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1108 CHAPTER 45 1957 Guide Table 6. Approximate Convetino Velocities Material Conveyed Design Velocity FPM 2,000 3,000 3.500 3,500-4,500 4.500 and over He a93 vp c*q.?2 PLAIN DUCT END =2- Ha 2.2 VP DUCT, OR BETTER, L78 VP ORIFICE + 049 VP DUCT 0.55 ORIFICE PUIS FLANGED DUCT 0*ANY SLOT TYPES) Iff , _c* c* He- F X VP Q-4009AC*VVP" F ENTRY LOU FACTOR VP- VELOCITY PRESSURE M DUCT SP SCATS PRESSURE AT THROAT, INS. WATER GAGE He* ENTRY LOSS, INS. WATER GAGE Q AIR VOLUME,CFM A CROSS SECTION AT THROAT, SO- FT. C* COEFFICIENT OF ENTRY ENTRY LOSS FOR COMPLICATED HOOD SHAPES L BREAK HOOD WTO SIMPLE COMPONENTS 2. CALCULATE He FOR EACH COMPONENT 3. ADD VALUES OP H HOOP ENTRY UPSS C062- 09 TAPERED HOOPS FLANGED OR UNTLAMGCDj ROUND SQUARE OR REC TANGULAR. IS THE MAJOR ANGLE ON REC TANGULAR HOOOS- cr > cni> --w4_j=p> TAPER R, (REGAIN) L, (LOSS) ANCLE, FRACTION OF FRACTION OF 0CGRQE3 VP DIFFERENCE VP DIFFERENCE W s K a 20 25 SO OVER 30 0.7 072 OS6 042 0.26 OIS 0.00 0.00 0.22 0.2 0.44 O.SO 0.72 0 67 1.00 1.00 DECELERATION LOSS L REGAIN TAPER L, (LOSS) ANGLE, FRACTION OF OCCREES VP DIFFERENCE s O.OS H5 006 IS OOO 20 0.10 2S Oil 30 0.13 45 0.20 60 03 ACCELERATION LOSS ANGLE LOSS FRACTION OF VP M DEGREES BRANCH M O.OS IS 009 20 012 25 OIS 30 aie 36 0.21 40 0J 43 0.26 SO 032 0 044 *> LOO BRANCH ENTRY LOSS -J EOUAULV spaced BRACKETS AS SHOWN H- NO OF DIAMETERS 1.00 D 0.73 D 0.700 o.eso 0.60D 055 D O.SO D 0.4SD LOSS FRACTION OF VP 0.10 018 022 0.30 0.41 0.S6 0.73 1.00 WEATHER HOOD LOSS Fig. 2. Exhaust System Design Data13 Tapered hood data from Reference 37 Weather hood data from Reference 38 ' Industrial Exhaust Systems 1109 exhaust system construction specifications and DESIGN DETAILS1* Correct design and competent installation of sheet steel ducts and hoods are necessary for the proper functioning of any exhaust system. The follow ing specifications are mainly those recommended in the manual Industrial Ventilation.13 General Requirements All exhaust systems shall be constructed with the materials recommended here with and shall be installed in a permanent and workmanlike manner. The interior of all ducts shall be smooth and free from obstructions; with joints either welded, flanged, or soldered air-tight. Materials a. Ducts shall be constructed of black iron welded or of galvanized sheet steel riveted and soldered unless the presence of corrosive gases, vapors and mists makes use of such material impracticable. Galvanized construction is not recommended for temperatures exceeding 400 F. Welding of black iron of 18 gage and lighter is not recommended for field fabrication. b. For average exhaust systems on non-corrosive applications the metal thickness in the following table shall be supplied. Diameter of Straight Ducts (Inches) Up to 8.................................................................. Over 18 to 30........................................................ Over 30.................................................................. U. S. Standard Gauge for Steel Duct ' Class I Class II Class m 24 22 20 22 20 18 20 18 16 18 16 14 Class 1. Includes ducts non-abrasive applications Buch as for paint spray, wood working, pharmaceutical, and food products. Class II. Includes ducts for non-abrasive material in high concentration flow pressure pneumatic conveying); moderately abrasive material; and highly abrasive materials in light concentrations. Typical examples are chemicals and wood dust; exhaust of foundry shakeouts and sanefhandling systems, grain dusts; coal crushing and screening; exhaust of grit blast cabinets; and grinding, buffing.and polishing. Class 111. Includes ducts for conveying all highly abrasive materials in moderate to heavy concentrations and moderately abrasive materials in heavy concentrations as in: low pressure conveying of tobacco; and exhaust systems from abrasive cleaning operations, rock and ore screening and crushing, dryers and kilns, and flyash from boiler stacks. Where aluminum duct is indicated, the following equivalent B & S gage thickness of sheets should be used: Steel--U. S. Standard Gage Aluminum--B & S Gage 26 24 22 20 18 24 22 20 18 16 c. Elbows and angles shall be a minimum of two gages heavier than straight lengths of equal diameter. d. Hoods shall be a minimum of two gages heavier than straight section of con necting branches. e. Where flexible piping is necessary, a non collapsible type of flexible piping shall be used and its length shall be kept at a minimum. Construction a. Longitudinal joints of ducts shall be lapped, and riveted or spot-welded on -in. centers, maximum. b. Girth joints of ducts unless flanged or butt welded shall be made with lap in direction of air flow with not less than 1-in. lap.