Document O1eK5ZxwNX6L0Ka7G60342D6K

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 The average jet angle (included angle in both planes, see Fig. 1) for an air stream as it emerges from a rectangular outlet of any shape without spreading vanes is about 19 deg, plus or minus 5 deg, .depending on the type of approach, type of outlet and velocity. The spread, increases slightly with velocity. A vaned outlet discharging air uniformly forward will result in a spread of about 14 deg. This is equivalent to a spread in any direction of about one foot in every 8 ft of blow. Throw The distance air will carry measured along the axis of an air stream from the supply opening to the position'ifi the stream at which the average frontal air velocity reduces to 50. fpm is termed the throw. The throw distance is based on an assumed terminal velocity, which cart be assigned any arbitrary value.- Since air striking a wall at too high a velocity may bring the air stream down within the occupied zone, the terminal velocity should be limited to 50 fpm. The maximum transverse velocity of the air stream is usually from 2.5 to 3.5 times the average frontal velocity. As suming no obstructions, the blow is affected by face velocity, core area, '4 Fig. 1. Spread of Air Stream with Various Vanes aspect ratio and included angle of effluent stream as determined by vanes. For low aspect ratios, the major variables of velocity^ area and effluent . angle are related1 approximately as given in Equation 4 when the-air stream is unaffected by obstructions of any kind. : kQ "; _. Xa = y Ooio ' : ' - " - (4) Where Xa' =* throw, feet.' -r ` Q '= air volume flow rate, cubic feet per minute. Co and bo -- grille width and height, inches. . ." I k -- dimensionless constant with the following approximate empirical values: Vanes set straight ahead................. ,,.......................................... = 0.77 Vanes causing a spread on each horizontal side of 15 deg = 0.66 30 deg = 0.45 45 deg = 0.34 lThe Rationale of Air Distribution and Grille Performance, by C. O. Mackey (Refrigerating Engineering. Vol. 35. No. 6. June. 1938, p. 417). 592 CHAPTER 31. AIR DISTRIBUTION Vanes For vanes to be mechanically satisfactory, the depth of the vane should be between one and two times the spacing between the vanes. If the ratio of vane depth to spacing is less than one, effective turning by means of the vanes cannot be obtained. Little improvement is obtained by increasing the ratio beyond two.. Straight Vanes. As mentioned previously, the included angle between both planes willbe in the neighborhood of 14 deg, for a straight setting of the vanes as shown in Fig. 1. 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 uniformly 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. 1. The throw will be reduced one-half for such a vane setting. Increasing the divergence of the vanes reduces the air quantity handled by an outlet for a given duct static pressure. - The primary function of the vanes is to spread the air horizontally. Little is gained by spreading the air vertically. Converging Vanes. The blow of an outlet may be somewhat increased by converging the vanes of an outlet as illustrated in Fig. 1. Even with converging vanes, the resultant angle of spread of an air stream will not be less than 14 deg. The air converges for a few feet in front of the outlet, and then diverges more than if the vanes had been set straight. Both the horizontal and vertical vanes of an outlet are important. After an installation has been made, many conditions of draftiness or stuffiness can be alleviated by'some vane adjustment, provided an independent means for regulation of static pressure behind the vanes is included-. Room Air Motion . 1 The air motion in the occupied zone is usually traveling across the room in reverse direction to^the blow of the outlet. The cross-sectional area of this stream is equal to the outlet wall area less the stream area and the area obstructed by furnishings. Equation 5 gives the average room velocity in the occupied zone as a function of the air volume sup plied per square foot of outlet wall area and the outlet velocity. Vr Ql AZ (5) where .. Vi = average room velocity, feet per minute. A = outlet wall area, square feet. Z = 0.6 (reduction factor to allow for supply air stream 20 per cent, furniture obstruction 20 per cent, at point where supply air stream occupies 20 per cent of the room cross section). Since Qs = Qir by definition, and r -- -V=yi- from Equation 2, and V3 is assumed to be about 200 fpm for total induction in actual practice, then Equation 5 results in: Vr A yX V, X --Z FVi 120. 120 Vr V,. 593 ' (6)