THE CONDENSATION OF COAL GAS 457
J?ig. 284.—Theater Chamber for Electrical Precipitation.
to provide against the failure of insulation, Davidson
and each. electrode—of the squirrel-cage pattem—has an effective lengtli of 5 feet 8 inches. So far as contact time is concerned, it is estimated that the speed of the gas passing through the electrode chambers is about 30 feet per second, and the experiments have shown that tlie time of exposme to the electrical discharge, which is under half a second, is ampie for the removal of the last visible traces of tar. Incidentally, it has been found that in addition to bringing down the whole of the tar this electrical purifier has certain desirable efiects in the direction of minimizmg the trouble arising from the deposition of naph-t halene. For tar removal the apparatus has been interposed at a point where the temperature is in the iieighbourhood of 60° C.—that is, at a very early period in the foul main. Where naph.-thalene exists in the crystalline state the solid particles may be expelled in the same manner as the vesicles of tar. For naphtlialene removal, however, a second electrode is placed at a point where the temperature is at it.s lowest.
A thorough examina-tion of the possibilities of electrical precipitation has recently been made by J. G. Davidson,1 who points to the faet that the main difficulty associated with the process is one of insulation. In order employed the apparatus
shown in Fig. 284. The treater chamber consisted of an iron pipe 21 feet high and 10 inches in diameter, and it was provided with fused quartz insulators as shown. From filtration experiments which were made to determine the efficiency of cleaning on a large coke-oven works it was found that the gas from the treater contained from one to three milligrammes per cubic foot, whereas gas from the outlet
1 Report No. 3, Canadian Council for Scientific and Industrial Research.