number of fire tubes

[nigel jones 5Participant @nigeljones5]

just finishing the design of my steel (yes…) 5" boiler. On paper I can get 4 big tubes to flow the same as 12 small ones. 4 is much easier to do than 12 – Ive made the firebox as bog as possible (22 sq inches grate), so will 4 big tubes be ok? Ive done a lot of reading and it seems that apart from the first inch or so the tubes contribute little to the steaming process. To aid steaming ive set the gap between firebox and outer at 1/4 inch. Tubes are to be steel. Any thoughts from outside the box welcome.

[nigel jones 5Participant @nigeljones5]

[JasonBModerator @jasonb]

While the 1/4" gap may aid steaming by compensating for the loss of grate area due to thicker materials it will not do a lot of good when it comes to keeping the boiler clean. I'd be looking for more like 3/8" and a washout plug in each corner.

What do you term big and small tubes. The 4.75" dia boiler on my traction engine has 21 tubes and they are quite free steaming. The risk with 4 massive tubes is that all the heat will just go straight through them and up the chimney. The theory with more smaller tubes is that they give a larger heating surface for the same total cross sectional area.

J

[nigel jones 5Participant @nigeljones5]

thanks

my point is that new computer models show that very little heat is transfered via the tubes (apart from the first inch or so from the fitebox. I have recently read about a boiler with only one tube which runs fine. I know it will run well with lots of small bore tubes but does anyone have experience of using less and bigger. Computer models are all well and good until it comes time to pay for the boiler….

[Ian FowkesParticipant @ianfowkes89537]

Only the hot flue gasses in contact with the tube wall will give up heat to the water,so as Jason has said the case with large tubes is that most of the flue gasses will not be in contact with the tube wall and so will stay hot and contribute very little in terms of transferred heat. Obviously turbulence within the tube complicates the issue but at least with small diameter tubes you can be sure that all of the flue gasses will be close to the tube walls which should maximise heat transfer regardless of tube length. As for the single tube boiler this is nothing unusual, many small scale models have a single flue as have had many full size boilers, it is all a question of how much steam you need, history has has shown that the multi-tubed locomotive type boiler produces the highest steam rates for a given boiler size but if such high steam rates are not required then a simpler boiler may well suffice.

Ian

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Nigel ,

I fear that some of the information that you have come by must have originated from Great Experts .

Boiler tubes don't just transfer heat – they are also an important part of the combustion system .

Effectiveness of boiler tubes is characterised (in a simplistic way) by a number – L/d^2 . If you work this out for a large number of succesful multitube boilers you will find that he answer usually comes out about the same . Any individual tube in your boiler will have to achieve about the same value .

In reality your boiler is a common size and you could just take example from others and copy the tube layout .

Mild steel tubes have somewhat poorer heat conduction than copper but more importantly in the conditions of a normal boiler they will corrode rapidly .

There is much more to all this . Ask specific questions if you want to .

Regards ,

Michael Williams .

[Ian FowkesParticipant @ianfowkes89537]

Michael has raised a very good point about the influence of boiler tube size on combustion. It is important in a coal fired boiler (at least in full size) that the total cross sectional area of the tubes has a minimum ratio to grate size otherwise it will be difficult burn fuel fast enough to achieve maximum heat output and also the flue gases will be forced to travel too quickly down the tubes and not transfer heat effectively. The use of a large number of small tubes is usually necessary to achieve this ratio and in a small boiler thin tube walls would most likely be necessary in order to simply get enough tubes in, something which would be a problem with steel tubes.

As Michael has said there is much more to all this, the boiler designers of the past did not have computer models but they did know a great deal about thermodynamics and did put all those tubes in for a reason.

Ian.

[nigel jones 5Participant @nigeljones5]

size proposed is grate area just over 22 sq inch, 6 no. tubes, 3/4 OD and 13" in length, if this helps?

[nigel jones 5Participant @nigeljones5]

probably worth pointing out that I have been making boilers for a good few years, horizontal, vertical and loco, up to 5" scale but always out of copper and always using convensional thinking – but following the norm doesnt excite me! Ive got books covering design of full scale and model boilers, all use the age old formulas (nothing wrong with that) but none tell me what to expect if I dont obey these principals – i suspect I know the feedback to this….

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Nigel ,

all use the age old formulas (nothing wrong with that) but none tell me what to expect if I dont obey these principals – i suspect I know the feedback to this….

(1) I am always interested in looking at new designs . I can tell you what result to expect in specific cases .

(2) This is a concept which is very difficult to explain quickly but here goes :

In any given design technology there are limit states – design conditions reached at which no matter what you do the end result cannot get any 'better' .

This is where we are with model boilers of the simple locomotive type – as commonly designed they are very near the best that can be done already . Alternative design proportions will either give a design which is different but no better or a design which is worse .

The only way forward if you want to experiment would be to design a boiler which was different in concept to the simple type . For example based on a fluidised bed combustion and forced draught system or a precombustion/gasifier system .

Regards ,

Michael Williams .

[julian atkinsParticipant @julianatkins58923]

hi nigel,

with the greatest respect i think you are about to make a complete cock up of a boiler. ive built a few in 5"g and done IMLEC. i think you are way off the mark. try it as an experimental design by all means, but be prepared to throw it in the scrap bin! i would recommend a copper boiler with 3 x 1" dia superheater flues as the top row, and 12 x 1/2" OD flue tubes underneath. this will give ample free gas flow (though i think your grate area is excessive… beware the 'Greenly' lazy grates problem…. in which case what size are the cylinders?) i would have thought a total of 15 square inches grate to be sufficient for a 5" dia boiler. the keiller formula (as amended by jim ewins ie slightly larger bore tubes) hasnt been bettered in 5"g as a formula. i would depart from it at your peril!

dont forget that the last 1/3rd of the tube length contributes nothing to heat transfer as discovered by jim ewins in a standard type boiler. 22" long tubes for a 5" dia barrel seems rather long to me…makes me wonder what type of loco it could possibly look like! i would be inclined to recess the front tubeplate quite a bit. if you have a wide firebox it is possible you could go up to 9/16" dia flue tubes but you wouldnt get so many in or have the even spacing as described above (from someone who has built a few boilers with 5" dia barrels). in your proposed design you are basically having a huge firebox with a very long barrel with enormous sized flues contributing nothing to heat transfer… the main heat transfer will take place only in the firebox… so what is the point of the long barrel? it does nothing at all for steam production. btw, if you consider heat transfer in the flues, the slower the gasses flow some say the greater the transfer…but if you take this to the extreme and reduce the flow to zero there is no heat transfer!

good luck!

 

(sorry for the edits… my spelling or keyboard playing up!)

julian

Edited By julian atkins on 21/05/2012 23:03:05

Edited By julian atkins on 21/05/2012 23:18:34

Edited By julian atkins on 21/05/2012 23:22:32

[julian atkinsParticipant @julianatkins58923]

apologies my mis-reading… though i still think 13" is too long for a 5"dia barrel (instead of my mis-reading 22"!). 1 /2" dia flue tubes at say 11" long should be ok. recess the front tube plate 2".

only 6 x 3/4" dia tubes is daft, and what about superheater flues?

foundation ring thickness should be 5/16", not 1/4" for this size boiler.

see jim ewins' research in the SMEE journals and at the end of martin evans' 'model locomotive boilers' book

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