136
MODERN GASWORKS PRACTICE
the exhauster driver (who is standing by waiting tlie signal) notices the movement of his chart. He then shuts off the steam on tlie exhauster engine, and immediately afterwards closes the by-pass, which was opened when the <£ draw ” was originally Teduced. Gas is still being evolved from the coal in the retorts, and an outlet must, of course, be provided for this, or some considerable pressure will be thrown upon the apparatus between the retorts and the condensers. This should preferably be arranged for by fitting a 4-inch pipe to the inlet of the condensers, the pipe dipping into a seal pot, and being sealed in liquor to a depth of about half an inch. The gas will at first bubble through the seal fairly freely; but as the quantity evolved
gradually becomes less and less, precaution is necessary, for after a time no gas whatever will be evolved, and that already in the apparatus will cool and give rise to a vacuum. In conse-quence, air is liable to be sucked in through the blow-off seal. In Order to prevent the possibility of this occurring, the seal on the blow-off pipe is deepened until a pressure of about 1 inch. is thrown upon the apparatus.
The Prevention of a Vacuum
As serious results might follow any intake of air and the probable
formation of an explosive mixture in the apparatus, it will be realized that the pre-vention of this vacuum is one of the outstanding points requiring special attention when temporary stops are made. Some engineers prefer the rnethod of slacking-up the levers of the retort doors, so as to allow the surplus gas to find an outlet in this way. In the rnethod described above, the lids are kept tightly sealed until the retorts are re-charged, and any gas which. is expelled goes out into the open air and not directly into the retort house itself.
The late Mr. J. Tysoe drew particular attention, to the liability of danger from the formation of this vacuum, and it is as well to recall his methods by quoting his remarks, which. were as follows : “ It frequently happens that sufficient gas is coiitinuously driven off after the exhauster is stopped to maintain a slight pressure up to starting-time in the evening; but should the pressure entirely disappear before