312
MODERN GASWORKS PRACTICE
to the size of the pieces; lience large lumps must travel more slowly than smaller ones. Under normal conditions the speed for coal purposes will be from 150 to 300 feet per minute, althoiigh with fine coal this may be increased up to 400 feet per minute. The following table, compiled by Zimmer, gives an indication. of the capacity of these elevators and thé horse-power required to drive them. The author, however, is of the opinion that the speed given (600 feet per minute) is too great, the maximum figure for capacity too liberal, and the B.H.P. for wider buckets not adequate.
Speed and Capacity of Band Conveyors for Coal
Width of Band. Speed. Capacity.
Inches. Feet per min. Tons per hour.
12 150 to 600 10 to 35
18 50 to 175
24 125 to 475
30 •• 250 to 900
36 350 to 1,500
Size of Coal. B.H.P. for 100 feet length of Conveyor.
From 2-inch to dust 3-2
4-inch to J-inch 4-8
6-inch to 1-inch 6-0
J, 7-inch to 2-inch 7-6
,5 9-inch to 2-inch 9-2
Band conveyors need not necessarily be operated on the level, but can also be used for elevating purposes. Wlien conveying up-liill, however, the inclination should not be greater than 25 degrees, otherwise capacity will be reduced by slip. When the band is inclined the power necessarily increases in accordance with the height through, which the load is raised. One of the chief advantages attached to this type of conveyor is that it does not injure in any way the material on which it is working.
The end rollers of a band conveyor should not be less in diameter than 5 incb.es for every ply of the beit; e.rj. for a 6-ply beit the pulleys should be 30 inches diameter. The working stress in the beit when loaded should not exceed 30 1b. per inch of width per ply. The following table (compiled by J. E. Lister) gives the efiective cross-section of the layer of material carried by belts of various sizes wlien running flat and when troughed, and it illustrates tlie great increase in carrying capacity to be obtained by trougliing the beit.
COMPABISON BETWEEN FlAT AND TrOUGHED BfiLTS
Width of Belt (indies). Effective Cross Section of Material (square feet).
Flat Belts. Troughed Belts.
18 0-08 0-19
24 0-16 0-34
30 0-27 0-55
36 0-40 0-80