Need some advice

[Dodo-steam]

So now that I have a pistion steam engine I have been looking at getting a steam turbine, but I know even less about them other then efficiency sucks on small scale and they need to spin up to have any usable torque. I been looking at this little turbine Link But I can't find any number on ho much torque or what size model this thing could even go into, any of you guys got any idea

 

[Jennifer Edwards]

I’m not sure about what you have in mind regarding getting usable work out of a model steam turbine.

I have seen them used successfully in driving model boats on two occasions,

One was model steam ship that included s reduction gear running around 80:1 swinging a rather large goir blade prop.

The other in a radio controlled hydroplane with direct drive turning a high pitch two blade prop. I was amazed at the speed she obtained once she got up on step, which I admit seemed to take a while,

The steam ship had a nice boiler with a feed pump, condenser and the like. It could run for quite a while.

In contrast the hydroplane had a compact boiler that had huge heating surface for its size, it could only run about four or five minutes at full throttle before running out of water. But it was quite exciting to watch.

Other than those two examples I have not seen a model steam turbine bring used to do “work”.

Perhaps a model electrical generation plant?

I’m just finishing up a Cotswold Heritage beam engine. My next project is a 1/4 scale three cylinder Anzani radial engine.

Rather then go with steam i thought I’d give internal combustion a shot.

Here is s really great article on model steam turbine design:

http://s3.cnccookbook.com/CCSteamTurbines.htm

Do not be put off by the CNC reference, I am a member of the GTBA (Gas Turbine Builders Association), a lot of our members machine complicated turbine wheels without it.

You can find tips on our website www.gtba.co.uk

Good luck!

Jenny

 

[goldstar31]

The name Charles Algernon Parsons and his factory and 'Turbinia' should spring to mind. In earlier days, he built and sailed model turbines on a little pond near to where I was later to be born in Ryton on Tyne.

So yes, it has all been done before.

I was drinking with one of his top mathematicians a couple of days ago

Norman

 

[Dodo-steam]

One of the things i am trying to find is a formula to at least get some idea of the power that a turbine would have similar to the one used in piston engine plan.
P=Mean Effective Pressure in Cylinder (MEP)
L=Length of Stroke in FEET (times 2 for Double acting engine)
A=Area of Piston in Inches
N=Revolutions per minute
By usable work i mean how much could the engine push or propel a model like for a example this one has enough power to pull an rc tank Link. Hope it is more clear now what i am trying to find

 

[Jennifer Edwards]

Good morning Dodo,

There are a couple things to remember with turbines:

* That they spin up incredIbly fast. I’m not sure what the average model steam turbines speed is but I’m sure 15,000 rpm is not out of the question.

* That Archimedes reminded us “with a long enough lever I could move a mountain”.

* That a gear is just a circular lever.

* The turbine wheel is also a flywheel, it can store quite a bit of kinetic energy.

So I believe you can get a close idea of available torque by:

* Taking the weight of the wheel.

* Determining it’s velocity by calculating its circumference and multiplying by RPM to get distance traveled over time.

* And finally multiplying by the gear ratio you selected.

Remember:

M=mass in kilograms ( one gram = .001)

V(velocity)=D/T (meters/second)

E(energy in Joules)= .05 x MV2

With the formula for “E” above all you need do is convert the result which is in “Joules” to whatever unit of force you care to work with.

Fortunately the standard torque unit of measurement is the “Newton Meter”and 1 Joule = 1 Newton Meter, Easy Piesy !

In case you prefer Imperial units of measurement then one Newton Meter = about .74 foot pounds.

The only guesstimate you will need to make is the mass of the turbine wheel as you have not yet made it.

It should not be too difficult to come close once the material(s) and dimensions have been selected.

If you must make an educated guess lean a little towards the “ lighter” side. That way, when actually built, you will tend to have more torque available than you anticipated, which is more a pleasant surprise than discovering that you do not have sufficient power to do the work you anticipated.

Hope this helps

Jenny

 

[tornitore45]

The mass of the impeller or more properly the "inertia moment" does figure in the stored energy and determines the "rev up" time but has nothing to do with the power output to the shaft once the steady regimen is reached. Once the impeller turn at constant speed the energy stored does not change until the steam is cut, then is returned to the shaft.

 

[Jennifer Edwards]

Hi Tornitore,

Now that you mention it you are absolutely correct. My mistake.

I do not know why I went down that path. How would you go about it?

I stand corrected,
Jenny

 

[fcheslop]

This may help also the book is worth looking at
https://steamautomobile.com:8443/ForuM/read.php?1,5080,5115
Good luck with the maths

 

[tornitore45]

How would you go about it?

Tough question, that is why I did not try to address the actual issue.

If I were playing with numbers, for my own benefit, where nobody could see how ridiculously wrong I might be, I would start from first principles:

People that make boiler have a way of predicting steam pressure and volume.
So the power available in the steam is Pressure X Volume / Time
Assuming no steam expansion ( same as compressed air) and assuming an atmospheric discharge the power available is all converted.
In reality steam expand and by cooling release more energy, on the other hand some residual pressure is present at the output and that is unused power plus the mechanical efficiency and the impeller ability to extract the power can't be that good.
In conclusion give and take due to the simplifying assumption the power available is an indication of the ball park of what is available.

Is a starting point, one build based on the assumptions as wrong as they may be. Then quantify the error and add a coefficient to the formula to correct. That is not the same as understanding the physic but on small size ratio it may get you to build the right size.