Modern Gasworks Practice

448 MODERN GASWORKS PRACTICE tube, and the interior surface of the outer tube. For gasworks purposes the washing liquid employed is water, although for special purposes oil or thin tar may be utilized. The flow of gas through the condenser may be reversed so that, instead of entering at the top and passing down the annular space, it may enter at the bottom when it passes vertically between the tubes to the upper part of the tube chamber. It will be observed that sprays are fitted at the top of the water tubes as well as at-equidistant points t hrough out their lengths ; but in normal instances the top sprays and the .upper side sprays only are put into operation. A water-cooled type of battery condenser is shown in Fig. 277. The condenser is rectangular in shape, preferably built up from steel plates, and is divided by baffles into separate compartments. Water is admitted at the bottom, passing in a zigzag course through the apparatus in an opposite direction to the gas. By means of a “ tumbler ” attachment at the top weak liquor may be sprayed intermittently on the exterior of the water tubes, thereby assisting to wash the gas. THE THEORY OF CONDENSATION The primary object of condensation is not merely that of cooling the gas to atmospheric temperature, but to ensure that the bulk of the constituents which are not permanent gases at normal temperatures are separated out. By the law of vapour pressures it is known that the Saturation content of a gas decreases corre-spondingly with any reduction of temperature, and that the gas is unable to hold in suspension any excess of vapours above the quantity corresponding with. its vapour pressure at the reduced temperature. Accordingly, as the temperature of the hot gas is reduced during its passage through the condenser, a considerable portion of the excess vapours (in the form of steam and liquefiable hydrocarbons) is deposited. As has already been pointed out, it is the aqueous vapour which accounts for the greater portion of the work of cooling entailed. In the case of coals containing a high proportion of moisture or which yield a large quantity of steam on distillation, the condensation apparatus may be almost entirely occupied in dissipating the heat arising from the formation of steam. Owing to the extremely finely divided condition in which maay of the particles are sus-pended in the gas it is usually impossible to bring about their entire separation by a reduction of vapour pressure by cooling. For this reason the gas is frequently accorded mechanical treatment with the object of removing all traces of solid or liquid matter before its arrival at the wet purification plant. Gradual cooling, with the strict avoidance of “ shock,” is generally looked ijpon as the ideal to be secured, for in. such cases the hydrocarbons are thrown down in. sequence, and there is but slight danger of depriving the gas of a portion of the more valuable constituents which it is capable of retaining at normal temperatures. In connection with this point Butterfieid says that with rapid cooling supersaturation of the gas with certain condensible vapours occurs, whilst other less readily condensible constituents are dissolved or mechanically abstracted by the flushing action of the condensate. He points, moreover, to the probability of the selective action of radiant heat coming into play, and says that radiation from relatively cool walls lowers the temperature