Modern Gasworks Practice

250 MODERN GASWORKS PRACTICE deposits taken from the pipes contained on an average 6’75 per cent, of volatile matter, 91'75 per cent, of fixed carbon, and 1’5 per cent, of ash. It having been suggested by some that these deposits were due to the formation of pitch by the distillation of tar in the ascension pipes, Coudelou made several tests to disprove the theory by raising the temperatures in them. The maximum deposit was found at 280° C. (536° F.), which. is too low a temperature to produce pitch. If the diameter of the ascension pipes is insufficient in proportion to the gas production, the speed of the gas in them is increased until the dust from the retorts is no longer deposited in them, but carried forward and left in the hydraulic mains, where a thickening of the tar takes place. Coudelou verifled this by a comparison on five different gasworks. In those where the gas travelled in the ascension pipes at the low speed of 5'5 inches to 6’1 indies, the dehydrated tar from the hydraulic mains contained only 2’4 per cent, of carbon, as against 27'9 to 29’6 per cent, where the speed of the gas was 8'9 to 9’4 inches. The tar in the first in.stan.ce was fluid, and the hydraulic mains needed no watching, whereas in the second case they needed cleaning daily the tar quickly becoming thick. The following table shows the results obtained at the works in question :— A B C D ) E Length of retorts, feet 9 9 18 18 Diameter of ascension pipes, inches 4-9 6-1 7-1 7-1 7-1 Length „ „ feet .... 13-9 15-3 14-4 15-3 13-8 Period of carbonization, hours .... 6 6 8 8 8 Charge per retort, cwt Average quantity of gas in an ascension pipe, 2-4 3-3 10-6 10-6 10-6 cubic feet 5-5 5-0 8-1 8-4 8-4. Mean hourly make per retort, cubic feet Mean period during which gas remains in 240 339 738 777 77? ascension pipe, seconds . . . . . Analysis of tar from hydraulic mains : 27 32 19 19 17 Water incorporated, per cent 6-4 4-6 9-5 10-6 Carbon, per cent., crude tar .... 22-4 23-3 25-3 26-5 Carbon, per cent., dehydrated tar 23-9 24-4 27-9 29-6 The thickening of the tar in the hydraulic mains, Coudelou says, could only result from the mixture of carboniferous dust with it. The theory of a distillation of the tar must be put aside, the temperature being too low for the formation of pitch. The conclusions are that the diameter and length of the ascension pipes should be such as to allow readily of condensation, and to prevent the gas passing through them too rapidly. They must be kept as clean as possible ; and variations in their sectional area, and sudden changes of direction of the gas in its passage, must be avoided.