Water Tubes
Fire-tube boilers sometimes have water-tubes as well, to increase the heating surface. A Cornish boiler may have several water-tubes across the diameter of the flue (this is common in steam launches). A locomotive boiler with a wide firebox may have arch tubes or thermic syphons. These increase the heating surface and give additional support to the brick arch.
Another technique for increasing the heating surface is to include internal rifling inside the boiler tubes (also known as Serve tubes).
Not all shell boilers raise steam; some are designed specifically for heating pressurised water.
Read more about this topic: Fire-tube Boiler, Variations
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