Modern Gasworks Practice

442 MODERN GASWORKS PRACTICE contains an. undesirable quantity of heavy tar vesicles, the outer cliamber is liable to obstruction from this cause. In the calculation of the area of cooling surface required it should. be remembered that the actual surface to be considered is that with wliicli the gas is in contact. Hence when the gas travels inside a tube the interior surface must be taken, but when the gas is surrounding the tube the exterior surface is con-sidered. From this it will be seen that the water-tube type gives more condensing area for the same number of tubes. Mention has already been made of the com-bined atmospheric and water-cooled condenser. This in reality amounts to nothing more or less than a condenser of the water-tube type. In this case it will be seen that the outer casing is available as an atmospheric cooling surface, whereas in the multitubular condenser this function is lost, owing to nearly cold water (instead of hot gas) being in contact with the shell. J. S. Haug has pointed out that the upkeep of the multitubular condenser as regards painting is much more expensive, due to what is commonly called “ sweating,” which is occasioned by the condensation of moisture from the air on the cold shell. In the case of the water-tube condenser the shell is hot, and condensation, therefore, does not occur in this männer. So far as the actual condenser capacity required for a given works is concemed, the general allowance for all types of water-cooled apparatus may be taken as 3 square feet of cooling surface per 1,000 cubic feet of gas passing per maximum diem. In the case of the multitubular type of condenser a deduction (calculated on the rule already given for air-cooled apparatus) may be made for the surface of the outside shell, whilst for all types the length. of foul main should be considered, as previously pointed out. To determine the efficiency of a condenser and its ability to deal with a given set of conditions it is necessary to consider:— (a) The average inlet temperature and the desired outlet temperature. (b) The condition. of Saturation of the gas. (c) The rate of transmission of heat from the surfaces employed. First, it has to be recognized that the aqueous vapour with which the hot gas is saturated accounts for by far the largest share of the total work of condensation. This is due to the faet that the water vapour has to be deprived of considerable quantities of heat, the same applying in some measure to the liquefiable hydro-carbons. A. F. Browne has estimated that of the total work of condensation 87 per cent, is accounted for in removing aqueous vapour, and 13 per cent, in cooling permanent gases and in condensing liquefiable hydrocarbons. Grude gas, although deprived of a large portion of the aqueous vapour it originally contained, is still in a state of Saturation when it leaves at the condenser inlet. If this were not the case the process of condensation would be a trifling matter. The actual number of heat units to be extracted can. be arrived at by ascertaining the weight of water vapour present in the gas passing at the inlet, and, similarly, the weight of water vapour present at the outlet. The difference will give the weight of water Condensed out of the gas. This figure multiplied by the thermal units to be liberated per pound of water between the inlet and outlet temperatures will give the total amouiit of heat which is to be eliminated. The ability of a surface to transmit heat varies to