BOILER ACCESSORIES 131
oxygen, is practically the same as that of the oxygen before it entered into combination, always providing the temperatures are the same. Hence the volume of the hot gases is practically the same as the volume of the air supplied to the furnace, and the whole of the gases, consisting of the carbonic acid formed, the unchanged nitrogen and oxygen, have to be raised to the same temperature, the result being that the temperature at which the hot gases leave the burning fuel is never greater than about 2400° F., instead of being in the neigh- bourhood of 4000° F. Chimneys are required for two purposes-to furnish the neces- sary draught, and also to carry off the products of combustion, the hot gases, etc., and to deliver them to the atmosphere at a height where they will do little harm. When the chimney is employed only for delivering the products of combustion harmlessly into the neighbouring atmosphere, it is evident that the height of the chimney may be very much less in a great many instances, than where it is also required to furnish the necessary draught, and one reason why chimney draught is being superseded by the other forms of draught, in places where comparatively low chimneys may be fixed, is because the cost of the chimney itself is often a serious item in the outlay, and because the chimney demands, for furnishing the draught, something like 25 per cent. of the total energy delivered to the hot gases at the furnace. It will be understood that the heat carried by the hot gases is measured by the product of their weight into their temperature, and into their specific heat, and as they set out with a temperature of 2400° F., and are delivered to the chimney at 600° F., 25 per cent. of their energy is used in the chimney; whereas if the chimney was merely an apparatus to deliver the hot gases harmlessly, the only expenditure of energy necessary in the chimney would be that required to overcome the friction of the hot gases on the sides of the chimney. The waste involved in delivering the gases to the chimney at 600° F. may be better appreciated by converting the heat units which they carry into mechanical energy. There is hardly space to reproduce the calculation involved, but remembering that each unit represents 778 foot-lbs. of work, and that each pound of the gas carries off approximately 143 heat units, the enormous amount of energy passing up a chimney will easily be understood. It has been computed that the actual work done by the hot gases, in creating the draught, is only 0.00056 of the amount of energy contained in the gases themselves. Sizes of Chimneys and Horse-Power of Boilers The chimney designed to furnish draught for a boiler, or battery of boilers, has two sets of dimensions, both of which are equally
