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[Capricorn]

Looking for info on model steam turbine engines.  Would appreciate any links or discussion on topic.
Thanks, Capricorn

[bogstandard]

Hi Cap,
I recently designed and made my own, this was designed to run from air, but should be able to run from steam with a little modification to material selection. It all depends on what you want to 'do' with it. They are fairly easy to make, mine took about 3 days for the basic engine plus another one to match it to the generator.

http://www.youtube.com/watch?v=KW6V7JWbQwk

If you are looking to power a model boat, maybe a few less pockets on the rotor to make it smaller, but you would need a reduction gearbox on it, as props don't really like running at over 30-40,000 rpm, which is easily obtainable from a turbine.

John

[Capricorn]

Thanks John,

I'm watching the video now, it will take an hour on our connection.  Have you seen the tesla turbine video on youtube too?  I haven't got the link handy but it was good, if you want to see it and can't find it let me know.  I thought about a tesla too, but it seems it may be a bit touchy

I do plan to power a boat with it, a 10 ft long, 100 lbs fletcher class destroyer.  I'm going to need something in the range of 1/4 hp (approximate).  It's preliminary, I wanted to use steam, decided to go with electric, but now am back on steam again.  I plan to use a couple 16.4 oz tanks of propane for fuel.  The principle is very simple, and the turbine can be done I think, but the system controls get complex fast.

[Capricorn]

John,

Finished watching the video, it screams.  Any info on it, like rpm, power output etc.  I've seen a few turbines and they usually run a generator like yours, but I've not found out how much power they produce, can you put a watt meter on it?  Cap

[bogstandard]

Hi Cap,
The main problem with turbines are the bearings. The ones I use are shielded stainless and are rated by the manufacturer at max 45K.
Even the ceramic bearings that NASA uses don't go to the full potential of a high speed turbine, and you would need a second mortgage to buy a pair. That is why I have put a load on mine to stop it reaching that sacred number.
If you can supply a watt meter ?? it can be connected.
At about 40psi mine runs at what I suspect is somewhere between 35 & 40k loaded with generator. My tacho only goes up to 29k so I can't measure it accurately above that. Without the load I am positive the motor would shatter within a few minutes running as I suspect from previous very short duration unloaded testing it would easily exceed 100k.
Turbines for model boats were still being manufactured up until I think the late eighties, so maybe someone has some info on how they performed.
Tesla turbines are a total waste of space as far as I am concerned, they were just a by product of his experimentation and are as far as I know very 'greedy' engines, it is that they are easy to produce coupled with impressive looks is the reason they have caught on.
For the size of ship you are contemplating I am sure that a couple, or even one of the pocket type turbines geared down to about 20 to 1 would power your model easily. Keeping them supplied with the volume of steam that is required would be your main problem.

John

[Circlip]

    For turbine bearings check the WREN site for model gas turbines cos in toy aero construction, SELF built
    model gas turbines IDLE at around 40K.

      Ian

[Capricorn]

John,  thanks for taking the time to respond.  It's a bit of a nutty project but I enjoy it. I'll definitely have to keep the revs down, in fact I wasn't anticipating over 20K, but I imagine that's where you get the power, low torque x many rpms.  (the watt meters are fairly common now, gadget with 2 wires on each end and a LC screen, you just put it between your generator and load, it reads out volts, amps, watts, and records high point, sort of expensive, about 50$ US I think).

Yes the steam supply is one of the big concerns, and then matching it with the turbine consumption.  I need to get out the old physics book and steam tables.  One item I'm not sure about either is whether it's worth putting in a condensor to try to retain warm water for feeding the boiler, or if I should just use that space for a second boiler.  

See attached "preliminary" drawing.  That's just one version of the turbine, though you can't see much, it's goofy, with the exhaust ducts, would be difficult to build.

I think the propane tanks can put out 11000 btu's for 4 hours, so two tanks could put out as much as heat as needed if it can be accomodated, and then converted to steam. (Like 176000 btu's for a half hour heh, heh, that's 34 hp) at 1% effeciency that's 1/3 hp, but I'm afraid the thing would melt or explode.  

What do you think?  Cap

[bogstandard]

Circlip,
I think you will find that the type of ceramic bearings used in model gas turbine engines have very limited life, around 30 hours at my last query about them. That is why they are so cheap.

Cap,
I am no expert on boiler design, but with the available output you have I would definitely consider a flash boiler. That would certainly be man enough to keep a small turbine running, in fact it just might be too much, your main problem as far as I can see is getting rid of the heat produced. If you want to see how I made my basic designed pocket turbine you should be able to find it here
http://freeforums4u.com/viewtopic.php?t=244&mforum=homemodelengine
You might have to register to view the pics, but if you want to build one at least it will give you a good basic understanding, but for your use I would consider building one with solid end shields, and duct the spent steam away thru a pipe, one can easily be knocked up in a couple of days. For gearing down from this sort of speed a good quality worm gear system could easily do it, that is 1 worm gear and a 20 tooth pinion.
No complicated gearboxes. This type can also be reversed either from another nozzle blowing the wheel from the opposite direction, not very efficient, to machining a set of reversing pockets on the side of the wheel, more efficient, or even two rows of pockets, one for forwards one for reverse on the top face, separated by a wall, most efficient, but would need two sets of tangential exhausts.

John

[Capricorn]

Ian,
Thanks for the link, I looked through it some.  I'll eventually have to select bearings but probably get relatively cheap ones, with an auto oiler and just replace them as needed.

John2, thanks for the info, I'll look.  It's exactly the sort of thing I need, see something that works and not stray too far from it.  A small computer fan(s) may not be adequate for taking the heat out, but if I insulate it hopefully most will go up the stack, may end up like a blow torch, direct it aft and get some more thrust.

Will look at both sites and respond, takes a while.  Thanks all for input/discussion.  Any other ideas info much appreciated.  Cap

[Circlip]

   Yes, thats the price of fame in the turbine world, I think if the aero set get up to 30hrs between rebuilds they
      would be ecstatic, they usually replace the bearings quite regularly but only in the interests of safety. This is
      even using forced lubrication and while the aero side has been 'sorted' it will still take a few years to make a
      Successful model marine unit. If you can trace links to the Saito units of a few years ago you can see the problems
      a 'Manufacturer' had. John has given you a clue as to the problem in terms of steam consumption of turbines
      by stating you would probably need flash steam at 'small' sizes, the high speeds associated with them are due
      to the need for EFFICIENCY. If you can get hold of some of the old Model Engineer mags dealing with this you
      will be amazed at the experimentation to try to achieve this, - nozzle size ,venturi shape, injection angle - there's
      no EASY quick fix. The peripheral speed of the rotor also tends to be a bit troublesome, if it runs too fast the rotor
      disk can explode - this is a function of rotational speed and disc diameter, are you getting a basic insight? If you
      can get hold of a copy of Experimental Flash Steam by J H Benson & A A Rayman it certainly makes interesting
      bedside reading cos due to temp and pressure of steam it makes even 'slow' reciprocating engines glow red hot!
       Hope I  haven't put you off, bearings are going to be the LEAST of your problems.

        Ian
     
     

[Capricorn]

Ian,  Thanks, I'm sure you are correct about the difficulty, I'm sure the nozzles are touchy as well as much of the rest.  On the other hand I'm not planning to run the thing for extended periods, or often, it's more the building of it that interests me.  Obviously a successful maiden voyage is hoped for but I'm okay with whatever the result, just hope I don't get seriously injured  :(.  Thanks for the sources too, thats what I need, I'm not a great researcher so I appreciate getting help finding these things.  Cap

[Circlip]

      Senility creeping in, - you'll get there eventually, but article on model steam turbines is in JULY edition of
       MODEL BOATS - THIS YEAR.

[Capricorn]

Ian, I'm working on it.  Senile... only in the early stages.  I'm working on finding the article but I don't subscribe so I don't know what the chances are (do you know the title of the article?).

I am getting a basic insight into the steam powered turbine world. I have an idea of the scope, it's big.  Any design suggestions, sketches?

[Colin Bishop]

I have the article. I can scan it and email it to you if you like. It's three pages.

Colin

[Capricorn]

Thanks Collin that would be great.  My home email is in my wifes name, [email protected], accessible through the forum I imagine.  You can send her gift ideas too, only gifts for me though .

Any collaboration on this project is welcome, I may not have the machinest skills for some parts of it, not to mention equipment.  Electronics/computers as well, although I'm working on that.

Cap

[Colin Bishop]

On the way! Hope you find it interesting.

Colin

[Capricorn]

Thanks much Colin, I do find it interesting and will keep looking for info myself as well.  It would be nice to bypass all the design, trial and error over the years.  Cap

[Jonty]

  Years and years ago someone used to advertise a model turbine every month in Model Boats. Cheap and simple, he said. Did anybody try one?

[Circlip]

    Jonty, I'll bet it was pre 'Trade descriptions act' ?  Cap, just to show how simple steam turbines are;-
     MODEL STEAM TURBINES, How to design and build them, by H H Harrison.   Originally published by
    Percival Marshall,  (stop laughing silver surfers!)  - re published by MAP - Argus books Ltd. Don't know
    why people have found it sooo difficult?

[dreadnought72]

Could someone explain to a person who knows nothing about steam why it's impossible/impractical and/or inefficient to make a model turbine that spins at sane revs?

I mean, if there was a solution that works around 2k-3k rpm then surely people would be ditching reciprocating engines for turbines at least as fast as the RN did...and possibly luring more people into (what for me, at any rate) is the most romantic and potentially most rewarding method of propulsion available.

Curious, Andy

[bogstandard]

Hi Andy,
I think you are getting slightly confused here. As far as I know (and I await correction), surface warships use mainly gas turbines, basically the same as used in aircraft, but adapted through gearboxes to drive the prop shafts. Nuclear submarines use steam turbines, again through gearboxes, driven by steam being produced by the reactor heating water.
To answer your question about why not have a slow running turbine. The power band of a turbine doesn't start until it reaches a high RPM. Very similar to an F1 racing car, imagine one running at 3k, you could most probably overtake it with a Reliant Robin (nothing against them by the way), but put at over 15k and the power band kicks in, you have great power at your fingertips, that is why they have so many gears, it allows them to stay within the power band being produced by the engine without slowing it down. 
So basically, what is being discussed is, run the turbine at fast speed to get to its ideal power band, then reduce that speed with the aid of a gearbox to a speed that can be used with a standard prop drive. The main problem is keeping the turbine running at high speed requires large amounts of steam (remember we don't have unlimited heat sources as in a nuclear sub), so generating that steam, and getting rid of the heat in producing that steam is the main issue.
I do hope that this explains it ok.

John

[dreadnought72]

Hi John!

I think you are getting slightly confused here.

Now THAT is highly likely! 

But what I mean is (and should have explained better) the RN of the early 20th century ditched triple expansion steam engines for a whole raft of reasons, adopting steam turbines with great rapidity, while the modelling steam fraternity have tended to stick with, at best, century-old technology.

Full sized steam turbines run around the hundreds of rpm's - handy for direct drive propellors or for low gearing. I can see that model steam turbines might well have to run at tens of thousands of rpm for scale and efficiency reasons, making them fairly impractical (short bearing life, massive gearing required) for models.

Please correct me if my assumptions are wrong - would I be right in thinking that (slow rev) reciprocating engines are powered mainly by the pressure of the steam created, while (high rev) turbines run mainly on the kinetic energy of the steam? If that's the case, is there not some engineering middle ground possible that could result in a "few thousand" rpm steam plant more suitable for model boat use? Something that doesn't result in wear and friction along the lines of the reciprocating engine, and avoids the high rev wear amd large steam loss of small turbines?

Andy

[Circlip]

   No correction John, just a simplification for Andy, If you run a low voltage motor and grip the shaft you can
    relatively easily stop the rotation? add a few gears and watch yer fingers

[Capricorn]

Hello, good to hear things moving along, I'm in a different time zone here (Minnesota USA).  It does seem any turbine worth its salt is high rpm.  In order to accomplish a reasonably useful one for model purposes (large model maybe) I'd like to reduce the rpm too, even if it means there's a lot of potential power lost.  Many of the turbines have the first gear reduction within which I'd also like to omit for simplicity, if it doesn't result in more problems.

To get maximum torque and lowest rpm I'd want the largest rotor possible (I think), which I'm assuming would be around 4" dia.  It could potentially go as large as 6" if the casing and other geometry can be kept within the space.  It seems I could burn gas to provide 10000 btu's per hour (4 horsepower of heat), maybe that could convert to 1 horsepower of steam (25% efficient), then if the turbine was 25% efficient I'd get 1/4 horsepower.  Overall that is 6% efficiency.  Maybe I could burn 20000 btu.

The props are rated around 4000 rpm, 70 ft-lbs per second per shaft is about 1/4 hp, I'm not sure exactly how the prop works out but I get about 2 ft lbs of torque on the shafts.  Direct drive on a 4" diameter turbine would require 4 ft lbs, or 12 lbs force tangential on the rotor (that is 70 ft lbs per second).

If the steam speed through the nozzle is close to the rotor speed 840 in/sec and the weight of steam (1 hp) going through in one second is 34.5/3600 = 0.01 lbs then the power is 1/2 hp.  If the steam speed going in is 1150 in/sec or 133% the speed of the rotor then power in is 1 hp.

By volume the rotor has to carry 13.7 cu in of steam through the rotor per second, divide by 840 in/s gives 0.016 sq in crossection effectively (or 0.2 cu in per one rev), that's only .008 cu in per bucket with 24 buckets.  And the nozzle diameter is 0.025 in if there are 24 nozzles.  So how many psi is required to force the 34.5 lbs per hours of steam through 24 - 0.025 in dia nozzles?

If there is one thing I know the above calculations are all bull, but I don't like to read so I'm guessing, can someone spare me and correct it?  Another problem might be the finite number of buckets and space between them when the steam gets cut off (maybe it will hum  :angel:)

Go for it...  Cap
density is .00007 lbs/ci  then .5pv^2 is 3556 in-lbs/s or 296 ftlbs/s, 1/2 horsepower

[bogstandard]

Hi Cap,
I noticed a chap had put this up on paddleducks, so I stole it and transferred it here.
To me, he has made the gearing too complicated, as stated before, a simple but good quality worm would do the initial stepdown gearing, then to a simple fwd/rev gearbox.
http://www.youtube.com/watch?v=irp004zw8pQ
The Jensen turbine looks to be a basic pocket type, but I think the exhaust is a bit haphazard in this design. I think it has been designed for the 'toy' market.
You are going a bit over the top with the theory bumph, most of us on here are the suck it and see brigade.
It is a shame that you are not a bit closer as I could most probably knock you up a prototype in a couple of days, but anyone in the UK within striking distance of where I live is quite willing to take up the offer to develop one, on the understanding it costs me nothing other than my time to produce, and I would "be in on the act" if it went commercial.
In fact I have nothing special on next week, so I just might knock up a basic prototype.

John

John