This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
262. The process of drying air is just the reverse of that for drying cloth and other wet materials.
In the ordinary process of drying, the materials are heated, in order to convert the moisture into steam and drive it off into the atmosphere; but in dealing with humid air, the only way by which the moisture can be reduced is by condensing a part of it.
Air cannot possibly be dried by heating it; on the contrary, if hot air encounters any water, it will impart heat to it and cause more evaporation, thus actually increasing the amount of vapor per cubic foot of space. The only practicable method, therefore, of drying air, that is, of condensing the atmospheric steam, is to lower its temperature.
263. The method usually adopted for this purpose is to pass the air over trays containing ice, or over pipe coils containing cold brine or other refrigerating liquids. It has been attempted in many cases to perform the work with coils which were filled with water at ordinary natural temperature, but the results were not entirely satisfactory.
An apparatus that employs natural water in the coils is useful for cooling purposes only: it is quite incapable of drying ordinary air, because the water seldom has a temperature in summer time of less than 55° or 60°, and, consequently, cannot lower the dew point below 60°. The humidity of air at 70°, having its dew point at 60°, would be about 71 per cent., which is far too high to be comfortable. In this respect, such apparatus is quite inferior to that in which ice or cold refrigerating liquids are employed.