Steamboating comes of age with the

DEVELOPMENT OF AMODERN STEAMBOAT

by Wally Mounster

PART 1 THE MONOTUBE BOILER

8 MODELTEC Magazine - May 1987

The design of my steamboat boiler owesa lot to experience gained with a Stanleysteam car which had been fitted with anundersized and inadequately-controlledmonotube boiler.The problem of controlling a boiler on a

boat is much easier than on a road vehicleas the boat does not need to climb hills orstop suddenly at traffic lights! The use ofsolid fuel on a conventional grate is notpractical for a motor vehicle; however, it issatisfactory in a boat where load changesare much less sudden, and in the event ofan unplanned stop, the energy containedin a hot fire may be stored by allowing thepressure to rise. Amonotube boiler, by vir-tue of the fact that it has no large diametervessels, may carry a very high pressure asthe stress in a pressure vessel increases inproportion to the square of it's diameter.By way of explanation of this, my boatruns best with steam pressure of 400 psi.The hydraulic test pressure for the tube is5000 psi. If pressures of this magnitudesound frightening, be reassured by the factthat the volume of steam and water in theboiler is very small and a burst tube is theworst that can happen, in which case theescaping steam is easily contained and con- .trolled by the casing.The real control problem with a

monotube boiler is to ensure that the rightamount of water is pumped into thecoil-too much and the boiler primes, toolittle and it runs out of steam and over-heats. I have devised an electronic devicewhich senses the steam temperature andalters the stroke of the feed pumps to givemore or lesswater as required (this will bedescribed in another article). The benefitof this little piece of complexity is that onedoes not need to constantly watch a gaugeglass.Many diagrams of monotube boilers

have been published at different timeswithout any explanation by their authorsas to how the tube is to be bent into theshapes shown and retained in that shape.When designing a monotube boiler,thought must be given to the following ~e-quirements:

1. It must be possible to bend the tube tothe required shape without complexand expensive formers.

2. Small-bend radii must be avoided.3. There must be a minimum number of

welds.4. Allwelds must be accessiblefor repair if

they leak when tested.5. Spacing and retaining devices must be

kept to a minimum.6. The layout of the tubes must encourage

efficient heat transfer by enclosing thefire as much as possible and by offeringthe hottest steam to the hottest part ofthe fire. By this I mean that thesuperheater should be close to the firewhile the economiser section containingthe cold feed water should be in thearea where the flue gas is about to leavethe boiler casing.After much deliberation, I decided to