In such a type of boiler it would be impossible for water to extract more than quarter the heat of the fuel. Usually it was much less. The next step was to make what is called a return tubular type in which the heat of the burning gases is conveyed to the rear end of the boiler, and then returned to the front end through tubes.
[Fig. 4] shows this construction. The head of the shell holds the ends of a plurality of tubes, and the products of combustion pass through the conduit, below the boiler to the rear end, and are conducted upwardly to the tubes. As all the tubes are surrounded by water, it will absorb a large amount of the heat as the gases move through, and before passing out of the stack.
The Cornish Boiler.—One of the most important inventions in the generation of steam was the Cornish boiler, which for many years was the recognized type for marine purposes. It had the advantage that a large amount of water could be carried and be subjected to heat at all times. Aside from that it sought to avoid the great loss due to radiation.
It will be seen from an examination of [Fig. 5] that the shell is made very large, and its length does not exceed its diametrical measurement. Two, and sometimes three, fire tubes are placed within the shell, these tubes being secured to the heads. Surrounding these fire tubes, are numerous small tubes, through which the products of combustion pass after leaving the rear ends of the fire tubes.
In these boilers the tubes are the combustion chambers, and are provided with a grating for receiving the coal, and the rear ends of the tubes are provided with bridge walls, to arrest, in a measure, the free exit of the heated gases.
These boilers would be very efficient, if they could be made of sufficient length to permit the water to absorb the heat of the fuel, but it will be seen that it would be difficult to make them of very great length. If made too small diametrically the diameter of the fire boxes would be reduced to such an extent that there would not be sufficient grate surface.