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HEATING VENTILATING AIR CONDITIONING GUIDE 1942 Table 8. Materials to be Used for the Protection of Exhaust Systems Against Corrosion Ttpk or Fume Conveyed Photectiyi Material to be Used Rubber lining or chrome-nickel alloys Hydrogen sulphide....... - . Aluminum coated iron, aluminum, high chrome-nickel alloys Iron or steel .' Sulphurous gases__________ High chrome-nickel alloys Rubber lining, chrome-nickel alloys Nitrous gases______________ Nickel-chrome alloys Condensed from data given by Chilton and Huey (Industrial and Engineering Chemistry, Vol. 24,1032). chemical corrosion. A list of the materials which may be used to resist the action of certain fumes is given in Table 8. Hoods and ducts when short, may frequently be constructed of wood and be quite effective. Rubberized paints are available and may be applied as protective coatings in handling such gases andfumes as chlorine and hydrochloric acid. 740 Chapter 41 DRYING SYSTEMS Drying Methods, Driers, Mechanism of Drying, Moisture, General Rules for Drying, Equipment, Humidity Chart, Com bustion, Design, Estimating Methods DRYING, in its broader sense, refers to the removal of water, or other volatile liquid from either a gaseous, liquid, or solid material. In practice, the process of direct drying gaseous material is referred to generally as dehumidifying, or condensing, and in some cases chemicals are used in the adsorption or absorption of moisture. Drying a liquid is called evaporation or distillation. The common usage of the word drying refers to the removal of water or other liquid, such as a solvent, by evapo ration from a solid material. When the solid to be dried contains large amounts of free water, the actual drying process is frequently preceded by the removal of part of the water by some mechanical means, such as filtration, settling, pressing or centrifuging. Removal of as much water as possible by such methods is usually advisable, as the cost of these operations, per pound of water removed, is generally much less than by evaporation. DRYING METHODS Drying may be accomplished in any one or combination of the following methods: 1. Radiation. 2. Conduction, or direct contact. 3. Convection. Radiation The source of heat for radiation may be either the sun, or heated surfaces. Sun drying is practiced where danger from rain is slight, and where sufficient time can be allowed. Where a strict adherence to a schedule is necessary, or where dusty atmosphere is present, this method is not in favor. Fruits are often dried in the sun. Radiation from hot surfaces (heated by steam, electricity, or other means) furnishes generally, from one-third to one-half the total heat required for evaporation. Convection currents set up by these hot surfaces and the cooler materials carry the balance of the heat. 741