Document 10pLdGQZQXEEZgw9XLjywXXKZ

654 CHAPTER 30 -1952 Guide '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 .' / .' .] ,''7, :The distance that thelower 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: (H, 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: . ; . ... r g, = LX tan (Spread2AD8le) (11) where ,;-gi = 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 ni(t, -- t,,)L"i 7, ' (12) Ht = additional drop due to temperature difference, feet. ni and nt = constants (tentative suggested values ri = 5, n, = 1.2). tt = room temperature, degrees Fahrenheit, t. = supply air temperature, degrees Fahrenheit. Vi = jet,1velocity, feet.per minute. . It should be remembered,'that the total' drop H = Hi -f Hi. Hl is positive for either heating or cooling; H2 is positive, for cooling, negative for'heaitihg. In consequence, there will always'be vertical'drop in coqling, and a vertical rise in heating only if If* > Hi. ' : Another empirical'equation for the total drop is:2'" . . '; where - g= m(tr -- l,,)L Ui (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.If the most important, problems in air distribution is to achieve air motion in the occupied zone within acceptable velocity limits.Therefore, Air .Distribution 655 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 Fj 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:2-4 , where. Q. 0.6 x A (14) V = .average room velocity, fpm. Qa = volume of room air in motion, cfm. Aw- area of wall in which outlet is located, square feet. Since Q3 = Qi x r, (by definition); and r -- Fi according to Equation 3; v.3 . . . the average room velocity is: .. . ... .... ... . . fvAv = ^-r =-9` 0.6A. 0.6AW\F,/ or, with V3 = 200_fpm.............. : F = QiF, 120A,, ' (15) When the volume'Qi; of primary air, the velocity Fi, of primary airland 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 heat-ed air to rise too rapidly above.the occupied, zone and thus create excessive vertical temper ature variation (stratification);, in cooling operation .it may cause cold air. to drop into 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 from 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