Document 36VkrMoG3jv0L8MjOj59qbKJ

Heating Ventilating Air Conditioning Guide 1938 Chapter 35. Drying Systems Conduction or Direct Contact Drying This method of drying is advantageous where the material can be flowed on to the drying surface and the dried material scraped off, or where the material to be dried can be handled in a sheet, and where there is no danger of subjecting the product to the full temperature of the heating medium. The source of heat for this method may be steam, electricity, hot oil or hot water. Convection The circulation of heated air or other gases about the material to be dried is generally termed convection drying. The convection may be either natural or forced. With forced circulation, the temperature of the T a b l e 1. D r ier s for E vapo r atio n of W ater Fig. 1. Relation Between Useful and Total Heat Supplied Fig. 2. Rate of Drying of Whiting Slab drier is more uniform and the rate of drying is much higher than with natural circulation. Where humidity is used, the control is much easier, and more accurate. The source of heat for a convection drier may be steam, electricity, hot water, oil-fired heater, gas-fired heater, or coal furnace. Where either oil, gas or coal is used, the type of heater may be direct or indirect; i.e., the products of combustion may be used (direct), or the circulated air may be heated through an interchanger (indirect). Where the direct type is used, there is naturally a higher thermal efficiency, but it can only be used where the odor, soot, or the chemical elements of the products of combustion do not affect the material being dried. When heat economy is an important consideration this method (Fig. 1) may be used, permitting a small amount of air to be circulated, if a sacrifice of accurate control of temperature and humidity can be justified. DRIERS The term adiabatic drier is applied to a drier in which all the heat is supplied by air externally heated. The temperature of the air in the 648 649