Document X8D3BEXrZyvMjGJg6bZ9L3n5K

Y-Y-: '[' '7_,Y' 'Y'"^;$ V7-: American Society 0/ Heating and Ventilating Engineers Guide, 1928 Table 1. Corresponding Pressures and Velocities of Dry Air at 70 Deg. and 29.92 In. Barometer Inches ok Water 0.05 0.10 0.20 0.25 0.30 0.40 0.43 0.50 0.60 0.70 0.75 0.80 0.87 0.90 1.00 1.25 1.30 1.50 1.73 1.75 2.00 2.17 2.25 2.50 2.60 2.75 3.00 3.03 3.25 3.47 3.50 3.75 3.90 4.00 4.25 4.34 4.50 4.75 Ounces per Sq. In. 0.0289 0.577 0.1154 - 0.1443 0.1730 0.2308 0.2500 0.2884 0.3460 0.4037 0.4326 0.4614 0.5000 0.5190 0.5768 0.7209 0.7500 0.8650 1.0000 1.0092 1.1535 1.2500 1.2975 1.4418 1.5000 1.5860 1.7300 1.7500 1.8740 2.0000 2.0185 2.1630 2.2500 2.3070 2.4510 2.5000 2.5950 2.7395 ' Velocity. Ft. per Min. 896 1266 1791 ' 2003 2193 2533 2637 2832 3102 3351 3468 3582 3729 3800 4005 4478 4566 4905 5273 5298 5664 5895 6007 6332 6457 6641 6937 6976 7220 7457 7492 7756 7910 8010 8256 8337 8496 8729 Inches of Water 4.77 5.00 5.20 5.50 6.00 6.07 6.50 6.94 7.00 7.50 7.80 8.00 8.67 9.00 .9.54 10.00 10.40 11.00 11.27 12.00 12.14 13.00 13.87 14.00 - 15.00 15.61 16.00 17.00 17.34 18.00 19.00 19.07 20.00 20.81 22.54 24.28 26.01 27.74 Ounces 1 per Sq. In. 2.750 2.884 3.000 3.172 3.460 3.500 3.749 4.000 4.037 4.326 4.500 4.614 5.000 5.190 5.500 5.768 6.000 6.344 6.500 6.921 7.000 7.497 8.000 8.074 8.650 9.000 9.227 9.805 10.000 10.380 10.960 11.000 11.535 12.000 13.000 14.000 15.000 16.000 Velocity Ft. per Min. ' 8745 8943 9134 9392 9810 9864 10210 10545 10595 10968 11187 11328 11792 12015 12367 12665 12915 13282 13445 13875 13950 14440 14913 14985 15510 15820 16020 16513 16675 16990 17456 17488 17910 18265 19012 19730 20420 21090 ' . Corresponding Velocity for Dry Air at Various Pressures and Temperatures and 29.92 In. Barometer PreSSURE Inches 0.25 0.5 0.75 1.00 1.25 1.50 1.75 2.00 2.25 Ounces 0.1443 0.2884 0.4326 0.5768 0.7209 0.8650 1.0092 1.1535 1.2975 50 1965 2778 3402 3929 4393 4812 5197 5556 5892 60 1986 2808 3439 3971 4440 4864 5254 5616 5956 70 2003 2832 3468 4005 4478 4905 5298 5664 6007 100 2059 2911 3565 4117 4602 5042 5446 5822 6174 150 300 2149 3038 3720 4296 4804 5262 5683 6076 . 6443 2399 3391 4153 4796 5362 5874 6344 6783 7193 500 2696 3812 4668 5390 6027 6602 7131 7624 8085 550 2895 4095 5020 5795 6470 7100 7655 8195 8690 306 Chapter XXI--Air Duct Design and Construction It is customary in proportioning ducts for heating and ventilating work to follow either of two methods: 1. Arbitrarily select sizes from assumed velocities, depending upon velocity of air at fan outlet. 2. Determine the velocity which will give an assumed resistance within fan capacity at noiseless operating speed. By decreasing the velocity in main duct as air is delivered through branch outlets: (1) uniform air delivery through outlets is accomplished, (2) friction in smaller pipes is reduced, (3) portion of velocity head is converted into static pressure. The two greatest losses in duct systems are dynamic losses and friction losses. The former are chiefly caused by changes in direction or in velo city of air flow and are expressed in pressure in inches of water gage as per Table 1. Friction losses due to friction of air against sides of ducts,'vary directly as the length of the pipe, directly as the square of the velocity, and in versely as the diameter. Friction is commonly expressed as equivalent pressure in inches water gage or in terms of velocity heads, (the ratio of friction loss to the theoretical pressure corresponding to the velocity in the duct). One velocity head is the pressure corresponding to the velo city of air in the duct. For smooth round pipes the friction loss is: F L (JLY 50 D \4005/ where F = loss of pressure in inches of water V = velocity in feet per minute L = length of pipe D = diameter of pipe in feet; = length of pipe in diameters. If a factor of safety is thought desirable the length 45 may be used though experiments show that the friction loss is equal to one velocity head in a length varying from 40 to 60 diameters depending upon the smoothness of the duct. The engineer's judgment and experience should prevail in this matter. For example, correction should be made for pipes with rough or uneven surfaces and in the case of brick or concrete ducts the friction loss should be increased 25 per cent or more. A formula for rectangular ducts is derived in a similar manner but it will be found very convenient to use the accompanying chart, Fig. 1. Other losses of pressure are at the entrance to the duct, through heater, air washer, etc. In ordinary practice it is. usual to keep the sum of the piping losses $ to Y and the loss through heater at less than Yi of the static pressure. The remainder is then available for producing velocity. The ideal duct system will take all factors into consideration and proportion air velocities so that the resistance will be practically equal in all ducts regardless of length. 307