The water is allowed to descend through 2 pipes, closed or opened at will by the valves k into the transverse pipe g; thence it rises through the pipes, and flows out by cocks into pans, from the overflow of which it drips on to the fagots. Berthier calculates that the average evaporation in ordinary fine weather by this means at Moutiers, in Savoy, where cords are employed instead of faggots, the other general dispositions remaining the same, is 13 1/5 gal. for every sq ft. of cord surface in 24 hours. At Kissingen, the sheds are nearly l 1/4 miles long by 25 ft. high. The water is raised.
A times la passing from one end to the other of the building, and by this, its strength is raised from 2} to 17) per cent. of salinity. Forbes has calculated that here Dearly 3 million cub. ft. of water are evaporated annually by this means. The first set of fagots are stained brown by ferric oxide which encrusts them, and they all have to be changed every 2 years or so, on account of a deposit of calcium carbonate (" thorn-stone") which coats them. By whatit needs evaporation to produce white salt.
Brine evaporating-pans are built of common boiler-plate, 1/4- 3/8 in. thick, the plates being about 4 ft. long by 2 ft. wide, and well riveted together. The plates are usually of rather smaller dimensions in the part immediately over the fire than elsewhere on the bottom or floor of the pan, as by this means some of the tendency to warp and buckle is supposed to be avoided. In England, the usual dimensions for fine and extra-fine salt-pans are 30 ft. long by 22-25 ft. wide, and 1 ft. 9 in. deep. This gives an evaporating surface of 720-750 sq. ft. Butter-salt pans are perhaps a trifle longer, say 35 ft. by 22-25 ft., and the same depth, with an evaporating surface of 770-875 sq. ft. Common and fishery pans range from 50 to 70 by 22-25 ft., and have the same depth, presenting an evaporating surface of 1100-1750 sq. ft.; some fishery salt-pans belonging to the British Salt Co. at Anderton are 90 ft. by 22, while at Stoke and Winsford, are fishery salt-pans ranging up to 130 ft. in length. Beyond 70 ft. in length, however, there really would not seem to be sufficient gain, at least with the quality of fuel used in Cheshire, to compensate the increased cost of construction and repairs.
In France, the common and fishery salt-pans are about the same sizes as ours, only perhaps a trifle wider; and at Dombasle, near Nancy, where Botta has carried the manufacture to as great perfection us is attained in perhaps any works, the pans (poeles) are 72 ft. by 29J ft. by 43 1/2 m., with an evaporating surface of 2124 sq. ft.
The floor of a pan is usually made slightly arched upwards towards the centre, so that a new pan is rather deeper at its sides than in the middle; but they soon flatten out and warp in various directions under the influence of the firing. On the Continent, cast-iron pans have been in some cases adopted, and cast-iron plates substituted for the smaller wrought-iron ones universally employed in this country in the part of the pan just over the fires. Besides the advantage accruing from the less tendency to buckle and warp, the cast iron has a much higher conductive power than the wrought iron, and the advantage of cheapness. The plates are not made much thicker than the ordinary wrought plates, and are cast with exterior flanges all round their edges, by which they can be bolted together beneath the pan. They also have grooves cast in their edges, to receive asbestos cord or cement, by which, when screwed up, they can be made watertight. Were it not for fear of their greater fragility and some difficulties of adjustment, they would doubtless be employed in this country, thus avoiding leakages into the flues, and the consequent production of large stalactites of salt, technically termed "cats," an intolerable nuisance to the salt-maker. In Austria, such cast-iron pans are actually now in use, and their advantages will be manifest from the following comparative experiments made at Berchtesgaden under like conditions of tiring, etc.: -
Temperature attained in the pan.
Cost of maintenance.
It is also sometimes the practice abroad to make the pans with plates riveted on to "T-iron bars running across the width of the entire pan, the central flange of the T-iron standing up between the edges of the plates, and these latter having the rivets countersunk into them. This seems somewhat to prevent the buckling.
Wooden pans have been and still are employed. One belonging to Thompson, of Northwich, is 4 ft. 6 in. deep, 12 ft. wide, and 75 ft. long. The 2 ends are of sheet iron, and a long sheet-iron cylinder, closed at the 2 ends by steam-tight doors, runs from end to end. This cylinder is about 18 in. diam., and is supplied from above at about the middle of the pan by means of a lateral pipe with waste steam from an engine and boiler near. By this, the pan is kept at a temperature of about 90°-100° F. This pan is said to produce 45-50 tons of extra fishery salt every 6 weeks or so.
In Cheshire and Worcestershire, the fire-places, usually 4 in number, measure about 4-5 ft. from the door to the back, and are about 3 1/2-4 ft. wide; from the bottom of the pan to the grate-bars is usually about 3 ft. In the case of very long pans, this height may increase to 3 ft. 4-5 in. The grates are formed of square wrought-iron bars, it being found inconvenient in salt-works to employ the improved cast-iron "fish-bellied" bars. This is on account of the great liability to choking with clinkers, and caking of the ashes with the brine which drips from leaks over the fires fusing into clinker, and clogging the grate-bars. The blows necessary to detach these masses would seriously endanger cast-iron bars; but certainly the shape of the bars might well be improved, and rocking-bars, such as those employed in pyrites-kilns and elsewhere, might be more generally introduced with advantage. The firing is usually done in a stoke-hole with steps on each side leading up to the pathway around the pan.