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[kiwimodeller]

Greetings, for a while now I have been running a boiler built to a design by A H Rayman, 9" by 4" dia. with a 1 & 1/2" I. D. flue. A friend and I now want to build a couple more for other boats. I would like to make one of the a return flue one, partly because that would put the funnel in a better position and also because I thought it might put more of the heat in to the water and therefore make the boiler more efficient. We were also going to make a smaller version of the single flue boiler, probably 8" by 3". Any ideas on what size flue to use in each of these would be welcome, we had thought about 1 & 1/8 or 1 & 1/4. Does a small flue slow the gas flow and allow more time for the heat to transfer to the water or is this just theory? How do you balance the need to draw air through against the need to transfer heat? It does seem that the big boiler is wasting a lot of heat up the funnel. Any input welcome. Thanks, Ian.

[SteamboatPhil]

Hi, just a quick point Alans boiler at 9 inches is too long and should be around 7 inches (his words not mine) so you could make a shorter one, would that help the postition of your flue ? (it also will steam better)
Return flue boilers do need a large flue to help draw the flame through.
Phil

[kiwimodeller]

Thanks for that Phil. I had wondered about that. The big boiler gives a good run time even if I am not using the engine pump but it does take a lot of getting up to pressure initially and a lot of heat to keep it there. I will try a shorter version and a return flue version. My friend Bill who builds the engines is on to his third Rayman engine so we have enough testbeds! Any thoughts on the best burner? I have been using a large Primus burner on the paddler and it woks well but I need to run two gas cannisters in tandem so they do not ice up as it draws gas off fairly rapidly. Even running a copper strip to the cannister does not stop the icing up if I run only one cannister. I have made some ceramic burners for smaller boilers and was thinking of making or buying one for the big boiler but I have some doubt about whether it would result in any more heat or any less gas consumption. Your thoughts would be welcome. I would love to try a Flash boiler but all the discussion here and on other sites scares me with the complication needed to control  one if it is not a fast boat. One day maybe> Cheers, Ian.

[malcolmbeak]

Ian V
I've made a number of return flue boilers. I usually try to make the area of the return flues slightly larger than the main flue, and the outlet flue slightly larger again. Using a small flue is just likely to make the burner blow back. I always use a poker type and take practically all the air in as primary (next to the jet) and only a little as secondary. Hopefully the sketch and additional notes shows what I mean.

I've also added a sketch for a typical return flue design. It shows two layouts of return flues. One thing to watch is not to place the flues too close together – water must be allowed to get to them. I did build one with as many flues as I could jamb in with very small space between them. It should have produced vast quantities of steam, but it didn't. I believe that water could only contact the tubed around the outside of the whole bundle!

This boiler which is based on the above design is for a ½” bore and stroke twin, so only has 4” heated length. As you can see, I’ve had to arrange the outlet so that it will line up with the funnel. I find the easiest way is to use plumbing fittings – there are three 90 degree bends in this one. If it comes back over the top of the boiler, remember not to get any of the fittings in the way,  (I seem to remember that there is a blanking plate just to the right of the safety valve on this one).


This is just a sketch of how I fit the burners. It was made years ago, but in general the notes are still relevant. Ignore the note about the Treetower jet – they went out of business years ago. Nowadays I use Primus jets . Or you could make up one with a needle valve. If you can get the Primus jets, I would suggest you get a range from No. 6 up to No. 20 and just see what works.
You may find it useful to fit a simple slide for the primary air holes. I light the burner down the funnel, and sometimes have found it best to close off some of the air supply, but once warm, there is no need. Another way is to fit a pizo electric lighter next to the burner tube – but be warned, this can make a noise like a rifle going off!

[kiwimodeller]

Malcolm, thanks for all that. Just to make sure my understanding of it all is correct this is what I take from your words and diagrams :-
1) You use several small return tubes with a total area just a little more than the area of the primary flue. (I had only thought of the type which has single large tubes for both primary and return with water cross tubes in one or both - what do you call that type of boiler?) Do you put water tubes across any of the flues?
2) You use the poker type burner with saw cuts inside the primary flue. I have used the complete Primus nozzle in the past with the gas/air mixture being set by the size of the jet in relation to the air holes in the nozzle. This is the device Primus sell to plumbers for heating up copper pipes for soldering or bending. An approx 1" dia heating tip is what I use in the flue but I have also used a smaller soldering version as a mixing device below a ceramic for small pot boilers. Obviously you are not blasting the flame in like I do, yours must burn more gently and rely on the draft to draw the hot gases through?
As discussed with Phil the engine I am primarily using is the Rayman 3/4 by 5/8 double acting single, the plans for which show a 9" by 4" single flue boiler but the designer is now saying that is too large which is good as the point of this exercise was to get a boiler which took up less room in the boat. Thanks for your help. Regards, Ian.

[kno3]

Hi Malcolm, I was wondering, what is the "inner dome" for, shown on that boiler sketch, where the pressure gauge is to be fitted?

[malcolmbeak]

Ian, The reason for having many small tubes is to increase the heating surface, and also on a relatively short length with a large tube, a lot of the hot gas can pass through without getting near the surface. The boiler I had in the picket boat is roughly the same size as in the sketch, and when running at 30psi, the temperature of the exhaust gas was around 140 C which I found surprising as the temperature of the boiling water at that pressure is 134 C. So it seems as though the efficiency is pretty reasonable.
There are no cross tubes in the main flue as that would stop me using the poker burner. Another reason is that built the way shown, all the joints are accessible. With this type of burner, there a whole lot of small fan shaped flames impinging directly onto the heating surface. In order that all the slots give a flame, practically all the air has to be taken in as primary, otherwise there would not be enough getting to the furthest slots. As you can see, the end of the flue is blanked off with only a couple of small holes for the secondary air. In fact, the burner will not light if it is out of the boiler. I feel that the blowlamp type of burner you are using just blasts down the main flue without doing much heating at the input end. 
I don't know about gentle heating, it's just possible to look into the flue through the secondary air holes, and when working well, all one can see is an intense purple haze.
As you can see from the photo a few posts ago in the picket boat thread, the picket boat goes very well. There it is running at about 30 psi and the engine is the 5/8" bore and stroke twin  also pictured.

Kno3, the "inner dome" is a baffle surrounding the water level probe. With fast boiling, the actual water level is rather difficult to detect, and as you can see there are a couple of 1/8" holes top and bottom so things are rather calmer around the probe.