The sketch on this page represents what is known as the Tar Tower. The rectangular box-like vessel is the hydraulic main, which acts as a collecting chamber for the gas from the separate retorts in the setting.
The gas passes from the retort to the hydraulic main by way of the ascension, bridge and dip pipes, three of which are shown.
The dip pipe projects downwards inside the hydraulic main, the lower end being immersed m ammoniacal liquor to the extent of about a quarter of an inch. This is known, as a "hydraulic seal," and its purpose is to prevent gas from flowing from the hydraulic mains back through the dip pipes to the retorts.
The gas is very hot when it leaves the retort, and carries with it tar and water vapour, much of which condenses in the relatively cool hydraulic main, and this condensate, together with other liquor which is added to ensure a sufficiency, provides the" hydraulic seal" for the lower end of the dip pipes.
The depth of the seal in each dip pipe must be the same to ensure equal pressure conditions in all retorts in the setting. Provision is therefore made for the removal of the (words missing)
The tower receives the condensate from a number of hydraulic mains by way of a connection known as the tar main, and the liquor, by reason of its higher gravity, sinks to .the bottom of the tower.
Inside the tower a cup or weir is fixed at the level it is required to maintain in the hydraulic mains, and the separated liquor flows through this cup and a scaled pipe to the storage tank. The tar is removed periodically by way of the pipe at the bottom of the tower and flows into the funnel and away to the storage well, liquor being supplied to maintain the level in the tower.
A small pipe, known as the “equilibrium pipe" connects the top of the towel to the “foul main" to maintain equal pressure conditions in the tower and the "foul main" into which the gas flows from the hydraulic main.
Co-partnership Herald. (date not known, probably 1920s)