3. Drying with Heat.—Outside-air at 10° as above, heated to 25° C., giving an effective capacity for moisture of 13·5 - 2·0 = 11·5 grm. per cubic meter.
100011·5 = 87 cubic meters or 3070 cubic feet. To warm this 15° requires 391 cal.; and 600 cal. added for evaporation gives a total of 991 cal.
Comparing 2 and 3 we see that the higher temperature is more economical, where it can be allowed, than the lower, both in air and heat, though this is partly compensated by the greater loss of heat by cooling of the building, etc., which it entails.
4. Air at 0° C. heated to 20° requires about 97 cubic meters, or 3430 cubic feet of air, and a total of 1180 cal.
5. Air at 0° C. and heated to 25° C. requires 63 cubic meters or 2230 cubic feet, and a total of 1075 cal.
6. Air at -15° C. (5° F.) requires 4·5 cal. per cubic meter to raise it to 0° C., and acquires a capacity for drying of about 2 grm. per meter.
We will apply these figures to a drying room arranged with a screw-fan with a central division, or two floors, so that the air can be either circulated or replaced with fresh air from the outside at will (see [Fig. 94], [p. 435]). Such a room with 100 feet of length clear of space required for fans, air passages, and heating pipes, and 20 feet × 8 feet in section, should hang about 800 medium butts, weighing say 121⁄2 kilo (27 lb.) each, and when wet from the yard, containing the same weight of water. A 48-inch Blackman fan, under these conditions would probably move say 20,000 cubic feet (565 cubic meters) of air per minute, at the cost of 2 or 21⁄2 horse-power. This, in a room of the section named, would give an average velocity of 125 feet per minute or rather under 11⁄2 miles an hour; not at all too much to keep the air freely circulating among closely hung leather. If we assume that these butts are to be dried in a week (practically 10,000 minutes) under the conditions of No. 2, the 10,000 kilos of water they contain will require 1,700,000 cubic meters of air, or about 170 cubic meters per minute, or about 3⁄10 of the air must be fresh every time it passes through the fan. 1 kilo of water requiring 1110 cal. must be evaporated per minute.
Under the conditions of No. 4, only 97 cubic meters of air per minute would be required, or about 5⁄6 might be circulated without change, but the total heat required would be about the same, 1180 cal. Under the conditions of Nos. 4 and 6 some 1620 cal. per minute would be employed. It is hardly necessary to provide for the full amount of heat required by No. 6, since in this country such conditions occur but seldom, and never for more than a few days at a time, and during such a period, much less heat would suffice to carry on the drying at a slower rate, and keep out the frost.
Beside the heat required for actual drying, it is necessary to provide for that lost by the building during cold weather, and this is much more difficult to calculate. If, by arranging the outlet for moist air on the pressure side of the fan, the internal pressure of the building be kept a little lower than the outside, there can be no loss by escape of hot air, any leakage being inwards, and supplying a part of the change of air which, we have seen, is necessary. In a brick building with glass windows, the loss of heat is far less than in the old-fashioned wooden louvre-boarded structure, and where fan-drying is in constant use, the brick structure is much to be preferred. Frequent windows, with casements horizontally pivoted at the centre, will supply enough air for favourable conditions of air-drying, and when the weather is bad, resort is had to the fan. Most modern drying rooms in the Leeds district are built upon this plan. Where louvre-boarded structures must be used for fan-drying, the sides should be made as tight as possible in winter by sheets of canvas or sail-cloth nailed on, for which purpose old sails can be bought in seaport towns at reasonable rates, a few louvre-boards only being kept open for the admission of air in suitable positions.
Box, in his ‘Practical Treatise on Heat’[183] puts the loss through walls in brick buildings for a difference of 30° F. (16·6° C.) between inside and outside temperatures, at the approximate amounts shown in the following table.