Document VG8rbpJqMb2aXYgpe42L0ybRq

American Society of Heating and Ventilating Engineers Guidi to a minimum. A factor often overlooked is that of suitable provi I for ventilation of the space around the dryer openings so that moist air which may escape from the dryer when the doors are one*^ may be carried away before it has a chance to condense on the c?^ or windows of the building in which the dryer is located or into it opens. Convenient arrangements for the operator should also be n ' vided where tests can be made, records kept, and clothes changed case it is necessary for the operator to.work inside the dryer. ' 111 EXPERIMENTAL TECHNIQUE One vitally important phase of dryer design has been ignored in thi preceding discussion, namely, the time required to dry the stock, or, what is equivalent to the same thing, the size of the dryer, and the character and extent of the contact surface between the stock and the air. In general, this cannot be determined except on the basis of experimental data on the specific material to be dried. The source of such data may either be the known performance of actual commercial drying installa tions handling the same stock or direct experimental determinations in the laboratory. Data of this type for many materials are given in Table 8. Where it is necessary to determine drying conditions and rate in the ' laboratory, it is vitally important properly to control the experimental conditions. Where possible, the material upon which the experiments are made should have the same shape and size as that to be treated'; commercially. Furthermore, the conditions of exposure to the drying air, the temperature and humidity of that air, and its velocity and dis tribution over the material should be identical with those used, in the full-scale operation. Where the commercial operation is by batch, it is ' relatively easy to duplicate commercial. conditions in the laboratory. REFERENCES Commercial Drying Apparatus, by L. P. Dwyer (Transactions, A. S. H. V. E., Vol. 22,1916, No. 418)./ Artificial Drying with Special Reference to the Use ofGas, by G. C. Shadwel) (Transactions, A.S.H.V.E., Vol. 23. 1917, No. 440). Drying by Evaporation, by F. R. Still (Transactions, A. S. H. V. E., Vol. 23, 1917, No. 441). Drying in Industrial Plants, by J. G. Rosa. , High Temperature Drying, by Burt S. Harrison (Transactions, A. S. H. V. E-, Vol. 24,1918, No. 472). The Temperature of Evaporation, by W. H. Carrier (Transactions, A.S.H.V.E., Vol. 24,1918, No. 473). Commercial Dehydration, by J. E. Whitley (Transactions, A. S'. H- V. E., Vol. 26, 1920, No. 678). Drying os an Air Conditioning Problem, by A. W. Lissauer (Transactions, A. S. H. V. E., Vol. 27, 1921, No. 600). A Chronological Survey of Drying and Dryers, by J. E. Bolling (Journal, A. S. H. V. E., October, 1921. t p. 716). Modem Drying Machinery, by H. B. Crenshaw, London, 1926. The Kiln Drying of Lumber, by A. Koehler and R. Thelen, New York, 1926. Drying, by W. H. Carrier (Marks' Mechanical Engineers Handbook, 2d ed., 1924). Drying, Kent's Mechanical Engineers Handbook, 10th ed., 1923. Calculations for Drying Design, by Grosvenor (Transactions, A. /. Chem. Eng., 1908, p. 184). The Rate of Drying Solid Materials. by J. Lewis (Ind. Eng. Chem., 1921, p. 427). Principles of Chemical Engineering, by Walker, Lewis, McAdams, 1923. (Chapters 12 to 16 on Evapo ration, Humidity and Drying. The Kiln Drying of Lumber, by H. D. Tieniann (Lippincott, 1920). Drying by Means af Air and Steam, by E. Hausbrand (D. Van Nostrand & Co., 1901). Principles of Drying Lumber and Humidity Diagram, by H. D. Tiemann (Forest Service Bui. 104,1912). Symposium on Drying. Articles by W. K. Lewis, W. H. Carrier, A. E. Stacey and Fleming, R. G. Mere, G. B. Ridley, C. O. Lavett, D. J. Van Marie (Jour. Ind. Eng. Chem.) - 494 Chapter 31 DUCTS, GRILLES, AND REGISTERS' - Resitance to Flow; Static, Velocity, and Total Pressures; Funda mentals of Duct Design; Friction Losses; Methods of Distribution; " Grilles and Registers; Measurement of Air Flow. tHE laws governing the flow of all fluids are based on the assumption -1 that the density remains constant throughout the flow. In consider- tjje gow of a gas such as air, however, the laws referred to do not strictly hold. The velocity in an air duct of uniform size varies due to a loss or decrease in pressure which causes an increase in volume and a jonTsheequfleonwt ionfcraeiar sdeueintothae lvaergloecditiyff.erence in pressure is most accurately stated by thermodynamic formulae for air discharge under.conditions or adiabatic flow. The flow of air in heating and ventilating ducts takes place under very small pressure differences, usually less than a pressure equivalent of 2 in. of water. Under this condition the change in density is very slight and for all practical purposes of calculation and design the density may be assumed constant and the laws governing the flow of fl.u' Fidosraoprdpliineadr.y duct work where flow exists under a low pressure difference but slight error is introduced, if the same formulae are applied to the flow of air as are commonly used for the flow of water. The basic formula for such calculation is; p_ V = 1096.5 W (1) where V => velocity in feet per-minute. p = head or pressure in inches of water. ' For staWnd=ardweaiigrh(t7o0f aFi,rainndpo2u9n.9d2s pbearrocumbeictefro)oWt. = 0.07495 ib per cubic foot. Sub stituting this value in equation (1) . JV = 1096'5 X _0_.07=0945005 J 1 (2) An inspection o'f Formula 1 T indicates that temperature affects the velocity of flow by changing the weight per cubic foot of air. Tables 3 and 4 (Chapter 2) give the velocity at various pressures and tem- peratures. RESISTA/inNuCuE, Ti O1/ F1' LAIOVWr , two types of loss that occur when air moves in a confined There are "--' --of the air with the walls of the