Boiler design ideas

[baffle]

Hello. I am new here and this is a first post - not sure I've posted to the right sub-forum?? Apologies for any incorrect terms I am about to use - I am not that familiar with the various acronyms and abbreviations that can get used - so please bare with me.
I am about to design and make a boiler for a wood burner on a narrow boat. The boat has been a long project, starting with design drawings six years ago, getting the steel work built, and then a self fit out (with gourgeous recycled teak from a late Victorian boat). We finally moved on last New Year. Our heat source is an Italian made stove/cooker (La Nordica Rosetta - 6kw rated) with an OK sized fire box, a great top cooking surface, and a tiny oven. I installed it in the knowledge that I would later fit a boiler, as I have done previously*. I want to use a top boiler so that I don't decrease the depth or width of the fire box (or make the fire box longer) and at that same time, I still want to let direct heat through to the hob (so we can still cook on it - it makes toast really well). So I am thinking of a boiler of tubes, rather than a box. When I was looking at buying the stove, the bigger La Nordica models had I think stainless tube top boilers as an option - but i am struggling to find any dimensional information, or line drawings to give me ideas. Same for other makers - line drawings of boilers are very thin on the ground.
I want to heat 12 gallons of water, and feed one radiator. I know I need to ensure that I have enough capacity to dump out unwanted heat in the system.
I intend to use 22mm copper to run to the cylinder which is 4m away.
Questions:
Can a tube boiler have a smaller volume than a jacket/box, as there is proportionally more surface area?
Has anyone made a similar boiler of tubes - what are the difficulties (harder to weld) - any unforeseen problems?
Any thoughts on bore/materials (stainless/mild)? I have ideas of pitching the tubes at an angle rising to the front of the stove.
* I installed a top boiler in a similarly sized fire box of a woodburner on a previous boat, and that heated our winter water for almost 20 years (via a calorifier and coil) using only thermosyphon to circulate.
thanks
Baffle

[baffle]

Well, sorry I have not triggered any responses.....   Having looked at the job again, I have decided to use a U shape made out of 2" bore heavy duty mild steel pipe which I am going to weld up with with weld elbows and 3/4 BSP studs at the ends that pass through the stove wall and will themselves receive BSP to 22mm brass elbows. I think I will tilt the U slightly so that there is a rise within the stove body. I have calculated that this will give a boiler volume of about 1.47 litres - which aint very much, but there will be lots of surface area.
Baffle

[dhaslam]

Making boilers is a bit of a black art. You have to compromise between cooling the fire too much and not having enough surface area to heat the water sufficiently.   The only way to do it is to make a test boiler and see how it works.   There isn't much advantage in copying commercial ones because most don't work properly.

One of the most successful was the Grant triple pass grant back boiler for open fires.  The inventor spent years making and remaking boilers before reaching near perfect results.  Each time he lit a fire in the latest version  and then made the surface area bigger or  smaller  until the best output was reached.

www.grantengineering.ie/product-area/triple-pass-solid-fuel-back-boiler/

[Fintray]

Baffle

Try asking the same question on www.mig-welding.co.uk and you might get more response.

[baffle]

thanks for replies -

Making boilers is a bit of a black art. You have to compromise between cooling the fire too much and not having enough surface area to heat the water sufficiently.   The only way to do it is to make a test boiler and see how it works.   There isn't much advantage in copying commercial ones because most don't work properly.

Sure - many variables involved, so suck-it-and-see. I was interested to see that some of the commercial ones had switched to pipes rather than a jacket/box - this might be economic/cheaper, or indicate that they had done some r&d ? Difficult to get any information out of them though....

One of the most successful was the Grant triple pass grant back boiler for open fires.  The inventor spent years making and remaking boilers before reaching near perfect results.  Each time he lit a fire in the latest version  and then made the surface area bigger or  smaller  until the best output was reached.

www.grantengineering.ie/product-area/triple-pass-solid-fuel-back-boiler/

Thanks for the link.
Cheers

[baffle]

Baffle

Try asking the same question on www.mig-welding.co.uk and you might get more response.

Fintray  - will do/have done - good call

thanks

[Baz]

Try searching around steam engine boilers/ model engineering boilers. The heat transfer per squ ft is well established. They tend to use units like lbs of steams generated per squ ft but you can do some calculations.

[Heinz]

I built a tube boiler a couple of years ago. Learnt two things. 1/ be sure to have plenty room between the tubes for welding and repair. 2/ roller welded tube will be porous. I spent many hours carefully welding up the tube boiler only to find that each tube was porous down the side where the weld/seam was. Past experience had taught me to expect the odd pinhole in the seam, but the entire length of the weld on all the tube was porous. Cheap foreign imported steel? One large, heavy and expensive boiler consigned to the scrap pile  

Heinz

[johnrae]

You might find 28mm a better choice for the gravity circulator circuit.  4 metres of 22mm there and back plus a few elbows will be quite restrictive.  You also need to ensure your brass components are suitable for "de-zincing" since the connection to the steel boiler will encourage the zinc in the brass to be sacrificed with the result the brass will "turn to" porous copper (the porosity being where the zinc was previously)