# Optimal number of boiler tubes.

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Macc Models have 1/8" 3.175mm 6" and 5" tube that I was planning on using. Certainly a lot to think about.

Hi Raygers. The BEST option is to contact Ernest Winter = and get a copy of the calculations for
1. the tube strength of the outer boiler shell tube - 6inch - in TENSION (HOOP stress) - with consideration for the stress raising factors (3.3 for ASME) to be applied for the shell tube penetrations.
2. the tube strength of the inner boiler shell tube - 5inch - in COMPRESSION (HOOP stress) - with consideration for the stress raising factors (3.3 for ASME) to be applied for the shell tube penetrations.
Ernest is the guy that is declaring the design to be good for 100psi Max working pressure, but before you buy material, you need to be sure it complies with appropriate legislation. That is, IF you want to be covered by Insurance if anything goes wrong. As you are in Canada, I don't know the laws, etc, that apply to you. But I am pretty sure the Canadians will apply ASME (The USA Regulations) or their own regulations that are basically the same. Your local Steam Model Engineering club will be able to advise, I reckon.
All I know is how to do the stress calculations at inner and outer surfaces of thick-walled tubes, from University papers, and other Engineering texts, and by applying some second-hand knowledge from ASME Regulations for Pressure vessels and boilers. Some of this is from magazine articles by Kozo Hiraoka, and other internet sources about ASME regulations.
A few simple ASME rules I use: see attached Word doc.
e.g.
• Copper at 100psi in tension must not exceed 3142psi tensile stress (by calculation) - including a stress concentration factor of 3.3 where ANY penetrations exist in the shell of the pressure vessel.
• In compression, there are complex tables to determine factors to develop the maximum permissible COMPRESSIVE strength that the Regulations will allow. Also, this must consider a stress concentration factor of 3.3 when penetrations exist - or perhaps something else? - when considering hoop stress.
• Rivetted boilers are NOT PERMITTED.
• Silver soldered COPPER boilers are not be permitted above 100psi NWP.
• A drawing dated 2018 must comply with whatever Canada have as their Regulations - including calculations. A boiler inspector will want to see these before he tests a boiler. Material certificates, components before and sub-assemblies during manufacture of the boiler may also be necessary. Talk to him FIRST. He may accept the design as drawn as complying with 2018 regulations? - Therefore OK?
• If you choose to change ANYTHING from the design selected, then it becomes a NEW design and must comply with LATEST Regulations. e.g. changing tube sizes to Tenor's optimised design?
e.g.
Thickness of shells under internal pressure:
Min thickness shall be that calculated, for the pressure, plus any additional loading stresses as per UG22.
Circumferential stress: (When t < Rinside/2, or P =<0.385 S x E):
T = PR/(SE-0.6P) or P = SEt/(R + 0.6t)

where:
E = joint efficiency (for seamless tube & good design of silver soldered joints, E = 1)
P = Design Pressure,
R = Inside radius of tube or part considered,
t = min thickness of shell or component,
S = max allowable stress value.

I reckon this works out as:
R/2 = 1.25"< which is greater than t (wall thickness = 1/8"):
So: 1/8" wall of 6" tube:
P = SEt/(R + 0.6t) = 3142 x 1 x 0.125 / (2.375 + 0.6 x 0.125)
= 392.75 / (2.375 + 0.075) = 392.75 / 2.45 = 160psi.
BUT applying a stress concentration factor of 3.3 (Because the tube has "Penetrations" - covered in a different part of the Regulations!) this is reduced to 48 psi NWP!! = less than half of the proposed 100psi NWP.
That is why I think the design is not suitable for current Regulations.

But the Bottom line is that Ernest Winter produced the drawings, so he may have the calculations appropriate to his design, which you can discuss with your "boiler inspector". Well worth starting there, as when you have spent many hours and the Boiler inspector says "Sorry, that is no good. It does not comply with the Regulations." it is an expensive lesson to learn.
There are many experts on this forum - but I do not know if there are any Pressure Vessel experts who can advise better than this?
I am NOT a Pressure Vessel expert, but have been in industry and designed systems that had to comply to pressure regulations. (Out-of-date, UK. regs.). So if any Experts from Canada or USA can correct me I shall be glad to learn.
Thanks,
K2

#### Attachments

• ASME SECTION VIII DIV-1 UG-21 to UG29 for compressive strength of Tubes in Copper boilers.doc
320.5 KB · Views: 47
Thank you for your effort mr steamchick.

Item #1 - - - has some formulas.
Item #2 - - - is, for me anyway, largely a collection of dead links (nothing to be found when following the link).
Item #3 - - - interesting - - - will have to root through it.

I am looking for formulas something like those you have been using for the firetube boilers or something so that I can create a spreadsheet to calculate results from tubes like this with heat from this configured like this . . .

What I have been finding is either almost purely theoretical (with mountains of calculus (mine is severely decrepit and from very long ago)) or the overly simplistic - - - sorta like - - - just do it like this.

One area of investigation that I've fallen into is trying to find a good highly efficient watertube boiler design is a Thornydyke better than a . . .

I don't really want to make a career out of this - - - just looking to create some 'good' stuff.

(Thanking you for your willingness to help and time - - sir!)

Hi Ajoeiam.
A Yarrow Boiler is also a very "fast" steamer, as a 3-drum water tube boiler. Used by many Naval vessels where maximum steaming was important, not fuel economy.
I don't know the Thorndyke design you reference...
Apparently, in a book there is a reference to the "Lune Valley" boiler, and Bolsover express boiler, both of which are supposed to be very fast steamers. - Reputed to generate up to 5cu,.in of evaporation per 100sq.in. of heating surface. This is a top limit, so whatever boiler configuration you use should consider this factor as a maximum for steam production (heat flow to water) and typically flue-tube boilers can only achieve 2 cu.in. boiling per 100sq.in of surface area, max. Many water tube boilers are a combination of the 2 designs, so considerations must ne given to the cooling of burner exhaust that proportionately reduces the heat flow into steam production.
Otherwise you are into the realms of coiled-tube and flash boilers for high-volume steam generation.
Babcocks water tube boilers have been a standard for Power Stations (coal and oil fired) and are generally water tube boilers.
For models, especially boats - with continuous return feed of condensate, a "Scott" boiler is proposed in some books. Silmply loops of tube mounted beneath the large Boiler tube. One end of each tube is low on the main tank, whereas the other end is higher, promoting fast circulation, and the water tubes are "in the fire".
Maybe this helps?
K2

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Useful for straight forward type substances.
Now what about for bio-mass - - - and that would likely also depend upon the type of (I would bet!)?
TIA
I would recommend staying as far away from biomass as possible. Its not the burning that is a problem, it is the ash softening temperature which is low. The ash melts and its a major design issue of getting it out of a unit if not impossible. Basically the bulk furnace temperature must be kept low. And the second problem is bio mass could be just about anything! But for this conversation we will define it as grass and other organic material. Wood is sometimes classified as bio mass but it can have the same issue.

Hi Ajoeiam.
A Yarrow Boiler is also a very "fast" steamer, as a 3-drum water tube boiler. Used by many Naval vessels where maximum steaming was important, not fuel economy.
I don't know the Thorndyke design you reference...
Apparently, in a book there is a reference to the "Lune Valley" boiler, and Bolsover express boiler, both of which are supposed to be very fast steamers. - Reputed to generate up to 5cu,.in of evaporation per 100sq.in. of heating surface. This is a top limit, so whatever boiler configuration you use should consider this factor as a maximum for steam production (heat flow to water) and typically flue-tube boilers can only achieve 2 cu.in. boiling per 100sq.in of surface area, max. Many water tube boilers are a combination of the 2 designs, so considerations must ne given to the cooling of burner exhaust that proportionately reduces the heat flow into steam production.
Otherwise you are into the realms of coiled-tube and flash boilers for high-volume steam generation.
Babcocks water tube boilers have been a standard for Power Stations (coal and oil fired) and are generally water tube boilers.
For models, especially boats - with continuous return feed of condensate, a "Scott" boiler is proposed in some books. Silmply loops of tube mounted beneath the large Boiler tube. One end of each tube is low on the main tank, whereas the other end is higher, promoting fast circulation, and the water tubes are "in the fire".
Maybe this helps?
K2

Thanks - - - I have read of the Yarrow and some others and have tried to chase things Naval as they were still using steam in some applications until even the 1950s (and past) see the Skinner Unaflow engines. From what I can determine these Unaflow engines (even looking at Dan Gelbart's very modern rendition) were actually quite efficient.
So then I come to what is an efficient boiler design - - - ie maximum steam at T (likely only 300 to maybe 600F) at a pressure of (again likely lower something like 300 to 500 psi (above atmospheric). And there everything grinds to a halt! No matter what combination of search terms I'm using I just can't find any comments regarding steam production efficiency.

Compounding the problem is that I'm looking in the 25 kW to maybe as much as 100 kW size for the boiler. That's been long considered 'tiny' by anyone working with steam.

So - - - any ideas for an efficient steam production design - - - or is this 'fast' have to do? (I am worried about fuel consumption!!!)

TIA

I would recommend staying as far away from biomass as possible. Its not the burning that is a problem, it is the ash softening temperature which is low. The ash melts and its a major design issue of getting it out of a unit if not impossible. Basically the bulk furnace temperature must be kept low. And the second problem is bio mass could be just about anything! But for this conversation we will define it as grass and other organic material. Wood is sometimes classified as bio mass but it can have the same issue.

I'd love to - - - but - - --- practically - - - its the only option left!
Anything from previous power production is today being excoriated as terrible (diesel, coal, propane, natural gas are some of the suspects) and I'm not stupid enough to believe that nuclear power is clean (!!!!!) so I am left with only the one option.
Biomass - - - I could use straw (don't like as its lower bulk weight, pricey on a per ton basis, don't like the way its produced, etc etc) and the wood chips.
A local pallet plant wanted to sell all of their non pallet stuff as landscaping material. Soon found that there just wasn't that much demand. They had previously been selling slapboard and other trim into the firewood market.
This biomass boiler thing is being done in the US north east where there is a strong local lumber industry more than just occasionally so whilst I agree that biomass is NOT the best option - - - well - - - - its my ONLY realistic option. It helps that I can design my boiler to use different forms of such (bulk cardboard, carpentry waste besides straw or wood chips).
My idea of biomass is the utilization of a waste stream from another industry as an input - - - note that your definition is quite different.

Sorry. I don't know much about biomass.... but I do know that in the "olden days", we in UK and Europe used coal. (My grandfather ran ships on coal, shipping to Belgium and Italy for their railways!). But the US of A used wood. The USA locos had huge (volume) tenders and huge grates, fire-boxes and flues to manage the much bigger fires needed, compared to coal, while they burned wood, which was plentiful and grew almost everywhere across the USA.
NOWADAYS, to avoid burning coal, in the UK, we have converted some coal-fired power station to burn wood-chip that we import from the USA. Works politically for CO2 production, if expensive and not very efficient compared to coal.
So what is the best boiler for wood chips? One with a huge firebox, grate, etc. 2 1/2 times the volume of the coal fire firebox.... which you have seen in Rayger's boiler... for similar output. You need a lot of flues to capture heat more efficiently, superheater tubes and economise tubers to preheat boiler feed water, etc..
So go for a 150 kW boiler and run it max at 100kW, using the flue gases for space heating as well. E.g. feedwater, central heating, air exchange heaters for space heating, etc.
Does this help?
K2

Thanks - - - I have read of the Yarrow and some others and have tried to chase things Naval as they were still using steam in some applications until even the 1950s (and past) see the Skinner Unaflow engines. From what I can determine these Unaflow engines (even looking at Dan Gelbart's very modern rendition) were actually quite efficient.
So then I come to what is an efficient boiler design - - - ie maximum steam at T (likely only 300 to maybe 600F) at a pressure of (again likely lower something like 300 to 500 psi (above atmospheric). And there everything grinds to a halt! No matter what combination of search terms I'm using I just can't find any comments regarding steam production efficiency.

Compounding the problem is that I'm looking in the 25 kW to maybe as much as 100 kW size for the boiler. That's been long considered 'tiny' by anyone working with steam.

So - - - any ideas for an efficient steam production design - - - or is this 'fast' have to do? (I am worried about fuel consumption!!!)

TIA
The size boiler you are considering is not considered a small boiler. Based on conversion from kw to btu and using 1000btu/lb steam its a small residential size boiler. ASME lists a minature boiler at less then 100 psi and no more than 16 inches in diameter. If you are looking at fuel efficiency then use 78% efficiency to back calculate the fuel consumption. Do not build a boiler for 100kw and run it at 25 kw. This is on the limit of a turndown ratio. You can build this thing but it will never be certified or legal in most places unless its done under an ASME stamp shop. The efficiency number is based on gas fuel and a well designed furnace.

Boiler design is usually in the back chapters of well written heat transfer books because the heat transfer calculations will include all types of gas to tube convection, radiation to wall calculations, draft loss across and through tube banks, nucleate boiling considerations and a number of other calculations There is no one chart to look at. Boiler design is both an art and a science. Once all those calculations are done then of course there is the material science part of thermal expansion and adequate margin for design pressures.

Now I do know a man who built a maple syrup evaporator without doing all that. But he overbuilt and rebuilt it at least four times to get it to work right. Trial and error so I never say what can not be done but he was able to build it because it was motivated and his materials were inexpensive for him.

Hi Raygers. The BEST option is to contact Ernest Winter = and get a copy of the calculations for
1. the tube strength of the outer boiler shell tube - 6inch - in TENSION (HOOP stress) - with consideration for the stress raising factors (3.3 for ASME) to be applied for the shell tube penetrations.
2. the tube strength of the inner boiler shell tube - 5inch - in COMPRESSION (HOOP stress) - with consideration for the stress raising factors (3.3 for ASME) to be applied for the shell tube penetrations.
Ernest is the guy that is declaring the design to be good for 100psi Max working pressure, but before you buy material, you need to be sure it complies with appropriate legislation. That is, IF you want to be covered by Insurance if anything goes wrong. As you are in Canada, I don't know the laws, etc, that apply to you. But I am pretty sure the Canadians will apply ASME (The USA Regulations) or their own regulations that are basically the same. Your local Steam Model Engineering club will be able to advise, I reckon.
All I know is how to do the stress calculations at inner and outer surfaces of thick-walled tubes, from University papers, and other Engineering texts, and by applying some second-hand knowledge from ASME Regulations for Pressure vessels and boilers. Some of this is from magazine articles by Kozo Hiraoka, and other internet sources about ASME regulations.
A few simple ASME rules I use: see attached Word doc.
e.g.
• Copper at 100psi in tension must not exceed 3142psi tensile stress (by calculation) - including a stress concentration factor of 3.3 where ANY penetrations exist in the shell of the pressure vessel.
• In compression, there are complex tables to determine factors to develop the maximum permissible COMPRESSIVE strength that the Regulations will allow. Also, this must consider a stress concentration factor of 3.3 when penetrations exist - or perhaps something else? - when considering hoop stress.
• Rivetted boilers are NOT PERMITTED.
• Silver soldered COPPER boilers are not be permitted above 100psi NWP.
• A drawing dated 2018 must comply with whatever Canada have as their Regulations - including calculations. A boiler inspector will want to see these before he tests a boiler. Material certificates, components before and sub-assemblies during manufacture of the boiler may also be necessary. Talk to him FIRST. He may accept the design as drawn as complying with 2018 regulations? - Therefore OK?
• If you choose to change ANYTHING from the design selected, then it becomes a NEW design and must comply with LATEST Regulations. e.g. changing tube sizes to Tenor's optimised design?
e.g.
Thickness of shells under internal pressure:
Min thickness shall be that calculated, for the pressure, plus any additional loading stresses as per UG22.
Circumferential stress: (When t < Rinside/2, or P =<0.385 S x E):
T = PR/(SE-0.6P) or P = SEt/(R + 0.6t)

where:
E = joint efficiency (for seamless tube & good design of silver soldered joints, E = 1)
P = Design Pressure,
R = Inside radius of tube or part considered,
t = min thickness of shell or component,
S = max allowable stress value.

I reckon this works out as:
R/2 = 1.25"< which is greater than t (wall thickness = 1/8"):
So: 1/8" wall of 6" tube:
P = SEt/(R + 0.6t) = 3142 x 1 x 0.125 / (2.375 + 0.6 x 0.125)
= 392.75 / (2.375 + 0.075) = 392.75 / 2.45 = 160psi.
BUT applying a stress concentration factor of 3.3 (Because the tube has "Penetrations" - covered in a different part of the Regulations!) this is reduced to 48 psi NWP!! = less than half of the proposed 100psi NWP.
That is why I think the design is not suitable for current Regulations.

But the Bottom line is that Ernest Winter produced the drawings, so he may have the calculations appropriate to his design, which you can discuss with your "boiler inspector". Well worth starting there, as when you have spent many hours and the Boiler inspector says "Sorry, that is no good. It does not comply with the Regulations." it is an expensive lesson to learn.
There are many experts on this forum - but I do not know if there are any Pressure Vessel experts who can advise better than this?
I am NOT a Pressure Vessel expert, but have been in industry and designed systems that had to comply to pressure regulations. (Out-of-date, UK. regs.). So if any Experts from Canada or USA can correct me I shall be glad to learn.
Thanks,
K2
Steamchick, thank you for taking the time to teach this old Geordie a few things about boilers and requirements. I like how you are making sure that I build a SAFE boiler.

After much soul-searching, I have a few options I think.
1. Give up, not happening.
2. Use a round firehole, it seems that the stresses in the corners are a problem. Would this fix it? Probably not, moving on. .
3. Keep the general plan but remove the water around the firebox by enlarging the lower tube plate to fit to the boiler shell, making the firebox a separate entity. I would have to line the firebox wall with some sort of firebrick material.
4. I would still like the option of using coal so the firebox and ashtray would be made basically as shown in the plans.
5. With the option of a separate firebox I can leave the fire tubes at 8" or make a full 12" long boiler with an addon firebox.

I'm liking the idea of a 12" boiler with a 4" firebox

As for certification. . . since this will be for my personal use only I'm not too bothered, but I do want it to be as safe as it can be.
Ernest Winter has been retired for a few years, I contacted the present owners to see if I can get in contact with him, but have yet to hear back.
There are a couple of model railroad clubs around here, the one that emailed back, they do mainly larger steel boilers for traction engines and the like.
By hook or by crook I will build this boiler or one like it, I need 1325 in3 /min

Hi Raygers, I have ben struggling with ASME VIII.. and some charts for a couple of hours... Bamboozled!!

I guess you mean "1325 in3 /min of STEAM at 100psi?"
Cheers,
K2

I use to work for a boiler company we use to build and repair boilers 25 years ago. There are formulas to calculate heat transfer through each tube. You need to know, tube diameter, length, type of metal, metal thickness, number of tubes, heat source, gas, wood, coal. Fire box should be surrender by water all 4 sides. I think machinist hand book has this information. You can probably find a boiler calculator online. Left drawing is side view of boiler, right drawing is front view of boiler. Math for fire box is 1 large rectangle tube.

I use to own a 4"x 4" factory steam engine. I built a 4.9 gallon boil so I did not need state inspection. If boiler is 5 gallons or larger there is a lot of, red tape, permits, state inspections, etc.

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ROCKET man: Is it true that in the US of A there are gun laws, laws against killing people, boiler regulations, etc. but you can make explosives and rockets?
I like the idea of boilers being unregulated up to 5 gallons - I assume US short gallons? - Not full gallons we use in the UK?
- If so, that's 25 pints - or 14Litres... In the UK we are permitted to call them "small boilers" at under 3 bar litres of steam & water. So we could call a 25 pint boiler "small" at 3psi... Duh! Maybe OK for a Newcomen or James Watt condensing engine, only... But not for a Trevethick high pressure (3 bar) engine!
Flippin legislation! - But "We, the people..." voted to have a government and rules...

K2

Hi Raygers:
As you need ""1325 in3 /min of STEAM at 100psi?":
That relates to 1325/237 cu.in of water/min. = 5.5 cu.in /min of water to be boiled.
Assuming the boiler is only 70% efficient, it should be planned for 8 cu.in./min. - in my books.
Say the boiler will boil 2 cu.in. of water per min. per 100 sq.in. of surface area, then we need 400sq.in. of surface area exposed to fire and flue gases to satisfy your engine. I think you have a lot more than that in your boiler.. 9.2 sq.in. /tube x 55 tubes = 506sq.in. = OK. (without even considering the firebox walls, and top (tube plate).
QED.
K2

Raygers,
Martin can do a "better job" than these scribblings... but an estimate of the heat needed for your boiler: We need 1152B.Tu. per lb. of water to make steam at 100psi. (I got this from a book). 1lb. of water = 27.68 cubic inches of water.
Therefore we need 41.62 BTU of heat to make 1cu.in of water into steam at 100 psi.
You need heat to convert potentially 8 cu.in./min. of water to steam so need 8 x 41.62 BTU/min = 19977BTU/hour.
This equates to (5.85kW.) say 6kW of heat input with the assumption of 70% efficiency I made in the previous post. (Which rules out the ceramic burner option, but a Hot Wire Matrix option could sill manage that?). You always need more "Burner power" than the calculations (in my experience) because you inject cold water into a hot boiler of steam and want quick pressure recovery, need steam for injectors, pumps, etc., need heat for superheating or steam drying, and waste more than "planned" through lagging and from unlagged bits like valves and pipes for gauges, etc.
Hope this aligns with your assumptions/calculations? (Post #5?).
SO:
Just a small matter of configuring the numbers for the thickness of the firebox tube... when I can figure out what ASME really means! (it is not simple!). Incidentally, a 1/2in x 18SWG tube is just OK by my reckoning for a flue tube (in compression with steam and water surrounding it). - You suggest you may use 30 of those, and Martin suggests 18 x 1/2" tubes + 4 x 1" tubes with superheaters in them, in post #26...
K2

Have you asked Macc models what they would recommend for your boiler? - They may suggest the tubes are "OK or NOT"? - Or not give a judgement... if you need something they cannot supply....?
Searching the web for thick walled copper tube is a nightmare... trying to get past "plumbers' common stuff".
I found an odd page:
https://lawtontubes.co.uk/copper-tubes-in-acr/working-pressures/- that gives an indication of "safe pressure" (Pressure rating as per BS 14276) - INTERNAL pressure that is - of 25psi for a 4 1/8" in pipe x 12 SWG.... for annealed copper tube. With hoop stress being a linear factor of Thickness/diameter, a 6in OD tube can safely work at 4/6ths this pressure, but a 1/8 wall (10 gauge) can increase the safe pressure by 128/104 times... = 20.5psi !!! - So maybe my calculations are "reasonable" - if not correct?? - And that is considering the tube at 100deg.C, not de-rating it for allowable stress at 200deg.C. - A further de-rating of 3000/5400 = which reduces the SAFE working pressure to 11psi....
But we do not use BS 14276 for rating boiler designs.... Do you know what is used in Canada? - I found this: which may give exemption to your boiler? (for Saskatchewan?).
(f) a high pressure boiler that has a heating surface with an area of two square metres or less;
(h) a pressure vessel that has a volume of 0.0425 cubic metres or less; (i) a pressure vessel that has an internal diameter of 152 millimetres or less;

So that puts a different perspective on things. - You are allowed to make a bomb!
But please excuse me for not attending the operation of the boiler...

K2

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There's a lot happened since I last checked in.
Steamchick,
I am a bit surprised by the numbers for tube wall thickness coming out for Rayger's boiler. What makes me wonder is that the boiler for the 3" scale Atkinson wagon is 7" barrell but specifies 1/8" copper for 100 psi W.P. The stress concentration factor you are quoting for any shell opening just sounds rather high to me. It may well be right and ASME are uber conservative - that certainly happens having dealt with the UK approval body on steel boilers. I specced 16mm plate which was OK in the calculations, but they then bump it to 20 mm. I am not sure it is clever engineering as extra thickness will not "breathe" as the boiler cycles hot / cold. Remember, the strongest trees bend in the wind........ Most boiler problems I have seen are localised cracking in firebox corners and such like, where there is no room for deflection again.

HMEL
I agree that a good starting point for guessing required grate area to produce a given amount of steam is a boiler efficiency. I think your suggested figure of 78% would be good for full size work, but optimistic for small work. As you get smaller, it gets difficult to cram enough heat exchange surface into a give volume without running into problems like insufficient space between tubes. Remember that most of the boiler codes stipulate a minimum value on tube ligament or spacing. For the British Standard that I designed to it was 1/2" and that severely limits how many tubes you can get in. That being so, the efficiency drops away quite sharply. Strangely, it can come up again on copper boilers because you can use thinner material and smaller tube spacings. I think Steamchick is nearer the mark at 70%, and I would go down to 60% on steel boilers.

AJOEIAM,
I understand your frustration about not being able to start. Most real engineers start from something else and adapt. (Some call it copying). In that spirit, I attach an outline of a largish design of Yarrow boiler from the Steam Boat Association of Great Britain. You might do well to make contact with the USA steam boat guys, they will be using similar boilers under similar conditions, so will have a better handle on the right tool for the job. The full design of the 3 drum would be OK to UK standards, not necessarily to USA standards - Even though we all have to work to API tube sizes over here, because that is all you can get in certified material in UK.
I also sympathise over the complexity of calculations. The problem is you may well find such a such a tube will evaporate X per square foot or similar "rules". What they really mean is that in a certain case that tube evaporated X per sq foot - but you might well be working with a different fuel, different size, different configuration, forced draught or natural draught etc. etc. So such guides are useful for a first cut, but can be widely out in the overall scheme.

Martin

#### Attachments

• 3 drum boiler.jpg
636.9 KB · Views: 27
There's a lot happened since I last checked in.
Steamchick,
I am a bit surprised by the numbers for tube wall thickness coming out for Rayger's boiler. What makes me wonder is that the boiler for the 3" scale Atkinson wagon is 7" barrell but specifies 1/8" copper for 100 psi W.P. The stress concentration factor you are quoting for any shell opening just sounds rather high to me. It may well be right and ASME are uber conservative - that certainly happens having dealt with the UK approval body on steel boilers. I specced 16mm plate which was OK in the calculations, but they then bump it to 20 mm. I am not sure it is clever engineering as extra thickness will not "breathe" as the boiler cycles hot / cold. Remember, the strongest trees bend in the wind........ Most boiler problems I have seen are localised cracking in firebox corners and such like, where there is no room for deflection again.

HMEL
I agree that a good starting point for guessing required grate area to produce a given amount of steam is a boiler efficiency. I think your suggested figure of 78% would be good for full size work, but optimistic for small work. As you get smaller, it gets difficult to cram enough heat exchange surface into a give volume without running into problems like insufficient space between tubes. Remember that most of the boiler codes stipulate a minimum value on tube ligament or spacing. For the British Standard that I designed to it was 1/2" and that severely limits how many tubes you can get in. That being so, the efficiency drops away quite sharply. Strangely, it can come up again on copper boilers because you can use thinner material and smaller tube spacings. I think Steamchick is nearer the mark at 70%, and I would go down to 60% on steel boilers.

AJOEIAM,
I understand your frustration about not being able to start. Most real engineers start from something else and adapt. (Some call it copying). In that spirit, I attach an outline of a largish design of Yarrow boiler from the Steam Boat Association of Great Britain. You might do well to make contact with the USA steam boat guys, they will be using similar boilers under similar conditions, so will have a better handle on the right tool for the job. The full design of the 3 drum would be OK to UK standards, not necessarily to USA standards - Even though we all have to work to API tube sizes over here, because that is all you can get in certified material in UK.
I also sympathise over the complexity of calculations. The problem is you may well find such a such a tube will evaporate X per square foot or similar "rules". What they really mean is that in a certain case that tube evaporated X per sq foot - but you might well be working with a different fuel, different size, different configuration, forced draught or natural draught etc. etc. So such guides are useful for a first cut, but can be widely out in the overall scheme.

Martin
Martin: Just for your information the boiler shown would be outside of ASME miniature boiler classification as it exceeds 250pig. I would call this an A frame quite common in larger sizes. The ASME , API, and ANSI standards often overlap and the material code will let you know what standards they adhere to. Sometimes the prefix will tell you which codes they comply with.

I start boiler design with the steam conditions, mass flow, temperature, and pressure at the out let. I add what is a manufactures margin to the mass flow of steam about 10% to account for unknown issues to ensure the design number is achieved. Physical space is ignored because it will be forced by what is required. Fuel is also specified which drives the burner or grate size. The efficiency number is variable as it is used to calculate the mass of fuel required. I never use volume calculations unless I size for piping and orifice flow conditions and pressure drops across equipment.

However for modelers who want the steam system to look like the real thing issues of scale can get into the way of the above calculations. And that is the problem. They need the same care structurally as well as from a steaming point. But even at a smaller scale heat transfer equations can be used and they should predict what will happen. Its a lot of work. And that is why I recommend if somebody has one that is working well copy its design. Do you have an idea of the limit in scale of a small boiler? Never had anyone address that issue.

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