In the case of horizontal centre flue boilers - like the standard Roundhouse / Accucraft etc., internally fired types, there are a couple of issues pro and con having these transverse water tubes.
The main "pro" is that these transverse tubes can certainly increase the circulation and improve the steaming rate, and lower gas consumption.
The "cons" mostly are about leaks. The repair of leaks is one - Harry and I discussed this here a year or two ago. Two problem possibilities - having a pinhole leak in a tube joint during construction - this may not show until the boiler is completed and tested, and the leak inaccessible. The second is having a leak develop after the boiler has been in service for a while.
The former can be generally avoided by cleaning the flue tube and cross tubes thoroughly before silver soldering, pickling thoroughly before silver soldering, and then cleaning and pickling well afterwards and inpsecting thoroughly with a lens. This will catch most pinholes, which can be fixed easily at this stage. I also always make the flue assembly too long, and silver solder a bit of copper plate across both ends, with a pipe conection in one, so I can hydraulic test the flue by filling it with water and pressurising from the inside before assembling the rest of the boiler. This usually shows any leaks. The excess length and the temporary ends are then cut off. The rest of the boiler is then completed normally. By the way, silver soldering in this context is using a torch to "Silver Braze" the joint at around red heat, using a silver solder rod with 25% silver or higher. It is not using the silver bearing soft solder that can be bought - that is a disaster in this context.
Developing leaks in service is usually the result either of not doing the above, or else overheating the boiler by running it dry, with a powerful gas burner. The cure for that is simple - Don't do it! Many commercial boilers do not use cross tubes simply to obviate possible problems with people unaware of this possibility of damage by running dry. It is also commercially cheaper.
The other "cons" are a bit more subtle.
If you pack too many or too large cross tubes in then you may throttle the flue tube and actually reduce the available heat, and upset the burner.
Do not use either horizontal or vertical cross tubes in such a centre-flue boiler. Vertical tubes tend to throw water into the steam space, leading to the possibility of water being carried over into the cylinders. Horizontal ones do not circulate, leading to surging and spurting, iwth similar effects to the vertical ones. I always use tubes angled about 45 degrees either side of vertical, and inclined alternately in opposite directions, so looking down the flue you see a sort of "X".
Jim Gregg.
.At 11:47 PM 4/1/04 -0600, you wrote:
At 10:52 PM 4/1/04 -0600, you wrote: > I know some don't like to use them because they are difficult to repair >if a leak should occur but I was not aware there were other 'concerns' >Pete H
That's pretty much it. But they can be made as relatively bullet-proof as any other part of the boiler by giving a little thought to the material used, location, and installation. I think cross tubes and porky-pine heat sinks have a great potential for Ga1 which hasn't begun to be utilized yet and coupled with the advances in burners can make ferocious little steam generators.
>does the trouble lie with the varied expansion rates of the differant sized tubes?
In small scale practice I'd say it usually has to do with heat working on too thin or poorly located tubes and/or poorly soldered joints.
>The reason I ask is because I have been looking at designs of vertical >stationary boilers and was leaning towards one with "L" shaped water >tubes.
Quite a common arrangement.
> . . . . . . there is apparently more to it than just drill/reaming holes and brazing tubes into place?
I would say one important consideration in that instance, in addition to using good boiler building technique, would be to use water tubes which are as thick-walled as you can find, certainly thicker than standard soft copper plumbing tubing.
Regards, Harry
