THE DRY PURIFICATION OF COAL GAS 605
So far as the various means of jointing are concemed the main difference lies in the formation of the rubber strip and as to whether it is applied to the upper or lower curb. Common types are shown in Figs. 363, 364, and 365.
LIFTING-GEAR
In small works the cover may be lifted by means of eyebolts or hooks attached to it, with pulley blocks running on a horizontal joist overhead and supjoorted at each end by columns of timber, cast iron or steel. In medium-sized and large works, a common lifting-gear is that composed of two “ A ” frames running on rails on either side of the purifiers. The apexes of the two “ A ” frames are then con-nected by a single joist or a lattice girder. Travelling can he efiected by hand, a purchase being obtained by means of suitable toothed gearing. The actual lifting of the cover may be carried out by pulley blocks, or in the case of very heavy covers by means of hydraulic or other power. "Where electric power is available an overhead ciane of the shop or foundry pattem can be used. with advantage.
THE PRESSURE ON PURIFIER COVERS
It should be borne in mind that in the case of purifiers with a water-luted cover, the internal gas pressure on the box and cover is limited by the deptih. of water in the seal. The pressure wliich the cover has to withstand, therefore, is the total pressure of the gas on its (the cover’s) area, less the weight of the cover itself. Holding-down lugs have, therefore, to be constructed accordingly. The bracirig of the cover must also be strong enougb. to avoid distortion of the members, with its consequent ten-dency towards leaking rivets. When abnormal pressure occurs the lute in this type of purifier forms the safety valve, and the seal “ blows.”
In dry-luted covers the pressure provided for must be greater, and a normal specified test would be 1^ 1b. per square inch, i.e. a water column of 3 feet 6 inches, As the holding-down bolts are doser together than in the water-lute purifier, no difficulty is found in designing them of sufficient area to withstand the upward tension. When domed tops are employed the pressure is less than with. the flat top, and an allowance for this can accordingly be made.
PURIFIER GRIDS
The most usual form of grid employed for supyorting the material ■within the
lii li
.. . .
Fig. 366.—Open-ended Purimer Grid.
Fig. 367.—Butt-bnded Gbid.