Document GmQ91nVgVZnNR1ODpMOJ9dKav

790 CHAPTER 40 1949 -Guide Since 55 effective diameters are equal to 30 ft as required, 1 effective diameter = fg = 0.545,ft or 6.56 in. On Fig. 3 vertically below intersection of 6.56 in. effective diameter line and equivalent round opening line read 8.5 in. in lower margin. Example 4-' A jet of air issues from a pipe or from air orifice having a well-rounded Fig. 3. Effective Diameters for Round and Rectangular Openings . (Plenum Approach) (entrance (coefficient of discharge = 1.0) and delivers air with the same velocities and with the same throw as in Example S. What is the required diameter? In this case since the coefficient of. discharge is unity, the effective diameter of the jet is the actual diameter of the pipe or orifice, or 6.56 in., as obtained in Example S. Spread' The induction-effect results in the spreading of the air stream. The total angle included by the air stream from straight flow outlets has been meas^ AirDistiibution 791 urba'andfound tobe-between 14and25 deg./.Tbeangle'will-depehdbh'the type of approach', type of outlet and velocity. : - - v-^ ; 'Tbe. effect of vertical bars placed in de face of the outlet to increase tl} Sttr'ead,, may also be deduced from the momentum theory. . Assuming that there are no horizontal deflecting bars and that the air spreads vertically through a total angle of 14 deg; thkt a uniform velocity exists at any sec tion of the air stream; and that the conservation of momentum principle applies down to a velocity of 60 fpm; the following approximate equations for throw are to be substituted for equation (4)J: For a spread of 15 deg on each horizontal side L .0.55 --7=. - ' -v A\ Qv For a spread of 30 deg on each horizontal side L = 0.37 ___ v^l ' (8) - (9) For a spread of 45 deg on each horizontal side L 0.28 Q__i _ vA i (10) i ! Converging Fig. 4. Spread of Air Stream with Various Vanes Guide Vanes Vanes should have a depth of one to two times the spacing between the vanes. If the ratio of- vane depth to spacing is less than one, effective con trol by means of the vanes cannot be obtained. Little improvement is ob tained by increasing the ratio beyond two. The effect of .various-types of vanes is given in following paragraphs. - Straight Vanes. As mentioned previously, the included angle between both planes will be in the neighborhood of 14 deg, for a straight-setting of the vanes as shown in Fig. 4. Diverging Vanes. Such vanes set for an angular spread will have a marked effect on'the direction and distance of travel of an air stream. An outlet having vertical vanes set straight forward in the center, with uni-, formly increasing angular deflection to a maximum at each end of 45 deg, will produce an air stream with a horizontal included angle of approximately 60 deg as shown in Fig. 4. The throw will be reduced one-half for such a vane setting. Increasing the divergence of the vanes reduces the air quan tity handled by an outlet for a given duct static-pressure. The primary function of the vanes is to spread the air horizontally.' Spreading the air vertically entails the risk of hitting beams or other obstructions or of blow ing primary air at excessive velocities into the occupied zone. . v . Converging Vanes. The blow of an outlet may be somewhat increased by converging, the vanes of an outlet as illustrated in Fig- 4. Even with converging vanes, the resultant angle of spread of an air stream will not