The coiled tube boiler is a type of Water Tube Boiler in which a cylindrical furnace envelope is formed by multistart, helically-coiled tubes in which water is boiled. Firing is by oil or gas and the burner is provided usually in the base, although sometimes in the roof of the furnace. The output from the boiler is usually saturated steam, but superheat may be provided by returning the saturated steam to a separate coiled section concentrically-mounted within the furnace envelope. Typical arrangements of both types are shown in Figure 1 and Figure 2.
Coiled tube boilers have also been used in nuclear applications, the most significant of which are the pod-boilers in the Hartlepool and Heysham Advanced Gas-Cooled Reactor (AGR) power stations in the UK. In these power stations, the boilers are of the Once-Through Boiler type and consist of concentric multistart helical coils, alternately handed around a central spine which houses the feed water tubes. Above the boiler bank, a coiled reheater is mounted. Hot gas from the reactor outlet is circulated down over the heating surfaces by high-powered blowers situated below the boilers. By using the pod-boiler concept, it has been possible to incorporate the units within the reactor's concrete pressure vessel. The general arrangement of the boilers is shown in Figure 3.