How big a boiler?

[Phelonius]

How does one figure how big a boiler to build for a given engine?
I would be looking at between 60 and 100 psi, and wondering how to figure one that would generate enough volume of steam to do the job without getting too big to be practical.
For instance, a small home made stem engine in a model boat that would have a boiler sized to fit the boat and yet be adequate to the job

Phelonius.

 

[Kermit]

http://books.google.com/books?id=3a...oilers"&lr=&as_brr=1&as_pt=ALLTYPES#PPA297,M1


Seems the higher the pressure you work with, the less amount of water you need to achieve the rated HP per hour.

Kermit

 

[steamer]

In steamboat work,

5 sq feet of heating surface/indicated HP for a water tube boiler

10 sq feet of heating surface/indicated HP for a fire tube boiler


Dave

 

[shred]

I'm no boiler expert (where's SandyC these days anyway?), but pressure and volume are different things when dealing with steam. You can make a tiny boiler that will make a lot of pressure, but without volume it's not going to run a big engine. A big boiler might be able to generate a lot of steam, but little pressure if the fire isn't sized right.

 

[Maryak]

Phelonius,

Attached is an excel file that I used to calculate boiler specs. I have not checked it with Sandy C and hope he comes on line soon. I have put your psi details, (60 & 100) in but the engine details are for my mill engine.

Hope this helps ??? ???

Best Regards
Bob

View attachment Blr2.xls

 

[Kermit]

This topic made me do some reading (like most topics here)

With the limitations of weight for scale boilers of the conventional type - what about water injection coil boilers like are used in a Stanley steamer automobile conveyance. They would lose efficiency at small scale but should be much more realistic as to the boiler weight - ounces to pounds!! versus water weight. The O.P. could even siphon boiler water from the liquid source in which he floats the model; freeing up even more weight for the engine. The whole coil and other required devices could all be housed inside a realistic looking boiler outer shell.


Sound like I'm making plans for me!! Sorry guys, I get carried away sometimes, ;D
Kermit

 

[IanN]

Hi,

A rough estimate of boiler size can be gained as below:

Estimate steam consumption:

Ideally this can come from measuring the area of the indicator diagram – of course, you will not have this, so take the worst case of swept volume of the cylinder multiplied by the pressure (i.e. assume no cut-off). This will give steam consumption per power stroke.

Multiply the steam consumption per power stroke by the number of cylinders, the RPM and a factor of two if it is double acting to give steam consumption in terms of volume per minute. Knowing the pressure that you want to work at, use steam tables to convert this volume to the equivalent mass of steam per minute.

You now have an estimate of the amount (mass) of water you must boil per minute. The estimate ignores condensation in the cylinders, the effect of dead space etc. and can be under estimating the real steam consumption by up to 50% - so double this estimate to be on the safe side.

Now you have to evaporate the calculated mass of water – estimates for the steaming rates (in cubic inches per minute per 100 sq inches of heated surface) for some of the more common boiler configurations are:

2.5 – 3.5 dia. Simple spirit fired pot boiler = 1.0
3 inch dia. centre flue spirit fired = 1.25
3 inch dia. centre flue gas fired= 2.0
3 inch dia. Cross tube gas fired = 2.5 – 3.0
2in x 6in horizontal with circulating tubes, gas fired = 3.0 – 3.5

(The above figures are from The Model Engineer’s Handbook by Tubal Cain)



Ian.

 

[RonGinger]

You can use a lot of science and arrive at a reasonable answer.

You can also use simple scale and proportion. If you are building a model steamboat say 36" long, a vertical boiler 1ft in diameter, or 1" in diameter is obviously absurd. Something about 4" dia would look about right in the boat, and Ill bet that wont be far off calculated values.

When I did my steamboat model I rummaged around my scrap pile and just tried several pieces until I got one that looked like it fit.

MODEL BOILERS AND BOILERMAKING by K.N. Harris published by the Model Engineer magazine company has a long chapter on exactly how to calculate all this for model sizes, as well as some great details on many forms of model boilers.

I happen to have 2 copies of the book, I will sell one for the best offer over $25 plus postage I get using the private message facility of this forum in the next 24 hours. Please dont post offers on this topic- PM only

 

[tel]

Alan Stepney also has some good info here;

http://www.alanstepney.info/page18.html

 

[Phelonius]

Thanks for all the answers. The only one I can disregard is the one about siphoning water from the body of water the boat is in. salt water in the bay is not a good boiler water.

phelonius