Spiral watertube boiler

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.

The outer casing of the boiler is of steel plates, lined with firebrick, and plays no part in their heating area.

[7][8] Each steam generator tube consisted of three turns, giving a greater heating surface for the number of pipe joints to be made.

An advantage for steamboat use was its rapid steam-raising from cold, the time to reach a useful pressure, rather than the rate of steam generation once hot.

This was a result of two features: firstly the low mass of metal in the heating surface rose quickly to temperature.

The central drum could remain cold after the tubes were in action and boiling, without putting excessive mechanical strain on their fittings.

As the tubes are also closer to vertical, circulation within them is vigorous, they have greater evaporative power per area, and a suitably fired boiler with a liquid fuel burner may still generate a large volume of steam.

[11] The tubes in the Illingworth boiler design are spirals, but wrapped around the central steam drum, rather than sitting alongside it.

There are thus fewer tubes, sixteen in this case, but each one is longer (between 260-280 bore diameters is suggested[12]) and so the boiler is still a dense arrangement of heating area in a small space.