Pillars of cast iron rising from the bottom of this lower pan support the upper pan, which is of the ordinary make, and demands no special description. The interval between the two need not, according to Pohl, be more than 3 in. In practice, however, 5-6 in. is not too much from the bottom of the upper pan to the surface of the brine in the lower one when completely filled. The length of these pans is about 60 ft.; breadth of the upper one, about 20 ft., and of the lower one, 22 ft., the space between the two being filled all around with brickwork.
Milner has made the lower pan in his arrangement much wider than this, or rather it may be said a lip or opening running all along each side of the lower pan permits of the salt as it collects being drawn to the sides by rakes, and lifted out by perforated scoops as it accumulates. According to Pohl's arrangement, this might easily be managed by continuing the sides of his upper pan downwards for say 8-9 in., the pan being placed at such a height above the lower pan that these sides may dip 2-3 in. below the surface of the brine in the lower pan, and thus constitute a flue / 4-5 in. deep, through which the furnace gases might pass. The lower pan might then be made say 3 ft. wider than the upper one, so as to leave a trough on each side about 18 in. wide, through which the salt might be drawn. As it is, when the pan has to be drawn, which, of course, must be done as soon as it becomes full of salt, the fires have to be let out, the brine run off, and the salt drawn by the door or manhole k.
The furnaces in Otto Pohl's arrangement are 4 in number; they are made about 4 ft. wide internally, and 4 1/2 ft. or even up to 6 1/2 ft. between the top of the arch and the grate-bars; a distance of 3 ft. or so is also left at the back between the end of the grate and the lower pan, the angle being filled up with a curve of masqnry as shown at e. This form of construction is intended to allow space for more perfect combustion, before the heated gases enter between the pans, where they tend to become rapidly cooled, with proportionate liability to deposit soot. Fig. 130 shows the front elevation and the arrangement of the sliding doors b. Pohl at first carried his upper pan right over the fires. He now stops short behind them, covering them in with arches of massive brickwork, so as to avoid as far as may be loss of heat by conduction in this quarter. He also proposed to make a sort of short circuitous flue, through which the products of combustion might, be made to pass on their road to the space between the pans, by building 3 arches over the fires, constructed so as to reach alternately to the back and to the front of the fire-place, like the shelves of pyrites-dust kilns.
These arches becoming strongly heated would aid in promoting the combustion of the smoke, while they served to catch the dust and ashes carried over from the fires. This plan, however, he appears to have abandoned. A further provision was made for getting rid of soot by keeping the lower pan always filled to the brim, making the end of it farthest removed from the fires a trifle lower than the fire end and sides, and keeping it full to the brim at that end. Much of the soot, falling on the surface of the brine in light flocks, would float thereon, and be carried off over the end of the pan by the draught towards the chimney.
Between that end of the pan and the entrance to the chimney, is a soot-boz or closet A, with a door for cleaning it out. Notwithstanding all these precautions, large quantities of soot are liable to become condensed, either upon the bottom of the upper pan, or between the 2 pans, and, falling on the surface of the brine, get carried down and mixed with the salt, rendering it black and totally unfit for food. This quality of salt, however, has been found specially suitable for the Hargreaves' salt-cake manufacture, so that the small quantities now produced find a ready enough sale, as the soot does not signify. The method shows an important economy of coal, and, according to Pohl, gives 3 tons of butter-salt with the same amount of fuel and labour as is requisite for producing 2 tons by the old methods. The use of gas from a Siemens producer would, obviate the soot completely, while it is probably preferable (according to Milner's plan) to do away altogether with the upper pan, employing merely a brick or tile covering as a reverberatory and radiating surface to throw the heat down into the lower pan, and so get rid of leakages, salt cats, and much cobbling and repairs involved in working by bottom heat.
According to some experiments by Pohl, while the temperature of the upper pan remained suitable for making common salt, or ordinary fishery-salt, that of the surface of the brine in the lower pan was maintained at fall boiling, and the produce, so far as grain was concerned, was very fine butter-salt, while no scale worth mentioning forma in the lower pan. He gives as a result of 16 days' boiling with brine containing 25.27 per cent. salt, for 57 tons of slack (from Little Houlton Colliery, Lancashire) burnt, - 82 tons of fine batter-salt, and 49 of common salt; while on the old system, the 82 tons butter-salt would have taken 54 tons 13 cwt., and the 49 tons of common salt, 26} tons, or a total of 81 tons 3 cwt., showing an economy of 24 tons 3 owt. Instead of the gases escaping into the chimney at a temperature of 600° F. (315° C.)ae during the manufacture of salt with the ordinary common salt pans, or at a temperature of 800°-1000° F. (425°-538° C.), as when making butter-salt, they never rose, even with the strongest firing, above 288° F. (142° C).