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Steam Turbines Large and Miniature

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Martin Johnson 102/08/2018 09:02:07
128 forum posts
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I wonder what other members are making of this article?

I must admit to having been involved with turbomachinery design most of my working life, so I am finding the basic theory a bit laboured. That said, I hope the author is going to get on to the reality of designing miniature steam turbines. It seems to me the major problems are:

1) Selection of type - radial inflow, outflow, axial, impulse, tesla. It seems to me that with the low flows of steam available in miniature the choice really comes down to impulse or impulse. There was an article in the seventies about using a jet pump to entrain air and then running the mixture through a radial inflow unit, which has always fascinated me.

2) We are all agreed that the efficiency of small units is very low, but how does one get a reasonable estimate of how low? Without that number, you cannot even estimate a required steam flow, hence no idea of nozzle size hence no idea of blade height.

3) Most agree that bearings can be a problem due to high running speed, but also will there be any guidance on calculating bearing loadings - particularly axial load?

4) Will there be any guidance on blade design - row height, solidity, blade number? It looks as though we might get on to blade angles, but there has been no sign of a velocity triangle yet!

5) Blade manufacture - there have been a number of methods put forward over the years for something approximating impulse blades such as cut using a hole cutter approach, or profiled using a small end mill and cams - even including pen nibs as blades! There doesn't seem to be anything on using insterted blades with a proper fir tree root.

6) Tesla - I am glad the author is not blinded by the mumbo jumbo on that one. I agree that quantitative data on EFFICIENCY is as rare as hen's teeth.

I hope the articles will allow us to move forward from grossly inefficient windmills, but I am yet to be convinced.

Hope this generates a bit of discussion.

Martin

Ady102/08/2018 09:22:52
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3463 forum posts
513 photos

Compared to some other experimental steam locomotives of the era such as the LNER Class W1, Turbomotive was relatively successful, showing a saving of coal compared to a normal reciprocating engine and no hammer blow on the track. Because steam turbines are highly inefficient when throttled (not a problem on steamships, where turbines typically run at constant output, but a major disadvantage for a railway locomotive which has to run at different speeds), power was instead controlled by turning on a different number of nozzles (from the six available) through which steam was admitted to the turbine. One disadvantage of the design was that the small reverse turbine only had sufficient power for manoeuvring "light engine" and the locomotive always had to be turned to face forwards in order to pull a train

**LINK**

Thing is, if you want a switch on switch off system, variable power and turbine simplicity then electric seems to be the sensible route

Edit2 Even the Royal Navy is taking this route now, albeit with early development issues

Edited By Ady1 on 02/08/2018 09:34:26

duncan webster02/08/2018 09:59:59
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2262 forum posts
32 photos

many years ago I worked for Greenwood and Batley on small impulse turbines, deLaval, Curtis and Rateau type. The major reference books are by Kearton and Stodola. The big problem with very small turbines exhausting to atmosphere is windage on the rotor, a lot of the power is absorbed just turning the wheel. The wheels should be as small and fast as possible to minimise windage, with a close fitting shroud against the wheel. Stumph type, whilst attractively simple to make, have the blades too widely spaced for efficiency. G&B's nozzle/blade design man reckoned that even with vacuum exhaust a reciprocating engine would give better efficiency at less than a few hundred horse power. It would however cost a lot more, and need a man to look after it full time, whereas a turbine would just run.. The smallest deLaval types ran at very high speed and had bulb root blades, but the Curtis type ran slower and had tee slots round the wheel into which blades were fitted, much cheaper.

When the DO stores ran out of drawing paper because they couldn't pay the bills I left and moved into gas turbines. Pity really, G&B could make anything bar a profit!

Speedy Builder502/08/2018 10:00:55
1840 forum posts
128 photos

The following link was the horrific end to the "Princess Anne" Turbolocomotive (Modified)
**LINK**

SillyOldDuffer02/08/2018 10:06:57
4836 forum posts
1017 photos

So far Mike Tilby's article is suitably pitched at my level of understanding and interest, which is certainly below yours Martin! I fancy it's both too much for those who don't care for theory or turbines and too simple (so far) for anyone with a deep interest. Suits me though, and I hope it continues to educate me to the point where I could, perhaps, have a go.

The Tesla turbine feels like a suitable target. I'm sure they work but my gut feel is they're inferior to the alternatives. It's a shame propaganda and conspiracy theories outweigh practical evidence. Same problem with some of Tesla's electrical work.

Dave

Neil Wyatt02/08/2018 10:20:26
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Moderator
16738 forum posts
689 photos
76 articles

I'm appreciating the series. There have been a few articles in ME over the last 20 years that were either empirical (Turbo - which clearly worked!) of about De Laval turbines which were so technical that I was totally lost - they needed a series like this one to get me started.

Neil

Edited By Neil Wyatt on 02/08/2018 10:31:45

Hopper02/08/2018 10:33:34
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3770 forum posts
79 photos

Posted by Martin Johnson 1 on 02/08/2018 09:02:07:

...

3) Most agree that bearings can be a problem due to high running speed, but also will there be any guidance on calculating bearing loadings - particularly axial load?

...

I wonder if magnetic bearings could be made/used at model sizes? They are used in industry on machinery such as centrifugal air-con chillers and in applications as small as domestic power consumption meters. Contact-free and thus friction-free, they should be good for very high rpm.

Would a model turbine produce enough power to generate enough electricity for its own magnetic bearings? Or could model-sized bearings be made with fixed magnets, using modern powerful rare earth magnets?

And there must have been at least one article in ME years ago about making a small model turbine. I remember my dad making it when I was a kid. Only ever remember it running on compressed air though. But you could blow in the inlet pipe and it would spin over so it probably would have run on steam ok. From what I remember the rotor was about 2" diameter, 3/8" thick aluminium. Blades were very simple, machined in the lathe by mounting the disc sideways on a piece of aluminium bar in the chuck with the bar machined down to half thickness minus 3/16". Indexing was via a series of holes drilled in the rotor body first so after one curved groove was machined, the blank was rotated one hole and the next groove machined, producing one blade. The tool bit was like a trepanning tool. Amazing what you remember after 50 years!

Edited By Hopper on 02/08/2018 10:40:54

Nick Clarke 302/08/2018 10:59:49
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419 forum posts
12 photos
Posted by Speedy Builder5 on 02/08/2018 10:00:55:

The following link was the horrific end to the "Princess Anne" Turbolocomotive (Modified)
**LINK**

But the only good thing to come out of that disaster was that Robin Riddles was able to show the need for a new class 8 loco to replace Princess Anne and so give us the wonderful Duke of Gloucester!

Mark Rand02/08/2018 12:19:19
798 forum posts

Why bother with fir tree roots on an axial flow impulse turbine when pinned roots are simpler to make and have a proven track record in full size machines?

As to efficiency, Mr Reynolds and his number don't approve of small blade passages, unfortunately. Countering that however, the first stage blades on a 9,000 hp boiler HP boiler feed pump turbine can be down to about 5/16" tall. That's pretty small!

Martin Johnson 103/08/2018 09:51:15
128 forum posts
1 photos

Ah - Greenwood and Batley what a company they were! How many of us remember GreenBat works trucks?

Anyway, Duncan, you are right that windage loss is a major loss, but capable of calculation without too much bother. I agree that the Stump rotor is simple to make, but grossly inefficient.

Mark,

As to fir tree roots, it is what I remember from apprentice days at W.H. Allen (steam turbines to discerning few - who became a bit too discerning and too few over the years). I guess pinned roots are fine as long as the centrifugal loading is kept to reasonable limits. Yes, small passages are an issue, but stupidly large blades with uncontrolled expansion and extra windage from "unused" areas of blade are also a big issue.

Neil,

You are probably right in that the technical level of the articles is a reasonable introduction starting from zippo. I just hope it moves on to something useful in actually designing and making a whizzer.

I guess I should explain why I am interested. I am considering a turbo alternator for my steam lorry. However, it is a 7" scale steam lorry and the turbo alternator set would be for battery charging. I think 5 Amps at 13 - 14 volts would be about right, so I would probably need to aim at around 100 watts shaft power...............Any suggestions? A recip. engine would also do the job, but at a weight penalty. Besides, a turbo alternator would be an interesting challenge.

All interesting stuff.

Martin

Andrew Nash 103/02/2019 18:49:56
2 forum posts

I am enjoying this series immensely. For a while I have been toying with electric power generation models, and for power generation, consistent hight speed is one of more important design criteria.

The level of engineering and science theory in these articles has been challenging but rewarding

The tolerances for blade to shroud manufacture, the work to produce large numbers of blades and the potential for failure have all been daunting. So I was delighted to see the conversation move from de Laval, to Parsons and now Rateau implementations.

It seems like the Rateau from a design and implementation perspective offer a reasonable trade-off with the potential advantage of running at lower steam pressure. I am hoping Mike will move on to provide some suggestions for builds and materials.

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