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Doble Water Tube Boiler (monotube)Doble Steam Motors automobiles, built from 1924-1931, are considered by many to be the finest steam vehicles ever produced, with the boiler figuring significantly in this evaluation. An electric fan and igniter made the burner faster and easier to start and the small volume of fluid in the water tube boiler reached operating temperature and pressure in much less time than the Stanley. Doble imagesThe Doble is a once-through boiler, water entering one end and heat applied until emerging from the other as steam in contrast to recirculating boilers which cycle water endlessly through a heating loop, extracting only that portion converted to steam. The Doble is also a monotube boiler, meaning there is only one tube for the water to pass through rather than a number of parallel tubes.*The small water volume provides little reserve power in excess of maximum steady state output.*Superheating in a tube only starts above the water level; minor level fluctuation changes the length of the portions generating and superheating steam, making accurate control difficult.*The peak firing rate is limited by DNB and a circulation ratio of 1. Notwithstanding the above tradeoffs, monotube boilers have been the favored steam car boiler for generations and only relatively recently are we seeing a shift.
The Doble monotube is composed of a number of helical ‘pancake’ coils, so named because the tube windings comprise flat discs stacked upon one another and connected end-to-end. Water entering a ‘pancake’ from the outside spirals inwards until reaching center, moves upwards to the next ‘pancake’ and then spirals outwards. See image o
