3” boiler project.

Steamchick · Apr 25, 2022

Hi Doug, Good. I like the 2-row idea, as it fills gaps so radiant heat is captured better.
I have made a few calcs, based on 40 x 4 in long water-tubes. and an 8" x 3" ceramic (or other radiant) fire between the 2 bottom tanks.
For your twin cylinder engine: A very crude analysis came up with:

Water tube heating surface = 188.5 sq.in.
Underside of top tank heating surface = 31.4sq.in.
SO it can raise 770cu,in per minute of steam at 50psi...
Engine needs 4.5cu.in of steam per rev.
At 1500rpm that equates to 6786cu.in of steam at 50psi = 12.5kW. of heat needed to raise that steam. (Cor Blimey! - That's a big coal fire with forced draught!), SO you probably need a wire radiant gas heater? - and 30lb. Propane tank to feed the gas? (3 or 4 times the power of a room heater?).
But the boiler can only raise 10% of the steam you need for that speed so, realistically, you may only get a maximum of 150rpm with this boiler. (1.2kW in steam power needed at the engine)
That is consistent with a ceramic (~70W/sq.in?) of 24 sq.in. = 1.7kW. = and gives a needed heat transfer efficiency of 1.2/1.7 = 70% = NOT UNREASONABLE. (I can design the sizes of ceramic/air intake/gas jet for you if you need?).

I think this may open your eyes as to what you are trying to do? - Even if my sums have some significant assumptions, I think the boiler is a worthwhile job for this engine, and will run it at a decent speed for you to enjoy its natural beauty.
Enjoy!
K2

DJoksch · Apr 25, 2022

Interesting. The 3” burner works nicely on the original boiler project and I was thinking I would do the same for the Yarrow. I may have to adjust the angle between drums to fit the burner. Might go back to my original idea and size the second cylinder bore to run a two stage expansion engine.
Thanks Doug

Steamchick · Apr 25, 2022

Hi Doug, Using a compound you will only need half the steam, but get a bit more power (I don't know,... maybe 10% more?) than a single of the same displacement.
Of course, on ships and in Industry, where economy counts top of the list, the extra size and weight for a compound was worthwhile. - If you make yours compound it will represent an industrial Mill design oft seen in the mills in the UK. Engines (ellenroad.org.uk) ; Tandem Twin Steam Engine - Bing images
BUT if you want to power a generator with lots of lights, then you'll be better served making the boiler as big as reasonable, and sizing the generator when you get the boiler running the engine as an in-line twin with steam to both cylinders.
Whatever you choose, we'll support! - and watch the fun as it happens!
On Burners, they are so easy to make, you can adjust the size to suit whatever you choose for the boiler. Of course, the more area the bigger the burner and more steam is generated. With your double row configuration for the water tubes, just make sure there are no visible gaps directly sideways from the burner that would allow "light" (radiated heat) to shine through. Spacing between tubes at 3/8" for 3/8" dia tubes on an equilateral triangle arrangement is about perfect. If you use the tin-plate from domestic food tins, or stainless steel (shiny) for the cleading to make the boiler casing, then any radiant heat will be well reflected back onto the outside of the water tubes, thus increasing the efficiency. (Proven on a simple Mamod boiler).
Without reflectors:

With reflectors:

This photo is out of focus, but shows how the boiler firebox sides lined in reflective steel sheet send much of the radiant heat (that is otherwise wasted by heating the outside of the casing) up to the boiler, and steaming was considerably improved as a result.
K2

Steamchick · Apr 25, 2022

Just had a thought... Could you simply add a larger cylinder, and arrange the same valve chest as you already have, on top? - Perhaps the valve rods would be off-set, but a shuttle slide should manage that quite easily?
K2

DJoksch · Apr 28, 2022

Once I get it together and running, I will substitute one of the cylinders. I will simply bolt the valve chest to the larger cylinder. I was going to make the out board cylinder smaller, but it would be easy enough to replace with a larger one. I’m cutting a new eccentric assembly for the valve timing. I’m also marking off holes for the Yarrow boiler.