I am working on my Plan C boiler - a yarrow style water tube. I would welcome your thoughts on the following:
1. The casing - I was proposing that the casing should be in s/s but some local enthusiasts have said why not use 2mm aluminium. It is aparently cheaper and would do the job just as well
2. Insulation - the basic casing is box like so board insulation would be easy to fit. Could one get away with 25mm or should one use 50mm? (I am currently doing a re-draw with 50mm)
3. Funnel diameter - The grate will be 300mm X 600mm (11.8" X 23.6"), about 0.18sq m or 1.94sq ft as drawn. Just how big a funnel diameter does one need? I have drawn 150mm, but the steam dome projects into it.
Stainless Steel would be my preference, it is stronger, and does not have accidental "dents" that tend to show up all over my Aluminum lagging. However Stainless is more expensive and harder to work with. That is the entire reason why I used Aluminum, but I an now cinsidering covering most of the aluminum with wood, OK for a VFT, but not really suitable for a watertube boiler casing.
It depends what the 'look' is that you want to achieve- do you want a polished, no nonsense finish; use stainless. Do you want a less polished finish that will need polishing to keep shiny, will dent easier and will scratch very easily; use aluminium.
My personal preference- use far cheaper 1.5mm steel, use a decent matt black or gloss black high temperature paint- I like matt black- it doesn't glare sunlight into your eyes, looks very professional for a long time as scratches don't show, and when they do you can touch up very easily. Also looks great if you finish the joints with polished brass...!
A 6" (150mm) flue (inner funnel if you will) sounds a little too much to me- a taller, thinner funnel would induce a better draft- around 5" (125mm) maximum would be about the job.
I am working on my Plan C boiler - a yarrow style water tube. I would welcome your thoughts on the following:
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3. Funnel diameter - The grate will be 300mm X 600mm (11.8" X 23.6"), about 0.18sq m or 1.94sq ft as drawn. Just how big a funnel diameter does one need? I have drawn 150mm, but the steam dome projects into it.
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Pete
The old books say that funnel internal area should be about 1/6 of the area of the grate. And, as the gases of combustion are hotter the closer one is to the smoke hood, this figure should be observed at the base of the stack. Just as needing to supply enough air for combustion governs the quality of the fire, being able to remove all the combustion gases also governs the quality of the fire. A stack blower might help. Remember to mount it above the stack damper - you will be using a stack damper for a solid fuel fire!
Ron,
So is that grate area the area of the openings where the combustion air gets through or the area of the grates all together?
I'm fairly green when it comes to launch steam but I've never noticed any dampers on the wood fired boats around Puget Sound. My ash door and fire door are very good fits and I can go from a roaring fire all the way to holding steady pressure with the engine stopped in about a second by closing everything up.
I like the performance of blowers but I'm so miserly about makeup water that I haven't fitted one. Soon enough, I suppose.
Mike
If you think you are too small to make a difference, try sleeping with a mosquito.
Dalai Lama
artemis wrote:
The old books say that funnel internal area should be about 1/6 of the area of the grate. And, as the gases of combustion are hotter the closer one is to the smoke hood, this figure should be observed at the base of the stack. Just as needing to supply enough air for combustion governs the quality of the fire, being able to remove all the combustion gases also governs the quality of the fire. A stack blower might help. Remember to mount it above the stack damper - you will be using a stack damper for a solid fuel fire!
Perhaps on a US wood fired boiler, but not on a coal fired UK boiler- the grate damper and a well fitting fire door are the only controls you need. A flue damper on a coal fired boiler gets sooted up and restricts flue gasses in no time at all.
The better funnels use a tapered flue as this induces a better draft and keeps any smuts higher above the boat.
Rule of thumb for a coal fired boiler is 6" dia. for every 2 sq.ft. of grate area.
Clearly the Stainless/Mild steel/Aluminuum choice comes doen to a matter of preference and, in my case, what the chap doing the bending would like to use. I will keep you posted on the outcome of that one.
It is nice to have some formulae or rules of thumb around for funnel aperture size. If my sums are right, I supect that Ron and Greg are actually quite close. Either way, the funnel opening as drawn is too small for the grate area. Some further sums suggest that a 200mm diameter opening would probably do, perhaps with tapering to 150 at the top of the funnel.
So how do you feel about the insultation? Should one go for 25mm or 50mm? Also pertinent to the discussion is how one should fix the insulation sheets to the casing. I imagine bolting through would create thermal bridges but would be easy.
I am envisaging the casing being finished in dark green paint, probably with three brass bands going down to over centre catches at the base. The idea is for the straps to hold together the top seam. The latter will have location holes and pins, but bolts/self tappers would make dis-assembly far too hard. Bolts & captive nuts or self tappers should fix the sides to the ends.
Discussing the use of the boiler and the size and style of the firebox door with another forum member, it became obvious that the drawn hole of 300mm X 250mm is too big. Essentially, in a Rose hull, a door that size hung from either one side or the other is a serious leg burn waiting to happen. Bottom hinged would get in the way of bunker access (under the seats).
The idea that was suggested was to use a locomotive style double opening door on a slide.
Does anybody have a drawing of the geometry and size ratios for such a system? That would save me re-inventing the wheel, but with a kink in it!
Regards
Pete
Last edited by PeteThePen1 on Sun Jul 15, 2012 4:27 pm, edited 1 time in total.
I think some Burrell traction engines also had the double sliding doors you describe . I believe they're called butterfly doors .
The only steamboat I know of which has them is PENDLEBURY , not sure who owns her now but I think Roger Mallinson made them . They work very well .
I'll almost certainly be seeing Roger in the next few days and I'll try to remember to ask him .
Many thanks indeed for your offerings.
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So how do you feel about the insultation? Should one go for 25mm or 50mm? Also pertinent to the discussion is how one should fix the insulation sheets to the casing. I imagine bolting through would create thermal bridges but would be easy.
...
Pete
Since the insulation is primarily to keep heat in and secondarily to prevent burns, then the more the better. Assuming you're using a "board" then 50mm (about 2" US) should keep the outside "burn safe". As to adhesive, I've used furnace cement. This is a "glue" that will hold up for many years (and many miles over bumpy roads). It's thermal resistance is such that a welding rod won't burn it away. Don't have a makers name, but I'm sure whoever you buy the insulation from should be able to supply you with it. Usually sold in one gallow containers (enough for about 10+ boiler fireboxes).
