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708 Chapter 40 1945 Guide 10.Table Corrosion Resisting Materials for Exhaust Systems*1 MATERIAL Mbtaia Acsnc Chromic Hydro chloric Hydro fluoric Nitric- Phos phoric Sul. PHUROU8 Sul phuric DiljConc. Dil. Cone. Dil. Cone. DiL jConc. Dil Cone. DiljConc. Dil. Cone. Dil Cone. Ahiminnm.. _ Magnesium and AUnya. Good Fur Poor No Data' Poor Good Poor No Data Good J Poor No Data Poorj Good No Data No Data Poor No Data Poor No Data Lead and Lead-Ccated--_- Poor ` Good Poor Poor Poor Poor Good Goot Poor Moly Alloy (60 Ni--2QMo --20 Fe),, Good No Data Fair No Data Poor ' Poor No Data Good Monel MetaL- Fair Poor Fair! Poor Goodd Fairj Poor Fur Poor Good s poor Bronze_____ Poor Silicon Iron.... ................. Fair | Good No Data Fur Poor Good Good Good No Data Good Stainless Steele (18 Cr8 Ni)- Enameled Steel.. Miscellaneous Aqhffrt/w Comp Good No Data Good No Data Poor Good No Data Poor Good Good Poor Poor Good except against strong adds and . Good Poor jOodO No Data Good WnnH Rnhha* Some woods are decomposed or softened faster than others. 1 1 1 Poor 1 1 - Poor In general plastics resist weak adds and are decomposed by concentrated acid. ^Standard Practice Sheet No. 115 (Division of Industrial Hygiene, New York State Labor Department). "Add mists in air are more corrosive than as liquid in storage tank. Galvanized iron not resistant to acid. cStainless steel of (24 Cr--10 Nf) fairly resistant at low temperature for HCl and Hi POt. d\Jnder most conditions. *At room temperatures. venturi ejectors, centrifugal exhaust fans, disc or propeller fans, and axial flow fans16. Manufacturers generally provide special fans for the collection of various industrial wastes. These are available for the collection of coal dust, wood shavings, wool, cotton and many other substances. When substances having an abrasive character are conveyed, the fan blades and housing should be protected from wear. This may be accomplished by placing a collector on the negative side of the fan or by lining the housing and blades with rubber. PROTECTION AGAINST CORROSION The removal of gases and fumes in many chemical plants requires that metals used in the construction of the exhaust system be resistant to chemical corrosion. A list of the materials which may be used to resist the action of certain fumes is given in Table 10. 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 and fumes as chlorine and hydrochloric acid. "The Axial Flow Fan and Its Place in Ventilation, by W. R. Heath and A. E. Criqui (A.S.H.V.E. Journal Section, Healing, Piping and Air Conditioning, February, 1944, p. 107). CHAPTER 41 S^dtemi Drying Methods, Radiant-Heat Drying, Conduction or Direct Contact Drying, Convection or Air Drying, Mechanism of Drying, Omissions in the' Cycle, General Rules for Drying, Humidity Chart, Dryer Calculations, 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. The subject of dehumidification is treated in Chapter 23. 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 evaporation 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. In some drying processes the evaporation of the liquid is accompanied by a chemical change, as in the drying of paint and varnish., DRYING METHODS Heat must be supplied in order to dry a solid by evaporation. Since this latent heat is large compared with the specific heat of. the materials, drying becomes largely a problem in heat transfer. Hence drying methods are . often classified according to the method of heat transfer used, as follows: 1. Radiant-heat drying. 2. Drying by direct contact, and conduction. 3. Convection or air drying. Radiant-Heat Drying Drying by sun heat is still practiced where danger of rain is slight, atmospheric pollution is negligible and sufficient time can be allowed. Radiating surfaces, (heated by steam, electricity or other means), afford a good method of heat distribution and control. Radiant heating sets up convection currents, and in low-temperature dryers only about one^third to one-half of the total heat for evaporation is actually supplied to the material by radiation. At high temperatures the radiation output in creases rapidly, according to the fourth-power law. The total radiation may be computed by the equations and tables given in Chapter 3. In general, fins and irregular- surfaces do not increase radiation, hence, the area to be used in calculations is the area of a smooth-surface envelope enclosing the radiating elements. A certain amount of air circulation is required through a radiant dryer, in order to carry off the vapor. 'jH.