Boiler for putting around in small dinghy.

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rolphill

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I'm working on a small* boiler for powering a small ~10ft wood dinghy or something similar. It'll probably all be mounted on a removable frame so it could probably do whatever. I'm looking for about 100 watts mechanical, or at least similar speeds as rowing. The engine is planned for 1.5in bore x 1.5in stroke double acting single cylinder.


So I found three of these things on marketplace:
Screenshot_20231220-082320_Facebook(1).jpg

20240103_163554.jpg

IMG_0983.JPG


I don't know exactly what they were, but they seem perfect for making a small boiler. Two of them even came with sight glasses. They're obviously pressure vessels, 3.5in by 12in, 3.35mm wall thickness. Subtract 1mm for corrosion allowance still gives a hoop stress of 2500psi at 150psi pressure. The ASME calculation for the flange gave ~8500lb bolt stress at 150psi. I thought that was a little high for 6x 1/4in bolts, so I drilled and tapped for 8mm bolts instead. I know the label on the outside talks about oil level, but the internal corrosion and the white deposits on the sight gauge tell me these things held water at some point. Especially since they have a spray guard of sorts attached to the top flange.

So the outer shell already has 1sqft of surface area, so I don't think I'll need that much more surface area. This is my preliminary idea so far:
Screenshot 2024-02-08 091647.png

Screenshot 2024-02-08 091717.png


The vessel will be held at an angle, with two legs. It's vaguely similar to a babcock wilcox style. This will give a good space underneath for a coal fire. Each leg is calculated at about 0.25sqft of surface. The shell will be rectangular, with the firebox door underneath the flange sticking out the front top. The fire will be in between the two water legs, and the gases will flow around the vessel and exit in the rear. I'll be silver soldering the copper pipes to the steel vessel.

Obviously the cad model is just a vague concept of what I want to do, I'm sure the actual piping shape will change as I start making it. Any recommendations as far as the water legs go?
 
It is my understanding that stainless steel should be avoided in a boiler, so you should verify that they are not stainless.

Generally they use continuous coils.
All those elbows/joints would be problematic.

.
 
They're galvanized mild steel. Should I remove the zinc from the area around the silver soldering?
YES!
You can weld or braze galvanized but first the zinc needs to be removed or it won't stick well with either method and welding on galvanized metals releases nasty vapors that if breathed affect the liver.
 
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Okay, I wasn't sure because some silver solders have significant zinc in them already.

I also have access to a bunch of A1 bronze brazing wire, phosphorous free, but I'll probably stick with the silver solder for the copper pipework since the bronze won't wick into the joints.
 
YES!
You can weld or braze galvanized but first the zinc needs to be removed or it won't stick well with either method and welding on galvanized metals releases nasty vapors that if breathed affect the liver.
If you get a good enough shot of zinc oxide id doesn't just sorta affect your liver - - - you get your funeral happening real quick.
(Was an oft repeated warning in apprenticeship training!!! (the other was welding on something cleaned with brake cleaner - - - see phosgene (sp?))
 
I'm working on a small* boiler for powering a small ~10ft wood dinghy or something similar. It'll probably all be mounted on a removable frame so it could probably do whatever. I'm looking for about 100 watts mechanical, or at least similar speeds as rowing. The engine is planned for 1.5in bore x 1.5in stroke double acting single cylinder.


So I found three of these things on marketplace:
View attachment 153835
View attachment 153836
View attachment 153837

I don't know exactly what they were, but they seem perfect for making a small boiler. Two of them even came with sight glasses. They're obviously pressure vessels, 3.5in by 12in, 3.35mm wall thickness. Subtract 1mm for corrosion allowance still gives a hoop stress of 2500psi at 150psi pressure. The ASME calculation for the flange gave ~8500lb bolt stress at 150psi. I thought that was a little high for 6x 1/4in bolts, so I drilled and tapped for 8mm bolts instead. I know the label on the outside talks about oil level, but the internal corrosion and the white deposits on the sight gauge tell me these things held water at some point. Especially since they have a spray guard of sorts attached to the top flange.

So the outer shell already has 1sqft of surface area, so I don't think I'll need that much more surface area. This is my preliminary idea so far:
View attachment 153838
View attachment 153839

The vessel will be held at an angle, with two legs. It's vaguely similar to a babcock wilcox style. This will give a good space underneath for a coal fire. Each leg is calculated at about 0.25sqft of surface. The shell will be rectangular, with the firebox door underneath the flange sticking out the front top. The fire will be in between the two water legs, and the gases will flow around the vessel and exit in the rear. I'll be silver soldering the copper pipes to the steel vessel.

Obviously the cad model is just a vague concept of what I want to do, I'm sure the actual piping shape will change as I start making it. Any recommendations as far as the water legs go?
This may not have acceptable materials for what you want to do. You could use ASME pipe and weld high quality steel and use one of these for a condensate recovery vessel. I would put the vessel up right and design a header to feed the wall tubes. You can use a pipe cap to make a steam dome. You may be able to use the site tubes if and this is a big if the glass is rated for your intended pressure. You really need to find out where and how these things were used along with any specs you can find on them. One of your issues is weight considering its going in a boat. But the idea is feasible just not so sure about those vessels you found. If you use boiler piping you would not have to worry about the zinc.
 
I'll be brief: Water tubes should not have any sharp corners in them. Smooth bends and continuous pipe from tank-joint to tank-joint are best. "Kinks" per your CAD sketch are horrible stress raisers, that basically de-rate the "certifiable" capability of the tube. Use boiler water treatment additive to minimise corrosion and always blow-down after using the boiler.
I like your ambition, and strongly recommend a superheater (in the hotter zone) and feed-water heater (after the boiler) as well in the boiler casing. I assume you'll be using continuous re-cycling of condensate (after oil separation)? I like the idea, so "well done" and please keep us informed of developments.
K2
 
Have you considered wrapping a single coil of tubing around the 3.5" pipe, then cover the coil and pipe with a sheet of thin steel plate. Use a small blower, to direct the hot combustion gases tangentially in-between the sheet metal outer shell and the 3.5" pipe such that combustion gases flow in a similar spiral pattern as the tubing, until the gases exit at the opposite end of the 3.5" pipe. Your 3.5" pipe would receive the steam from one end of the coiled metal tube, and function as a steam reservoir.
Just a suggestion :)
 
I'll be brief: Water tubes should not have any sharp corners in them. Smooth bends and continuous pipe from tank-joint to tank-joint are best. "Kinks" per your CAD sketch are horrible stress raisers, that basically de-rate the "certifiable" capability of the tube. Use boiler water treatment additive to minimise corrosion and always blow-down after using the boiler.
I like your ambition, and strongly recommend a superheater (in the hotter zone) and feed-water heater (after the boiler) as well in the boiler casing. I assume you'll be using continuous re-cycling of condensate (after oil separation)? I like the idea, so "well done" and please keep us informed of developments.
K2

That was just to get a general idea of the shape. In reality it'll all be silver soldered fittings. Should be easier than an ofeldt or clishay style boiler.
 
I see what you mean. For fast steaming I think you'll be missing a trick as there looks to be a lot of metal to heat with the steel cylinder... But that just means "Time" while you raise steam.
I feel some calculations stirring in my brain.... so I'll see what I can deduce from some numbers?
You have an interesting idea here.
K2
 
Hi Rolphil:
I note you have "a preliminary idea so far - the outer shell already has 1sqft of surface area, so I don't think I'll need that much more surface area. "
I attach some numbers that suggest a variation on your ideas? = at least 40 water tubes needed...
See attached.
I hope this is useful? - You can change the numbers and recalculate for your ideas. The principles of my calculations are from books and -
https://www.vicprop.com/free-propeller-sizing-calculators
Enjoy!
K2
 

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Hi Rolphil:
I note you have "a preliminary idea so far - the outer shell already has 1sqft of surface area, so I don't think I'll need that much more surface area. "
I attach some numbers that suggest a variation on your ideas? = at least 40 water tubes needed...
See attached.
I hope this is useful? - You can change the numbers and recalculate for your ideas. The principles of my calculations are from books and -
https://www.vicprop.com/free-propeller-sizing-calculators
Enjoy!
K2
That is quite a lot of work for my random project. In return I have a spreadsheet of my own you can have and use. I made it as part of my college thesis. Let me know what you think of it. I've never gotten any opinions of it from other steam enthusiasts yet.

It calculates the thermodynamic stages of the steam piston engine cycle. Green boxes are for data entry, blue boxes are useful info. Data entry is in inches, but it displays both english and metric. It also has a graph to give a rough visual idea of the cycle. It was a huge amount of work and took a lot of digging to find the right equations, since all the modern thermo books only talk about turbines and the system as a whole.

For my planned engine, at 200rpm, 80psi steam, and 70% cutoff, it should put out about 100 watts and consume somewhere between 7000 and 13000 btu/hr depending on actual steam efficiency. I am not fit in the arms, and I was able to row the little boat I'm planning on using at about 3 knots. So I think 100 watts would be in the ballpark of my expectations. If it's too low, I can always add another cylinder like I did my last engine. The design is very modular.

This spreadsheet is also very good at showing how inefficient our little engines are at low pressures and temperatures, even if they are achieving 50%+ of the ideal carnot limit.

I also attached a picture of the engine design so far. It's very similar to the last engine I built, except it will use a slide valve instead of a piston valve. My last engine worked well, but I think the piston valves had a lot of friction, even though they used X-rings.
 

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Thanks!
I'll study that.
I had used a "calculator" off the interweb that suggested the screw size, based on a sample boat size, engine power, speed, etc. as I have no idea of the calculations involved. 200rpm is only 1/10th of my best guess (2000rpm).... so I am not sure where my calculations (guesstimates!) are wrong?
I'll study the spreadsheet when I get time (other stuff today).
Thanks, Rolphil.
K2
 
Thanks!
I'll study that.
I had used a "calculator" off the interweb that suggested the screw size, based on a sample boat size, engine power, speed, etc. as I have no idea of the calculations involved. 200rpm is only 1/10th of my best guess (2000rpm).... so I am not sure where my calculations (guesstimates!) are wrong?
I'll study the spreadsheet when I get time (other stuff today).
Thanks, Rolphil.
K2
I'm not sure either. Running the classic "PLAN" calculation at 200 rpm and 60psi MEP gives 0.164HP, which is in the ballpark of my spreadsheet.

60psi MEP * 0.25ft stroke * 1.8in2 area * 200 rpm / 5400

On another note, I drilled milled three sets of holes in one of the vessels with the CNC. I sanded off the zinc around the holes with a dremel, but I think I'm going to soak it all in vinegar this weekend to remove the zinc, as I can't reach the inside.

Unfortunately, my mind is wandering as it does to alternatives, and was pondering making it a vertical bolsover boiler instead. I need to actually get started on the pipework and set this in motion before I get decision paralysis.
 

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Thanks!
I'll study that.
I had used a "calculator" off the interweb that suggested the screw size, based on a sample boat size, engine power, speed, etc. as I have no idea of the calculations involved. 200rpm is only 1/10th of my best guess (2000rpm).... so I am not sure where my calculations (guesstimates!) are wrong?
I'll study the spreadsheet when I get time (other stuff today).
Thanks, Rolphil.
K2
I’m watching this one intently! I have an idea for a reciprocating steam powered generator and have no outright knowledge regarding boiler capacity, etc., although I have several 19th century books (somewhere) in my library to dig out.

John W
 
I think I might take a closer look at a bolsover design. If I had 2x 5/8od coils, I'd have about 2.25square feet of heating surface, and it would be way simpler to build. My cad software says I could fit 3 coils, but I think I'd have trouble. The only issue is that I'd have to plug the now extra holes, and I'd have to cut that mounting flange off the side of the vessel. However the flange is held on by only a few small weld beads, which should be easy to safely remove with a dremel or similar.

In this case, the only things I'd need to buy are some copper tubing, and a couple pipe nipples to extend the water gauge outside of the shell. that'll be much cheaper than a double dozen elbows and tees. This would also greatly simplify the construction of the firebox / shell. I even have an old 10lb propane tank that's just about the right size and shape to fit this in.
 

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Hi Rolphil:
I have just been looking at your excellent spreadsheet - "Beyond my ken" but I follow the reasoning.
The calculation appears to work with 80psi and 21deg.C Superheat. Not quite the same as the PLAN quick calc. Above. Also, you considered 200rpm, whereas I worked on 2000rpm. And now I read the Spreadsheet develops a max rpm - based on passage sizes and velocity limitations (?) -of just under 1500rpm.
Is the interpretation something like right?
And how does the PLAN calculation relate to the spreadsheet 3.8kW input to the steam and 0.14HP (105W?) output? PLAN doesn't consider the engine efficiency, just work "INPUT" to the engine, I think?
But I may be all muxed-ip?
I appreciate you are looking at the "easy" way to convert the steel tanks to boilers. And I think you have decided "How much steam" you need. (Please ignore my twisted perspective in my sums - probably all wrong?).
But if I put 200rpm into the on-line steam calculator instead of 2000rpm, you will be doing 1/10th of the 3mph that my "calculation" predicted.
Try it yourself and see if it works for you, with your parameters. Boat size and planned speed is critical - how big is yours?
https://www.vicprop.com/free-propeller-sizing-calculators
I agree a large coil gives lots more surface area for steam generation. I reckon the 3 coil arrangement would be well worth the trouble! An undersized boiler just means all the work is wasted. An oversized boiler just means you get the steam you want - and with more fire can actually run to the limit of the engine - even if the calculations are a bit shy of the mark... or run with just enough fire for the steam you are really using, and the boiler isn't running "flat-out". Think of your car... when do you really run it for prolonged periods flat out? I last did that decades ago traveling South on unlimited European roads. - Never in the UK! Although I did have a motorcycle with side-car in the 1970s that went everywhere "flat-out" as top whack was just over 60mph - unless I tailgated a large wind-break truck at 70mph!
looking forward to see what develops here....
K2
 
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Maybe expending 4 bolts from the bottom flange will give you somewhere to fix the support brackets in the fire-box?
Re: silver soldering copper tubes into the steel central tank: You'll need a really big blowlamp! - So bury the lower 2/3rds of the assembly in DRY sand, to reduce heat losses while soldering the pipes at one end, and surround the area mostly with fire-bricks.. Then after pickling the un-soldered end, repeat. As per my little boiler set-up here. (2 different boilers being modified!):
  • 11in. long 3 1/1in dia boiler half buried in sand: Note the "grey circles" on the shell where 2 blowlamps have been heating the shell while my hand-held blow-torch is used to do the silver soldering of the stays. I should have added the second tin so I could have covered more of the shell of the boiler (almost up to the white band?) as this only just managed to get hot enough to silver solder. - Or I could have added a 3rd petrol blowlamp! - Advantage of those blowlamps - they just sit there, needing "no hands" while providing enough support heat to the boiler doesn't draw all the heat away from the zone I want to silver solder. The only red-hot zone is under the hand-held Propane blow-torch.
20240212_174209[1].jpg


  • Shorter 3in dia boiler almost all buried: just enough sticking out of the sand to have 2 1800W (petrol) blowlamps on the sides, as shown, with a 4kW hand-held blowtorch on the end to do the silver soldering. This one was easy.
20240214_161702[1].jpg

I have also buried boilers near horizontally, with just the end exposed where I want to silver solder. >2 inches of sand n gravel seems to be adequate insulation - also holds the job nicely! Firebricks can get red-hot in parts. DO NOT use concrete or ordinary clay bricks. Wear welders' gloves to keep hands from burning with radiated heat from the job. Even so some fingers get flippin hot and I have wrapped them in aluminium foil before now!
Always pickle copper-work, pre-flux and add lots more flux during pre-heating and soldering. Over-heating flux will simply burn it and give a dirty, bad job that leaks/cracks. So be as quick as you can! Afterwards, cover with a can or something (Maybe an aluminium foil cover? NOT a plastic bucket!) to retain heat and slow the cooling to relieve stresses. Quenching will crack some joints, requiring further repair work! Then pickle before doing the other end (another day).
It is a good idea to double check this won't set fire to anything around it... (e.g. an overhead shelf....?). Easy to spot it after the event! Get a competent mate to assist, and brief him beforehand on the operation and his duties - and when! (e.g. Watching your back! - Keeping a phone to hand for emergencies, turning off blow-lamps when you shout, passing rods of silver solder, or the flux application rod. etc.).
Adequate Ventilation is very important!
I have a CO monitor on the shelf above. This is just next to the open garage door (For shelter! Keeps wind out, blow-lamp heat on-the-job. Door frame shown in second photo). Anything less than the fully open door will set-off the CO alarm. (CO is a killer! Flux fumes just permanently damage lungs.)
Hope this illustration helps? - Have SAFE fun.
K2
 
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You might want to review the article in the HSM Steam and Stirling book V1 on a boiler/engine design for a 14 foot boat two teenage boys took from Washington state to Alaska via the inside passage back in the 1930's.
 

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