were readily accessible, which made for easier inspection and maintenance or replacement Water-tube boilers with longitudinal steam drums, as in Figure 3,[4] were developed to allow increases in generated steam pressure and increased capacity. The water-tube boilers, in which water flowed through inclined tubes and the combustion product gases flowed outside the tubes, put the desired higher steam pressures in the small diameter tubes which could withstand the tensile stress of higher pressures without requiring excessively thick tube walls.[1]
The relatively smaller steam drums (in comparison with the fire-tube shells) were also capable of withstanding the tensile stress of the desired higher pressures without needing excessively thick drum walls.
The water-tube boiler went through several stages of design and development. The steam drum was arranged either parallel to the tubes (as shown in Figure 3) or transverse to the tubes, in which case the boiler was referred to as being a "cross drum" rather than a "longitudinal drum" boiler. Cross drum boilers could accommodate more tubes than longitudinal drum boilers and they were designed to generate steam pressures of up to about 100 bar and at rates ranging up to about 225,000 kg/hour.
The next stage of development involved using slightly bent tubes, three to four steam drums and one to two mud drums at the bottom of the tubes (see Figure 4). The three sets of bent tubes, as shown in Figure 4, each represent a bank of tubes extending from the front of the steam drums back to the rear of the drums. Thus, the longer the steam drums, the more tubes were available and the more heat transfer surface was available. The tubes were bent slightly so that they entered and exited the steam drums radially. Baffles made of firebrick forced the flue gas to travel upwards from the mud drum to the right-hand steam drum and then downwards from the middle steam drum to the mud drum and finally upwards to the left-hand steam drum and out the flue gas exit in the upper left-hand corner. in essence, as shown in Figure 4, the baffles created a multi-pathway for the flue gas.