The admission of the minimum of air necessary is therefore easily managed. The fulfilment of this condition alone, however, is not enough to ensure good working, for harm may be done by the introduction of more than the above-indicated excess as well as by the want of air. The evils arising from an excess of air are not so great as those caused by a want of it, but still they are sufficiently important to warrant every care being taken to avoid them. Primarily, an excess of air does harm by reducing the heat of the gases, and thus interfering with the process. Then it fills a part of the chamber space unnecessarily and renders it useless. Finally, it acts injuriously on the formation of the acid by thinning the gases and weakening the energy of their chemical action. The regulation of the draught is one of the greatest difficulties in a sulphuric acid works, because of the daily changes in the condition of the atmosphere. Hence it is of primary importance for the manufacturer to take note of these changes, and immediately to lessen their effect on the process by opening or closing the dampers in the connection pipes, &c., and by regulating the doors and ventilators in the kilns.
Though the admission of air will be reduced by shutting the outlet damper as well as by closing the kiln ventilators, still the two things are not quite synonymous. By the first means the pressure inside the chamber will be increased for the moment, by the last it will be diminished. If the kiln ventilators be shut too much, the chambers will draw in air from the other end, unless the dampers there be correspondingly closed.
The sectional area of the exit flue must always be in direct proportion to the size of the ventilators which admit air. In general the former is made equal to of the latter. No rule call possibly be given respecting the allowances to be made for the changes of the atmosphere, their degree can only be ascertained by actual experience with the works. In all well-administrated factories the escaping oxygen is measured as we shall indicate further on.
Tee Ste.(m.—The amount of steam admitted to the chambers must be regulated quite as care fully as the air. We have already mentioned that in Very hot countries it is not imperative that all the water introduced should be as 'steam in order to keep up the heat. Experience teaches us that the best results are obtained when the amount of steam injected is just sufficient, or but a trifle more than sufficient, to form 4 HO or acid of 1.55 sp. gr. (110° Tw.), at which strength chamber crystals do not form, and which absorbs far less nitric acid than does acid containing more water. To form the tetrahydrate 4 equivs. of water are necessary for every 1 equiv. of sulphur, forming 1 equiv. of sulphuric acid, or for 16 parts of sulphur, 1 x 9 = 36 parts of water, as the atomic weight of sulphur and water are 16 and 9 respectively.
From this it may be estimated that for each kilogramme of sulphur 2.25 kilos. steam must be introduced to the chambers. It is of the greatest importance for the success of the process that the
steam should only condense in the mass of the gases existing in the chambers in order to form the tetrahydrate, for by the condensation of great quantities of simple water an unnecessary thinning or weakening of the acid is caused, and part of the nitrogen compounds withdrawn from action. Hence arises the question, how much water can exist in the chamber gases as steam ? This can be ascertained by the following calculation.
On a previous page we have shown that from the volume V of a gas at 0° C. and 760 nns. pres, sure, when saturated with water b mme. and at temp. t C. arose the volume V' (273 ± °IT ; 760 273 (b — e) We have also seen that for each kilo. of sulphur burnt there are required 6199 litres of air at 0' C. and 760 fume. pressure, the volume of which will not be altered by the formation of sulphurous acid. On saturation with moisture at the same pressure and at a temperature of 50° C., which may be taken as the ordinary temperature of the gases entering the first chamber, these 6199 litres of (273± 50) 6199 ; 760 = gas with a difference of pressure of 92 mme. will give the volume 8345 litres 273 (760 — 92) of gas with a tension of 668 mmc. Now 1 litre of steam at 0° C. and 760 mme. weighs 0.801343 gems., and gives, according to the formula (273 1)V; 50° C. and 92 mme. pressure, a volume of 273; b (273 + 50) 760 273 ; litres.
According to the proportion 9 7739: 0104343 = 8345 : x, we get 8345 r = = 686.8 grin.
Therefore the steam needed for 1 kilo. of sulphur, yielding 8345 litrce of chamber gases, which steam they at 50° C. and 760 mm. can take up, is grm. or • 06868 kilo. ; while according to our former calculation altogether 2.250 kilos. of steam are necessary for this quantity of sulphur. From this it might be thought that for every kilo. of sulphur only 2250 grm• of steam, or about 30.5 per cent. of the necessary quantity, should be introduced at the commencement of the process. This conclusion would, however, be erroneous, because the excess of steam beyond the saturation point does not immediately condense in drops, but is conducted in great part through the chamber system in the form of mist, in which state it probably assists in the formation of the acid. The amount of steam which spreads through the chambers in this state is not known. In the figures of works which we have given, the greatest part of the steam is introduced at a single spot near the entrance of the gases into the large chamber. In many works ou the other band there are several steam pipes leading to the large chamber. Occasionally they are not placed in the most advan tageous spots, but only a little above the bottom of the chamber, and without taking into consider ation the direction in which the gases are passing. The influx of the steam should always be regulated in accordance with the directions pursued by the gases.