# Optimal number of boiler tubes.

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#### Raygers

##### Active Member
HMEM Supporting Member
Forgive me if this has been discussed before. I have Quinn's boiler calculator which I am using to plan a design for a 6" boiler. I do have a professional set of plans for a 6" that states 55 tubes, I'm not sure of the diameter at the moment, I'm going to guess at 1/4"-3/8". Using Quinn's calculator and these figures I get a certain amount of steam, more than enough to power my big engine.
However, 55 tubes is a formidable amount to assemble and solder, so I changed the number of tubes and came up with a lot fewer tubes of a larger diameter. Easier build.

My question is, is there a benefit from having smaller tubes, I'm thinking of it slowing the passage of gasses/heat to aid in the transfer of heat to the water in the tube as opposed to a rapid flow with larger tubes. Is my thinking correct?

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Forgive me if this has been discussed before. I have Quinn's boiler calculator which I am using to plan a design for a 6" boiler. I do have a professional set of plans for a 6" that states 55 tubes, I'm not sure of the diameter at the moment, I'm going to guess at 1/4"-3/8". Using Quinn's calculator and these figures I get a certain amount of steam, more than enough to power my big engine.
However, 55 tubes is a formidable amount to assemble and solder, so I changed the number of tubes and came up with a lot fewer tubes of a larger diameter. Easier build.

My question is, is there a benefit from having smaller tubes, I'm thinking of it slowing the passage of gasses/heat to aid in the transfer of heat to the water in the tube as opposed to a rapid flow with larger tubes. Is my thinking correct?
Heat transfer is all about temperature difference and surface. Its actually more complicated, but they are the big ticket items.
More tubes gets you more surface area, more heat transfer and more steam.
Downsides are blockage problems, cleaning, construction and more draught needed to push the flue gases through smaller passages.
So that in simple terms is the design compromise.
I have my own self written boiler calculator for loco boilers, but have not heard of Quinn's before. Have you a web link?
Martin

55 tubes sounds a lot. Are we talking about fire tubes or water tubes? What is Quinn's boiler calculator?

There is a formula found by a study of a wide range of full size locomotive boilers relating fire tube length to bore. It has been found to work for boilers of any size.

For length L, and bore diameter d, both in inches: d squared = L / (50 to 70). Martin Evans recommends a factor of 65. So for example, for a 1/4" bore tube the ideal length would be about 4", and a 12" long tube would want to have a bore of about 7/16". Because you can't have a linear measure on one side of an equation and a square on the other, the factor actually has units - units of 1 / length. So the 65 is actually "65 per inch". This is why you have to specify the units in the formula. It is not a dimensionless ratio.

If you are working in mm then you have to translate the factor. It works out, near enough, to 2.5/mm.
So: d squared = L / 2.5 , or d = sqrt (L / 2.5) mm. [2.5 x 25.4 = 63.5] For a 100mm long tube, L / 2.5 = 40,
and the square root of 40 gives 6.3mm bore.

This formula applied to locomotive boilers, in which there is a fierce forced draft. More gently drafted boiler types will benefit from flues with a relatively larger bore.

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Heat transfer is all about temperature difference and surface. Its actually more complicated, but they are the big ticket items.
More tubes gets you more surface area, more heat transfer and more steam.
Downsides are blockage problems, cleaning, construction and more draught needed to push the flue gases through smaller passages.
So that in simple terms is the design compromise.
I have my own self written boiler calculator for loco boilers, but have not heard of Quinn's before. Have you a web link?
Martin
Quinn Dunki otherwise known as BlondiHacks on YouTube has several files for her Patrons and the calculator is one of them.

55 tubes sounds a lot. Are we talking about fire tubes or water tubes? What is Quinn's boiler calculator?

There is a formula found by a study of a wide range of full size locomotive boilers relating fire tube length to bore. It has been found to work for boilers of any size.

For length L, and bore diameter d, both in inches: d squared = L / (50 to 70). Martin Evans recommends a factor of 65. So for example, for a 1/4" bore tube the ideal length would be about 4", and a 12" long tube would want to have a bore of about 7/16". Because you can't have a linear measure on one side of an equation and a square on the other, the factor actually has units - units of 1 / length. So the 65 is actually "65 per inch". This is why you have to specify the units in the formula. It is not a dimensionless ratio.

If you are working in mm then you have to translate the factor. It works out, near enough, to 2.5/mm.
So: d squared = L / 2.5 , or d = sqrt (L / 2.5) mm. [2.5 x 25.4 = 63.5] For a 100mm long tube, L / 2.5 = 40,
and the square root of 40 gives 6.3mm bore.

This formula applied to locomotive boilers, in which there is a fierce forced draft. More gently drafted boiler types will benefit from flues with a relatively larger bore.
This is my latest calculation using Quinn's calculator. Using the designed 55 tubes @ 7/16" diameter will give me 2280 cubic inches per minute, more than enough for my engine that needs 1325 cubic inches per minute.
30 0.5" tubes will give me 1493 cubic inches, still enough for my engine, and also easier, I would think, to manufacture.
My question is I guess, larger tubes have more surface area and in theory, heat more water, but does more heat, in practice go up the chimney as opposed to smaller tubes?

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This is my latest calculation using Quinn's calculator. Using the designed 55 tubes @ 7/16" diameter will give me 2280 cubic inches per minute, more than enough for my engine that needs 1325 cubic inches per minute.
30 0.5" tubes will give me 1493 cubic inches, still enough for my engine, and also easier, I would think, to manufacture.
My question is I guess, larger tubes have more surface area and in theory, heat more water, but does more heat, in practice go up the chimney as opposed to smaller tubes?
As you go to larger tubes, you really need more length to make full use of the heat. There are various formulae, one for models has already been quoted although it's theoretical parentage is very dodgy (He said, politely). In full size, a length of 100 inside diameters is typically used, but this gives stupidly small diameters for models.
Thanks for the lead on Quinn's calculations. I cannot comment on what formulae are behind the cells but will attempt to make contact with blondihacks. My own investigations on this topic have been going on for around 10 years now. My program is based on accepted heat transfer formulae used in industry (I am a retired chartered mechanical engineer) and I have carefully avoided a whole bunch of empirical rules of thumb.
Martin

Thank you Martin, that answers my question, smaller is better in this case. I think that Quinn's calculations use surface area only, not taking into account what percentage of heat is lost up the chimney, so to speak.

My question is I guess, larger tubes have more surface area and in theory, heat more water, but does more heat, in practice go up the chimney as opposed to smaller tubes?

No. Because you can fit in more small tubes. In a given area of tube plate, a bank of small tubes will give you more total heating surface area than a bank of larger ones.

Yes, if the tubes are too large in diameter, heat will be wasted. And if they are too small - Tenor answered this in his first reply.
As you go to larger tubes, you really need more length to make full use of the heat. There are various formulae, one for models has already been quoted although it's theoretical parentage is very dodgy (He said, politely). In full size, a length of 100 inside diameters is typically used, but this gives stupidly small diameters for models.
Martin, C M Keiller's formula does not have any theoretical parentage. It is a result of empirical comparison of successful full size British locomotives. That it appears to work at any scale, when a length to diameter ratio does not, should tell you something. Of course, a proper theoretical approach would be interesting. As another retired mechanical engineer, I have to admit I really struggled at uni with the maths of heat transfer theory. But good engineering is surely a judicious mix of theory, empirical data, art and experience.

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I have read that "the length of a flue tube for a loco (forced draught) boiler should NOT exceed 80 x diameter of bore". Unfortunately, for gas fired (Unforced boilers) these ratios (given in various formulae above) all choke the gas exhaust. The ensuing back pressure seriously affects the indrawn air at the airhole by the jet, so the performance of the burner is dramatically affected and dangerous CO is made in the boiler due to lack of air.
Therefore for a GAS (or vaporised liquid fuel) fired boiler you need 3~4 times the cross sectional area of the flues compared to a Solid fuel (coal) fired and forced draught boiler. Example: a 2.2kW gas fired boiler needs 4 x 1/2" tubes - a bigger gas supply just can't get the gas exhaust to the chimney from the smoke-box end. This also needs a 30mm Bore dia chimney - as the 25mm bore chimney choked the burner, so a 1.5kW burner was all it would take. The chimney is about 1 foot/12in/30cm high to develop draught. The 25mm earlier chimney only worked by using the exhaust from the engine through a 1/8in bore pipe, fed up the chimney as forced draught.
Conclusion: Check the design to understand total CSA of flues and chimney compared to Burner size. It is too late when you have made a boiler "to a prescribed design" to find your burner strategy simply doesn't work so you can't develop the steam as expected. It is more complicated than just as explained above.
Please write directly to me with your proposed design details (Steam requirement - pressure and volume/minute, Boiler design: Horizontal, vertical, Burner strategy: Coal, oil or gas fired, blowlamp or radiant burner, etc.) and I can advise any modifications based on my own experience, if you wish.
Cheers,
K2

An interesting aspect, Steamchick as I have mainly considered coal fired locomotive boilers (is there any other sort?), but what you say makes perfect sense to me. One question though, there a reasonable number of locos running with "Marty" burners in an array in place of the grate in otherwise coal fired boiler designs. Presumably, the boilers happen to have enough tube area and forced draught to cope?
Having been on here a few days now and starting to get to grips with the features, I realised I could post a complete article which I wrote up some years ago for "Model Engineer" in the UK. It has also been translated and published by Stoomgroep in Holland in their magazine "Onder Stoom".
I hope the article proves interesting and puts a bit of background to the OP's original question.
I am about to start a fairly major upgrade to my program, which should better cope with questions of what air ratio, how much lost fuel, what quality of solid fuel........ The upgrade comes on the back of analysis work I have been doing on published full size test work which I have published in the ASTT newsletter (UK again).
Not really on topic in this thread, but Stoomgroep Holland published an excellent article on a finite element stress analysis of a typical copper locomotive boiler. It is not mine to give away, so I can't post it on here, but if you can get a copy it makes for very interesting reading.
Martin

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Interesting! Right in my realm of interest.
1. "Heat release above the firebed would typically be due to volatiles burning in the firebox volume." - I understood this was also from CO combustion when CO meets secondary air from fire-door or other vents? - This combustion is generally extinguished as it cools in flue tubes, so may be incomplete, and "wasted" CO goes up the chimney? Also Yellow flames (from Volatiles) can burn to a degree within the flues.., especially with "yellow" glowing carbon, and transfer more heat to flues than completely burnt fuel exhaust? - But wasting unburnt fuel in the process?
I must read this a few times, as it is a deep - and deeply rewarding - paper! (NOT that I am likely to design a loco boiler!).
I have previously corresponded with Joan Llutch - A Spanish engineer who does a lot of calculation in his designs of loco boiler. He uses Gas firing, and I assisted him in developing an optimum size of venturi for his gas burner air intake. (My ideas from a 1920s US Gov't paper! - Adapted by him and proven in his final design.). Some odd pics attached.
His Freelance Pacific loco can be seen on web pages:
https://modeleng.proboards.com/thread/10937/freelance-pacific-loco-lsfornells-build?page=16And a comment on his burner: Noise resolved by altering the gas-air ratio with using a better venturi design. - More air, faster combustion and defeated the "noise" issue.
K2

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Interesting! Right in my realm of interest.
1. "Heat release above the firebed would typically be due to volatiles burning in the firebox volume." - I understood this was also from CO combustion when CO meets secondary air from fire-door or other vents? - This combustion is generally extinguished as it cools in flue tubes, so may be incomplete, and "wasted" CO goes up the chimney? Also Yellow flames (from Volatiles) can burn to a degree within the flues.., especially with "yellow" glowing carbon, and transfer more heat to flues than completely burnt fuel exhaust? - But wasting unburnt fuel in the process?
I must read this a few times, as it is a deep - and deeply rewarding - paper! (NOT that I am likely to design a loco boiler!).
I have previously corresponded with Joan Llutch - A Spanish engineer who does a lot of calculation in his designs of loco boiler. He uses Gas firing, and I assisted him in developing an optimum size of venturi for his gas burner air intake. (My ideas from a 1920s US Gov't paper! - Adapted by him and proven in his final design.). Some odd pics attached.
His Freelance Pacific loco can be seen on web pages:
https://modeleng.proboards.com/thread/10937/freelance-pacific-loco-lsfornells-build?page=16And a comment on his burner: Noise resolved by altering the gas-air ratio with using a better venturi design. - More air, faster combustion and defeated the "noise" issue.
K2
Hi Steamchick, the statement in italics is a generalisation, but broadly true. Certainly some CO may be burning to CO2, in addition to suspended coal particles. The lighter hydrocarbons from methane all the way up to light bitumens will be flaring off and burn as "gases". Exactly where it all extinguishes is up for discussion, but at entry to tubes seems a fair working assumption.
I have just been doing some calcs in support of the next version, and in full size at reasonable load the transit time through the firebox is less than a second. In models it is significantly less.
Glad to hear you find it of intersest and happy to discuss.
A good looking venturi design (fluid dynamics was my day job).
Martin

Hi Martin, Glad to be able to talk to an expert (which I most certainly am not!).
You mention "Marty Burners" - Yes, they do function well, and produce very compact combustion. "there a reasonable number of locos running with "Marty" burners in an array in place of the grate in otherwise coal fired boiler designs. Presumably, the boilers happen to have enough tube area and forced draught to cope?"

I'm sure you could work these into your calculations an an "interesting" exercise? - The LACK of radiant heating in the firebox amazes me - must burn more fuel and pass hotter gas through the tubes to get the required performance, perhaps? I am sure they will be using a lot of forced draught to do this.
http://ibls.org/mediawiki/index.php?title=File:Marty_Burners_original_drawing.jpg Note the gaps between rows of gas piping in the bottom of the firebox to get air in, which is drawn into the burners below the layer of combustion in the firebox. Marty Burners - IBLS
You can easily buy similar burners sold as Chinese Wok burners, - to build into your own manifold. (Shown here in a Wok Burner manifold).

This is a completely different approach to that of Joan Llutch - who identified a type of "knitted wire" burner - and managed to obtain a couple of samples for testing - one of which is used in his Pacific loco - very successfully. I have tried making one (I can't knit - which means my wire matrix is too variable!) and made 8kW for 5 mins before ir flashed-back. Not good compared to the factory unit of over 27kW - from a 4in cylinder, 3 in tall. The radiant power is most of the heat broadcast from the burner. Therefore the gas temperature is lower? So a lower volume to pass through the flue tubes?
See Bekeart brochure.
I have a friend at the local Model Eng club (The Chairman) who has a Steam wagon, and has converted the boiler to gas firing using a Nystrom design of flare burner: see attached. 3 of those (powered around 25kw) power his wagon as capably as a coal fire, but he needs full blower to force the flue gases through all the time he is running at full gas. - so he uses more steam for the blower, and more water as a result. - Again, I reckon the flue gases must be hotter than from a coal fire, to get the heat into the boiler via gas heating flue tubes, with the low radiant heat from the flames giving less radiant heat to the firebox.
On a static boiler, my father used 3 of these Bray burners with a fan flame (converted from a domestic gas boiler to use Butane at pressure: Dia 0.25mm jet):
- BUT they only rated at maybe 500w each. A ceramic burner (possibly ~1.8kW, using Dia 0.3mm jet) was much more effective at generating heat in a 3in horizontal boiler with 4 x 1/2"dia flue tubes.

BUT Ceramic burners cannot produce the power (temperature) required to get the power density for a firebox on a loco. Limited to about 1000deg.C max, you simply cannot get lots of flue gas heat added to the radiant heat they produce. And as a coal fire is more like 100~1200deg.C, and much deeper, the radiant power from that is much more than from the flat surface of the ceramic burner. (T-fourth effect).
It is an interesting balance!
K2

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Or maybe I am just spouting "a load of hot air"? - Correct me where my meanderings are wrong.
K2

I know that Steamchick said to message him directly, but I thought that with such a gathering of gods (experts) here I would post a photo of a portion of my blueprints, who remembers when they were actually blue and faded away to blank paper?
They are copyrighted, but posting a small view I think is ok, someone tell me if it is not.
The boiler will be coal-fired if I can find some.
I do have another boiler that I am drawing up which will be gas or spirit-fired, but that's for another day.

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Thanks. Yes, I was concerned about plans on the Open Web and imtellllectual rights. I'll study. And if I spot anything odd I shall ask.

Thanks
. K2

But I didn't spot my spelling mistakes (using a "Smart" phone!! - Usually changes at least 2 words I type - that are Engineering terms, etc., not in its "common use" dictionary... Not so "Smart" this time! Could not spot those spelling errors!).
K2

Steamchick,
You are right that the balance of heat exchange changes on gas firing due to the very low emissivity of clean flue gas. Thankfully, that delivers hotter gas to the tube bank, which in turn increases the heat exchange in the tubes. There is predicted drop in performance due to lower heat transfer in the firebox, but not large due the effect just described.
I have zero experience of gas firing, but theory certainly suggests that something that can radiate in the firebox would certainly help. I assume the orange part in the firebox section.jpg in your post of yesterday is some sort of woven mesh? You are way ahead of me on the use of ceramic burners. I know the Gauge 1 guys use them and that is the extent of my knowledge. That kind of makes sense if the power input is limited and space is obviously an issue in such small locos.
My program takes account of the emissivity of the flue gas - I did some back calculations to determine appropriate values for the soup of soot and ash particles that you get with coal firing. There are some standard values for water vapour and CO2 from work by Hottel. The rest is mostly N2 which has effectively zero emissivity. It's ages since I used Hottel's data, so would have to flog through to find some appropriate values.
A correspondent on another forum has put me on the Wok burners - they are cheaper than the brass bar to make them!

Raygers,
I have downloaded your boiler design and will run the numbers. My first impression is that some superheat or at least steam drying would be a good move. There is a paper here:
Bill Hall was a UK professor and model engineer. If you look at Fig 5 it shows that with good superheat an engine (in our sizes) will consume about 25% more than the theoretical cylinder volume at cut off. With saturated steam that goes up to 150% more. In effect you would have to boil 2.5 times as much water to supply a given engine which means you will spend a lot more time shovelling coal and pumping water.
It may take me a few days to work the numbers. Feel free to give me a nudge if other things get in the way.
Martin

Thanks Martin. Good info!
I always add a "DRYER/SUPERHEATER" in my boilers as the benefits of energy transmitted from burner to cylinder using superheat are just something I new to be worthwhile from being a teenager and seeing the Coal fired boilers in a power station (for 150Mw generator turbines), with superheaters, water pre-heating economisers, condensers, etc. - I worked as a "shutdown" boiler cleaner in the summer hols as a student. Crawled in and outside Babcock's finest coal-dust fired monsters..
On my titchy gas fired boilers the superheater/dryer is at least a steam pipe from the take-off fed through a flue and back - or through the firebox, or a coil in the smoke-box. Depends on physical constraints as much as how much superheat. My 3in vertical Boiler with a coil in the firebox is super-hot! Tarnishes brass and copper pipe easily. at 20psi it really makes a difference to the performance of little engines, typically powering bicycle dynamos.
Many people forget Physics... and think "Pressure" is all you need. But the steam is just an energy transfer medium, so "Enthalpy" is what makes it work (in my head): I.E. Energy as the steam leaves the boiler minus energy as it leaves the exhaust of the engine. I.E. More energy "IN" means more power generated. - But if I am wrong, please teach me, as I was hopeless at Thermodynamics in college!
Must get around to studying your paper again, and having a look at real numbers of Coal fired versus Gas fired, flue sizes, etc.
Incidentally: I have a diagram showing temperatures of radiant Media: but I can't find it! Wire mesh works up to 1200deg.C, but Ceramic only 900deg.C. Higher and they degrade. New foam matrices work up to 1400deg.C - but cost hundreds of £ for a slab!
Cheers!
K2

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• REZNOR Infra-red heating tech info.pdf
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"The boiler will be coal-fired if I can find some."

Perhaps you could contact the Toronto Society of Model Engineers or the Richmond Hill Live Steamers. They should have coal to burn, as it were.

If that fails, you can PM me. I have a fair amount of Welsh steam coal in the size you'll need, though it's near Belleville.

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