Flash Steam Boiler Project

[vtsteam]

Waiting for the end of winter here and a usable shop again, I'm revisiting project ideas I've wanted to pursue -- for years in some cases -- which I don't immediately need a shop to imagine, discuss, gather materials for/ etc. with maybe even some simple fabrication possible in the house.

And primary on the list of those long held project interests is a flash boiler. Or monotube steam generator if you like. Anyway, I thought maybe I'd throw this out there as an initial topic and see what discussion comes out of it. Has anyone here built one, for instance?

My own interest is in moderately low pressure steam (not racing hydroplane superheated) say ~ 90 PSI output. Stationary boiler sized for at least my 1-1/2" bore utility horizontal oscillator steam engine being built in another thread. I'd like it to be wood fired for fuel.

I know that's not very conventional for the type, but that's what I'm interested in.

I have lot's of background magazines, papers and books on the subject (and have read most probably 3 times at least) including Benson and Rayman's Experimental Flash Steam, Edgar T Westbury's Flash Steam, and a lot of M.E articles.

Right now I'm thinking a vertical orientation, stainless steel tubing. Partially worked material I have presently is of 3/16" O.D. tube 18 ft long, wound to about a 3-1/2" x 7-1/2" coil.

[crueby]

Sounds like a fascinating  project. Will be following along!   

[vtsteam]

Thanks Chris. I'm going to start out as simple as possible, but still, my ideas are somewhat vague on how this will eventually work out. I do know I will want insulation, and so I've just ordered some 1/2" ceramic blanket and rigidizer. I'm kind of leery of some of the off brands offered on the big river company these days at low prices, so I'm getting some genuine Lynn Manufacturing blanket for a little more.

With 1/2" blanket and a 3-1/2" coil diameter it looks like I'll need a 5" diameter housing. I'm thinking in interests of simplicity for an initial experiment, maybe just 5" black stovepipe.

Another thought... I felt like I was at the limit of bending the stainless steel steam tube without kinking at 3-1/2" dia. after springback. But I bet it can be done tighter. (I just don't know how --so any suggestions are welcome.) Anyway, if I continue with what I've got it seems like the heat from the firebox might largely pass through the center of the coil inefficiently. I was thinking maybe I could add a baffle in the center to direct the flow over the coils. I think a tube or pipe of say 2-1/2" dia centered in the coils might do that.

The initial water supply system I'm imagining is something I've read about in a Model Engineer flash steam locomotive description. It consists of a smaller pressure tank containing water and air, and a  second larger water tank to supply the first tank through a hand pump.

Water is added and drawn from the bottom of the pressure tank. The upper portion of that tank retains the pressurized air that it was initialized with (from a bicycle pump). As water is drawn off, the pressure in the tank drops, but can be made back up by pumping more water in from the larger water supply tank.

I was once given a really nice stainless pressure tank that I think would be perfect for the purpose.

I'm not sure what I will use for the firebox. I could try initially just using the lower portion of the stovepipe. Although a rectangular section would fit a wood fire better. I'm just not sure how soon I could make that. I'll have to think about it.

[vtsteam]

More thoughts...... there should be a way to stop heating the coils. With a liquid or gas heated boiler shutting the supply off does that easily. But a wood fire will continue to burn until the fuel is exhausted.

[Kim]

Sounds like an interesting project, Steve!  I'll be interested to follow along with your project 

As for how to shut off a wood or coal fire... I've heard that on steam locomotives, they would 'dump' the fire out of the bottom of the firebox if it was a coal fire. If it was wood, they would spray it with water to stop the fire quickly.  Not quite sure how they did this, but I seem to recall reading that somewhere.

Kim

[crueby]

Could you make the grate so it could be lowered out? Or the upper shell and coil lifted off? 

[paul gough]

Given the smallish dia. of your firebox wrapper your grate is going to be fairly small and may only fire well with smallish kindling size timber burning fairly fiercely to get good heating. Such a ‘light’ hot fire would likely respond rapidly to air restriction/denial. It may be possible to incorporate a damper that shuts off air supply to kill the fire and reduce heat output to non damaging levels. Increased water injection to the coil can also ‘cool things down’ so a combination the two could be arranged. Flash steam is a matter of finessed balance. Regards, Paul Gough.

[vtsteam]

Thanks Chris, Kim, Paul.

Just some additional considerations ---that the reason I was thinking about overheating the coils is in case of loss of water, and thinking that in the future I might want to try an automatic management system on a larger boiler. This is sort of a test boiler. So, Paul, flooding the coils would not be possible if the problem was actually loss of water.

But re. the other ideas -- dropping the embers works -- I once made a short charcoal cupola furnace, which ultimately didn't work out for melting iron, but had the usual trap doors under to drop the spent fuel, slag and embers (photo below). But for that I'd want some kind of ash box containment on a boiler and it doesn't extinguish the fire.

Wetting down can work though it does create a mess and lots of steam and smoke (I've done that on a chip furnace).  Also, if the boiler water pressure loss is only temporary and you wanted to start up again after pumping water into the pressure tank, a water doused fire is a full stop condition requiring clean-out, and fire starting.

I've been fooling around with an idea this morning of a hinged drop down damper flap ahead of the round boiler section in a rectangular section firebox. I sketched something (below) and the proportions are wrong but just to give an idea.

It both blocks off the round boiler section and opens an alternative route for exhaust in the top of the firebox. That could be extended upwards My sketch shows that as a rather too large opening, but the the opening can be smaller than the damper flap. In fact the opening could be round to fit an exhaustt pipe instead of rectangular. The flap just needs to fit the firebox.

What do you guys think?