THE steam boiler, in the same way as the engines it drives, has undergone many and great changes since the days of Newcomen. The different forms owe their origin to a variety of circumstances. The marine water tube boiler, for example, has to be as light as possible, economical in fuel, able to supply steam at a high rate and capable of withstanding high pressures.
One of the best known forms of water tube boiler is the Yarrow boiler, of which six examples are seen above in the maker’s works. These particular boilers were made for the Southern Railway Company’s cross-Channel steamers. The two boilers at the left hand, which are only partly erected, show the principles of construction. Three steel drums are arranged at the corners of a triangle, the two bottom drums being connected to the top one by a number of straight tubes.
The tubes and drums comprise the water space, the working water level being about halfway up the top drum. The holes in the ends of the drums, closed by strong covers when the boilers are working, enable men to get inside the drums and clean the inner surface of the tubes without difficulty. The furnace occupies the tent-shaped space under the tubes, the grate extending between the lower drums but at a lower level.
The boilers are enclosed in steel casings lined firebrick. The casings are in place in the four boilers to the right and are finished at the top with a rectangular opening which is connected by a trunk to the funnel. The hot gases from the blazing fuel pass outwards between and over the tubes and thus reach the upper part of the casing. The gases, however, do some more work before they leave the boiler.
The small drums carried on the water drums have a number of tubes extending from them into the stream of the hot gases. The steam from the upper part of the top drum passes into the small drums, through their tubes, and emerges at the other end of the drums to go to the turbines perfectly dry and hot. This part of the boiler is the superheater. Above the superheater is the air heater, another mass of tubes situated in the gas stream. The air supplied to the furnaces passes through these tubes and the gas flows outside them. The object of heating the air is to ensure better combustion in the furnace, and thus to secure greater economy.
The boilers are fired with coal, but not by hand in the old way. Instead, the furnaces are fitted with automatic mechanical stokers, a recent arrangement in a ship, and the stokeholds are “closed”, that is, air is supplied to them under pressure by means of fans. Each boiler is capable of providing 16,000 lb of steam an hour at a pressure of 250 lb. per sq. in. and a temperature, on leaving the superheater, of 500° Fahr. The air is heated to about 380° Fahr. by the time it reaches the furnaces. The fuel used is Kent coal, the collieries being close to Dover, from which the steamers - train ferries - start for Dunkirk, France. The Yarrow boiler is extensively used also on land.