As regards the heat consumed in evaporation; it requires about 1000 times as much heat to convert 1 lb. of water into vapour, as it does to raise the temperature of the same quantity 1° F. At least as much heat as this must be supplied if the air which has been used in drying is to retain the same temperature it had at the outset, and therefore if a turret is to keep at a higher temperature than the air, which is necessary to create a draught, this is the minimum amount of heat which must be supplied per pound of water to be evaporated. In practice much more will be needed.

The following table shows the heat given out by different sizes of pipes at different temperatures, and steam pressures, in units equal to the heat required to raise 1 lb. of water 1° F., and the cubic feet of air which they will heat.[V]

[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.

Temp. of Pipe.Pressure
of Steam
per In.		Units of Heat per Ft.-run
of Pipe per Hour.		Cub. Ft. of Air at 60° F.
(151/2° C.) heated 1° per
Ft.-run of Pipe per Hour.
2 in.3 in.4 in.6 in.2 in.3 in.4 in.6 in.
° F.lb.
3005340354566993822235287133691951760
2803535548058782519582264903238745521
2602131242151572317218232332842139952
2401027136644862714946201992471734594
2202·523331338453712858172712118429629
200..19526332245210775145071778024967
180..1602162643698830119201457320368
160..128172210295707094871159016300

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.