Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 91 (a) B.Th.U. = 12-76 X 0-246 (1200—60) = 3580 if gases are oooled down to atmospheric temperature. (b) or B.Th.U. =12 76 X 0'246 (1200- 400) = 2510 if gases are cooled down to 400 deg. Thiis if the waste heat is to be recovered from a battery of six horizontal retort settings, the approximate quantity of coke bumt in the producers per hour will be (6 X 1-6) = 10 cwt. = 1,120 Ib. The heat available, therefore. will be :— (c) Gases cooled to atmospheric temperature, 1120 x 3580 = 4,009,600 B.Th.U. per hour. (d) Gases cooled to 400 deg. F., 1120 x 2510 = 2,811,200 B.Th.U. per hour. Accordingly, neglecting the dryness fraction, the theoretical quantity of steam at 100 Ib. pressure whicb. may be raised from feed water at 60° F. may be ob-tained direct by dividing the above figures by the total heat required per Ib. of saturated steam. To obtain. steam at 100 Ib. pressure the sensible heat required. per Ib. of water amounts to 268 B.Th.U., while the latent heat of evaporation is 888 B.Th.U., or a total of 1,156 B.Th.U. Therefore, under condition (a) the theoreti-cal quantity of 100 Ib. steam obtained per hour would be 3,470 Ib. and under (6) it would be 2,430 Ib. It is questionable, however, whether it is ever possible to obtain more tlian 65 per cent, efficiency from an installation of the kind ; hence the actual quantity of water evaporated would be in the neighbourliood of 2,250 Ib. per hour in the first instance, and 1,580 Ib. per hour with partial heat extraotion as considered in the second case. Whole or Partial Extraction ?—When a waste-heat boiler alone is employed it is not possible, nor would it be advisable, to reduce the gases to a temperature approach-ing that of the atmosphere at the outlet of the boiler. An alternative arrangement is a combination of boiler and feed water heater, the waste products passing first through, the boiler and tlien through, the feed heater. In this way, by arranging for a coimter-current flow, the gases may be brought down to a temperature little above that of titte, atmosphere. The problem which. arises in connection with feed water teaters, however, is that the surplus heat leaving the boiler is considerably greater m. quantity than that which. is required to heat up the maximum quantity of water which. the boiler can evaporate. Hence, some loss is inevitable, unless a portion, of the pre-heated water can be drawn ofE to give a hot-water supply for some auxiliary purpose. The question resolves itself into whether (a) complete cooling should be effected, in which. case natural draught is eliminated, and induced draught by fan h.as to be employed; or (6) whether it is more economical to reduce the temperature to only such, a limit that natural draught will still be preserved. In. this connection it should be remembered that in order to retain sufficient natural draught it is not advisable to work with a waste gas temperature of less tb.an 400° F. This method is, perhaps, the better of the two so far as gasworks are concerned, for on such establishments a supply of pre-heated water may nearly always be obtained from the gas condensers or from the cooling tanks of large gas engines. On the author’s works one boiler is supplied with. water taken from the outlet of water-gas condensers,