Optimal number of boiler tubes.

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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....?
snip
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?).
https://www.tsask.ca/uploads/File/PDFs/Legislation/boiler_pressure_vessel_act.pdfExcepted:
(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...
o_O
K2

OK - - - so if I use a water wall (tubes with steel welded in between so a sealed surface do I use the overall area or is it necessary to calculate the exact amount of perimeter of the tube's fire side facing or is it something else?

(This is where more information almost doesn't help - - - its like I'm needing definitions of the the definitions of the definitions. It shouldn't be that way - - - for pete's sake we've been doing this for at least a 100 years. But then I guess obfuscation makes for more work for those with the 'stamp'. )

When I look at the Raygens (sorry likely mis-spelled) spreadsheet - - - -there are absolutely NO calculations.
I was hoping that I could determine how those numbers were arrived at. Rather the spreadsheet just presents the values as fait d'accompli !
 
There's a lot happened since I last checked in.


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

I had long ago found out that in engineering there is NO such thing as a clean sheet design that's why I've been trying to find something close!

Are you up for questions on that design?

With your last paragraph - - - that's why I've been known to ask 'what is the most efficient boiler design?' except so far all I hear is a resounding thunder of 'nothing'.

Its a number of years ago now but I worked through the design of a not really simple machine. What I found interesting was when I ran into the differences between shaft materials and how that would impact the design (and cost). (You know do I use 1018, 1045, 4140, 8620 or perhaps even do something to the material - - - like case hardening, or even through hardening and then to what level . . . (when you get into it there is a very large range of options in this one sentence!))Realized that no one would actually want such a beast (something designed to be bullet proof) largely because they could get a pos from a known brand for less. I was actually quite surprised at the support I received from a couple bearing manufacturers.

Maybe I just need to stop trying to work through any particular problem and wait until I have plenty of $$$$$$$$$$ to throw at any issue and then toss money at them (likely ain't going to happen - - - - no one has yet given me a winning major lottery ticket and I can't afford them!). I also like to make things so I suppose that as I get up there eventually the ideas just won't matter - - - I just won't be able to do the stuff.
 
OK - - - so if I use a water wall (tubes with steel welded in between so a sealed surface do I use the overall area or is it necessary to calculate the exact amount of perimeter of the tube's fire side facing or is it something else?

(This is where more information almost doesn't help - - - its like I'm needing definitions of the the definitions of the definitions. It shouldn't be that way - - - for pete's sake we've been doing this for at least a 100 years. But then I guess obfuscation makes for more work for those with the 'stamp'. )

When I look at the Raygens (sorry likely mis-spelled) spreadsheet - - - -there are absolutely NO calculations.
I was hoping that I could determine how those numbers were arrived at. Rather the spreadsheet just presents the values as fait d'accompli !
That was just a copy-and-paste from the spreadsheet that Quinn Dunki "Blondihacks" produced, available to her Patrons on YouTube for a small monthly donation. I don't feel comfortable sharing without her blessing.
Sorry
Ray
 
AjoeIam.
Picking up on some of the questions here... from your posts #101 & #102.
  • it IS necessary to calculate the exact amount of perimeter of the tube's fire side facing the fire. The "steel wall " interface does not count for generating steam. Better is probably a wall outside of the tubes, but make a staggered double row of tubes so radiant heat hits all in some part. (Like the Yarrow design below). That way hot gases surround the tubes so you get "double the heat uptake" of your proposed "tube and filler" firebox wall.
  • "what is the most efficient boiler design?" - Generally a flue tube boiler - from the books I have read. (K.N.Harris talks about "Scotch",
    20231129_093801[1].jpg
    "Cornish" or "loco" boilers being "the best" because they encompass a firebox that has a wet wall, and flue tubes to extract as much heat as possible from the residual flue-gas heat). To burn "organic" fuel as you suggest (wood waste?) I should recommend a boiler with an external fire, as this is able to have a much bigger (low efficiency) fire like a wood fire.
    20231129_094233[1].jpg
    Or a "loco" style with large firebox per "USA 4-4-0" locos, etc.. But the jury is out as it depends on "EXHAUST" temperature from the boiler. You cannot have exhaust gases leaving the boiler at a lower temperature than the Steam boiling point at pressure. - So a 100psi boiler at 205deg.C cannot release exhaust gases cooler than that. But a feedwater heater CAN take that heat and pass it into the boiler, so the SYSTEM efficiency is improved without the BOILER efficiency changing. Also a condenser, after the engine has used the steam, can improve efficiency OF THE ENGINE, and some of that heat can pre-heat the boiler feed water too. Martin has offered an improved design using SUPERHEATERS - which technically reduce the efficiency of the boiler, but increase the efficiency of the ENGINE's use of steam... K.N.Harris proposes that a "larger" boiler will naturally be more efficient than a smaller boiler of the same design, because it has a better Volume to external Surface ratio (heat losses), and can be worked "less hard" so the flue gases can be fully "cooled " to Boiler temperature in a boiler not worked to full performance. Sorry, all this sounds like "Obfuscation", but you asked...
  • "I just won't be able to do the stuff." - Therein lies failure, before you start. But we don't do that (often) until we have had a go and got something worthwhile out of the exercise. So take a breath, shake your head, have a cup of tea and go back to where we are trying to help. A Flue tube boiler with "wet-walled" is pretty good for efficiency and performance. Martin's solution for flue tube and superheater sizes is best for "overall system performance" of Rayger's boiler. A water-tube boiler is generally much more powerful, size for size, so in "building power stations" that is what they use. And they are "cost optimised" for Capital versus running costs. Finally, Flash-boilers (coils in the flames, etc.) usually require "better than copper" tube and are not "efficient", but use huge "fires" and produce huge amounts of steam at much higher pressures - for racing, etc. applications.
  • Water tube boilers are generally half-way to "flash-boiler" technology, requiring constant controlled water feed at full boiler pressure. But for "domestic sizes, generally a flue-tube boiler is selected as it is proven technology and fits "the Regulators thinking".... (And surely they are not completely stupid or corrupt?). Regulators for "Model boilers" do not ban water-tube boilers, but the Federation Rules for clubs "do not permit the club inspectors to inspect and certify Water-tube (drum type) boilers" like the Yarrow. - So we stick to flue tube boilers, because we can do those... and we are not on a Domestic application, budget, or efficiency target.
  • Attached a design for a small Yarrow boiler.
    20231129_085811[1].jpg
    But change ANYTHING and you need to do a full set of calculations (e.g. to ASME) for your (available?) tube sizes, etc. to get any certification for your design. (Model club inspectors will usually accept an accepted published design without needing calculations, but not always...).
  • I suggest you get a book - like K.N.Harris "model boilers and boilermaking". Because he does the sums in a way many people can follow... from "how much steam?" etc. to a finished design. BUT: ASME do not agree with his sizes: Better material sizing and tube spacing, etc. comes from Kozo Hiraoka: "Safety of Copper Boilers" article in "Live steam and outdoor railroading" magazine: Issue Vol. 40 no.6: Nov/Dec. 2006 - a back-copy available on-line from the magazine. - THIS IS MY BIBLE. ;)

Now follow some of the advice from those cleverer than I, and get on and do your stuff. - Because you can. :):):):):)
K2
 

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Hi Raygers,
Re: "basing my calculations on running at 60psi":
Revised calculations:
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 the proposed 60psi NWP.

Your proposed pressure does not change the calculation that works out what the tube can manage.. It just changes the conclusion...
I am still bamboozled by all the different ways that the strength of tubes "in compression" are deduced.
As far as I understand it: Simply:
ASME state "Any penetrations require a Stress Concentration Factor of 3.3." - But I was told that, and haven't found it yet....
(On one boiler I had calculated penetrations etc, with SCFs of 1.7, 2.3, 2.8 - but 3.3 by ASME will be the blanket figure THAT THEY REQUIRE to be used in calculations, covering most cases adequately!).
But tube distortion factors, variations in actual wall thickness, etc. all degrade tube strength in compression, and while there are many "mathematical" predictions to calculate, ASME etc. do not appear to specify these as a requirement.
Hope this helps?
K2
 
Steamchick,
You seem bamboozled as to why tubes in compression need such a different calculation to tubes in tension. The answer is that in compression, they fail by buckling which is a very tricky failure to predict and certainly depends on how "perfect" the circle is. That is probably why ASME rely on charts and graphs to do the sums.
I am concerned that without access to the full ASME code (a weighty and expensive tome!) we are not getting the full picture here. I certainly cannot help in that regard, having only worked in detail on British Standard codes. Anyway, would it really be that bad if Raygers had to silver solder and butt strap a 4mm (5/32") wall shell?
The problem in tension is much easier to solve. BTW you quoted calculations for thick walled tubes, but most boiler design is done on thin walled tube assumptions. The (much more complicated) thick walled calculation is only relevant where there is significant change in stress between inner and outer walls of the tube.
HMEL,
I agree that basic heat transfer sums must apply to boiler both large and small. There is a snag, though. At roughly 5" to 7.5" gauge, typical flows etc. in a typical loco boiler the flow patterns change from laminar <5" gauge to turbulent >7.5" gauge. This transitional regime which covers a lot of "model" boilers is a swine to calculate with reliability. I have reviewed a number of papers dealing with prediction of Nu under such conditions and all I can conclude is that the uncertainty band is "large"! So while I respect your obvious experience on industrial scale boilers, the little ones can present their own problems.
I was interested in your warnings about low ash fusion temperatures for biomass combustion. It may be of interest to some on here but this guy claims to have it sorted:
Mackwell Boilers
Just in case anybody thinks I am promoting his product. I am not.
AOJEIAM,
I don't think we have any indication of how much steam you want to generate. You have quoted 300 - 500 psi, which seems very high to me. Those pressures are likely to lead you into an area where specialised steels would need to be specified. That's all fine in industry where you are buying tons of material for megabucks. Not so fine for a hobby where nobody will be interested in supplying a hundred lbs, as the certification will cost far more than the job is worth. That is not theory, just based on my own experience of designing and getting approval for a "large" boiler (by ASME definition) for home use. So I think you need to reign in your ideas a bit on pressure and give us an idea of just how much steam (or even power) you want to generate, then the assembled brains might have a chance of getting you off the starting line. As I suggested, a trawl round the steam boating community is likely to turn up something that will at least provide inspiration.
Martin
 
Ajoeiam: re-reading your posts #88 & #89.... I am out of my experience zone with the best design of boiler to cope with the ash and smoke and residue that deposits inside flues and om water tubes. Ash is a weird combination of chemicals.... Some particles that do not melt in the fire, but some that do.... As the flue gases cool, from fire temperature of maybe 800 - 1200C, to maybe 200 - 350C, the non-solid ash content becomes solid, usually as a sticky coating around solid particles, and sticks lots of particles together to make clinker. If the cooling is at the Syria ce of a tube full of colder water, or steam, then it builds-up as a coating that drastically affects heat flow. In fact on coal fire boilers it is common to clear a few blocked flue tubes at the end of a working shift. And ash also fuses to and blocks the grate. Depending on the nature of your biomass.... you may be much better with water tubes as in a Yarrow boiler, as these can get a coating/fouling of ash on one side and still have a clear side. Without flue tubes that block completely. But then clearing water tubes on a Yarrow could be much harder than flue-cleaning on a Scotch boiler...
I should be inclined to size the boiler >>20% larger to give myself scope for the loss of performance due to fouling....
K2
 
Martin, I am not familiar with the 3 in Atkinson, but I understand your point. Yet there are many "old" designs that were acceptable in their day, that would not be considered acceptable by today's standards. Or by the standards in a different country.
SIMPLY: The factor of safety may not achieve what we require using today's standards.
Pre-1960, I doubt that many steam boilers used a real FOS of 8, or applied stress concentration factors to boiler shells, assuming that the reinforcement made it safe. Also, I have done some sums on old designs and have calculated FOSs of as little as 3..... which pass a design hydraulic test of 2 x NWP, and nobody knows any better.... I have also seen blown boilers that failed hydraulic test at 2 x NWP. Much to the Owner's dissapointment.
K2
 
Thanks Martin, for post #107. I appreciate advice from someone with better experience than mine. (I read Physics at university, which taught me to look at things in the finest detail, but I missed the real world experience!).
While I use the thick-walled analysis for my own understanding of hoop stress, I usually use whatever it says in the Regs/standards for certification.... which is why in post #87 I have used the calculation from ASME for the 6in outer tube of Rayger's boiler. As Raygers is in Canada, he may not need certification... or may need it to an equivalent standard to ASME....?
Thanks for the advice.
K2
 
Hi Martin,
I am bamboozled that everyone (in the boiler books and things) assumes the tubes in compression can be proven by calculations of tubes with internal pressure.... same hoop stress, and LIMITS for strength.
A simple table shows that copper (Annealed - like when you have just silver soldered your precious boiler and it is new and shiny) has a min strength of 45MPa, but tensile is 210MPa... If applied to hoop stress, it means many tubes are overstressed - but "get away with it" because they are only tested to twice NWP when they should have a FOS of 8... and they are carefully pumped up (by a good tester) so they work harden a bit to increase their compressive strength... - That still doesn't give one a calculation saying it is OK... or a Factor Of Safety of >8.
https://www.google.com/search?q=compressive+strength+of+copper&sourceid=chrome&ie=UTF-8And that is before one considers stress concentration factors, wall thickness variation, distortion from circular, etc.
Maybe "Bamboozled" wasn't the right word, but I am "puzzled"?
I am.
K2
 
Last edited:
I was interested in your warnings about low ash fusion temperatures for biomass combustion. It may be of interest to some on here but this guy claims to have it sorted:
Mackwell Boilers
Just in case anybody thinks I am promoting his product. I am not.
AOJEIAM,
I don't think we have any indication of how much steam you want to generate. You have quoted 300 - 500 psi, which seems very high to me.
Hmmmmmmmmmmmm - - - interesting - - - - looking at the website!!!

You're finding 3 to 5 hundred psi very high pressure.

ultra super critical fluidized bed combustion runs to well over double that.
A lot of contemporary hydraulics systems are running at 4k and 4.5k psi pressures.
That's without any thinking of possible shock loading.

Need to get a copy of the article you recommend.
Already have a copy of the Harris book.

Regards
 
AjoeIam.
Picking up on some of the questions here... from your posts #101 & #102.
  • it IS necessary to calculate the exact amount of perimeter of the tube's fire side facing the fire. The "steel wall " interface does not count for generating steam. Better is probably a wall outside of the tubes, but make a staggered double row of tubes so radiant heat hits all in some part. (Like the Yarrow design below). That way hot gases surround the tubes so you get "double the heat uptake" of your proposed "tube and filler" firebox wall.
  • "what is the most efficient boiler design?" - Generally a flue tube boiler - from the books I have read. (K.N.Harris talks about "Scotch", "Cornish" or "loco" boilers being "the best" because they encompass a firebox that has a wet wall, and flue tubes to extract as much heat as possible from the residual flue-gas heat). To burn "organic" fuel as you suggest (wood waste?) I should recommend a boiler with an external fire, as this is able to have a much bigger (low efficiency) fire like a wood fire. Or a "loco" style with large firebox per "USA 4-4-0" locos, etc.. But the jury is out as it depends on "EXHAUST" temperature from the boiler. You cannot have exhaust gases leaving the boiler at a lower temperature than the Steam boiling point at pressure. - So a 100psi boiler at 205deg.C cannot release exhaust gases cooler than that. But a feedwater heater CAN take that heat and pass it into the boiler, so the SYSTEM efficiency is improved without the BOILER efficiency changing. Also a condenser, after the engine has used the steam, can improve efficiency OF THE ENGINE, and some of that heat can pre-heat the boiler feed water too. Martin has offered an improved design using SUPERHEATERS - which technically reduce the efficiency of the boiler, but increase the efficiency of the ENGINE's use of steam... K.N.Harris proposes that a "larger" boiler will naturally be more efficient than a smaller boiler of the same design, because it has a better Volume to external Surface ratio (heat losses), and can be worked "less hard" so the flue gases can be fully "cooled " to Boiler temperature in a boiler not worked to full performance. Sorry, all this sounds like "Obfuscation", but you asked...
  • "I just won't be able to do the stuff." - Therein lies failure, before you start. But we don't do that (often) until we have had a go and got something worthwhile out of the exercise. So take a breath, shake your head, have a cup of tea and go back to where we are trying to help. A Flue tube boiler with "wet-walled" is pretty good for efficiency and performance. Martin's solution for flue tube and superheater sizes is best for "overall system performance" of Rayger's boiler. A water-tube boiler is generally much more powerful, size for size, so in "building power stations" that is what they use. And they are "cost optimised" for Capital versus running costs. Finally, Flash-boilers (coils in the flames, etc.) usually require "better than copper" tube and are not "efficient", but use huge "fires" and produce huge amounts of steam at much higher pressures - for racing, etc. applications.
  • Water tube boilers are generally half-way to "flash-boiler" technology, requiring constant controlled water feed at full boiler pressure. But for "domestic sizes, generally a flue-tube boiler is selected as it is proven technology and fits "the Regulators thinking".... (And surely they are not completely stupid or corrupt?). Regulators for "Model boilers" do not ban water-tube boilers, but the Federation Rules for clubs "do not permit the club inspectors to inspect and certify Water-tube (drum type) boilers" like the Yarrow. - So we stick to flue tube boilers, because we can do those... and we are not on a Domestic application, budget, or efficiency target.
  • Attached a design for a small Yarrow boiler. View attachment 151796But change ANYTHING and you need to do a full set of calculations (e.g. to ASME) for your (available?) tube sizes, etc. to get any certification for your design. (Model club inspectors will usually accept an accepted published design without needing calculations, but not always...).
  • I suggest you get a book - like K.N.Harris "model boilers and boilermaking". Because he does the sums in a way many people can follow... from "how much steam?" etc. to a finished design. BUT: ASME do not agree with his sizes: Better material sizing and tube spacing, etc. comes from Kozo Hiraoka: "Safety of Copper Boilers" article in "Live steam and outdoor railroading" magazine: Issue Vol. 40 no.6: Nov/Dec. 2006 - a back-copy available on-line from the magazine. - THIS IS MY BIBLE. ;)

Now follow some of the advice from those cleverer than I, and get on and do your stuff. - Because you can. :):):):):)
K2



Re: boiler efficiency - - - - it would seem that only firetube boilers are on your radar - - - - which is most specifically NOT what I want. Safety factors with a water tube boiler are quite a bit better than a firetube boiler by its very nature!

It would seem that copper is the only material you will consider for a boiler.

I have experience (and knowledge) regarding the fabrication of 'steel' boilers. Likely could do the copper one as well but really not interested. Don't know how Hiraoka's article will help for a 'steel' boiler.

There is a local 'steam' association but am waiting on a response. It may be somewhat moribund - - - - no web activity in almost 2 years.

Not just trying to be argumentative but you seem to keep circling back to a copper firetube boiler and that's NOT on my radar.

Regards
 
OK Ajoeiam, stay calm, please. I have never had anything to do with steel boilers specifically, so I am happy if I just respond to Raygers about his plans for a silver soldered copper boiler on this thread, and not offer you any more advice as it doesn't suit your needs. Except "read ASME Regulations" - American Society of Mechanical ENGINEERS - as they are a body of ENGINEERS - who wrote their rules to "Help" people like us.
I don't do water tube boilers, as I recently found out that the local Federation rules prohibit my club's tester from examining and certifying such designs. - They would need a registered and certified professional company examination, testing and certification which is not appropriate to my HOBBY. Also you wanted recommendations for the most efficient boiler - for the fuel you choose - and Flue tube boilers are considered "very efficient" in the books I use.
If you are a certified steel welder for BOILERMAKING then you have far more expertise than I. I bow to your expertise. Perhaps you can use a separate thread about your different boiler interest, so as to not confuse anyone? e.g. there is a thread on "Welding a copper boiler" - you could start a thread on "Designing and Welding a steel boiler"?
K2
 
AJOEIAM,
I have 35 years as a professional engineer under my belt, including Fellowship membership of our UK institution. Clients could either accept my advice or ignore it, but my fee was still the same. It's a system I still use.
My efforts to assist are clearly inadequate, so I shall bow out now.
Martin
 
OK Ajoeiam, stay calm, please. I have never had anything to do with steel boilers specifically, so I am happy if I just respond to Raygers about his plans for a silver soldered copper boiler on this thread, and not offer you any more advice as it doesn't suit your needs. Except "read ASME Regulations" - American Society of Mechanical ENGINEERS - as they are a body of ENGINEERS - who wrote their rules to "Help" people like us.
I don't do water tube boilers, as I recently found out that the local Federation rules prohibit my club's tester from examining and certifying such designs. - They would need a registered and certified professional company examination, testing and certification which is not appropriate to my HOBBY. Also you wanted recommendations for the most efficient boiler - for the fuel you choose - and Flue tube boilers are considered "very efficient" in the books I use.
If you are a certified steel welder for BOILERMAKING then you have far more expertise than I. I bow to your expertise. Perhaps you can use a separate thread about your different boiler interest, so as to not confuse anyone? e.g. there is a thread on "Welding a copper boiler" - you could start a thread on "Designing and Welding a steel boiler"?
K2
Hmmmmmmmmmmmmm - - - thought I was quite calm.

ASME - - - - regulations were written to mollify a whole steaming heap of insurance companies and, imo, to largely obfuscate what they were actually doing.

Pity about your local Federation - - - sounds like another group that could do with a modernization (watertube boilers are NOT intrinsically weird!!!).

(Wasn't looking for anyone to 'bow to my expertise' just trying to get some way of finding what appears to be unobtanium.)

Hmmmmmmmmmm - - - - thought the thread was on 'optimal number of boiler tubes' - - - oh well.
Time to dig into the obscure land.
 
Hi Martin,
I have been enjoying your submissions, and appreciate your expert advice. I am looking back at your posts to try and align what I have been doing with what I now think I should be doing... But I'm still not clear on what I should be doing.
  1. Should I be using 45MPa as the compressive strength from which to derive the safe stress (ASME based) relating to temperature (NWP steam temp) and Factor-of-safety?
  2. I picked up the "blanket FOS" for any vessel wall penetrations to be used as 3.3? Should I apply something different?
  3. I have equations for distortion of tubes, elliptical variation (out of round max and min), wall thickness variation, etc. but have not applied them to my calculations - yet... (I have not come across anyone doing so for model boiler design?).
  4. I have used thick-walled calculations to allow for some things that have had thicker walls... as once the spreadsheet calculation is set up on Excel it is easy to bang-in the basic sizes and get the answer for the more complex equation for hoop stress at the inner or outer surface.. I admit that the average stress in the material from the thin-walled Hoop stress calculation is simpler. (Nice for anyone who cannot handle a bit of arithmetic). But failures don't understand "average stress", just the extreme case... which is partly the skill and expertise of the Design Engineer to identify and calculate... before it fails.
  5. I saw (a decade or more ago) a boiler at an exhibition, for a 5in or 7 1/2in loco.... where the firebox (wet) was a very thick walled cylinder 1/4" or 5/16" thick wall - with the fire above a grate and the (dry) firebox end wall gave access to the ash-pan/air-intake and coal-firing door. So the complete inner firebox wall was a 5 or 6in tube without penetrations. When I asked the rep about "why" that particular design, he said "Do the sums on strength of the firebox - a copper tube in compression from the steam/water pressure at elevated temperature."... which is why I started this long tale of "doubt". I have failed to find this applied in any boiler design "text book". But some authors mention that fire-tubes containing a furnace (like a Scotch or Cornish Marine arrangement) have to be "carefully made".. But often there is no comment at all about their strength as "a tube in compression".
As an ex-Design Engineer (not pressure vessels) I have a curiosity towards the "best design practices" as I was taught by a Structural Steelwork Chartered Engineer. - e.g. "Calculation first, not as an afterthought". And "Design to manage stress, not create it", and a few other memos.
  1. I have de-rated some boilers I had earlier made, because my latest calculations require a lower NWP. Always due to my new rating of tubes in compression, not the outer shells or ends.
  2. I have repaired/rebuilt and had re-certified a few boilers that I had been given as "failed" (leaking). In studying the failure modes, I have reasoned that there are a lot of "Amateur" copper-smiths who think they understand boilers, then make one with good workmanship, but of a "corrupt" design that must fail due to the way the stresses work the metal/joints.
    1. One boiler had failed around the firebox door frame, where the inner had deformed where the inner tube met the firing tube and split the silver soldering. I reasoned the stress concentration had caused this. Later I determined a safe NWP that allowed the boiler to work after I had repaired it. This was <1/5th. of the NWP used by the previous owner, which would (according to my calculations) have been adequate reason for the joint to fail on a 2 x NWP applied by the previous owner. (80psi NWP with 160psi hydraulic failed the boiler. => 15psi NWP is all my calculations of the firebox inner tube permit!)
    2. Another boiler failed because the rivetted and soft-soldered end plates were peeling the joints apart due to the internal pressure, as well as the failure of the solder at the temperature that the boiler was used at for the pressure the guy thought was safe. I stripped this to re-use some of the tube, and scrapped the rest.
    3. Yet another had a fire tube for a blowlamp, with cross-tubes, that had always failed when the boiler was steamed. I deduced that the tube used was "too thin" and was compressing (distorting) at the stress concentration where the hole for the cross-tubes existed. No amount of silver solder appeared to hold the tube from failure at the cross-tubes (multiple repairs!), and my sums explained that it was due to the stress concentration factor at the cross-tube joint. (I was using about 2.6.. which pushed the stress to fail the copper tube in compression, with the elevated temperature from the overly high NWP that was being used!).
Hence my search for the "right sums" to develop a "design" for the tubes in compression... so I can identify the "proper" NWP for the strange boilers that come my way...
Sorry to go on a bit...
I hope you can advise further? - Then I can do the sums properly for boiler designs from Raygers et al.
K2
 
I looked at the Mackwell boiler design. It is a simple flash-steam boiler, but not clear of the number of flash-steam water tubes, size, length, etc., pressure, feed pump, engine size or anything else. I guess coiled stainless steel tubing? Where the flue gases from the fire are made to pass through the centre of coils of tubing and away from the outside? (Water feed from chimney end towards firebox - hot-end).
K2
 
Steamchick,
You seem bamboozled as to why tubes in compression need such a different calculation to tubes in tension. The answer is that in compression, they fail by buckling which is a very tricky failure to predict and certainly depends on how "perfect" the circle is. That is probably why ASME rely on charts and graphs to do the sums.
I am concerned that without access to the full ASME code (a weighty and expensive tome!) we are not getting the full picture here. I certainly cannot help in that regard, having only worked in detail on British Standard codes. Anyway, would it really be that bad if Raygers had to silver solder and butt strap a 4mm (5/32") wall shell?
The problem in tension is much easier to solve. BTW you quoted calculations for thick walled tubes, but most boiler design is done on thin walled tube assumptions. The (much more complicated) thick walled calculation is only relevant where there is significant change in stress between inner and outer walls of the tube.
HMEL,
I agree that basic heat transfer sums must apply to boiler both large and small. There is a snag, though. At roughly 5" to 7.5" gauge, typical flows etc. in a typical loco boiler the flow patterns change from laminar <5" gauge to turbulent >7.5" gauge. This transitional regime which covers a lot of "model" boilers is a swine to calculate with reliability. I have reviewed a number of papers dealing with prediction of Nu under such conditions and all I can conclude is that the uncertainty band is "large"! So while I respect your obvious experience on industrial scale boilers, the little ones can present their own problems.
I was interested in your warnings about low ash fusion temperatures for biomass combustion. It may be of interest to some on here but this guy claims to have it sorted:
Mackwell Boilers
Just in case anybody thinks I am promoting his product. I am not.
AOJEIAM,
I don't think we have any indication of how much steam you want to generate. You have quoted 300 - 500 psi, which seems very high to me. Those pressures are likely to lead you into an area where specialised steels would need to be specified. That's all fine in industry where you are buying tons of material for megabucks. Not so fine for a hobby where nobody will be interested in supplying a hundred lbs, as the certification will cost far more than the job is worth. That is not theory, just based on my own experience of designing and getting approval for a "large" boiler (by ASME definition) for home use. So I think you need to reign in your ideas a bit on pressure and give us an idea of just how much steam (or even power) you want to generate, then the assembled brains might have a chance of getting you off the starting line. As I suggested, a trawl round the steam boating community is likely to turn up something that will at least provide inspiration.
Martin
Well I am really glad someone has sorted it out. Cause I have seen ash fusion temperatures so variable based on the salts it was running like lava in the hoppers or on the grates. So I did look at the boiler on their website and it would have a snowballs chance in hell on some of the biomass fuels I have seen fired. One such fuel was sunflower seed shells. As far as small boilers I have pretty much stayed away from those designs because one takes on a bit of liability when they design them. Little hand grenades I call them.

I guess I would challenge the turbulent flow numbers because it depends upon the Reynolds number to be less than 2000 to enter the laminar flow region. And reaching this number is pretty hard to do as most flows are turbulent flow. The calculation is the same as for big units.

As far as buckling, steamchick is correct the equations are very different because the stress is using the hoop stress equations when pressure is from inside the tube. The buckling will come from not enough room for expansion due to heat. Tubes with pressure on the outside have to be treated differently because hoop stress can not help. The recent tragedy of the titan submersible is a good analogy. Inappropriate design for handling the external pressures. There is a lot more experience with loaded columns on the buckling equation, however as soon as internal or external pressure is applied the game changes.

As far as the method of silver solder and butt strap method I think it depends. On such unknown construction the only way to be sure is a destructive test by over pressure. Then you know.

And as far as the ASME code I do have it and its located in about four large boxes. Its not that helpful a better help is a good engineering book on stress analysis. And the reason I have that code is a good friend of mine gave me his copy when he retired otherwise there was now way I would spend that much on all the code.
 
Looks like there is quite a lot we agree on HMEL. I have never worked with the ASME code, but I know it runs to several volumes and needs a barrow to carry it around. So REALLY knowing your way around it is a lifetime's work.
If anybody is struggling with visualising a cylinder with external pressure failing, this link (among others) is pretty good:
Rail tanker implosion
I read last night that "Locomotion" a very early UK locomotive suffered two furnace tube failures (+1 fatality) in the early days.
That "crinkly" failure is very difficult to predict mathematically. On the other hand, a cylinder with internal pressure fails when simple hoop stress exceeds material strength. No crinkly bits involved.
I am pretty sure the Australian model boiler code - which includes copper boiler designs - would have design factors for made up barrels with butt strap joints. In fact, I think the Aussie code (1 relevant volume, I think) would probably be the best bet for Raygers, apart from the fact he may struggle to get the calculations accepted in Canada! The Aussie code has become the "go to" in the UK - which possibly says a lot about the state or our engineering expertise.
Martin
 
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