Spiral watertube boiler
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Spiral water-tube boilers are a family of vertical water-tube boilers. Their steam generating tubes are narrow spiral tubes, arranged in circular fashion around a central vertical water drum.
The water tubes are long, numerous and of 2 inches (50 mm) diameter, or smaller. This gives an extremely large heating area, and so good steam raising capabilities. "A maximum fire surface is obtained in a given space, and great economies in fuel are thereby made possible."[1]
As both drum and tubes are of small diameter, the boilers are also suitable for high pressure use. However the boilers entered service early on, as one of the first small-tube water-tube designs and so they have rarely been used for high pressures or with steam turbines.
Differences between these types of boilers are in the arrangement of their water tubes. They all share the same basic layout. The outer casing of the boiler is of steel plates, lined with firebrick, and plays no part in their heating area.