_The facility of the fuel's_ descent is dependent largely upon the nature and the size of the coal employed. Porous coal gives better results than dense and compact coal. It is therefore preferable to employ screened coal free from dust in pieces each the size of a hazel-nut. The various sections given to the interior, including as they do cylindrical forms, truncated at the summit or the base, partially truncated toward the base and the like, would lead to the conclusion that this question is not of the importance which some writers would have us believe. Still, it must be considered that if the fuel drops slowly, its prolonged detention within the walls of the hopper and its transformation into fusible slag may result in a disintegration of the refractory lining of the furnace.
The quantity of steam injected, greater or less, according to the nature of the fuel, renders it possible to obtain friable slags and consequently to prevent grave injury to the retort. Red-ash coal is in general fusible, containing as it does some iron. Its temperature of fusion varies between 1,832 to 2,732 degrees F.
_Cleanliness_ is most important so far as the operation of the generator is concerned. It should be possible to scrape the generator during operation without changing the composition of the gas, when the incandescent zone is chilled, or an excess of air is introduced, or the steam-injector be momentarily thrown out of operation. Mechanical cleaners with movable grates or revolving beds have the merit of causing the ashes to drop without interfering with the operation of the apparatus. The same meritorious feature is characteristic of ash-pits having water-sealed joints.
Pressure gas-generators need not be as perfectly gas-tight as suction apparatus. Leakage of gas, which is usually manifested by a characteristic odor, results in a loss of consumption and renders the air unfit to breathe.
A generator should be provided in its upper part with openings through which a poker can easily be introduced in order to shake up the fuel and to dislodge the clinkers which tend to form and which cause the principal defects in operation, particularly with fuels that tend to swell, cake, and adhere to the furnace walls when heated. Many apparatus, moreover, are provided with lateral openings having mica panes through which the progress of combustion can be observed (Fig. 79).