Are water-tube boilers actually easier to build?

[fredrosse]

Marinesteam is providing proper advice. The technical aspects of pressure vessel design, especially in boiler design, is best left to already proven designs, or those designed by professionals. Yes, many homemade boilers do exist, and fortunately accidents are rare, but that might change easily.

It seems that your last post describes a Roberts Boiler, very popular in steam launches for over 100 years. There are many examples already built.

"Could this work to power a small single or two-cylinder compound for a small steam launch? ANS: yes, this has been done, almost certainly hundreds of times. If you look into the FAQ section of this forum, you can determine the size of things required starting from scratch. I have seen Roberts boilers suitable for steam engine powers ranging from a small fraction of a horsepower all the way up to the 100 horsepower range. But the FAQ section of the forum will not in any measure, teach you how to design a pressure vessel.

"This is a theoretical exercise in finding what is the simplest and smallest boiler for this purpose." ANS:
The smallest (and simplest) boilers I know of are in the high speed (well over 100 MPH) steam hydroplanes, getting about 5+ horsepower out of a boiler with outside dimensions about 4 inches in diameter and 12 inches long. That is however not anything close to a proper choice for a steam launch. Many factors need to be considered to arrive at a reasonable boiler design, and following accepted practice is highly recommended.

[hartleymartin]

Copper end caps rated to 3,500psi are available in Australia only up to 5" diameter... drat. I rather wanted a fairly large steam-drum which would act as a steam reserve. Theoretically, this means that the boiler would be able to withstand up to 35 times working pressure.

At the moment I'm looking around to match ready-made components to a proven design if I can find something suitable.

[barts]

Copper's strength disappears very quickly with elevated temperatures.

The ASME boiler code says:

Copper and brass pipe for water and steam service may be used for design pressures up to 250 psi (1750 kPa) and for design temperatures to 406°F (208°C).

I don't have a copy handy here, but if I remember correctly the allowable stress is on order of 2500 psi at 406F, so large cross- sections are out.

- Bart

[Lopez Mike]

O.K., I'll be the bad boy here.

Flame on:

DO NOT DESIGN YOUR OWN BOILER!!!!

It is an exercise for an experienced and educated and trained designer.

And then it should be turned over to an experienced and trained welder with expertise in this field. You will have grief, frustration and at the minimum extend your project such that we will never get to go steaming with you. And if, on the odd chance, you have a really bad experience like a catastrophic failure, you will attract the attention of the authorities in way we do not want or need.

You are frightening the daylights out of us and we have been trying to get the message across to you politely. You have no idea of the forces you are dealing with.

Flame off:

[hartleymartin]

I should explain that I am an INTJ personality type. I love to theorise about it. I'd get a proven design built, or at least get a proper engineer to ok any ideas that I have before actually trying this sort of thing.

I AM NOT HACKING MY OWN STEAM BOILER!

I might be crazy, but I'm not stupid.

[hartleymartin]

I'm rather attracted to the first boiler on this page:

http://www.nmpproducts.com/boiler.htm

[Lopez Mike]

Good that you are not crazy. Not that it's any qualification for being a steam nut. Most of us are unbalanced in our own different ways.

Keep in mind that a match between the boiler and the fuel is something to be considered. A boiler with a smaller amount of water reacts more rapidly to variations in firing. A solid fuel fire is a sluggish thing and if there is an interruption in feed water it can be difficult to reduce the fire as much as you would like! I'm being sly about this. It can be terrifying.

So solid fuel and a FT unit work well together, especially when there are other things to do besides tend the power plant. It is easy to forget that a steam boat is a tool of navigation and that you will be black gang and deck crew and navigator and tour guide and I don't remember what all else. You will have attractive women on board. Amazing how the situation (the power plant, I mean) can get out of hand!

The advantages of a WT boiler with its usual smaller reserve of water seem to fit better with a quickly adjustable heat source like oil. Suddenly need to stop? Got a feed water glitch? Just turn the beast down.

My love affair with launches is connected with experience with donkey boilers in stationary service, running locomotives and, of course, watching The African Queen. So I'm partial to round boilers and wood. Nothing much rational about it.

[barts]

If we're going to discuss crazy, I continue to be fascinated by the potential of pumped circulation boilers. Using a schedule 80 A106 pipe pressure washer coil 18" in diameter, it is possible to absorb 400K btu/hr w/ forced circulation of 3.5 gpm. This is in excess of 300 lbs steam/hr at 300 psi - rather more that most of us need. Combined with a simple (ASME code approved even!) tank w/ dry pipe, etc. and a pressure balanced electrically driven reciprocating pump (using stainless plungers and Teflon packing), you could end up with a reasonably compact and efficient light weight boiler.

Yes, this isn't a traditional boiler, and the use of electricity will definitely offend the purists and the preppers .... but for larger boats such a design definitely has its attractions. Yes, the pump would be of large displacement so it could run at a reasonable
speed, both for reduction of noise and to prevent cavitation due to low NPSH, and the packing might take some time and effort to sort out - but it remains an interesting alternative. Such 'La Mont' boilers have been used with good success in steam cars, and of course have been used in industry.

- Bart

[Lopez Mike]

I knew that as soon as the subject of craziness came up, it would drag Bart out of the woodwork.

I can see the new version of African Queen. "Keep your head down Rosie while I reboot the microprocessor!"

[barts]

I knew that as soon as the subject of craziness came up, it would drag Bart out of the woodwork.

I can see the new version of African Queen. "Keep your head down Rosie while I reboot the microprocessor!"

The only microprocessors will be in the GPS, the refrigerator, the voltage regulator and the solar charger, and

A traditional code VFT capable of 200 lbs/hr steam at 200 psi weighs prob. 1000 lbs in running trim. I think being able to
reach the same level of performance w/ a lot less weight and still meeting code is attractive. Note that there's nothing
keeping one from having a nice large steam drum other than a desire to keep weight down.

The fact that an electric motor is needed to start the boiler seems to me to be an acceptable tradeoff.
With a low rpm pump, you can even have a hand crank to start it ! If you prefer to run the pump w/ steam while
underway, that's quite feasible, too.

- Bart