Document 2J64eO9rBqrMZEd2nVE2eonLb

Heating Ventilating Air Conditioning Guide 1938 Table 8. Materials to be Used for the Protection of Exhaust Systems Against Corrosion3 Ttpb or Fume Conteted Protective Material to be Used ' Rubber lining or chrome-nickel alloys Aluminum coated iron, aluminum, high chrome-nickel alloys High chrome-nickel alloys Hydrochloric acid________ Rubber lining, chrome-nickel alloys Nitrous gases______ __ ___ Nickel-chrome alloys Condensed from data given by Chilton and Huey (Industrial and Engineering Chemistry, Vol. 24,1932). 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 and fumes as chlorine and hydrochloric acid. PROBLEMS IN PRACTICE 1 What is the most common method of reducing total air volumes handled in cases employing large hoods over apparatus covering a large area? The use of the petticoats on large hoods which permits a comparatively high air velocity at the rim of the hood and controllably small velocities in the center. 2 Why is it not permissible to connect up emery wheels and buffing wheels to the same exhaust system? Emery wheels and buffing wheels should be handled by separate systems because of the fire hazard, as it is possible for sparks from the emery wheels to ignite the lint and dust from the buffing wheels when both are carried through the same system. 3 A tank, 4 ft by 8 ft, contains a fluid which gives off injurious, vapors. A large hood is located 30 in. above the top of the tank and extends slightly over its edges. Assuming that a velocity of 60 fpm is required to adequately control the vapors near the edges of the tank, calculate the air flow, required. Using Equation 4, P = 2 X 4 + 2 X 8 = 24 ft; 0 = 30 in. = 2.5 ft; V = 60 fpm. Hence, Q = 1.4 X 24 X 2.5 X -60 = 5040 cfin. 4 Silica dust with a specific gravity of 2.65 is being conveyed in a duct system. The velocity measured in a vertical portion of the system is found to be 2700 fpm. What is the mHomnm diameter particle 'transported at this velocity? 2 65 Using Equation 5a, 2700 = 13,300 X X <?>" - - : from which d = (0.281k = 0.11 in. 5 . What special materials may be used to resist chemical corrosion .in a system exhausting gases and fumes? Various protective materials are available for exhaust systems depending largely upon the type of fumes conveyed. Nickel-chrome alloys, aluminum coated metals and rubber linings are extensively used. Also protective rubberized paints are available which may be applied for conveying chlorine and hydrochloric acid fumes. 646 Chapter 35 DRYING SYSTEMS Drying Methods, Driers, Mechanism of Drying, Moisture, General Rules for Drying, Equipment, Humidity Chart, ' Combustion, Design, Estimating Methods DRYING, in its broader sense, refers to the removal of water, or other volatile liquid from either a gaseous, liquid, of 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.