While the use of such computations will serve where approximate results only are required, that they are approximate should be thoroughly understood.
Calorimetry—An ultimate or a proximate analysis of a fuel is useful in [Pg 184] determining its general characteristics, and as described on page [183], may be used in the calculation of the approximate heating value. Where the efficiency of a boiler is to be computed, however, this heating value should in all instances be determined accurately by means of a fuel calorimeter.
Fig. 23. Graphic Representation of Relation between
Heat Value Per Pound of Combustible and
Fixed Carbon in Combustible as Deduced by Wm. Kent.
In such an apparatus the fuel is completely burned and the heat generated by such combustion is absorbed by water, the amount of heat being calculated from the elevation in the temperature of the water. A calorimeter which has been accepted as the best for such work is one in which the fuel is burned in a steel bomb filled with compressed oxygen. The function of the oxygen, which is ordinarily under a pressure of about 25 atmospheres, is to cause the rapid and complete combustion of the fuel sample. The fuel is ignited by means of an electric current, allowance being made for the heat produced by such current, and by the burning of the fuse wire.
A calorimeter of this type which will be found to give satisfactory results is that of M. Pierre Mahler, illustrated in Fig. 24 and consisting of the following parts:
A water jacket A, which maintains constant conditions outside of the calorimeter proper, and thus makes possible a more accurate computation of radiation losses.
The porcelain lined steel bomb B, in which the combustion of the fuel takes place in compressed oxygen.
