VERTICAL RETORTS AND CHAMBER OVENS 151
producer and regenerator in each setting. Each seven-ton retort is a self-contained unit with its own regenerator situated at the lower end of the retort.
Producers of the step-grate type are now installed.
Until about six years ago the five-ton retort was the only size in use in this-system, but it was- then found that a demand existed for both smaller and larger sizes, consequently the three- and seven-ton retorts were designed to meet the. requirements of small and large works respectively.
For small works with. a daily output up to about half a million cubic feet of gas retorts of three- and five-ton capacity are installed, for medium sized. works up to about two millions per day the retorts are usually of the five-ton size, while for larger outputs the seven-ton size offers many advantages.
Coal Feed Roli
Gas Va/ve
Au vi Ha ry _ Coa! Hopper
Rodding P/ugt
Platform
Gas Offtake Va/ve
Gas Main
Seal Pot
WeirS/uice Valve
Tar Strainer
Gas Retort
Retort Setting
Tar Drain Pipe
Ftc. 73.—Gas Ojttake, Woodall-Duokham System.
On the smaller plants, lifts are provided for handling coal and coke to the over-head bunkers and for larger plants bucket elevators or conveyors are installed for this purpose.
Details of Settings
At the top of each retort is a cast-iron mouthpiece (Fig. 73), to which is attached the coal-feeding device and the gas offtake.
A gastight auxiliary coal hopper constructed of mild Steel plate and having a capacity of from two to four hours’ coal supply is mounted on a special casting attached to the mouthpiece. The casting is so fornied that the hopper in no way obstructs the free access to the retort for inspection, scurfing, or filling with coke.
At the top of the auxiliary hopper is fitted a circular gastight coal valve which is operated by hånd every two, three, or four hours. The time required for this