Modern Gasworks Practice

452 MODERN GASWORKS PRACTICE THE EFFECT OF CONDENSATION Purely from tlie standpoint of quality there can be no doubt that the gradual condensation as efiec'ted by the air-cooled condenser is preferable to the more drastic effect of water-cooled apparatus. This is instanced by the faet that, for the same temperature drop, the condensates from the two types of condenser show considerable difference in character. The tar thrown down in the water-cooled condenser usually contains a comparatively high, proportion of benzene and tohiene with a small proportion of solvent naphtha. With the air condenser, however, the proportion of benzene hydrocarbons is usually quite small, while the condensate is more or less rieh in fractions of the solvent naphtha description. Coudelou has referred to some interesting experience gained while he was comparing the relative efficiencies of two distinct systems of condensers. Both were of the water-cooled type. The first was fitted with vertical tubes and the speed of gas travel was 8-20 feet per second, the average time of passage being 24 seconds. In the second case the speed of gas travel was 1-2 feet per second, and the time of passage about 40 seconds. In the first case naphthalene troubles were rife, and in the second case they were absent. This may therefore be taken as an interesting illustration of the defects resulting from too great a speed of gas travel through the condenser, a condition usually arising from the apparatus being too small for the duties it is called upon to perform. The effect of sudden condensation upon the quality of carburetted water gas has been well illustrated by experiments conducted by C. S. Heath. in America. The following table gives a comparison between the distillation tests of two condenser tars taken from different works. In the case of the first works the gas was taken in the ordinary way first through hot scrubbers and thence to the condensers. In the second instance the scrubbers were absent, and the gas was Condensed immediately on leaving the superheater. The effect of this abrupt cooling on the presence of light oil in the tar will be readily seen from a comparison of the analyses :— Percentage of oils in tar. Gasworks No. 1. Gasworks No. 2. Water 1-8 3-0 Light oils to 170° C 1-2 4-5 Heavy oils to 230° C 23 0 26-5 Oils to 270° C.’ 23-5 18-0 Oils to 315° C *. . . . 17-0 13-2 Pitch, residue 33-5 34-8 Speciflc gravity 1-067 1-066 Free carbon 0-60 0-71 As would be expected from the high proportion of light oils in the tar collected from No. 2 Gaswerks there was a sudden decrease in both calorific power and candle power of the gas after the latter had passed the condensers.