Document J6bMX2Br5094M35D8wV1nEoO

792 CfiAPTER 4d , . 1949 Guide be less than. 14 deg.,: The air converges for a few feetin 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. Vertical Drop and Rise The distance that the lower edge of the air stream drops below the bottom of the outlet is important, since the air stream should not reach the occupied zone until the velocity has fallen to about 60 fpm. The drop (H, ft)- is influenced by two forces; the natural vertical spread of the stream and the gravitational force due to the difference in density between supply air and Fig. 5. Thbow of Wall Outlets room air. For air emerging at room temperature, the drop will be a func tion of the spread only and will be. equal to: g, = LXtan(Spread2Angle) (11) where Hi = drop due to spread (when emerging air and room temperature'are the same); feet. '- . L = throw, feet. When there is. a temperature difference between the air stream and. the room, there is an additional drop which is approximately*: where H, i(t, - Ufi " V, (12) Hi =,additional drop due to temperature difference, feet, ni and nt = constants (tentative suggested values n, = 5, n = 1.2). tr = room temperature, degrees Fahrenheit. Um = supply air temperature, degrees Fahrenheit. VY = jet velocity, feet per minute. It should be1 remembered, that the total drop Ii = Hi + Hi. H\ \s , . positive for either heating or cooling; Hi is positive for cooling, negative for heating. In consequence, there will always be vertical drop in cooling, and a vertical rise in heating only if Hi > Hi. i \ Air Distribution 793 Another empirical equation for.the total drop is* where m(t, -- Ut H= Vi (13) m = constant (tentatively suggested value of m = 16). In other words, for a given throw L the drop or rise increases as the tem perature difference increases and the outlet velocity decreases. This, equa tion is only valid, if a temperature difference exists between room air and supply air. . Room Air Motion (Wall Outlet) One of the most important problems in air distribution is to- achieve air motion in the occupied zone within acceptable velocity limits. Therefore; outlet performance and characteristics of the'space have to be related to this air motion. ,- The air moving in the occupied zone is (for a side wall outlet), equal, in quantity to the total air contained in the outlet: stream at the. end of the throw and it is generally moving in a direction opposite to the stream. Assuming that the maximum volume of air is in circulation when the air stream velocity V> drops to 200. fpm, that the free area for return flow is 0.6 of the area of the wall in which the outlets are located, then, according to the momentum theory*'4: where >j V -- average room velocity, fpm. Qt = volume of room air in motion, cfm. A= area of wall in which outlet ia located, square feet. Since Qi -- Qix r, (by definition) ;.and r the average room velocity is: Qir V= 0.6A, or, with Fj = 200 fpm Vi V, according to Equation 3; A,v = 9Xa 120 .(IS) When Qi, the volume of primary air, Vi, the velocity of primary air and Aw, the wall area are known, the average room velocity may. be calculated from Equation 15 in order to determine the acceptability of the air dis-' tribution system. ... - OUTLET PERFORMANCE " The factors of outlet performance, throw, drop, room'air motion, capacity, temperature differential, dirt and noise place considerable limita tions on the design of a satisfactory distribution system. Si