Document nNm9drbr3RzGe9exK0XjovOGG

626 CHAPTER 30 T950: Guide eah 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 50 fpm. The drop (if, feet), 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 room air. For air emerging at room temperature, the drop will be a func tion of the spread only and will be equal to: where fSpread Angle\ Hi -- LX tan t 2 ) (11) 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 :4 \ where H,< ` ' Vi r (12) Hi = additional drop due to temperature difference, feet. _ m and nt = constants (tentative suggested values n, = 5, n, = 1.2). t, = room temperature, degrees Fahrenheit. Um = 8upply air temperature, degrees Fahrenheit. F, = jet velocity, feet per minute. .. .... . - t -T .'It. should be remembered that the, total 'drop H = Hi +' lit- ' H, is positive for either heating or cooling; is positive fqr;cooling,.negative for heating. Inconsequence, there willalwaysbe vertical drop ih cooling, and a vertical rise in heating only if Hi > Hi.. . Another empirical equation for the toial drop is? , m(fr -- iJ)L , , : ' Vi : . (13) where ' ; m -- constant (tentatively suggested value of m = 16). In other words, for a given throw L, the drop :or. rise increases as the tern-; perature difference increases and the outlet;velocity decreases: 'This eqiia-. tidh.'.iB, onlyVyalid if a temperature difference'exists between room air and' SUPPly; air. .i-' ,, , ...... ... ` Room Air Motion (Wall Outlet) .. . , ....... UOne ofjtlie mqst(important problems in air distriButioh is td-achieve air motion in the"6ccupied zone within acceptable velocity limits^' Theraore, Air 'Distribution 627 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 Vi 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 theory4 where V Q, 0.6 ill. (14) V = average room velocity, fpm. Qi => volume of room air in motion, cfm, Aw= area of wall in which outlet is located, square feet.. Since Qi = Qix r, (by definition); and r the average room velocity is: YlV, according to Equation 3; or, with F = 200 fpm 0.6A* 0.6A,,\V,/ , OtF.. 120A. (16) When the voliune Qi, of primary air, the velocity Fi, of primary air and the wall area A,,, 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, (1) throw, (2) drop,. (3) room air motion, (4) capacity, (5) temperature differential/ (6). dirt and (7) noise, place considerable limitations on the design of a satisfactory distribution system. ' .. 1 .Throw. The throw of- a wall' outlet must be sufficient to produce satisfactory conditions .over the area to be conditioned. Underblowing may cause heated air to rise too. rapidly above the;occiipied zone and thus.create excessive vertical temper ature variation (stratification); in cooling operation it may cause cold air to drop mto the occupied zone before a satisfactory mixing of supply and room air has been accomplished by induction and thereby create a condition of acute discomfort (draft). On the other hand, overblowing will result in: objectionable downdrafts fronr'any surface the primary air stream may strike. On the average, it is considered most practicable to select a throw which is threefourths of the distance toward an exposed wall or window, as shown in A of Fig. 5. However, structural characteristics,.mounting height, temperature differential and resultant drop or rise', or location'of greatest heating or cooling loads strongly affect the selection of the .optimum throw.In spaces with- beamed ceilings, the-outlets should be located below the bottom of; the lowest beam level, and-' preferably low enough so that an upward or arched blow may be employed. The blow should be arched sufficiently to miss the beams and, at the same time, in such a manner as to