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256 CHAPTER 21 Table 5 .... Exhaust Requirements0 for Grinding, Polishing, Buffing, Scratch Brushing Abrasive Cut-Off Wheels, Grinding and Polishing Belts (Air Vokm*: 4500 fpai m off brandies.) (2-in. wafer wcfion at avareg* hoodj 1962 Guide And Date Book * Increase breach diameter QJ In., but not lea* m in width over noted. yi in., far each t in. in- the air flow. For conveying of solid particles, velocities shown in Table 8 include a reasonable factor of safety and, consequently, design velocities may therefore be ad justed as required to permit use of commercial pipe sizes that are available generally in Vt-in. diameter steps through 6-in. diameter, 1-in. steps through 16-in. diameter or larger, and 2-in. diameter steps for large diameters. For contaminants other than solids, conveying velocities 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 procedure 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 stub branches, in the 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 the system layout so that exhaust from future points can be picked up by a separate main that will run directly to the tan or air cleaning equipment inlet. CALCULATION OF SYSTEM PRESSURE LOSS The pressure loss of the system includes entrance loss4* as air is accelerated through the hood to a branch duct connection, resistance losses of ducts,44 elbows, and junc tions, and acceleration or deceleration losses from ve- Fig. 2 .... Exhaust System Design Datau locity changes within the system. Chapter 12 of the 1961 Guide And Data Boos includes data on duct resistance and on resistance of elbows, and provides sample calculations. In exhaust systems, either the equal friction method or the use of blast gates in the branches to equalise pressure loss of all runs is employed.4,11,1*'1* Data oh losses for hood entrance, branch entry, transi tions, 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 branch velocity pressure and tbe 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 oi any exhaust system. The following specifications are those recommended in the Industrial Ventilation Manual General All exhaust systems dull be constructed with the materials recommended herewith and shall be installed in a permanent Industrial Exhaust Systems 257 Table 6 ... .Ventilation Rates for Open Surface Tanks*8-*7 Miarmem VcnSfation Rate Mtntmwtn VentdaSoo Rafe Cfm Hr Sq ft Hood Cfa For Sq Ft Tank Ana Opening lateral fxfieud (Nofe I) [W/l- . Coopy Hood Took length Process Minimum Ventilation Rafe Cfa Per Sq Ft Hood Opening r ... Canopy Hood " Hood Minnaurn Ventilation Rafe Cfm For Sq Ft Tank And Lateral Cxhoud (Nofe 1) Tn--L 1 Tank length On* Two {Opan [Op*n Side Sldat W/l 0-0.24 W/l 0.250.49 w/i 0.50- One OfeA Two OpA Three Open four Open W/l 0-0.24 Sid* Sides A8 W/l 0.250.49 A8 W/l 050- 1.0 A8 Plating Chromium (Chromic Acid Mist) . Arsenic (Ar sine) Hydrogen Cy anide................ Cadmium..... Anodising.............. Metal Cleaning (Pickling) Cold Acid......... Hot Acid............ Nitric and Sul furic Acid... - Nitric and Hy drofluoric Acid................. Metal Cleaning (Degreasing), See Note 2) and Bef. 38 and 39 Metal Cleaning (Caustic or Electrolytic) Not Boiling. .. Boiling.............. Bright Dip (Ni tric Acid)... Stripping Concentrated Nitric Acid.. ,130 no iso! !125175 150 200}: jl25 175 150 1125 1175 Stripping Cone. Nitric and Sul furic Acid... 75 Salt Baths (Mol ten Salt).... 50 Salt Solution (P&rkerise, Booderixe, etc.) Boiling................ 90 75 Hot Water (if vent, desired) Not BoilingBoiling............ 50 75 100 125 175 125 175 150 200 175 225 75 75 125 60 90 75 100 90 110 90 100 150 90 130 110 150 130 170 100 125 1/5 125 175 150 AX) 175 225 no9d 75 75 125 60 90 75 100 100 125 175 125 175 150 200 176 225 Note l--Column A refer* to tank with hood aloes oat tide or two parallel aide* when ooe hood ie afxinst a wall or a baffle raAninc lensth at tank and a> hiefa aa tank b wide; alao to tanks with ezhauat manifold alone canter line with jf -- becomint tank width In W/L Batin. i! Column B refers to freestanding tank with hood elan* one aide or two parallel idea. Note S--Complete control of the npm sod mat from degreaei&f operation* requires tbe aaiae ventilation rates aa recommended for pickling. However, the advents employed in degreaei&gopeniioaaare relatively volatile, and thesolvent loss caused by evaporation increase* rapidly as the exhaust rate at the tank' ita-iuauui. For this reason perfect control ia usually sacrificed in favor of lower solvent evaporation rates. Where solvent lass docs not present an important cost at operating problem, the exhaust rate* given for pickling should be adhered fey but when solvent koa mat be kept at a minimam, so exhaust rate of 60 cfm per *4 ft of task area is commonly employed. Where this rats is used, control will be adequate only if the tank is located ia an area tree of drafts and il tba dsgnsm&g i with a rigid schedule.* aod workmanlike manner. The interior of all ducts shall be smooth and free from obstructions; with joints either welded, flanged, or soldered airtight. Materials 1. Ducts shall be constructed , of block iron welded or of galvanized sheet steel riveted and soldered unless the presence of corrosive gases, vapors, and mists or other conditions make use of such material impracticable. Galvanised construction is not recommended for temperatures exceeding 400 F. Welding,of black iron of 18 gage and lighter is not recommended for field fabrication. 2. For average exhaust systems on non-corrosive applications the following metal thickness shall be supplied: Ofeerefer of Straight Oacls (Jnehei) U. S. Standard Gog* for Sfeef Dad Gass 1 Oats 11 Goss PI Up to 8... ....................... 24 22 20 Over 8 to 18..................... 22 20 18 Over 18 to 30................... 20 18 16 Over 30 18 16 14 Class 1. Includes non-abrasive applications such as paint spray, woodworking, pharmaceutical and food product, and dis- -- charge ducts from dust collectors. Class II. Includes applications for non-abrasive material in high concentration (low-pressure pneumatic conveying); mod erately abrasive material; and highly abrasive materials in light concentrations. Typical applications are conveying of chemicals and wood dust' exhaust of foundry shakeouts and sand handling systems, grain dusts; coal crushing and screening; exhaust of grit blast cabinets; and grinding, buffing, and polish ing. Class III. Includes applications for all highly abrasive ma terials in moderate to heavy concentrations and moderately abrasive materials in heavy concentrations such as low-pressure conveying of tobacco; exhaust systems from sand and gnt blast ing and abrasive cleaning operations, from rock and ore screen ing and crushing, from dryers and kilns, and fly ash removal from boiler stacks. Where aluminum duct is indicated, the following equivalent BAS gage thickness of sheets should be used: Steel--U. S. Standard Gage 26 24 22 20 18 16 14 Aluminum--B St S Gage 24 22 20 18 16 14 12 3. For exhaust systems on corrosive applications, considera tion should be given to non-corrosive materials or coatings, (Continued on p. 6$)