THE PRELIMINARY PURIFICATION OF COAL GAS 523
SCRUBBERS
Serubbers of some form or other are almost universally employed for removing the last traces of ammonia from coal gas. They consist either of rectangular or cylindrical vessels, the height of which is comparatively great in relation to the width or diameter. The ratio of height to diameter usually varies between 2| and 5 to 1, although in some instances it may be as mueh as 7 to 1, particularly in the case of serubbers of comparatively small capacity. The cylindrical section is nearly always adopted, and the majority of serubbers are constructed from cast-iron plates bolted togetlier so as to form tiers. At the bottom of each tier ot bay a grid is fixed to brackets cast on to the side plates whilst the gas outlet pipe, in many forms, passes , down the centre of the vessel. Steel has been employed to some extent in the construction of serubbers, the ring sections being riveted up
Fie. 328.—Typical Joints in Cast-ibon Wobk.
by lap or butt joints, with external angles at the top and bottom of the sheet. Owing to its superior endurance against corrosion cast iron is, however, still preferred by the majority of engineers. Cast iron possesses, moreover, a secondary advan-tage in that the vessel is easily taken to pieces, and may be sold and re-erected elsewhere when it may have been outgrown. by the inereased output of the works.
In cast-iron work, as in common use for the construction of serubbers, washers, tanks, purifiers, etc., three types of joint are found. These are shown in. Fig. 328. It must be emphasized that although. rust joints, composed of iron borings and cement (see page 248), are extremely strong they are not altogether desirable when such substances as ammoniacal liquor are being dealt with, so that faced joints should preferably be employed. For smaller serubbers (say up to 5 feet diameter) the depth of the cast-iron plates is 4 feet, whilst above this size 5-feet plates are used. When the diameter of the serubber is exceptionally large (over 30 feet) the flat cast-iron top becomes decidedly eumbersome, and is liable to sag. In