FIG. 66.—Horizontal Fire-tube Boiler. ] [Illustration: FIG. 67.—Babcock and Wilcox Water-tube Boiler. ] Not less than two boiler units should be used in any power station, regardless of the demands for power, and if the feed water is bad, three or even four units should be provided, as two units may be down at any time. An appreciable factor of safety is provided by the ability of a boiler to be operated at 30 to 50 per cent overload, if sufficient draft is available, but with resulting reduction in the economy of operation. The number of units provided should be such that the maximum load on the pumping station can be carried with at least one in every 6 units or less, out of service for repairs or other cause. TABLE 28 EFFICIENCIES OF STEAM BOILERS From Marks’ Mechanical Engineer’s Handbook ────────┬───────────┬──────────┬──────┬────────┬──────┬──────┬────────── Type │Horse-power│ Furnace │ │ │ │Evap. │ │ │ │ │ │ │ from │ │ │ │ │ │ │and at│ │ │ │ │ │B.T.U.│ 212° │ Combined │ │ │ Sq. │Per Cent│ per │ per │Efficiency │ │ │ Ft. │of Rated│ Lb. │ Lb. │of Boiler │ │ │Grate │Capacity│ Dry │ Dry │ and │ │ │ Area │D’v’l’d │ Coal │ Coal │ Furnace ────────┼───────────┼──────────┼──────┼────────┼──────┼──────┼────────── Babcock │ 300│Hand-fired│ │ │ │ │ & │ │ │ │ │ │ │ Wilcox│ │ │ 84│ 118.7│11,912│ 8.81│ 71.8 Babcock │ 640│Hand-fired│ │ │ │ │ & │ │ │ │ │ │ │ Wilcox│ │ │ 118│ 121.5│14,602│ 10.83│ 72.0 Stirling│ 1128│B. & W. │ │ │ │ │ │ │ chain │ │ │ │ │ │ │ grate │ 187│ 198.3│12,130│ 9.51│ 76.1 Rust │ 335│Hand-fired│ 68│ 210.5│13,202│ 9.42│ 68.9 Heine │ 400│Green │ │ │ │ │ │ │ chain │ │ │ │ │ │ │ grate │ 83.5│ 123.8│11,608│ 8.79│ 73.5 Maximum efficiency recorded │ │ │ │ │ 83 ───────────────────────────────┴──────┴────────┴──────┴──────┴──────────