[Footnote V: To illustrate the use of such tables, the following example may be given. To dry 100 butts in a turret, each containing 20 lb. of moisture, at least 20 × 1000 × 100 = 2,000,000 units of heat will be required to replace the loss by evaporation alone. As a 4-in. pipe at 300° gives off 669 units per foot per hour (see Table III.), about 125 ft. would give off 2,000,000 units per day. If we compare this with Mr. Hepburn's practical experience, supposing the 4 working floors of his turret to hold 100 butts each (a low estimate), and to dry in 10 days; we have 540 ft. for 40 butts or 1350 ft. for 100 butts a day; showing that more than 10 times the minimum is required in practice. Of course this allows for weather in which the air must be heated considerably before it will dry at all, for heat that escapes uselessly at the top and sides of the building, and for the fact that the pipes are not heated the whole time, and probably, on the average, to a much lower temperature.] Table III.--Heating Effect of Pipes freely exposed to Air at 60° F. ──────┬─────────┬──────────────────────────┬──────────────────────────────┐ │ │Units of Heat per Ft.-run │ Cub. Ft. of Air at 60° F. │ │ │ of Pipe per Hour. │ (15-1/2° C.) heated 1° per │ Temp. │ Pressure│ │ Ft.-run of Pipe per Hour. │ of │ of Steam├──────┬─────┬──────┬──────┼───────┬───────┬───────┬──────┤ Pipe.│ per In. │2 in. │3 in.│ 4 in.│ 6 in.│ 2 in. │ 3 in. │ 4 in. │ 6 in.│ ──────┼─────────┼──────┼─────┼──────┼──────┼───────┼───────┼───────┼──────┤ ° F. │ lb. │ │ │ │ │ │ │ │ │ 300 │ 53 │ 403 │ 545│ 669 │ 938 │ 22235 │ 28713 │ 36919 │ 51760│ 280 │ 35 │ 355 │ 480│ 587 │ 825 │ 19582 │ 26490 │ 32387 │ 45521│ 260 │ 21 │ 312 │ 421│ 515 │ 723 │ 17218 │ 23233 │ 28421 │ 39952│ 240 │ 10 │ 271 │ 366│ 448 │ 627 │ 14946 │ 20199 │ 24717 │ 34594│ 220 │ 2·5 │ 233 │ 313│ 384 │ 537 │ 12858 │ 17271 │ 21184 │ 29629│ 200 │ .. │ 195 │ 263│ 322 │ 452 │ 10775 │ 14507 │ 17780 │ 24967│ 180 │ .. │ 160 │ 216│ 264 │ 369 │ 8830 │ 11920 │ 14573 │ 20368│ 160 │ .. │ 128 │ 172│ 210 │ 295 │ 7070 │ 9487 │ 11590 │ 16300│ ──────┴─────────┴──────┴─────┴──────┴──────┴───────┴───────┴───────┴──────┘ It may be taken that 1/20 of the above volumes may be heated 20°, from 50° F. to 70° F., and so on; but if the average temperature is higher than 60° F., the duty will be less, and to obtain the same effect the pipe must be heated so much hotter as to keep the same difference as before between the pipe and air. Thus a pipe at 300° F. will only heat as much air at 80° F. as one of 280° F. will of air at 60° F. [Illustration: Fig. 67.]