Document 6w4Z4XN9EdQG9BjDyVzLZoy93

HEATINC VENTILATINC AIR CONDITIONING GUIDE 1944 Table 1. Values of Room Circulation Factor (F) in Equation 6 Outlet Velocitt Fpm F, 200 300 400 500 600 700 800 Average Room Velocitt Fpm, Vr 10 20 30 40 50 6.0 12.0 18.0 24.0 30.0 4.0 8.0 12.0 16.0 20.0 3.0 6.0 9.0 12.0 15.0 2.4 4.8 7.2 9.6 12.0 2.0 4.0 6.0 8.0 10.0 1.7 3.4 5.1 6.8 8.5 1.5 3.0 4.5 6.0 7.5 Outlet Velocitt Fpm Vi Averaob Room Velocitt Fpm, Vr 10 20 30 40 50 900 1.3 2.7 4.0 5.3 6.7 1000 1.2 2.4 3.6 4.8 6.0 1200 1.0 2.0 3.0 4.0 5.0 1400 0.9 1.7 2.6 3.4 4.3 1600 . 0.8 1.5 2.3 3.0 3.8 1800 0.7 1.4 2.0 2.7 3.4 2000 0.6 1.2 1.8 2.4 3.0 where F is the room circulation factor expressed in cubic feet per minute per square foot of outlet wall area. Thus room air motion is directly a function of outlet velocity and air volume per square foot of outlet wall area. Hence Equation 6 and Table 1 can be used to determine the accept ability of a particular installation from the standpoint of proposed air volume, outlet wall area, and outlet velocity. Vertical Drop and Rise The vertical distance the lower edge of an air stream moves between the outlet and the end of the blow is termed the drop or rise (H). This drop or rise is influenced by the difference in density between the air stream and the room air, resulting from the temperature difference and the spread .of the air stream. For air emerging at room temperature, the drop or rise will be a function of the spread only and will be equivalent to: H = LX tan (SpreadAngle) (?) where H = drop due to spread, feet. L = throw, feet. -_____ When there is a temperature difference between the air stream and the room, the additional drop or rise is approximately given by Equation 8: where 7-7 nl (If las) L 2 H---------- vt-------- Bi and nt = constants (tentative suggested values m = 5, n2 = 1.2). <r = room temperature, degrees Fahrenheit. <as = supply air. temperature, degrees Fahrenheit. (8) For cooling application H is subtracted from the outlet height, for heating H is added. Duct Approaches to Outlets . Assuming that proper supply openings for a given installation have been selected, unsatisfactory performance may still result due to the con- 584 CHAPTER 31. AIR DISTRIBUTION struction of the duct work immediately back of the supply openings. Performance data on the grilles and registers of various'manufacturers are based upon results obtained with the air approaching the grille perpen dicularly and at uniform velocity over the entire duct cross-section. Where this condition does not exist in practice, performance predictions based on published data cannot be realized. Every precaution should be taken to secure as nearly ideal conditions in the approaching air stream as are possible. In addition to disturbances due to the construction of the duct work itself are those which may be created by dampers immediately behind the grille. Where either multiple louver or single blade dampers are used for throttling, considerable deflection of the air stream may result. This is particularly true when the fins of the register core are perpendicular to the damper blades. If the core has sufficient depth and the fins are ' ~~~ s Fig. 2. Effects of Expanding Duct Fig. 3. Unequal Face Velocities Fig. 4. Effect of Turning Member parallel to the blades, there is a marked tendency to straighten the air stream, although some deflection may still result. Any attempt to secure a low face velocity and high duct velocity by. the construction of any expanding chamber immediately behind the grille is likely to be unsuccessful. In order to expand from a small duct to a larger one, and have the air stream fill the duct at the end of the diverg ing section without turbulence, angle A in Fig. 2 should be about 7 deg. From this it is.apparent that an attempt to secure equivalent results with a short connection would be futile. What actually happens when this is attempted is illustrated by. the arrows in Fig. 2. When localized high velocities through the supply opening exist from this cause or any other, the noise produced will naturally exceed that which the supply opening area and average face velocity would lead one to expect. This fact.should be remembered in considering the use of register dampers, particularly in those cases where there must be considerable throttling with the damper to balance a poorly designed system. Where reduction of noise is im portant, it is recommended that balancing dampers be placed in the duct ahead of the acoustic duct lining. Similar unequal face velocities, aggravated by a deflection of the air stream, are obtained with the arrangement shown in Fig. 3. The latter may be corrected by inserting a turning member in the elbow back of the outlet face as shown in Fig. 4. The importance of straightening the air 585