boiler insulation / wood staves

[Lopez Mike]

I would rather make the firebox taller and sacrifice tube length. And go in the direction of a shorter and larger diameter boiler. The limit in that direction is how to get past the beast in a small launch.

The last twenty years of fire tube design trends went very much that way. I'm talking about locomotives which is where essentially all new development was going on in fire tube design towards the end fire tube use commercially.

There fireboxes got longer and longer and the tubes got shorter and shorter. Shell diameter had long before reached the limit of tunnels and bridges.

Radiant heat is your friend. Convective heat transfer as within tubes is much less effective. If the surface can see the flame, it is working hard.

The wet leg doesn't have to hold much water. An inch or so of water thickness protects against excessive temperatures and differential expansion issues.

[steamdon-jr]

I am all for short boilers but wider does not help in our typically small launches, our boiler is 21" diameter, 2" wet leg , 39'" tall and has 61- 1.25" tubes that are 14" long and she steams very well

[Lopez Mike]

Yeah. Width is a problem.

My shell is 18" diameter but by the time I add 2" of insulation and another 1/2" of wood the diameter has increased to 23". And there it sits like a big fat passenger. Fortunately Folly is an open boat so it isn't too bad getting past.

Now if I can just persuade people to not grab the stack that first time. Sigh.

[cyberbadger]

Yeah. Width is a problem.
My shell is 18" diameter but by the time I add 2" of insulation and another 1/2" of wood the diameter has increased to 23". And there it sits like a big fat passenger. Fortunately Folly is an open boat so it isn't too bad getting past.

The ASME boiler for the Nyitra I launch I'm building is 24" OD in the firebox area. Luckily it's a pontoon boat so the rectangular deck is almost 5" wide.

Now if I can just persuade people to not grab the stack that first time. Sigh.

I enjoyed that quip. Reminded me of my first time riding on my steam mentors 6HP Russell Steam Traction engine sitting on a water tank. It's a rough ride on a traction engine and this 6HP Russell is small for a full scale traction engine. I was slipping so I grabbed a globe valve handle bare handed to steady myself. Made me learn the lesson. If it's anything metal on a steam contraption, don't touch it without a glove or rag until you understand the steam contraption. Luckily my hand was fine after a day or so.

You'd think they'd know Mike - you don't grab the tailpipe on a car after it's been running for a few minutes.

Maybe tell the next adult person who tries it that you may forward them as a potential candidate for the next Darwin award?

-CB

[cyberbadger]

I seriously question what that water leg is worth and how much it is costing. Considering combustion begins halfway up the firebox - cold air being drawn in the lower portion. I ran some numbers on it a while back and I think I'd be getting much more heat transfer from another ring of tubes.

DetroiTug/Ron,

I can't quote you an exact source. But I've heard estimates that almost 70% of the heat transfer in a Wet firebox firetube boiler is in the firebox. (Fueled by solid fuel like wood or coal)

You can see in IR images I put in a previous post - all that heat is just heating 3/8" thick steel shell and not doing anything really to heat the water to make steam.

In a steam car like a stanley, weight is so much of a premium, and the practical steam cars were never fired with wood/coal. In a steam boat the boiler weight is very important but not critically so.

Solid fuels like wood or coal needs more time unless you pulverize it into dust and have a delivery method. Modern coal power plants blast in coal powder into the combustion chamber.

If you have well thought out/engineered insulation strategy - that's a different story. The Philips Brothers Mill in California is huge 3 story sawdust fueled vertical firetube boiler. It appears to be a dry furnace/firebox. They have firebrick (I think the porous kind) lining the inside of the firebox/furnace. The heat is mostly kept inside the the furnace and runs up the flues/tubes, instead of heating the furnace steel shell.

-CB

[DetroiTug]

CB,

I'm not seeing how radiation from combustion is more beneficial than the path of the flame. Pretty simple to prove really. Light a candle, lay a piece of paper along side the flame, now put it in the flame, where does combustion of the paper happen?

If we were wagering, my bet would be on the majority of the steam production in a wetleg VFT happens at the crown sheet AND the lower portion of the firetubes.

The FLIR pics show a very hot firebox (which has nothing to do with steam generation in a dryleg VFT) because it is not being cooled by the water like the rest of the boiler. They really only show the heat signature or heat loss on the outside surface of the boiler. A cross section view (which is impossible) would be conclusive.

Another test would be to simply insulate the inner wall of a wetleg and fire it. Then compare it against normal operation.

One of the reasons I question the VFT wetleg. When I purchased my boiler it had only been fired one time. New steels - the kind we use for boiler construction are covered with mill shale. A hard ceramic like coating which gets burned off over time. In my boiler it was still on the inner wall of the wet leg 1/2 to 2/3rds the way up. The entire crown sheet and lower portion of the firetubes had a layer of rust.

-Ron

[Lopez Mike]

There's very little flame in the tubes. The flame radiates away enough heat to drop its temperature below incandescence almost immediately after it enters the tube if not before.

The tubes are mostly there to get the combustion products out of the firebox. Any energy we get while doing that is great.

This really shows up in water tube designs where getting a sight line to the fire is everything.

I'm probably making a stronger case for radiation over conduction than is justified just for making arguing points but I believe it is true.

I'm appalled by dry leg boilers. The idea of letting all of that energy be spent heating up fire bricks instead of water. Ugh. I understand the need for combustion chamber volume with low density fuels like wood but think the problem is not that big. I do have to keep my firebox pretty full when showing off though.

On a related subject, I remember there being water cooled grates on coal burning locomotives. I wonder of building a grate out of small diameter black iron pipes and running the feed water through them would be a workable notion? Bugs me to think of the wasted heat going down.

Fred??? Jump in here! Ron and I are squabbling.

[DetroiTug]

Fred is too smart to get involved in my debates

Much of my opinion is based on what little research I've done, limited experience and a hunch .

BUT, I'm not agreeing that firetubes aren't doing that much and are more so merely vents for combustion gases. My firetubes are 1.75" internal diameter, and fire is assuredly entering them. There is evidence of it. When I clean the flues there is very little soot in them which indicates it is being burned up. I'm exhausting up the stack and inducing the draft, at high speed the firebox looks like a forge.

An efficient boiler will take most of the heat from the combustion gas and that happens in the flues as well. The steam car has a 150-200 thousand BTU burner under it. Just 14 inches above that is a wood seat which barely gets warm. The boiler is absorbing most of the heat. These are tweaked by adding or subtracting economizer or water wall to keep the gases near 300 degrees at the boiler exhaust(stack).

Heat can be stored and that is what is happening in an insulated firebox, the heat makes it's way in the tubes and generates steam as well. Uninsulated, the heat is dissipated through the external surface.

The only way to prove this is to build a boiler with multiple internal temperature sensors up the waterwall across the crown sheet, then up one of the firetubes and map the temperature.

-Ron

[Lopez Mike]

Big flues! Mine are only 1". And I seldom use forced draft unless provoked (like there is another launch within sight).

And I don't agree that leaving out a water leg is a good compromise. It seems like those boilers without a water leg were designed with the idea that it added too much complication. Not true.

A minor advantage of the water leg is that it concentrates the sludge to that blow downs are more effective.

I suspect that all of our boilers are wildly more efficient than our engines. That why I cringe when I see a steam pipe or cylinders un-insulated. One condenser on a boat is enough. On that issue, I'm not convinced that multiple expansion make much difference in smaller engines. The old surface area v.s. volume ratio is killing them. Those with gauges on the receiver(s) often show startlingly low pressure.

[cyberbadger]

I'd like fred to chime in too.

As noted - if it's a dry leg VFT the legs are not considered in a the square feet surface area for heating surface calculations, at least in the ASME code as I understand it.

I think we are a little bit arguing over different fruits. Part of it is the fuel type - solid (wood/coal) or atomized oil/kerosene/propane what have you. Another part is application. Another part is insulation, and were the insulation is.

Another semi unfortunate problem with _V_ Vertical firetube boilers is due to the natural convection, so much heat just goes up the stack barely used. That's why the tube-ulators (twisted metal strips placed in the firetubes/flues) help so much. They slow it down and let the hot gases impart more of their heat to boil the water. If I recall correctly Mike installed some on his launch with good success.

However I think it's false to assume that it's all the hot gases or flame that give off all/most of the heat.

Radiant heat is a big potential factor that should not be under estimated.

-CB