Modern Gasworks Practice

310 MODERN GASWORKS PRACTICE elevators are erected at so great an angle as 70 degrees, or in some cases vertically. The speed should not exceed 130 feet per miaute under any circumstance. In the De Brouwer combined hot coke elevator and conveyor the normal working speed is in the neighbourhood of 40 feet per minute, and the angle of slope is usually no more than 30 degrees with the horizontal. It is necessary to bear in mind that when a friable material such as coke is being dealt with the speed of an elevator of the bücket type should be low, because a high velocity of delivery entails considerable breakage of the material, in addition to cuitailing the life of the receiving shoots. In general, it may be said that coke elevators should not be operated at a greater speed than 60 to 90 feet per minute. Capacity, usually based on tons delivered per hour, depends, of course, largely on speed, but also upon the size and pitch of buckets, or —in the draw-bar types—upon the depth, width, and pitch of the bars. With bucket-conveyors much depends upon the extent to which. each bücket is filled. Where average practical conditions are considered, the following formula. may be taken as a useful guide to capacity :— „ SxBxW, , Capacity =------------- tons per hour. 1 J 96,768 X P where B = Capacity of buckets in cubic inches. S = Speed of conveyor in feet per minute. W = Weight per cubic foot of substance carried (see page 363). P = Pitch of buckets in feet. Fig. 227.—Band Conveyor WITH TBOUC4HED Leading Portion. Band Conveyors Band or beit conveyors have attained a certain degree of popularity on gasworks, and for conveyhig the lighter materials, such as oxide of iron or sulphate of ammonia, they are probably to be preferred to other types. For such materials less power is required by a band conveyor than by any other, and the speed may run up to as much as 600 feet per minute. For heavier substances, such as coal and coke, the whole design must necessarily be of a much stifter nature, whilst the band itself must be considerably stronger in order to take the greater weight imposed upon it. It is not generally realized that when used for coals the centre of the band, and not the outside edges, shows the greatest tendency to wear ; consequently the middle portions of the width require special care in their preparation. In a general way, leather material as used for ordinary driving belts is to be avoided, and the band is preferably composed of special insertion covered with india-rubber where it comes in contact with the rollers or guides. The upper portion which carries the load should be made from solid india-rubber, which may be reduced in thickness towards the edges. Canvas or balata is often used. There are two main types of band conveyors, namely, those in which the leading and return portions of the beit pass over parallel guide rollers so that throughout its travel the beit is perfeetly flat, and those in which the rollers are so arranged as to cause the leading portion of the beit to assume a trough-like shape (Fig. 227).