, so the centrifugal forces are very high and must be accounted for in any design.

There are further tricks involved in the generator portion, but the above will keep anyone going for quite a while.

Richard Trounce.

Asteamhead
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Re: Steam Turbine Question

Post by Asteamhead » Wed Apr 17, 2019 6:08 am

Hello Phil, RET,

Just let me add some photos to show how a simple construction will do, too.
Using a standard gear wheel (made of aluminium, if you can get) will work just fine :idea: . The steam is directed tangential along the wheel and is exausted after about 90°towards the generous exaust.
Force produced by the steam from the nozzle will be [speed x mass(of steam) per second], no matter of the sort of a single wheel, - just physics 8).
Steam should exaust after hitting the wheel (see TESLA-turbine) as easy as possible, of course.
Turbines shown above need less than 1 pound of steam / h to produce 1 W electric power.

Good protection and no axial force to the bearings in combination with very precise balancing of that simple wheel is achieved. An elastic fit to the case by means of O-rings will absorb some vibrations when running up from standstill.
You must not use superheated steam, just dry steam from the turret! Thus the turbine will never exceed 100°C due to the saturated exaust steam. Just right for a (brushless) little (servo) generator.
Good luck!
Asteamhead
Attachments
A Pyle National TG, wheel and steam nozzle,red.jpg
TG-Pyle National with brushless gen. Diameter of the venturi-nozzle is just 0.8 mm - for about 8 W electric p.
Turbo generator, system to seperate the turbine by means of piston rings,red.jpg
Turbine and generator will be pushed into the case as unit. Outsider O-rings will reduce vibrations. Flat centrifugal disc in front of the bearing will be fine, too (not shown yet).
Brushless turbo generators,red.jpg
Two tiny piston rings separate the (exaust)steam and bearing/generator. Wheel is pressed on and may be removed by means of a central thread

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NP317
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Re: Steam Turbine Question

Post by NP317 » Wed Apr 17, 2019 10:21 pm

Nice and simple!
Thanks for sharing.
~RN

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