This is a class of boiler characterized by water flowing through arrangements of tubes over which the heating fluid is constrained to pass. Water-tube boilers are best suited to relatively high pressure operation (>40 bar) and have good response to changes in load demand.
Tubing is arranged to suit the type of heating fluid, its temperature and pressure, and heat transfer characteristics coupled with the operating conditions of the boiler water. For example, fossil-fuel fired boilers for power generation are normally of the water-tube type, as are Heat Recovery Boilers and those associated with nuclear power stations. Most modern industrial boilers are also of the water-tube type. Typical layouts of units not specifically described in other entries are shown in Figures 1 and 2.
In cases where the heating fluid is a liquid, the internal and external heat transfer coefficients can be arranged to be high and plain tubing can produce economic designs. However, when the heating fluid is a gas, the external heat transfer coefficient can be an order of magnitude lower than the internal one and a degree of surface enhancement is desirable on the outside of the tube. Such enhancement is normally achieved by use of helical (either continuous or serrated) fins in clean gas environments as in heat recovery boilers or by use of large plate fins as are often found in economizers when heat is being recovered from dust-laden gases (see Figure 3). Illustrations are by permission of Mitsui Babcock Energy Ltd.