Document EoYwDpgrXG0myY08EYQY4YeN

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 Table 4. Recommended Sheet Metal Gages for Ducts3 U. S. Std. Gage . 26 24 24 Round Ducts Diameter, Inches Up to 18 19 to 30 Maximum Side, Inches Up to 12 13 to 24 25 to 30 Rectangular Ducts Type of Joint Connections S or Drive Slips 5 or Drive Slips 5 or Drive Slips Bracing None None 1 in. Angle. 4 ft from Slips 22 31 to 45 31 to 48 Bar or Drive Slips V/i in. Angle 4 ft on Centers 20 46 to 60 49 to 60 1}4 in. Angle Bar Slips or 1 in. Angle Connections 18 61 and up 61 to 90 lj/ in. Angle Connections 1J4 in. Angle 2 ft 8 in. on Centers 13^ in. Angle 2 ft 8 in. on Centers 18 91 and up 2 in. Angle Connections 2 in. Angle 2 ft 8 in. on Centers aif flat sides are not cross-broken two gages heavier material should be used. Table 5. Weights of Sheet Metal Used for Duct Construction u. s. Std. Gage Black Sheets Thickness, In. Weight Per Square Foot Steel Iron Ounces Pounds Galvanized Sheets** Approximate Thickness. In. Weight Per Square Foot Steel Iron Ounces Pounds 30 0.0123 0.0125 8 0.500 0.0163 0.0165 10.5 0.656 28 0.0153 0.0156 10 26 0.0184 0.0188 12 0.625 0.750 0.0193 0.0196 0.0224 0.0228 12.5 14.5 0.781 0.906 24 0.0245 0.0250 16 22 0.0306 0.0313 20 20 0.0368 0.0375 24 18 0.0490 0.0500 32 1.000 1.250 1.500 2.000 0.0285 0.0346 0.0408 0.0530 0.0290 0.0353 0.0415 0.0540 18.5 22.5 26.5 34.5 1.156 1.4()6 1.656 2.156 16 0.0613 0.0625 40 14 0.0766 0.0781 50 12 0.1072 0.1094 70 11 0.1225 0.1250 80 2.500 3.125 4.375 5.000 0.0653 0.0806 0.1112 0.1265 0.0665 0.0821 0.1134 0.1290 42.5 52.5 72.5 82.5 2.656 3.281 4.531 5.156 10 0.1379 0.1406 90 5.625 0.1419 0.1446 92.5 5.781 ^Galvanized sheets are gaged before galvanizing and are therefore approximately 0.004 in. thicker. Table 6. Weights and Thicknesses of Standard Copper Sheets0 ____________ Rolled to Weight _____________________________ c Weight per Square Foot Ounces Pounds Thickness, Inches Decimal Equivalent Nearest Fraction Nearest Gage No, B. & S. Stubs U. S. Std. 10 0.625 0.0135 Hi 12 0.750 0.0162 He 14 0.875 0.0189 Hi 16 1.000 0.0216 H2 18 1.125 0.0243 H2 20 1.250 0.0270 Hi 24 1.500 0.0324 Hi 28 1.750 0.0378 Ha 32 2.000 0.0432 Hi 36 2.250 0.0486 Hi 40 2.500 0.0540 Hi 44 2.750 0.0594 Me 48 3.000 0.0648 Me 56 3.500 0.0756 Hi 64 4.000 0.0864 Hi Variations from these weights must be expected in practice. 27 26 25 23 22 21 20 19 17 16 15 15 14 13 11 29 27 26 24 23 22 21 20 19 18 17 17 16 15 14 29 28 26 25 24 23 22 20 19 18 17 17 16 14 13 624 CHAPTER 32. AIR DUCT DESIGN Example 3. If the rooms and offices of the hotel building of Example 2 are to be served from a manufactured unit with a capacity of 22,935 cfm against an external resistance of 0.35 in., the known resistances are calculated as: (1) Outdoor air inlet.................. ....................... .......... .............. ................................ ..... 0.094 in. (2) Allowance for damper adjustment....... _........................................ ...................... 0.033 in. (3) Supply grille resistance (from manufacturer's tables)............ ..........................0.036 in. Total known resistance.................................... ................................................ 0.163 in. Subtracting this from the total available resistance: 0.35 in. -- 0.163 in. = 0.187 in. available for duct resistance. Known length of run......... ................... ...... ..................................... 150 ft The duct width is then estimated for the following elbow calculations: Four 150% ratio elbows, 4 x 13 x 3.5 ft....... ............................ 182 ft Three 75% ratio elbows, 3 x 35 x 1.5 ft--................................. 158 ft Total estimated length................................................................ 490 ft The duct friction per 100 ft is then 0.187 -5- 4.90 = 0.0382 in. and the mains and branches are sized from the 0.038 in. friction line in Fig. 2. If it is desired to size each branch for equal resistance, the total resistance back to the point of juncture is calculated and the branch is then sized in a manner similar to that outlined in Example 3. SOUND CONTROL Frequently the problem of sound prevention in a heating, ventilating or air conditioning system imposes more severe restrictions than the pre vention of excessive pressure drop. Tendencies toward higher duct velocities have produced noise control problems which require con sideration of enumerable factors in air duct design.. Naturally some types of occupancy and application permit relatively higher sound levels to be maintained than others, .but the design trend is progressively directed towards noise reduction wherever possible. Sound absorbent materials have been successfully applied to duct construction to reduce noise. The basis used for the selection of the proper amounts <?f absorbent materials will be found in Chapter 33. DUCT CONSTRUCTION DETAILS Straight sections of round duct are usually formed by rolling the sheets to the proper radius and grooving the longitudinal seam. Rectangular ducts are generally constructed by breaking the corners and grooving the longitudinal seam, although some fabricators still use the standing.seam due to lack of equipment. Elbows and transformation sections are generally formed with Pittsburgh corner seams because this seam is easier to lock in place than the double seam, but complicated fittings such as double compounded elbows are usually constructed with double seam comers. The.construction of these various seams as well as the types of girth connections are shown in Fig. 8. The application of the various slips and connections are outlined in Table 4. The end slip may be used wherever 5 slips are recommended. Where drive slips are used the end slip may be applied on the narrow side of the duct and only the drive slips on the 625