Having been cooled, the gas passes to the washers and scrubbers for the removal of the ammonia gas which it still contains. In the washer the gas is caused to bubble through water, while in the• scrubber it is caused to pass in thin streams over wetted surfaces, the object in each case being to expose the gas to intimate contact with water. Scrubbers are of two general types—tower scrubbers, vertical steel cylinders filled with bundles of thin boards which are wet by water caused to flow over them by the force of gravity, and rotary scrubbers, fitted with bundles of wooden rods mounted on a horizontal shaft and kept wet by being rotated through the water or ammoniaeal liquor, with which the lower part of the scrubber is kept filled.
From the scrubber the gas passes to the puri fiers. These are usually four in number, and the gas passes through three of them consecu tively, while the fourth is cut out for cleaning and refilling. They are cast-iron boxes with open tops, which are closed by means of removable covers made of light steel plates. When in place over the boxes the sides of these covers are sealed in water contained in 'cups' cast on the sides of the boxes, and the escape of gas is thus prevented. The purifiers are filled with one or more layers of slaked lime or oxide of iron, the latter being the most commonly used in the United States.
From the purifiers the gas passes to the station meter, where it is measured by means of a drum divided into either three or four com partments. The meter is partly filled with water and the inlets and outlets to the different compartments are so arranged in connection with this water that. gas cannot simultaneously enter and leave a compartment. The pressure of the gas causes the drum, which is mounted on a shaft, to revolve so that each compartment is alternately filled and emptied; and since each is filled with a definite volume of gas, the volume of gas passing through the meter is accurately measured, and is recorded by suitable mecha nism.
After passing through the station meter the gas is conveyed to the gas-holder, a cylindrical vessel open at the bottom, but closed on top, made of steel sheets. The lower edge of the gas-holder is always kept sealed in water con tained in a masonry or steel tank, in which the holder is free to rise and fall, being so guided in the tank and along columns rising above the tank as to move freely up and down while being prevented from tilting. The guiding is performed by rollers or wheels attached at equal distances around the top and bottom of the cylinder, and in the case of telescopic holders at the top and bottom of each of the sections, working against rails or channel irons fastened to the inside of the tank wall and of the columns. All large gas-holders are telescopic,
that is, are made with one or more outer sections, which are merely rings, in addition to the inner section closed on top. At the bottom of each of the sections, except the outermost, is a 'cup,' an annular trough having its inner side formed by the bottom course of the section, its bottom by a channel iron, and its outer side by a course of sheets riveted to the outer edge of the channel iron. At the top of each of the outer sections is fastened a 'grip,' which is a cup turned upside down. Gas is admitted to and drawn from the holder by pipes passing down on the outside of the tank under and through its foundation, and up on the inside to a point above the water level. When gas is admitted it enters the space between the closed top, or crown, and the water in the tank. As it continues to enter, the pres sure increases until it is sufficient to overcome the weight of the holder, which then begins to rise and continues to do so as long as gas is entering faster than it is passing out. When the inner section is completely filled with gas the cup filled with water engages the grip of the next section, and as gas continues to flow into the holder, raises this section, the water in the cup forming a seal which prevents the escape of any gas. When the holder descends the outer section lands on the bottom of the tank, and, the inner section continuing to go down, the cup and grip separate. The columns, by which the holder is guided and prevented from tilting as it rises above the tank, are built up of structural steel and are connected together at the top and intermediate points by girders, and also by diagonal ties, so that the whole of the guide framing is bound together into what is practically a rigid cylinder. Originally built in very small sizes, and with only a single lift, gas-holders have been made larger and with more lifts, until the largest holder yet built, one at the East Greenwich Works in London, consists of six lifts, and contains, when full, 12,000,000 cubic feet of gas. The largest holders built in the United States have five lifts and a capacity of 5,000,000 cubic feet. In the ease of one of these holders, the upper lift, and in the ease of the 12,000,000 cubic feet holder, mentioned above, the two upper lifts are allowed to the ends of each rope being attached to the holder at two points diametrically opposite each other, one at the top and the other at the bottom of the same section, passing over pulleys in such a way that as the holder rises and falls the rope is taken up at one end and paid out at the other at an equal rate, so that the two points to which it is attached are forced to travel along vertical lines.