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Monotube boiler construction

Posted by oliverjd 
Monotube boiler construction
June 03, 2016 03:31PM
Hello: I am assembling a monotube boiler based on the Richard Smith plans for a steam powered bicycle. It is constructed of 21 feet of 1/8 in schedule 40 black pipe as an inner coil with 42 feet of the same material as the outer coil (two 21' sticks tig welded together). I need some advise regarding the connection between the two coils. The plans call for the connection to be welded, however the space is too limited to get good position for tig welding. I am considering machining a mild steel tube about an inch long with an inside diameter equal to the OD of the 1/8" pipe plus about .005 Both ends of the two coils would slide into this tube and then be bronze brazed using Harris 15 bronze rod. (melts at about 1700 degrees) Is this a safe method to join monotube boiler tubes? The boiler will operate at about 300 psi at 700 degrees. I seek the opinion of those more knowledgeable than myself before proceeding.

Thanks, Jerry
Re: Monotube boiler construction
June 04, 2016 07:53AM

The method you describe should do the job if you get a hi-quality join on both sides of the sleeve. I've used both methods-tig and hi temp solder...and I go to great lengths to be able to tig whenever I can. I feel like a weld will take more abuse....If you don't have enough room for your tig torch-how about doing a gas weld? Use the tig torch on the accessible areas and the gas torch to get the back side....

I've used silicon bronze tig rod quite a few times also, and it's quite easy to get a good join with it.

Good Luck with your project-keep us updated on the progress-eh?

Re: Monotube boiler construction
June 04, 2016 06:38PM
Hows about welding it as it is wound? Wind half, Weld to the second half, pressure test, proceed winding.

I'm with Chuk on the welding versus brazing. If it can be welded it's a stronger joint (butt joint) that will take much more heat in use.


Sidenote regarding brazing. I'm in the process of restoring a 1900 Conrad chassis. Typical tubular bicycle-type frame construction. The steel chassis components are brazed to the tubing as is typical with these. It was necessary to bore the old tubing out of the frame joints. In doing so, I noticed that they were brazed completely through. Some of these have about three inches of contact - tubing in to the joint and there are no voids in the bronze. I can't figure out how they got brazing bronze all the way through the joint like that. Anyone know how they did it?
Re: Monotube boiler construction
June 05, 2016 12:00AM
When these were made at Smith's shop, he welded the two lengths of iron pipe together and then wound them up.

Karl Petersen

Edited 1 time(s). Last edit at 06/16/2016 07:57PM by Karl Petersen.
Re: Monotube boiler construction
June 05, 2016 06:28PM
Thanks for sharing your thoughts and suggestions regarding the best method to join the two boiler coils. I am leaning toward trying to heat and bend the ends of coils to gain enough room to tig weld the joint. Then (after welding) heating it again to tuck the joint back in place. I'll let you know how this works out, and will post a picture if I'm successful. I would like to share with you a picture of the freshly rolled 1/8" schedule 40 pipe as well as a picture of the device I welded together to do the rolling. As you can see from the picture, it is a manual process. My son was nice enough to provide the muscle to turn the crank. The small roller is the mandrel for the inner coil and the large roller is for the outer coil. Originally I had planned to fill the pipe with Wood's metal prior to rolling, however, after experimenting on short pieces of pipe I found that rolling it without the Wood's metal only reduced the diameter about .015. Thus I rolled both coils cold and empty. Both came out pretty nice.

Thanks again for your help. I'm pretty sure I'll need your advise again before I complete this project.


Re: Monotube boiler construction
June 06, 2016 09:35AM
Every brazing alloy weather silver or bronze has a minimum clearance for the alloy to flow.
Harris publishes a very nice chart for their alloys.
The attached photos are of a Stanley rear end and front end. I used Harris 56 silver and 0.005 clearance space. The tube was center punched in six places around where the castings were to go to keep them cantered and leave the required space. The filler metal was feed into a center hole in the casting.
These castings were about 2.5 inches long. You can see by the photos how the silver ringed around the outer edge of the castings.
I always use the black flux on steel as it takes much higher heat for a longer time.

Re: Monotube boiler construction
June 06, 2016 11:17AM

Thanks for the tips, that is a good method. Still wondering how they did it back then without Acetylene type torches. They must have used an oven. These had no feeder holes for filler and they were pinned with nails in position. And there doesn't appear to be much clearance in between the tubes and bores, the bronze is very thin, clean out the bronze and the bore just fits the tube. I was wondering if the did it with an autoclave under vacuum? I found an old British film on making bicycles from the 40's? and they show furnace brazing, but not very much of it.

Re: Monotube boiler construction
June 06, 2016 12:31PM
I also used a tapered pin to hold my parts in place till the job was done.
They were making copper castings 6000 years ago, I know they were brazing metal in 1830 or before,
How old are the parts you’re talking about.

I just found this site.

Edited 1 time(s). Last edit at 06/06/2016 12:37PM by Rolly.
Re: Monotube boiler construction
June 06, 2016 02:20PM

Oh I'm aware that they have been working bronze for thousands of years. Even Steel, as a steel knife was discovered in the sarcophagus from the tomb of the Egyptian ruler, Tutankhamen. There was a media report just a few days ago regarding that.

These parts are from 1900. I was just curious how they were able to braze the components together so efficiently. The process that they used.

These are definitely steel nails smiling smiley I have the broken boring bar inserts to verify.

Re: Monotube boiler construction
June 09, 2016 05:35PM
Thanks for your good suggestions and advise. I joined the inner and outer boiler tubes today. Pulled one of the outer coils out about an inch, tig welded the joint and then heated the pipe and tucked the coil back in place. The hydrostatic test went well. The boiler was pressurized to 3000 psi and it held that pressure for 1 hour. I plan to operate it at 300 to 350 psi. A couple of pictures are attached. My next project will be making the burner assembly. The Smith Bicycle used propane at full tank pressure which presents problems when operating it during cool weather. I would like to use a regulator to eliminate that problem, however, I need to design a new burner. What are your thoughts regarding tube burners like the ones used to fire a propane forge? Do you think that might work if I size it correctly? It must fire directly down the center of the inner coil which is 1-3/8" ID. Any thoughts or comments regarding a burner for this project would be appreciated.

Thanks again for your help. Jerry.

Re: Monotube boiler construction
June 10, 2016 09:08AM

That's the way it's done-just like you did it!

When using propane for a burner, I always use an adjustable regulator so that I can tweak the fire to make it do just what I want. I'm sure a forge-type burner would do the job if you make it big enough. The problem I see with the coil setup you have would be the distribution of the heat-the coils immediately in front of the burner are gonna get most of the fire. If you can achieve a long narrow flame that goes quite a way thru the stack you'll do well!

It'll be a screamer when you get it right-are you going to bring it to the Fall Meet in Michigan?

Re: Monotube boiler construction
June 10, 2016 04:08PM
A suggestion - this site


describes what he calls radiant poker burners for model steam locomotives. These are basically like a Stanley pilot burner, with the gas flowing along a perforated tube and emerging to burn through the small holes.The difference here is that the perforated tube is surrounded by a sleeve of fine mesh stainless steel) gauze which acts as a radiating element as the flame burns in the gap between the tube and the gauze sleeve which glows red hot. I have tried this and the flame is well controlled and the hot area of gauze can cover the full length of the burner which would help to avoid overheating the first few coils of the surrounding monotube. The flame and hot area does concentrate towards the upper side but with suitable placement of the holes in the tube it is possible to cover 180 degrees of the circumference of the tube.

I don't know how well this would scale up to the heat output you will need but it is probably worth experimenting with the idea. Inconel rather than stainless steel for the gauze would last longer. The model locomotive use is fired by propane but quite probably some kind of kerosene/gasolene adaptation could be made to work.


Edited 1 time(s). Last edit at 06/10/2016 04:12PM by Mike Clark.
Re: Monotube boiler construction
June 11, 2016 09:06AM
A similar idea that might work at full scale is the ceramic burner. I've been working on a model version of Chuk's burner. It works very well with propane, but it will run on liquid fuel eventually when I get the pump and blower controls sorted out. The pictures show the assembly with the blower, the inner can, the outer can, the coil nozzle test on low pressure propane, and the coil assembled in the inner can with the baffle plate. The last picture shows the burner assembled on the monotube boiler.

Lohring Miller

Re: Monotube boiler construction
June 12, 2016 07:14PM
Chuk: Your point about the first few coils getting the most heat is well taken. I want to think about this burner design. No, I won't make the Fall Meet in Michigan. I've never attended before but hope to be able to do so next year. Perhaps I'll have more of this project sorted out by then. Thanks, Jerry

Mike: I've never heard of radiant poker burners before, but they sure look worth looking into. Looks like it would supply a very even distribution of heat (except for the bottom). I'd like to do some experimenting and I am confused regarding the stainless mesh I should start with. The link that you provided recommends 40 to 50 denier tightly wrapped around the poker burner's slots and a strip of 20 mesh stainless steel cloth over that. I found stainless mesh at MSC, but they offer various wire sizes for both the 40-50 mesh and the 20 mesh. Could you recommend the best wire size to start with for both? I will attempt to build a poker about 1" in diameter and about 16" long. Any recommendation regarding the size of the holes to drill for the gas jets as well as the location and quantity would also be welcomed.
Thanks, Jerry.

Lohring: Your blower burner looks like it would work good too. Does the blower used to direct the heat up the stack, or does it provide the air for combustion? Also, I am unclear where the ceramic is located. Is it in the inner can?
Thanks, Jerry.
Re: Monotube boiler construction
June 13, 2016 12:55PM
The ceramic burner was another idea. There is no ceramic in my burner. The blower supplies combustion air to the inner can through vanes at the base and holes in the sides of the inner can. The vanes give swirl to the primary air going around the burner nozzles that are under the baffle. Secondary air enters the inner can through the holes further down the can. The combustion products exit the inner can and flow around the boiler coils that are enclosed in the long tube in the last picture. Below is a picture of the boiler coils. Water enters the coil at the right end of the picture. The burner is at the left end of the coils where steam exits.

Lohring Miller

Re: Monotube boiler construction
September 07, 2016 05:24PM
The trick in winding any kind of a coil is to put some drag on it. We use a drilled nylon block that is then split and held together with two bolts. The bots are tightened until there is a good drag on the pipe. This is all done upstream from the winding. If you put enough drag on the pipe, which we do not do, you can exceed Hook's Modulus of Elasticity. When that is done there is no spring-back when the tension is released . With partial drag there is enough stretching so that the outer half of the pipe stays coiled. When making hair-pin bends, which is what the late Jim Tangeman did all of the time as he could make a monotube easily, one needs to clamp the pipe solid before pulling it around in the 180 degree bend. Otherwise the pipe flattens. There are several tricks to use when welding pipe or tubing together so that the weld does not leave a lot of slag and junk blocking the flow, another story for another time. Tom Kimmel
Re: Monotube boiler construction
May 25, 2018 12:07PM
Hi Jerry,
We just got back from the Spring Mini Meet last weekend. When connecting boiler tubes, Tom's secret is to wrap up some wadding and straddle this inside the tube before weld. This prevents the slag from entering inside the tube. After welding complete, blow out the wadding.

Do you have more pictures on the joining the inner and outer tubes?

Re: Monotube boiler construction
May 26, 2018 12:15PM
Hello Rick: Sounds like I missed a real good time at the Spring meet. Wish I could have been there. I don't have any more pictures of the process of joining the boiler tubing. I beveled the ends of both pipes to allow full penetration and, after practicing of scrap tubing, tig welded the boiler pipes. The finished boiler hydro tested at 3000 psi with no leaks. If I had known about using a ceramic wool "plug" inside the joint I sure would have tried it. Using a ceramic plug makes sense to me for joints where one pipe is still straight (so the plug can be blown out the straight pipe with compressed air). Not so sure it would work for joining two pipes that are already coiled, as the plug would be more difficult to remove.

Tom Kimmel's site has a new article on how to coil steel pipe. It is very interesting and worth looking at. That article also talks about using a ceramic wool plug when welding pipe.

The boiler I built now resides in a steam motorcycle that is nearing completion. It runs, but still needs a lot of tuning. A link to a youtube video appears below showing the first attempt at running the bike with steam.

Thanks, Jerry.

Youtube Link: [youtu.be]
Re: Monotube boiler construction
May 26, 2018 02:48PM
Where did you get your tubing?
Re: Monotube boiler construction
May 26, 2018 03:34PM

Looks good, good job.

Re: Monotube boiler construction
May 26, 2018 05:19PM
Zimirken: Made the boiler from schedule 40 1/8" steel pipe. Bought it in 21' lengths from my local steel supplier. If you order any, be sure to take your calipers with you to measure ID, OD, and wall thickness to verify that it is actually schedule 40 pipe before you pay for it. There is foreign tube on the market that looks similar to schedule 40 but has thinner walls and the welded seams split wide open when you try to bend it.

Ron: Thanks for the kind words.

Re: Monotube boiler construction
May 29, 2018 11:48AM
Hi Jerry,
I second Ron's comments...Good Job!

I see you put some dead tubes in between coils. I'm thinking for spacing? Are the dead tubes removable? Please elaborate?

For Zim,
I purchased 21-22 length 1/8" for use as vaporizing tubes. Make sure you get seamless. My tube came from Canada, good stuff and kind of pricey. Don't settle for the inexpensive stuff.

Kind regards,
Rick H.
Re: Monotube boiler construction
May 29, 2018 04:39PM
Hi Rick: Right you are, there are three spacer tubes between the inner and outer coil. One tube is the tube that connects the back end of the small coil with the front end of the large coil (where I welded them together). The other two spacer tubes are not removable. They are welded to the small coil at one spot only (at the center of the small coil) so as to not interfere with the expansion of the small coil. The expansion of the small coil is used to shut off the propane if the coil gets too hot. I have attached a picture which shows the boiler with the temperature control.


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