Document zdj8av2rpwkx7L5dgXKmXRmn0

1132 CHAPTER 45 1958. Guide . . Table 8. 1 Approximate Conveying Velocities Material Conveyed Design Velocity FPM Coarse particles.................................................................................... 2,000 3,000 3.500 3,500-4,500 4.500 and over L- * 2.3 Pv DUCT OP BETTER 17a84P9*PORotFuICcET C = O.S5 Le=aso pv C 0.62 , Le = o.65Pw C* - 0.78 ORIFICE PI-US P1RECT BRANCH STANDARO ' FLANGED DUCT BOOTH CRINDER HOOP 120 180 a tHCLUDEO ANCLE IN DECREES i-c,2.. _ u,* c>2 <p, rSENTRY LOSS FACTOR ~ VELOCITY PRESSURE IN DUCT, IN. WATER P, a STATIC PRESSURE AT THROAT, IN. WATER L*= ENTRY LOSS, IN. WATER FACE AREA AT LEAST TWO TIMES DUCT AREA NT Q = AIR VOLUME, CFM C2 l*r x pv q = 4005 AtCs/T7' At= CROSS SECTION AT THROAT, SQ FT C = COEFFICIENT OF ENTRY ENTRY LOSS FOR COMPLICATED HOOO SHAPES L BREAK HOOO INTO SIMPLE COMPONENTS 2 CALCULATE Le FOR EACH COMPONENT 3.ADO VALUES OF L HOOD ENTRY LOSS Cs 0.82-0.98 TAPERED HOOPS . . FLANGEO OR UNFLANCEO} ROUNO, SQUARE OR RECTANCULAR. 6 IS THE MAJOR ANGLE ON RECTANCULAP HOOOS. CE2a- ->*420 MlNj*- i ---n--^ R. (REGAIN) P* (LOSS) FRACTION OF FRACTION OF DEGREES P. DIFFERENCE P,, DIFFERENCE 0.78 0.22 S 0.72 0.28 O.S6` " 0.44 0.42 058 0.28 072 25 0.13 0.87 aoo 1.00 OVER SO 0.00 1.00 DECELERATION LOSS & REGAIN TAPER Pk (LOSS) ANGLE FRACTION OF DEGREES P. DIFFERENCE S O.OS 10 0.06 15 0.08 O 10 25 0.11 30 an 45 0.20 80 0.3 . ACCELERATION LOSS ANGLE LOSS FRACTION e OF Pv IN DECREES BRANCH 10 IS 20 25 30 35 ' 40 45 . .50 - 60 90 0.06 0.09 0.12 0.15 0.16 0.21 0.25 0.28 0.32 044 1.00 BRANCH ENTRY LOSS `..It -5 EQUALLY SPACED BRACKETS AS SHOWN /#-NOior LOSS' FRACTION DIAMETERS OF Pv A 7SD O 70 D ' 0.65D 0.60 D 0 55 0 : 0.500 0.450 010 O 18 0.22 0.30 0.41 as* 0.73 1 OO -- -- WEATHER HOOO LOSS Fig. 2. Exhaust System Design Data15 Tapered hood data from Reference 43 TI7^.iL. flofa (mm P(fAnni i4 * ! ,:r ! Industrial Exhaust Systems 1133. 1-in. steps, through 16-in. diameter or larger, and 2-in. diameter steps for large diameters. For contaminants other than solids, conveying ve-` locities are based on a consideration of the higher resistance and lower first cost of smaller diameter ducts against higher first cost and lower resistance of larger diameter ducts. Main ducts connecting two or more branches are sized by the same pro cedure as for branches. The main should be designed for the total exhaust volume to be handled through all branch ducts. Where exhaust systems handling solids are to provide for a substantial' increase of future capacity, required conveying velocities can be maintained" by: 1. Providing open end Btub branches in main through which air will be admitted into system until future connection is made. The volume admitted into the main, can be adjusted to required air flow- rate, cfm, by use of a blast gate or orifice plate in such stub branch. 2. Arranging system layout so future points can be picked up by a separate main which will run directly to the fan or air cleaning equipment inlet. CALCULATION OF SYSTEM PRESSURE LOSS . The pressure loss of the system includes entrance loss4.3 as air is accelerated-through the hood to a branch duct connection, resistance loss of ducts,44 elbows, and junctions, and acceleration or deceleration losses from velocity changes within the system. Chapter 31 includes data on duct resistance andon resistance of elbows, and-provides sample calculations. Tn exhaust sys tems, either the equal friction method (illustrated in Chapter 31) or the use of blast gates in the branches to equalize pressure loss of all runs is em ployed.8, 11 16 19 Data on losses for hood entrance and branch entry, transitions and weather hoods are given in Fig. 2. The static pressure in branch ducts close to the hood connection is frequently referred to as hood suction. It is the sum of the velocity pressure in the branch and the hood entrance loss. CONSTRUCTION SPECIFICATIONS FOR LOCAL EXHAUST SYSTEMS Correct design and competent installation of sheet steel ducts and hoods are necessary for the proper functioning of any exhaust system. The following specifications are those recommended in the Industrial Ventilation Manual.15 General w th" e^aust systems shall be constructed with the materials recommended here of he installed in a permanent and workmanlike manner. The interior shall be smooth and free from obstructions; with joints either welded, linked, or soldered air-tight. Materials rivetertUCt3 3*>ah be constructed of black iron welded or of galvanized sheet steel condfr ant* s.^ere<J unless the presence of corrosive gases, vapors and mists or other recoin10ns make use of such material impracticable. Galvanized construction is not and ?j1?an"<l for temperatures exceeding 400 F. Welding of black iron of 18 gage Enter is not recommended for field-fabrication.