The usual form of ventilating furnace is a plain fire grate placed under an arch, and communicating with the upcast shaft by an inclined drift.
The circular grate G can be turned round K by means of the crank E from the outside.
Dr Grierson well describes its movement: "As a work of art, it has for European readers prolixities and episodes which grate against occidental tastes, but no one can read it in the original without being impressed by it as the work of a great genius.
Deep. B is the chimney-damper, C the grate, D the door for removing the slags, E the ash-door, F the inlet of the air-blast, G the upper, G 1 the lower outlet for the water-gas which is removed alternately at top and bottom by means of an outside valve, steam being always admitted at the opposite end.
The open grate still holds favour in England, though in America and on the continent of Europe it has been superseded by the closed stove.
Az¦awea/ clined" step-grate "; D, a;?
Ft., that the rate of combustion is 150 lb of coal per square foot of grate per hour, that the calorific value is 14000, and finally that n =0.06, the maximum indicated horse-power which the engine might be expected to develop would be o 06 X 150 x14000 X24 X 778/1980000 = I 190, corresponding to a mean effective pressure in the cylinders of 59.5 lb per square inch.
C. Bischof at Magdesprung (both in Germany), consisted of simple perpendicular shafts of masonry contracted at the top and the bottom, with or without a grate for the coal.
And burning too lb of coal per square foot of grate per hour would Table Xx require that 60,000 ib of air should be drawn through the furnace per hour in order to burn the coal.
The effect of this arrangement is that the great body of coal reaches a higher temperature than in an ordinary fireplace, and this, together with the reduction of the carbon dioxide formed immediately above the grate by the red-hot coal in the upper part of the furnace, leads to the formation of carbon monoxide which later on, on the spot where the greatest heat is required, is burned into dioxide by admitting fresh air, preferably pre-heated.
These are almost invariably air furnaces, though sometimes air under pressure is used, as, for example, in the combustion of small anthracitic coal, where a current of air from a fan-blower is sometimes blown under the grate to promote combustion.
Per pound, the maximum indicated horse-power is given by the expression I.H.P. maximum - CcA X778 1980000 where A is the area of the grate in square feet, and is the combined efficiency of the engine and boiler.
_ rying space or urry Kiln Lower shaft containing hot clinker Grate.,› Upper shaft containing raw material FIG.