In those works where the nitrogen compounds leaving the chamber system, are not recovered in the Gay-Lussac apparatus, hereafter to be described, the steam is introduced generally into the individual chambers in such proportions that the last small chamber receives considerably more than is necessary for the formation of tetrahydrate, the acid being there produced at 32° Tw., and in the last chamber but one at 52° Tw. When working in this manner the steam that would be necessary for the formation of the acid in the large chamber must be reduced in proportion to the amount replaced by the superfluous water contained in the acid flowing in from the small chambers. The object of this mode of procedure is to decompose the nitrous or hyponitric acid obtained from the small chambers, and to form the greatest possible amount of nitric acid, which is again brought into use by the action of the sulphurous acid in the large chamber. It is by no means claimed that a great saving is effected by this plan; it possesses rather a great defect, to wit, that the weak acid containing nitrogen compounds attacks the lead very energetically. Hence the strength of the acid in the last chamber should never be allowed to get below 32° Tw.
Taking all things into consideration, it is probably more profitable so to distribute the steam among the chambers as to produoe in each of them an acid containing not more, or very little more, water than the tetrahydrate.
The introduction of the steam is far easier to regulate than the amount of air admitted, especially when the chambers are worked with a slow draught. When once the steam has been raised to the proper pressure, and the taps have been carefully adjusted, it is only necessary to keep the head of steam at a constant point. Much greater disturbances of the regularity of tho process are likely to arise from an excess of steam than from a lack of it, bemuse in the former case the gases oxidize a large quantity of the nitrons and byponitric acids, which condense as nitric acid. This may become so serious as to cause stoppage of the works.
Want of steam, which acts injuriously by permitting the formation of chamber crystals, may continue for a long time without causing an actual interruption of the process, because they will be decomposed, and their nitrogen compounds recovered for use so long as there is acid on the floor of the chamber. The want of steam may continue until the sulphuric acid has become so strong that it will no longer effect their decomposition.
The Direction which the Gases follow through the Chambers.—It is evident that upon the manner in which the chambers are arranged will greatly depend the passage and distribution of the gases through the chamber system, and that any tendency to check or hinder their flow must be guarded against. The gases rising through the substantially built flue from the kilns enter the chambers in n heated condition and gradually cool during their passage through them until they escape into the atmosphere at about its own temperature. As the gases become cooler according to the time they
have remained in the chambers, one method of estimating the rate at which they are passing through presents itself in the form of temperature observations at regular intervals. Observations of the temperature in the first chamber (D) of the series already described showed, at a poiut immediately under the ceiling at the end where the gases enter from the kilns, 53° (127° F.), and at the opposite end, where they leave to enter the second chamber, 49° (122°F.). In the horizontal layer of gas at about 6 ft. above the bottom of the chamber an even temperature of 47° (116° F.), vevailed throughout the whole length, while at a level of about 4 ft. 6 in. a constant temperature of about 451° (114° F.) was noticed.
According to these observations, then, the moment that the gases enter the chamber they spread along under the ceiling and afterwards sink evenly over the whole expanse. We may therefore picture to ourselves the contents of the chambers as being so many horizontal layers of gas sinking slowly as their temperature falls, and being constantly replaced by new supplies.
Following out this line of reasoning, a chamber was divided, as shown in Fig. 47, by a vertical partition a b, rising from about 18 in. from the floor quite up to the ceiling. In the two compartments thus formed the following tempera tures were observed. Under the ceiling of the first half, and just over the inlet pipe e, the gaaes had a temperature of 60° (140° F.), and at about 18 in. above the floor, and near the partition b, they were 52i° (126° F.). At the same height in the second part, and 18 in. from tbe partition, or say at d, they were only at 50° (122° F.), while at the top of this half, dose to the partition, say at e, they reached 51i° (125° F.); but at the opposite end, at f, only 48° (118° F.). In the level 5 ft. above the bottom, shown by the dotted line g h, at more than 5 ft. from the partition, a constant temperature of 461° (115i° F.) was found.
From the foregoing we must deduce the fact that as soon as the gases which have descended to the floor of the first part have passed under the partition into the second part, they ascend along side of tbe partition directly to the ceiling, then spread themselves anew along the under surface of the ceiling, and thence descend regularly, so that in the neighbourhood of the partition there are two currents, one ascending, the other descending. The nature of gaseous bodies does not admit of the two streams being sharply divided, but where they impinge on each other they doubtless commingle to some extent, which is also shown by the fact that at the point d, 18 in. from the partition, a medium temperature of 50° (122° F.) was observed.