In order, therefore, to utilise the entire space of the drying-room, it is necessary to place the heating apparatus in such a position that the whole of the room will be warmed as uniformly as possible. The stove should therefore be situated in a chamber underneath the drying-room proper.
Fig. 20.
Because air that is already saturated with moisture cannot take up any further quantity, care must be taken to remove the damp air continuously from the drying-room, and to replace it by dry air. This may be effected by suitably designed ventilation, on the lines shown in [Fig. 20], which represents a drying-room arranged in such a way as to provide for all the above-mentioned contingencies, and ensure continuous drying.
The heating apparatus is located in the cellar, and consists preferably of a slow-combustion stove comprising a cast-iron cylinder, with an air inlet A (with sliding regulator T), for the air of combustion, and a shoot F at the top, through which the stove is fed with fuel—preferably coke, on account of its great heating power.
The stove is surrounded by an iron or brick shell M, having two flues R and R1 leading to the chambers I and II, where they terminate in register cowls K, which can be adjusted, by turning the handles h, so that when the slots o in K coincide with corresponding slots in the end of the pipe, the maximum amount of hot air from the stove is delivered into the drying-chambers; and, by suitably adjusting the cowls and the draught through the fire-door T, it is possible to regulate the temperature of the chambers to within one degree of the thermometer scale. When only one of the drying-chambers is required to be heated, the register in the other is closed, and the whole of the hot air is delivered to the first one. With this arrangement none of the heat is wasted, and the contents of one chamber can be dried while those of the other are being removed and replaced.
The moisture-laden air from the drying-chambers can be led direct into the stove chimney. When coke is used, the flue gases consist almost entirely of carbon dioxide. If the vent pipes are led from the top of the drying-chambers into the chimney, the hot gases ascending the latter induce a strong draught in the chambers and carry off the moist air into the open. These pipes, also, are fitted with registers, which, when suitably adjusted, assist in the maintenance of a uniform drying temperature.
The colours to be dried are spread on trays laid on suitable racks in the drying-chambers; and, by carefully planning out the available space, a very large quantity of colour can be quickly and completely dried in a comparatively small plant. The cost of the fuel is so small as to be more than counterbalanced by the saving of time.
The heating arrangements in drying-rooms are capable of improvement in many respects, especially where steam is at disposal; and in such cases, it is better to substitute steam heating for a fire. It will then be necessary to put in a good fan, or other device, to ensure the removal of the moist air. An excessive room temperature—above, say, 50° C. (122° F.)—is not only superfluous, but in many cases injurious, because, apart from the fact that some colours change in shade when over-warmed, an unduly high temperature causes the surface layers to dry very quickly and form a crust which prevents the escape of water vapour from the interior of the material.
Another form of drying-plant for earth colours is the drying-floor, a large room with a rammed concrete or stone floor, intersected with brick flues (about one foot square), covered with iron or concrete slabs and conveying hot flue gases from a furnace. These floors are particularly suitable where there is a possibility of utilising an existing supply of hot flue gases.