Finned fire tube boiler

I have these finned copper tubes I salvaged from a swimming pool heater, some of you may remember me posting about it a while back. I looked into the use of finned tubing for boiler use, and found out that as a water tube they are not practical due to the vulnerability of the thin copper fins to high heat. Even when used for relatively lower temperature pool heating, the fins would still melt if the water flow rate wasn’t adequate enough. Ok...no good for steam use, right? What a shame, all that heat exchanging surface area yet it’s useless for anything besides maybe some preheating at the very end of the boiler or for condensing. Then it hit me; why not use them as fire tubes? That way the vulnerable fins would be immersed in water and protected from overheating, yet it’s great heat exchange potential would still be utilized. Not really anything on the internet on this, and some steam folks I’ve talked to really couldn’t say if it would work or not. Time to find out for myself.

I did a bunch of research on traditional fire tube boilers, the Stanley type in particular. I made some sketches on how I thought it could work, keeping it small for possibly using it on the mini chopper. I decided on using a 1’ long piece of 4” schedule 40 pipe for the shell...and found I could fit 7 of these finned tubes, nested close together, inside the pipe. I chose 3/4” thick steel plate for the crown sheets, a bit overkill considering a Stanley boiler uses only 3/8” thick steel, but with the extra danger that a fire tube boiler brings and the lack of enough tubes to give it a stay rod type support (like a Stanley) I didn’t want to take any chances. I also welded (3) 1” x 1/4” bar stock reinforcing bands around the ends and middle of the shell. These bands also serve as a platform for drill / tapping pipe threads for fittings.

The finned tubes are one piece spiral wound, with a 5/8” inside diameter and 1 1/2” fin diameter. The gap between the fins is about 1/8”. They are designed to overlap each other and they nest together perfectly, creating very small gaps between the fins when combined. I like this because flowing water would be spread very thin between these heat exchange surfaces and would, I would imagine, flash to steam rather quickly. With the tubes cut to length and assembled inside the pipe, they fill up much of the inside volume. This boiler would not have much reserve water / steam space, so it could not work like a mini-Stanley...it would have to be used below the water level of a separator drum and would always be flooded with water, monitored by a sight glass. That way there is a reserve of preheated water and the steam can rise and collect in the drum as well.

I have a few concerns so far:

1. Heat flow: Because of the width of the fins, the amount of tubes are limited and the fire might not have enough flue volume to properly flow through. To help with this potential issue, I am also installing (6) 3/8” steel fire tubes in the gaps that are between the fins and the pipe wall. This should allow for extra flow and heat transfer.

2. Water flow: The fins create a physical barrier that limits water / steam flow in one direction, perpendicular to the tubes. That means while the fire will be running inside the length of the tubes, the water / steam flow would be on the tangent. This means the boiler would probably have to be laid horizontally, locomotive style. Because the fins are spiral wound....the water flow would presumably cork screw in mainly one direction. I plan on taking advantage of this by locating the water input on the tangent, going with the screw of the fins with the steam exiting the opposite side. Because of how congested the inside of this boiler is...there may be issues with getting feed water properly distributed.

3. Insufficient heat transfer: A well know issue with fire tube boilers is the amount of heat that is able to just pass through the fire tubes. Some Stanley boilers have twisted metal turbulators installed to force the heat against the tube walls. I could do this, too...but I was thinking on adding 50’ of 3/8” or 1/4” economizer tubing, in pancakes, at the outlet of the fire tube boiler. This would hopefully pick up any heat that passes through and would preheat the water going in.

4. Having to make a copper to steel transition: Because the finned tubing is copper, and the fins prohibit doing the traditional method of expanding the tube ends onto the crown sheet, I felt I had to somehow connect a steel tube to the ends of the copper so that it could be welded to the crown. I noticed that the inside of the tubes could be tapped for female 3/8” pipe threads. I did this and screwed in some steel 3/8” pipe nipples. I was told that may be a problem because copper expands more than steel, and may cause loose connections / leaks when heated up. To help mitigate this issue...I tightened the nipples in the copper tube ends....heated the ends up with a torch....then tightened them again. That way the connections are tight in the already heat expanded state. I am hoping this will work.

I have the crown sheets cut, marked for the holes and ready to be drilled and welded on the ends. Once this is done...the boiler is pretty much sealed shut and any inside changes would take cutting and drilling out all the welds to get back in. Because of this, I would really appreciate any input on what I have so far and any possible issues that might exist before I commit to welding it. Below are pictures of the project so far, with the first two showing the finned copper tubes and steel tubes partially pulled out of the shell:

Jamison

only possible problem I can imagine is in vigorous operation the steam generation could be so fast as to push water away from the tubes allowing the tubes to overheat and blow out. Careful testing needed, keeping things safe in case a tube blows. Or keep the fire at a moderate rate so it never runs that hard. But it's kind of a guessing game how fast it could be run without having some sort of tube temperature monitoring device. Is there a temperature gauge available with very small remote sensor that could be attached between fins on a tube? reading to 1000F or over. If it starts going too far above water temp you know it's in risk of blowing out.



Edited 1 time(s). Last edit at 07/30/2018 03:26PM by ReubenT.

There was one type of boiler, I think it was the Bonecourt, which had iron catalytic combustion elements inside of firetubes, giving massive radiant heat input to the inner surfaces of the tubes. Maybe that could be used with firetubes finned on the water side -- if enough water circulation & steam flowpath over/around the fins could be guaranteed? Just a thought.

Reuben,

I could see that happening as well. Kind of a good problem to have...fast steam generation. It’s like a fire-tube flash boiler, and like a monotube flash boiler requires a constant water flow when fired...so would this. It would work great, I believe, in a forced recirculating LaMont type system, where the main steam generating circuit is always flooded and flowing. Lots of potential and will need some safe testing to see exactly how it behaves. I drilled / tapped several fitting points on the ends and in the middle, on two sides. That way I can experiment with different entry and exit points for water and steam. I agree on being careful with the temps, will have to gradually turn up the fire while visually monitoring the tubes to make sure they don’t go red, while keeping track of the water level with a sight glass. I haven’t messed that much with thermocouples...I’m kinda old school when it comes to steam.

Peter,

I looked into the Bonecourt boiler, it was designed to be a waste heat boiler and originally used refractory material in the fire tubes. That made too much dust, so they switched to an iron spiral inside the tubes instead. Interesting stuff.
I think a more ideal version of a finned fire-tube boiler could utilize stainless steel finned tubing...much more forgiving if accidentally overheated. They make it for base board heaters, a bit pricey though, and the ones I saw online have kinda large diameter tubing...like 1 1/2”. Might not be suitable. If this copper tube version works well...then perhaps an upgraded stainless one might be in order.

Jamison



Edited 1 time(s). Last edit at 07/31/2018 05:03PM by Arch-Tone.

Some info that may be useful. The issue with firetube boilers is they have less than optimum internal circulation. The firetubes generate steam at the crown sheet - great, the issue is the water has to be replenished on that surface to continue the process, low water level and hard firing the water can't get there in time and the boiler can be scorched even with some water in it because of this. Often referred to as "lifting the water off the crownsheet", the more scientific term is DNB or departure from nucleate boiling which allows a surface to overheat and fail.

Considering the above, I'm quite certain the external fins on the firetubes as they are now intended to be used would only serve to exacerbate this issue. Think about steam being created between the fins, how would the water be replenished, imagine this condition on your center tube in the bundle. As Tom Kimmel joked once, "it's all up and no down" Think about how the water would react to a surface that is overheated, it would act like a flash boiler in that area and the steam would push water away, and the longer it is fired, the hotter it gets and larger the area gets. Possibly with forced circulation cross-wise parallel to the fins, with vigorous flow it could work. Just need an adequate pump, that no one has found at the hobby level that I'm aware of. Vigorous internal circulation like that of a water tube boiler is why they can produce roughly twice as much steam per square foot of heated surface.

The other issue is with pool heater tubes large diameter, they consume a lot of the area of the crownsheet and limit the amount of flue gas/fire thru the tubes, with the amount of fire you're getting out the vaporizing burner, I think the draft would be restricted considerably. In other words, it would be better to use regular firetubes and a lot more of them for the same diameter boiler, it would probably do better, but the only way to know for sure is, try it.

-Ron



Edited 1 time(s). Last edit at 08/01/2018 09:24AM by IronChief.

The heat transfer bottleneck is almost always flue gasses -> metal anyways, not metal -> water. Hence why the fins are always on the outside.

Honestly, I can't see any advantages to finned tube in a fire tube boiler. Water is far more dense than combustion gasses, and can absorb a much greater amount of heat for the same surface area. For the fins to improve heat transfer to the water significantly, you would need to apply far more than enough heat to the inside of the tube than it could ever withstand. This is ignoring, of course, the obvious fact that the outside of a tube already possesses greater surface area than the inside. A turbulator inside the tube improves combustion gas heat transfer a bit but this is simply due to the higher Reynolds number … the gain isn't as significant as you would get from jacking up the surface area.

Now, if you had your own rolling mill, you could make a heavy billet with internal longitudinal fins and roll that out into a seamless internal finned tube...

Actually, increasing the surface area where it counts, on the inside of the tube, is a relatively dauntless task. Even if you succeed, the gains are going to be modest … for simple geometric reasons. A tube with external fins that are half the tube diameter is going to have much, much more surface area than any internal fins could every achieve simply because pies are square.

When it comes to internal circulation of a fire tube boiler, that's been attacked a few different ways in the past. The most straightforward method was a gland, packing and a shaft connected to a propeller that would circulate the water. Naturally, the more modern take on that was centrifugal impellers. Thermopressers have been used to speed up circulation. Unequal distribution of fire tubes, so as to promote more heating on one side of the boiler, tends to accelerate circulation.

Honestly, the better solution seems to be finned water tube boilers and some kind of drum to give you the same reserve.

Regards,

Ken

Zimirken and Ken, Agreed.

One thing to keep in mind and always do when the system is operational, is shoot the flue gas exhaust with an infrared thermometer, the exhaust temperature should be somewhere just above the steam temperature. If it is, perfect and you're not going to get anymore out of it, if it isn't, then you need more heated surface to take the excess heat to the water by adding boiler area, with superheating, or an economizer etc.

-Ron

Successful boilers using finned tubing have always put the fins on the gas side, at least all the ones I know of. And usually at the cold end of the boiler, close to the exhaust gas outlet, with unfinned tubing in the zones exposed to radiant heat and/or the hottest combustion gases.

Rapid water re-circulation in heavily-fired un-finned tubes can deliver a lot more steam per square-foot/pound/$ of tubing than finned tubes with once-through flow.

My design studies indicate that going from 80% boiler efficiency to 90% efficiency, required _doubling_ the square footage, weight, and cost of the boiler tubing. Beyond 80% boiler efficiency, additional efficiency increases give "diminishing returns". EG, twice the boiler weight/cost for 10% better efficiency. If the goal is 95 or 98% fuel-to-steam conversion efficiency, then the boiler weight/size/cost is _multiplied_ even further. Something like 4-8x the boiler size/weight/cost for the same steam output.

With enough economizer tubing, boiler exhaust-gas temperature can be well below the output steam temperature. But would such a design be practical or economical, especially for a steam road vehicle?

Peter Barrett noted that finned tubing costs 20% more per square foot of finned-side area than unfinned tubing. For a given output/efficiency, and with the right design, it can cut the volume and weight of the boiler, but at a higher cost. There are many tradeoffs to consider.

V.I. Ganapathy & others have written about the difficulties of careful heat-exchanger design with finned tubing. A tip for further research there. It gets deep.

Peter

Hi Jamison,
Seams like the conversation has gone around a little bit. That's OK though.

I would advise that don't worry about boiler efficiency at this point regarding the use of finned tubing as fire tubes. What you're proposing, to have the fins in the water has merit. Also, to incorporate a monotube with it has excellent potential. I would call your boiler a hybrid - fire tube/mono tube. The acid test is to see if the prototype will power your engine. Please let us know how this works?

As an aside note, on Chuk's Speed machine he did use finned tubing as an economizer like device. To me the jury is not out on how it works to help heat transfer. My thought, not substantiated, is that the fins increase the gas flow. Like I mentioned before, in the heat transfer equation, if you increase m-dot, you increase heat transfer. Not sure what the Reynold's number is in this situation. The point with high Reynold's No., higher turbulent flow, you get better heat transfer. I'm not sure if spiral fin in a firetube is increasing the turbulent flow. I would surmise that the fin is increasing the distance (> area) and increases the gas velocity (M-dot...mass flow rate). These two factors increase heat transfer.

Hope this helps in Boiler design.

Kind regards,
Rick

Quote: "I'm not sure if spiral fin in a firetube is increasing the turbulent flow. I would surmise that the fin is increasing the distance (> area) and increases the gas velocity (M-dot...mass flow rate). These two factors increase heat transfer. "

The turbulator, the twisted strip of metal in a firetube disrupts laminar flow and changes it to turbulent flow resulting in a higher Reynolds number. Laminar flow is the hotter gases resisting the cooler wall of the firetube resulting in layers or laminae of increasingly warmer air to the center. The twisted turbulator prevents this from happening, disrupts and forces the hotter gases in contact with the cooler wall.

I put these in my tugboat boiler and did see a performance increase, a bit hard to gauge firing with wood, but there was on overall increase.

As Zimirken pointed out, the heat transfer between the hot gas and the metal barrier is the bottleneck or longest part of the process, many time longer. Hiram Maxim determined that condensers submerged in water versus air flow were 2400 times (not a typo) more effective at condensing steam, it's the other end of the spectrum, but it does serve to give one an idea of why the fins are typically on the outside of the tube on the gas side.

Small residential heating boilers of the water tube type, they use finned tubes as well for efficiency. Only good for gas firing, with wood or coal, they would clog in short order.

-Ron

To all,

Thanks for your response. I hear and understand what your saying, and I know that the real battle with fire-tube boilers is getting the heat from the hot gasses to the tubes not the tubes to the water. I agree it would be better off with no fins and more tubes for tube to water heat transfer in general. However, a major factor to this design is that the water would be spread out into very thin layers between the heat transferring surfaces of the fins and the tubes, reducing the volume of water to be heated at any given time. I believe this would give it a very quick, flash type response. Crucial to this design, because of the need to rapidly remove steam bubbles in these tight channels, an emphasis must be placed on getting water to flow fast and evenly distributed throughout the boiler. Remember....these fins are spiraled like a screw and not individual discs like some finned tubing. That means that water flow, when pumped in on the tangent at one end, would spiral towards the other end... guided by the fins. For even better water circulation, I recently came up with a great way to distribute the incoming water. The smaller 3/8” steel tubes that I originally placed in the gaps created by the fins, around the perimeter of the finned tube cluster as extra fire-tubes (with the intention of reducing draft restriction) may be better served as incoming water nozzle arrays. Each of the 6 tubes would have multiple holes (nozzles) drilled across each tube, all aimed in the direction of the fin spirals. This would spread the incoming water across the length of the boiler and swirl it tangentially across the fins. The six tubes would then be closed at one end and the other end connected to a distributing manifold outside the boiler. To address the flue draft restriction concerns...I plan on running a controlled amount of fire / gasses around the outside of the boiler shell, guided into a twist as it passes over it. This will transfer heat close to the swirling water flow inside and allow the torch to “breathe” better as well. This flue path would be dampened to ensure a balance between it and the fire-tubes. The left over heat at the boilers exhaust would be picked up by economizer coils on the other side.
I agree with Ron that it probably needs a high flow recirc pump to really shine, I plan on using the old LaMont drum I have (with the built in centrifugal pump) and converting it to the “Turbo LaMont “ with the internal turbine driven pump idea I’ve had for a while....it might be a good candidate for that.

Geez....so much for “keeping it simple”...but I think this generator has potential, will hopefully have it finished by this weekend for testing.

Jamison

Quote
arch-tone
I believe this would give it a very quick, flash type response.

Just remember a flash reaction cools everything. Yes with fins you could never compete heat density/speed unless the fins were thick as the wall tubing. Even so this would force the reaction out of equilibrium.

Got the boiler welded together and did a crude fire test over the weekend....no economizer section, just cold hose water behind a check valve going in to it and the output open to atmosphere. I used the gasoline torch for the burner with the boiler mounted vertically. I started it cold, filled about 3/4 with water. It took about 15 - 20 seconds to make steam. After letting the water level drop to about 1/4 full, I turned the hose back on. It chilled the boiler...as expected...and took another 15 seconds to make steam. Some pin hole leaks. There was no enclosure to contain the fire, so most of the heat escaped around the sides. The most interesting part of the test was the spiraling nature of the outgoing steam. It looked like a waterspout, or “steam-nado” I guess you could call it. Very interesting and fun to watch. I didn’t make a video because it was dark outside and the steam was very hard to see on camera. Next test will be with the economizer coils in place and a shell around everything. The down side of this boiler design (at least so far) is the weight of the thing. Very heavy for its size, especially when full of water. This means it might not be a good match for the already over-encumbered mini chopper...but still worth further development and testing due to it unique design.

Jamison

Jamison,

Sounds promising, keep us posted.

-Ron

Yes Sir,
Do keep us posted and would like to see the Steam-nado video.

Rick

A bad leak developed on the fired end of the boiler where the middle tube is. Apparently, the steel pipe nipple that makes the transition to the copper tube burned through when welded and damaged the copper on that end. Repairing it at this point requires drilling out all the tube welds and cutting the welds off the crown sheets. I regret not taking a video of the spinning steamnado when the leak was not so bad. Now, it sprays water out and can’t keep a fire going under it. So....for now it’s on hold and I’ve already moved on to the big double drum, 100’ 1/2” stainless main coil Turbo LaMont boiler project. I’ve already assembled the lower drum turbine / pump section and did a test (with video this time) on it over the weekend. I will start a new topic on it and post a link to the video once it’s uploaded. It was my 50th birthday on Saturday and I picked up a new toy as a present to myself. It is a used go kart someone was selling online for cheap and I couldn’t pass it up. It needs new tires and a drum brake band, but is otherwise in good shape and has lots of room to put steam stuff. This will be the vehicle for the new boiler system. Here are some pictures:

Jamison

Hi Jamison,
This looks like an excellent experimental platform. It gives you chassis, wheels and drive adaptability.

You'll be able to prove out a lot of concepts with this one...Steam On!

Oh and happy birthday!

Kind regards,
Rick

Rick,

Thanks. Yeah...it’s nice to finally have room to put stuff! All I have to do is trade out the existing gas engine with the mini choppers converted Predator 212 engine, boiler size / weight is not as restricted, so that make things easier there. Lots of room for controls, etc. too. Right now it works as an ICE vehicle and have been having fun driving it around the yard. Maybe by the fall meet it will be fully steamified...but you know how that goes...

Jamison

Oh BTW...your boiler experience reminds me of my Super Fire-Tube, Ofeldt Boiler. I just couldn't fabricate it. It's on the shelf as a learning experience.

So another correlation is: "To be loved and be in love, then breakup...better than not to fall in love at all". I guess what I'm saying is that you tried and failed...better than not trying at all. Keep trying my friend and I put you in the category of Thomas Edison (inventor of the Light Bulb). Or maybe James Watt is a better choice, you know what I mean

Another BTW, I just booked the room for the September Meet. I will be bringing my Steam Scooter, I wasn't going to for a while there.

Kind regards,
Rick

Wish I could remember how many different filament materials Edison tested before he finally found one that gave a reasonably long-lived light bulb. It was a LOT. As the ancient Chinese book "I Ching" said, "perseverance furthers". Lots of SACAnauts keep trying, me included, and I applaud all y'all.

I believe it took him a 1,000 tries to create the light bulb. Edison is my hero.

I recall the number of 42? if memory serves me correct, horsehair with carbon applied worked the best. That is how I remember it.

It should be noted, that he only invented a practical light bulb, the design had been around for some time, but using platinum etc for filament were too expensive to produce commercially.

It's like the Wright Brothers, they were the first in "controlled" flight, many people achieved powered and unpowered flight before them, but only in mostly a straight line with little to no control. Again made an existing invention practical or "Yankee-virtuous"

His quote of 10,000 tries was just an exaggeration I'm sure. It's like one place I worked, the machinist aptitude test during job application process was "How many thousandths are there in an inch" one guy supposedly answered "Man I don't know there's gotta be a million of em'"

-Ron

The same with James Watt. He perfected the steam engine, did not invent it. One of his stellar improvement is to create the double acting steam engine of which seems to be the standard.

Another invention of his was the planet crank. I particularly like this one to increase shaft output speed.

Another idea to incorporate into a Steam Car.

Sorry Jamison, deviation from the Finned fire tube boiler.

"In their authoritative life of Edison, Dyer and Martin state that Edison tested no fewer than 6,000 vegetable growths" [while searching for the best incandescent light bulb filament material]
--page 570 of "Modern Wonder Workers", Waldemar Kaempffert, editor, 1924

One Hollywood version of the story, from the movie "National Treasure". Better written and more inspirational, though less historically accurate in several ways:

[www.youtube.com]



Edited 1 time(s). Last edit at 08/28/2018 02:11AM by Peter Brow.

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