Document v1wVyE5X97ndb0Mbp4B495wjb

844 CHAPTER 46 1946 Guide ' damper for purpose of inspection, cleaning, or for renewal of fusible link. All exhaust piping to range hoods, commonly called grease ducts, should be provided with tight fitting cleanout doors of adequate size to permit easy removal of grease. Some engineers use filters to advantage in hoods which are subject to grease conditions. Hoods over steam tables should be of similar construction to range hoods. It is good practice to design such hoods with a face velocity of 60 to 70 fpm. Hoods over dishwashing machines are usually relatively small and generally 1500 to 2000 cfm per hood is allowed, which is equivalent to a velocity of approximately. 100 fpm per square foot of face area: Range hoods in diet kitchens are constructed the same as restaurant range hoods but with less exhaust air per square foot of face area, depending upon the nature of the food cooked. Hoods are not often used in private residences unless they are quite large and the consideration of expense is not important.' For such residences the hoods should be designed on the same basis as diet kitchens. Most all residence kitchens can be effectively and economically venti lated by the installation of a built-in kitchen ventilator, which should be located in an outside wall and in close proximity to the kitchen range. It has been found that the capacity-of the built-in kitchen ventilator should be at least 350 cfm regardless of the size of kitchen. This can be justified on the basis that the smaller the kitchen the more concentrated the heat will be thus requiring a more rapid rate of air change. Standard size built-in kitchen ventilators are generally available in three sizes, namely 350, 500 and 800 cfm. The proper size to use will depend on design conditions and available wall space. DUCT SYSTEM DESIGN In' designing a duct system it is necessary to recognize a few funda mental principles (see also Chapter 41). Knowing the quantity of air required, the size of the duct may be computed from Equation 5: where A = cross-section area of duct, square feet. . Q = air quantity to be handled by the duct, cubic feet per minute. V = velocity of air, feet per minute. . Air Velocities in Ducts Where it is necessary to transport the particulate material collected in an exhaust system, minimum carrying velocities must be maintained in the ducts preceding the collector. It has been found that good results are obtained when design air velocities in horizontal runs are not less than 2000 fpm or not greater than 5000 fpm. When the dust being carried is organic and other than wood flour, or similar material, a velocity of 2500 fpm is adequate. Approximate required conveying velocities are given in Table 6. . ', . ..For duct systems wherein the air has no dust .or solid load, a lower velocity is desirable, which may range from 1200 to 2000 fpm. In view ' of the fact, that the horsepower required by a system depends, directly ori the resistance and the resistance is a function of the velocity, economicardesign requires velocities of this magnitude. Exhausting and Conveying Systems 845 The equal friction method is generally used for designing a duct system as this insures equal resistance to air flow in all branches throughout the System (see Chapter 41). Long main ducts do not generally provide the most economical layout. Where it is necessary to ventilate a large number of machines, or machines which are widely separated, it is desirable to locate the fan at approximately the center of the system. With this arrangement it is possible to choose a fan which will deliver the required air quantity against a lower resistance pressure, and this will generally result in a horsepower saving. When a system carrying dust is designed with'an oversize main duct to allow for future extension; the air velocity may be found to be too low to carry the dust, and serious plugging may occur.. In this case it is desirable to install an orifice in the end of the pipe to allow for the lower air quantity. Construction The interior of all ducts.should be smooth and free from obstructions at joints and soldered air-tight. Other sealing mediums are permissible where soldering is impracticable. Ducts should be constructed of galvanized sheet metal except when the presence of corrosive fumes or gases, temperatures above 400 F, or other factors would make galvanized material impractical. For the usual exhaust systems the metal thicknesses shown on Table 7. are recom mended.' Elbows and angles should be a minimum of two gages heavier than straight lengths of equal diameter. Hoods should be a minimum of two gages heavier than straight sections of a connecting branch. Longitudinal joints of ducts should be lapped and riveted or spotwelded on 3-in. centers maximum. Girth joints or ducts should be made with lap in direction of air flow, with 1 in', lap for duct diameters through 19 in. and in. lap for diameters over 19 in. Elbows and angles should have an inside or throat radius of two pipe diameters whenever possible. Large radii are recommended for heavy concentrations of highly abrasive dusts. Elbows 6 in. or less in diameter should be constructed of at least 5 sections and, if over 6 in. in diameter, of 7 sections, with angles pieced proportionally. Hoods should be free of sharp edges or burrs and re-, inforced to provide necessary stiffness. 'Transitions in mains and submains should be-tapered with a taper 5 in. long for each 1 in. change in diameter whenever possible. All branches should enter the main at the large end of the transition at an angle not to exceed 45 deg or preferably 30 deg. Branches sho.uld be connected only to the top or sides of mains, with no two branches entering diametrically opposite to each other. Dead end caps should be provided within 6 in. from last branch of all mains and sub-mains. Cleanouts should be provided every 10 ft and near each elbow, angle, or duct junction in horizontal sections. Ducts should be supported sufficiently to place no loads on connected equipment and to carry weight of a system plugged with material. The Table 6. Approximate Conveying Velocities Matebial Conveyed Design Velocity FPM Vapors, gases, fumes, very fine dust.................................................w... Fine drv dusts.... ........... .............._......... Averaee industrial dusts Coarse oarticles.......................... .......... Large particles, heavy loads, moist materials - 2,000 3,000 3.500 3,500-4,500 4.500 and over