The pans are fitted with conical covers of sheet iron, through the centre of which pass iron spindles, geared above to the pinions of the shafting by bevel-wheels, and resting on the bottoms of the pans, in which they are free to turn. These spindles are attached at their lower parts to arms or stirrers, carrying scrapers swinging loosely beneath them, and resting on the bottoms of the pans. The covers are fitted steam-tight upon the tops of the pans, and each is provided with one or more manholes, by which workmen can enter to clean the pans. Those parts of each cover corresponding to the parts left open in the sides of the pans are brought down so as to partially close the openings and come just low enough to dip into the brine about 2 in., when the pans are about } filled, while the spindles passing through the covers turn in stuffing-boxes. Thus, when the pans are closed, they are steam-tight, and there is no exit for the steam unless by forcing the water out of the pans into the troughs, or passing off by the flues. Each pan is fired by 3 fires, and boiled as for fine salt, while the spindle carrying the arms and scrapers is made to rotate.
The incrustation of the -pans is thus for the most part avoided, while very fine salt is produced, and is swept by centrifugal motion into the troughs, whence it is continuously ladled with a scoop, drained on "hurdles," and sent to the stove or the butter-salt bins, as the case may require. The gases from the fires under the pans, and perhaps from the fire of the engine, are made to pass to the flues beneath the outer pans. Both the pans which are heated by the steam stand on short brick or iron columns without flues; the pans taking the waste gases are set upon winding flues such as already-described as being in frequent use in France.
Sometimes an ordinary boiling-pan is mounted with a fishery-salt pan behind it, so that the flues from the former passing beneath the latter, this pan also becomes heated by the waste gases. The Cheshire Amalgamated 'salt Co. have some interesting and peculiar composite pans, known as "clay" or "tank" pans, also working on this principle. Fig. 121 represents a ground plan of this arrangement, and Figs. 122, 123, 124, are transverse sections on the lines D E, F G, B C, respectively. The boiling-pan a is placed with its upper edge on a level with the ground or barely above it. It is of the usual depth of 1 ft. 9 in., and of the form shown. The fishery-salt pan b utilises the waste heat -of the furnace-gases, after they leave the flues beneath a. There are 3 fire-places /, and 3 flues e, beneath a, together with 2 dead flues. Alongside of and parallel with the pans a b, is a pit or trench c, about 4 ft. deep, 10-12 ft. wide, and 38-40 ft. long. It is puddled with clay, and lined with bricks throughout the sides and bottom. The upper edges of this trench are about 4-5 in. below the level of the upper edge of the pan a. A parting wall of brickwork also divides this trench c longitudinally into 2 compartments of equal width.
This wall, however, only goes to within about 10 ft. of the end of the trench farthest from the fires, and to within 2 ft. of that end which is in a line with them. The side of the pan a turned towards the trench is cut out at the end farthest from the fires, and a shallow channel of sheet iron, just as deep as the pan, connects it with the double trench, while the space k contained between a and the trench is filled up with a bed of masonry, the surface of which slopes gently from the upper edge of a towards c, so that the waste brine from any salt drawn on to it may drain into c. k is connected with d by a short wall, and a pump is placed at h, while another sheet-iron channel, only 2 ft. wide, but of the same depth as a, leads between the pump and the pan a. There is a small pit g, made of masonry, at the end of this channel; and at the end of the parting wall, at d, is a flat space just large enough for a man to stand upon to look after the pump when requisite. With this arrangement, if brine be poured in by the brine-pipe i, c will be filled, and if the influx of the brine be continued, a and b may be filled till c is nearly overflowing, and a becomes full to within 4-5 in. of its upper edge.
If then the pump h be worked so as to lift the brine from c and cause it to fall into g, it will flow back into a, and, circulating through a, will pass again into c; thus a steady circulation of the brine may be maintained in the directions shown by the arrows on the ground plan, so long as the pump is kept going. If then the fires f, Fig. 124, be lit, the brine will be heated in a, and, circulating in the manner described, expose a large evaporating surface. The heat is so managed in these pans as to produce butter-salt in a and common salt in c; while at g, where the pump produces constant agitation, very fine salt is formed. Around the clay pan, the butter-salt pan, and the fishery-salt pan, are the usual paths for the circulation of the workmen, and the places for the so-called "hurdles" m upon which the salt is thrown to drain. The stoke-hole is below the level of the ground. The fishery-salt pan 6 may be mounted on columns of brickwork or cast iron without separate flues, and the chimney at the end of this pan carries off the furnace-gases. These pans seem to produce very fine qualities of salt, particularly the common salt from the pit c.
The yield is about the same (as regards weight of salt to weight of coal consumed) as with the ordinary pans, but the repairs are somewhat less, and certainly the qualities of salt produced are very fine. The chief drawback to them is a rather greater tendency of the pan a to become coated with scale, than in the case of the ordinary butter-salt pans.
Otto Pohl's arrangement consists of 2 superimposed pans,at one end of which the fires are placed; the heated gases, passing between them to the chimney at the other end, heat the upper pan from below in the ordinary way, while they sweep the surface of the brine in the lower pan, which thus constitutes the bed of this portion of the flue. Figs. 125 to 130 show this arrangement in ground plan, longitudinal and transverse sections, and in side and end elevations. Milner, of Marston, near Northwich, has a pan mounted on this same principle, which Pohl states to be an adaptation of the principle of the salting-down pans of the alkali-makers. His arrangement, however, differs from that of Pohl in that the upper pan is dispensed with, being replaced by an arch of brickwork. According to Pohl's system of construction, the lower pan is 5 ft. deep. It may be made of boiler-plate or of cast iron, or, for that matter, the bottom and lower parts of its sides might very well be made of elm or pitch-pine with cast-iron ends and framing. Pohl tried brickwork for the construction of this lower pan, but abandoned it on account of leakage. In the pan figured, however, he has formed the bottom of tiles embedded in clay.