The disadvantage, too, of the deposits mentioned in the plain cylinder is, to a great extent got over in the Cornish boiler, for the bottom, where the deposit chiefly takes place, is the coolest instead of being the hottest part of the heating surface.
But the disadvantage of unequal expansion also exists in this type of boiler, as the internal flue in the Cornish system is the hottest portion of the boiler, and consequently undergoes a greater lengthways expansion than the flues. The result is to bulge out the ends, and when the boiler is out of use, the flue returns to its regular size, and thus has a tendency to work loose from the ends to which it is riveted and if the ends are too rigid to move, a very serious strain comes on the points of the flue.
Even while in use the flue of a Cornish boiler is liable to undergo great changes in temperature, according to the state of the fire; when this latter is very low, or when fresh fuel has been thrown on, the temperature is a minimum and reaches a maximum again when the fresh fuel commences to burn fiercely. This constant expansion and contraction is found in practice to also so weaken the tube that it frequently collapses or is pressed together, resulting in great disaster.
This led to the production and adoption of the—
Lancashire Boiler, contrived to remedy this inconvenience and also to attain a more perfect combustion, the arrangement of the furnaces of which is shown in [fig. 19 and fig. 20].
It will be observed that there are two internal furnaces instead of one, as in the Cornish type. These furnaces are sometimes each continued as a separate flue to the other end of the boiler as shown in the cuts; but as a rule they emerge into one internal flue. They are supposed to be fired alternately, and the smoke and unburned gases issuing from the fresh fuel are ignited in the flue by the hot air proceeding from the other furnace, the fuel in which is in a state of incandescence. Thus all violent changes in the temperature are avoided, and the waste of fuel due to unburned gases is avoided, if the firing is properly conducted.
LANCASHIRE BOILER—Fig. 18.
The disadvantage of the Lancashire boiler is the difficulty of finding adequate room for the two furnaces without unduly increasing the diameter of the shell. Low furnaces are extremely unfavorable to complete combustion, the comparatively cold crown plates, when they are in contact with the water of the boiler, extinguishing the flames from the fuel, when they are just formed, while the narrow space between the fuel and the crown does not admit the proper quantity of air being supplied above the fuel to complete the combustion of the gases, as they arise.