Boiler size

[Dark KnightParticipant @darkknight31337]

Hello,

I recently made model with volumetric ratio between cylinder and boiler = 1 : 20. Could the ratio be sufficient, I know it depends on steam production rate but does anyone have any experience with this I dont want to lose time by doing unnecessary calculations. I aatached image for reference.

[SillyOldDufferModerator @sillyoldduffer]

Nice model!

The question of boiler capacity is hard to answer because it depends on what the engine is intended to do, for how long, and what the boiler pressure is.   Driving a dynamo all night will need a lot more steam than ticking the engine over for 5 minutes to show it works.

My guess is a 20:1 boiler is too small for anything other than a short demonstration, but it should be fine for that.

I can’t measure my Mamod traction engine cause it’s in deep storage, but if remember the dimensions roughly and did the sum OK, it’s boiler to cylinder ratio is about 300:1, considerably more than your model.   This allowed it to move it’s own weight at a slow walking pace for about 10 minutes.   This on one charge of meths in the burner after filling the boiler with hot water.

In full-size operation the size and type of boiler is determined by the energy consumption of the engine it’s intended to drive.  The sums are done in Watts, for example:

Required Output at Shaft = 10kW

Efficiency of typical rotating steam Engine = 5%

Therefore heat input to engine  = 200kW which is also the output of the boiler

Assuming the boiler is 80% efficient, the energy input at the firebox would have to be about 250kW

From that the size of the firebox can be calculated from the Heat Value of the Fuel, and the size of the firebox gives a reasonable estimate for the exhaust diameter of the firetubes needed. and from them comes the size of the boiler shell.   This would be tweaked depending on the service, for example locomotive boilers are topped up with water intermittently, perhaps designed to run for a few hours before picking up more water, whereas marine boilers are refilled continuously.

All this is too elaborate and unnecessary for model engineering, so most engines and their boilers are based on experience, not calculation from first principles.   Try it and see: if a 20:1 boiler won’t run your model for long enough, measure how long it does go, and scale up from there.   If the 20:1 boiler runs for 1 minute, and ten are wanted, then a new boiler scaled up to 200:1 will be in the right ball park.

For extended running provide a way of refilling the boiler on the fly so it keeps going until maintenance is needed.  Refilling is done with either a force pump or a steam injector.  On a small model boiler a manual force pump would be my preferred option.

A safety valve is highly recommended.

MythBusters – Exploding Water Heater

Dave

 

 

 

 

[JasonBModerator @jasonb]

A quick calculation of my 2″ Fowler traction engine gives about 70:1 but as it is a double acting cylinder then boiler to displacement would be 35:1, your engine looks to be single acting so 20:1 still stands.

The Fowler is capable of pulling two people on solid ground so for a lightly loaded or display model your ratio should be OK.

[duncan webster 1Participant @duncanwebster1]

It’s heating surface that matters, not volume. KNH book on boilers has some figures, but I can’t vouch for their accuracy

[Dark KnightParticipant @darkknight31337]

Thanks, I will try to reduce some mass of smaller fly wheel (bronze). Model was meant to run on relatively low pressure about 0.3-0.6bar. Also thanks for advice I will do test run, i hope it runs for 1min maybe 2. If needed I can then unsolder 1 part of boiler and replace it with longer one if needed. Another thing about safety valve; its already on the boiler – smaller attachment on boiler. Also thanks for reply 😀

[JasonBModerator @jasonb]

Don’t reduce the flywheel it will not have any effect on steam usage but keeping it as it is will help the engine to run slowly.

[noel shelleyParticipant @noelshelley55608]

An option often not thought of for small low pressure engines is to use a domestic pressure cooker as a boiler on the stove ! They are set to 15PSI on the safety valve have controllable heat and ample water capacity. I have run a Stuart No 1 this way and thats a biggish engine. They oftern show up in charity shops at a fair price, far cheaper than a kit of boiler bits and you can cook dinner in it later so SHMBO will with luck be happy to ?  Have Fun ! Noel.

[Dark KnightParticipant @darkknight31337]

I was thinking only to partially remove material on sides so gravity will help piston little bit to move to starting position because of friction. What do you think?

[JasonBModerator @jasonb]

It is unlikely to be self starting.

[Dark KnightParticipant @darkknight31337]

Ok thanks 😀

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