Boiler for putting around in small dinghy.

[rolphill]

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

I just did the "PLAN" calculation to reality check my and your calculations. It's only useful as a ballpark figure at these scales anyways.

The spreadsheet calculates what would actually happen in the cylinder (assuming ideal conditions, instant steam flow, spherical chickens in a frictionless vacuum, etc.) So the input pressure should be the actual pressure that reaches the steam chest. The spreadsheet calculates the actual work done by the steam including the expansion (and calculates how steam behaves as it expands since it doesn't behave like an ideal gas) It also includes exhaust and compression stroke. It analyzes the engine from a thermodynamics standpoint, and just happens to spit out horsepower as a side effect. After all it's transforming heat into physical work.

The valve passage calculator is its own separate thing. I found some equations for estimating valve passage sizes in an old engineering book and just added a calculator to the bottom of the page. It's not connected to the rest of it.

I didn't get a chance to pickle the zinc off the vessel this weekend. However since I have three of these vessels, I'm going to put away the one I drilled holes into, and grab a fresh one to drill the bolsover holes. I have a third one I cleaned up and won't modify (besides the flange bolts) that I would like to use in the future on a lamont boiler that I have partially built. I'll keep the one i already drilled holes into, in case I want to build a horizontal boiler in the future.

I just got super lucky finding these on marketplace. Otherwise I was looking at building my own vessel out of a 4in sch 40 steel pipe and machined end caps held together with tie rods. A local metal supply place sells pipe up to that size for a reasonable price.

That was my original idea to go with my lamont firebox I built a couple years ago. Now that I have these, I'm glad I can make something a bit more conventional, and avoid the electric circulation pump and nest of tubing.

You can see the lamont firebox I built here: Link

I've got the tubing, some silver solder and flux, and everything I need to make this now. I'll be using an oxy acetylene torch, which I have some experience silver soldering with. It's not too hard as long as you're gentle. It's better to heat around the joint rather than directly on the joint itself.

 

[rolphill]

Finalizing the design now. I made three models:

3x coils, 2 turns, 12 degree angle, 1.44sqft
2x coils, 4 turns, 6.2 degree angle, 1.92sqft
2x coils, 3 turns, 8.2 degree angle, 1.44sqft

Minimum coil angle is around 6 degrees from what I've read.

In the real world, I was able to wind my 10ft section of tubing into just over 6 coils of the right size. So I'm going to do with the third option. I'm not a fan of the ~15mm gap in between the coils, but that's fine. This should be enough surface area, especially with radiant heat. The bottom of the vessel is ~1/2in thick, so I'm not too worried about overheating it. Overfiring is much easier with solid fuel anyways if necessary.

One question is do I add a water feed pipe into the bottom, which has NPT holes in it, or do I add water somewhere else? Any water pipe going to the bottom will be exposed to the full radiant fire heat. Related to this is the water gauge, I don't think it's original location will be ideal. I'm going to have to move it up a couple inches as well as extend it outside the casing. I might not even use that one, and just use the one I made for my old boiler. I'll be able to tee the water feed into that lower pipe then.

In any case, the coils are coiled, and I'll try to remove the zinc with vinegar tonight. Then I'll try assembling it tomorrow.

 

[Steamchick]

Hi Rolphil,
From your table, I should select the double 3-coil arrangement... if you are troubled by access for silver soldering the double 4-coil job... But really, the more coils, and tubes the better! There is a coil pipe-length consideration. length = 50x internal diameter is a traditional good rule, and definitely not exceeding length = 80 x internal diameter. - I think? So perhaps 3/8 pipe can do a better job with more tubes and fewer coils? But you have already drilled holes I see...
Water feed. It is usual to feed water into a zone that is not directly heated, so to avoid a cold-shock on a hot surface that can create horrid stresses in the metal locally. Maybe a single pre-heat coil "above" the boiler (to grab some otherwise wasted heat) then feeding into the top plate/zone?
Water gauge. I suggest for this design that the water gauge is only less than the top half of the boiler in length. So if the water gets low on the gauge, you still have half-a-drum of water to give you time to get more in without dropping steam pressure too much. The top quarter is even better. But remember the top of the gauge should be a level - maybe 1 1/2" below the top plate so you have the same effect for steam space. Priming (over-filling so water is pumped from the boiler with the steam) damages engines. - and you see the power loss immediately. On a 12" long boiler, the glass could be 1 to 1/2" below top of internal face to 4" below the top of internal face.
Of course, you can take the bottom feed of the gauge from the existing bottom connection, but both top and bottom connections to the drum should be long enough to have the gauge outside the casing - and that means a top and bottom connection that cross the gap where hot flue gases are passing. The gauge will need a blow-down valve, and preferably at stop cock at both top and bottom in case a glass ever breaks while in steam. You can arrange this from a new tapping for the bottom gauge glass feed pipe. Then use the old bottom tapping for the boiler blow-down - a pipe to the outside that will have a valve and exhaust pipe so the boiler can be emptied (when finished, but still hot and with a small pressure remaining, after the fire has been extinguished). Blowing down is necessary to avoid long-term corrosion.

 

[rolphill]

Ah yes, I forgot about blow down. I'll hook that up to one of the bottom ports, and use the lower side port for water gauge and feed water. I'll plug the upper side port and hook the top of the water gauge into the piping for the pressure gauge and relief valve. I did this on my last "boiler" and it worked well.

I do have to think a bit about steam space, I only have about 2 inches from the top of the coils to the top of the vessel. However, the flange cap has two NPT ports, a 1/4 and a 3/4. The 1/4 will go to the pressure gauge, water gauge, and relief valve. I think I'll add a 3 inch or so piece of pipe to the 3/4 port, which will give me extra steam separation space.

I'm thinking the water gauge will go from an inch or two below the top of the coils to an inch below the top of the vessel. I probably won't use the gauges that came with the vessels. I'll use the one I made.

 

[Steamchick]

Sounds good, except, I worry the injection of cold water at the bottom of the water-gauge could cause thermal shock and break the glass... - or rapidly cool the fitting and tighten the seal cracking the glass...
Water injection should be into the top water away from heat and areas where cold water can cause material stresses... e.g. the top port you were considering blanking-off? - Screw-in a reducer to something like "car brake pipe" sized feed pipe - or whatever is suitable? You may be able to add a short length on the inside of the boiler that bends downwards squirting the feedwater into the reservoir water in the central tank? = quick mixing promoting circulation, and less "thermal shock"?
Just good practice.
K2

 

[rolphill]

Well, it's together. I had a lot of issues.

The first issue was that bending the tubing makes it super oval shaped. I had a heck of a time getting the ends back into a shape that would fit in the elbows. I ended up making a tapered dowel to get the inside opened up. Then the model shop guy had a really good idea that worked quite well.

Take a (5/8 in this case) collet, jam the end of the tube into the collet, and then tighten the collet into a holder. This was able to force the oval shaped end back down into the proper sized circle. This worked really well, as long as you're able to get the end of the tube into the collet in the first place.

I started by just silver soldering the short stubs of pipe into the steel vessel. This went very well. The concentrated heat of the oxy-acetylene torch meant that was able to just wire brush the zinc off the area around the holes and didn't have issues with having to heat the whole vessel.

The next step was where the trouble began. First, I accidentally blew a small hole in the tubing when I didn't notice the torch flame got too oxidizing for a minute. The torch is old and the regulators drift a bit occasionally. I think I was able to fix it with some bronze brazing wire. If it fails during hydro I'll try to make a hole and braze in a small bolt or something.

Then I had issues with the joints between the tubing and the elbows. There's a gap on the outside, where the tubing bends, that took a few tries to properly fill with solder. Next time I'll try to bend and cut it in a way that leaves some unbent tubing at the ends.

I did a dunk test and at 50psi the only leak now is this joint between the elbow and the straight pipe. I'll try to fix this later today. I'm surprised that's all, considering the horrible mess I made of the silver soldering.



Yeah, I think I'll pipe the feedwater into the blowdown pipe. I'd like to add a feedwater preheater coil anyways, so it shouldn't be too cold. Depending on how fast this boiler loses water, I might install a spark plug in the upper side port and set up an automatic electric feed pump. Fun fact, model T sparkplugs have a 1/2npt thread. You just have to cut the side electrode bit off and you have a $6 water level sensor with no machining.

 

[TimTaylor]

Sounds good, except, I worry the injection of cold water at the bottom of the water-gauge could cause thermal shock and break the glass... - or rapidly cool the fitting and tighten the seal cracking the glass...
Water injection should be into the top water away from heat and areas where cold water can cause material stresses... e.g. the top port you were considering blanking-off? - Screw-in a reducer to something like "car brake pipe" sized feed pipe - or whatever is suitable? You may be able to add a short length on the inside of the boiler that bends downwards squirting the feedwater into the reservoir water in the central tank? = quick mixing promoting circulation, and less "thermal shock"?
Just good practice.
K2

I agree. In water tube boilers, the feed water is typically injected about 1/3 from the bottom of the steam drum. In fire tube boilers, the injection point is typically near the mid-drum level.

To minimize thermal shock, fabricate a simple economizer tubing coil that is located in the boiler exhaust stack - passing the feedwater through the coil will raise the temperature significantly, significantly reducing or eliminating any thermal shock issues.

 

[rolphill]

That leaky joint just needed a reheat with fresh flux. It passed the dunk test, so I filled it with water and it holds at ~100psi shop air pressure! (Ignore the gauge, it's kinda broken)

I'll have to do a proper hydro test to 150psi at home, but the hard part is completed. Now I have to build the firebox and plumbing and stuff. I now have a 2.5sqft boiler.

 

[Steamchick]

Can you add another coil? - The more the merrier!
I would have used smaller bore stuff, and a bender to avoid the flattening of the tube. One trick is to blank-off one end, fill the tube with water, then fit a compression fitting blank end on the open end so there is virtually no air inside. Then bend and it shouldn't collapse so much?
K2

 

[rolphill]

2.5 square feet is enough for this particular boiler. It just looks weird because the proportions are off compared to most boilers. Most ofeldts have the vast majority of heating surface in the coils, while this one is more balanced. Remember the minimum goal is only 100 watts mechanical output or so to match human rowing speeds. More would be great, but isn't in the original requirements.

Yes, next time I think I'll use 1/2in o.d. tubing. I made a chart of tubing $/sqft from my local hardware store and 1/2in and 5/8in tubing was the same $/sqft, whereas 3/8od was more expensive, and 1/4od was only a little bit cheaper. The larger sizes are harder to bend, but are otherwise much less work for a given surface area. Less joints and thus less expensive silver solder too. I am far more constrained by material budget than I am by time for things that I can tinker on during spare time at work.

In any case, Now it's time for the firebox. I have a spare 10lb propane tank that the boiler would fit in nicely. It's the tall narrow shape. Alas, it's a little too short to also fit the combustion chamber into as well. Also it was a ton of work to build the curved fire door on my lamont firebox. I think I might cut the bottom off, fit the boiler inside it, and build the combustion chamber separately and weld it on the bottom. I might make the combustion chamber U shaped, so it's round but with a flat face to make the door easy to build and fit. I also need to think about how to make this portable and mountable. I'll probably weld it to some L channel rails that have bolt holes to mount to the boat.

Since the boiler was vaguely inspired by the clishay boiler and similar in physical size, I'll also study the clishay book for inspiration on how to build the shell and firebox.

This may be where things slow down, as it's a bit hard for me to get a lot of time out in the barn to do metal fab work at home, especially while it's still cold.

 

[WisJim]

In the past I made a coil of copper tubing to fit around the stovepipe of a wood stove to heat water. I made a form using a coffee can filled with cement, and I filled the tubing with dry sand and left the ends long so I could crimp them and later cut off the crimped ends . It worked well and could dump out the sand and then flush it with water. The tubing bent well with no flattening.

Jim

 

[Steamchick]

Rolphil, Sounds like a good plan! While you have a boiler outside the firebox, you really should do an hydraulic test at around 220% of your max. working pressure. Then set the safety relief valve to lift at less than 4% over your max working pressure. After setting with hydraulics, then check it with an air compressor that delivers something like your expected max steam production. (Usually the only one in the garage?) And tweak the setting as required. Then you'll be in a safe situation to set the pressure relief valve finally, by sitting the boiler at max water on your barbeque, but using coal, with the surrounding tank you propose to use. The safety relief valve on full blow should prevent your exceeding max W. P. + 4%.
Hope this helps? It should ensure a safe boiler.... before you build the firebox.
You don't want to have to swim, or be blown out of the boat.
Have fun. Do tell how it goes....
K2

 

[Steamchick]

Just a thought on the spark-plug level detector. I presume resistance sensing? With steam = 1 value, with water a different value? - so a controlled voltage across the gap plus a resistor will show a different voltage across the resistor in the 2 different situations, so that can trigger a transistor to switch on or off the pump? - I am fascinated to see if this can work...
Fun!
K2

 

[rolphill]

Just a thought on the spark-plug level detector. I presume resistance sensing? With steam = 1 value, with water a different value? - so a controlled voltage across the gap plus a resistor will show a different voltage across the resistor in the 2 different situations, so that can trigger a transistor to switch on or off the pump? - I am fascinated to see if this can work...
Fun!
K2

Basically. Although it works better with a mosfet. Boiler is grounded, center electrode is tied to a mosfet gate that's tied to positive with a 100k or so resistor. mosfet switches your load on the ground side (source is grounded, drain goes to load -, load + tied to positive). I think I used a LED light, and just controlled the pump manually on one project. It didn't seem to work very well on a monotube boiler, since monotubes don't do water level very well. However it worked better on a regular water level boiler.

Commercial systems usually use AC between two replaceable electrodes as any current flow through the boiler water will corrode the positive electrode. That's why you use a high resistance value to reduce the corrosive current to ignorable(in hobby usage) levels.

 

[rolphill]

I've decided to basically copy the clishay firebox and shell with some minor changes. First, the base plate will be 1/8in instead of 1/4in. Unlike a loco, the added weight is a negative for me and the baseplate isn't directly exposed to the fire anyways. I'll be welding the firebox together out of mild steel. My ashpan will be much simpler, since I don't have to fit it in a loco chassis. Instead of round bars for the grate, I want to use angle iron facing up. I saw this somewhere on one of these forums, the channels fill with ash and supposedly help protect the steel.

It just so happens that Alro steel now has a storefront a mile from work, so I was able to pick up all the cutoffs I needed for a very reasonable price by the pound. It shouldn't be too much effort to machine all these out, and I might even add holes for pins to hold it together for welding.

 

[Steamchick]

A more refined set of firebars would use a 10 degree tapered slot - narrowing at the top - to introduce air acceleration to create a higher velocity of air through the fire, thus enabling a slightly thicker fire and faster combustion - I think?
I also have a hint of doubt, but these firebars were introduced quite widely in industrial hand-fired boilers, or so I seem to remember from somewhere - as a late Victorian efficiency improvement. Necessary for cost effective manufacturing!
I doubt your slots will be doing that though.
K2

 

[Steamchick]

Have you considered removable firebars? Weld stubs onto the frame instead of complete firebars, so the individual angles can sit in the stubs and become removable firebars.... Easier for cleaning and maintenance?
K2

 

[rolphill]

The grates will be removable. In the clishay book they are in two halves that can easily be removed through the firedoor. I wouldn't want them to be permanent anyways.

I modeled the V grates and now I'm not so sure about it. It both takes up more vertical space and has less space for air flow than regular round bars would. Vertical space is important as I only have access to anthracite coal (nut or rice from tractor supply co), and anthracite loves a deep fire. I doubt I'd ever care about the wear rate on them anyways, since making new grates wouldn't take long at all.



The original clishay design allows for the whole grate and ashpan to be easily dropped out in case of an emergency. There are two bars held in with cotter pins that the grates rest on.

I'm working on the firebox now. The base plate came out very nicely.

 

[Steamchick]

I would suggest you "try it and see" with the angle iron firebars. I assume you'll be forcing the fire with exhaust steam blast from a nozzle up the chimney to give a pretty effect while steaming, and to force the anthracite fire? Usually people have dampers to employ to balance the air intake to demand for steam = the fire, but your schematic looks OK from other grates (on model locos) that I have seen...
Somewhere there will be a ratio of "bar" to "gap" for the fire you plan to use. (My guess is 60% : 40% of grate area? - But an expert will tell you "what is right" for anthracite.)
Just a loose opinion though. No hard facts.
K2

 

[HMEL]

The grates will be removable. In the clishay book they are in two halves that can easily be removed through the firedoor. I wouldn't want them to be permanent anyways.

I modeled the V grates and now I'm not so sure about it. It both takes up more vertical space and has less space for air flow than regular round bars would. Vertical space is important as I only have access to anthracite coal (nut or rice from tractor supply co), and anthracite loves a deep fire. I doubt I'd ever care about the wear rate on them anyways, since making new grates wouldn't take long at all.

View attachment 154513

The original clishay design allows for the whole grate and ashpan to be easily dropped out in case of an emergency. There are two bars held in with cotter pins that the grates rest on.

I'm working on the firebox now. The base plate came out very nicely.
View attachment 154514

I would be tempted to reverse the angle iron to open up the air flow area and get rid of the dead spots where ash can accumulate. And if possible would make that grate so it can be moved a bit to shake clinkers lose so they go to the ash pan. This would allow more air to be evenly distributed across the grate. Cast iron would be a better material and there are some barbecue grates that might work for your purpose.