Document 2DqQ33LdnOkoQX9LXrXawk4a

772 For a long slot CHAPTER 30 :1958; Guide : (8a) where Qx = total volume flow rate at distance X from face of outlet; cubic: feet per minute; `' ' Qo "= discharge from outlet, cubic feet per minute. X = distance from face of outlet, feet. Fiq. 11. Nomogram fob Entrainment Ratios of Rounded,. Rectangular Jet Sources K' = proportionality constant. [" >- A o = effective area of stream at discharge from an open-end duct or at a con tracted section, square feet. ' Ho = width of slot, feet. -I'tif.unJ Equations 8 and 8a havo been used in plotting Fig. 9 for entrainment ratios. ; .vuil The nomograms (Figs. 10 and 11) for center velocities and entrainment ratios for jets emitted from circular sources, and from rectangular sources of all aspect ratios, are based upon recently developed theory.11 Air Distribution 773 OUTLET PERFORMANCE The results of the studies undertaken in room air distribution' which have been presented in the foregoing section'can be used as a basis for engineer ing design of supply outlets if the following is kept in mind: 1. The method for finding jet velocities is based upon several approximations and the two recommended equations must be used with caution for extreme axial and radial distances. 2. The characteristics of the low-velocity regions of ventilating jets are not yet well understood, and for both axial and radial jetB the effect of the Reynolds number is not fully known. 3. The quantitative treatment of the forces which govern room air distribution phenomena has been limited, and non-isothermal conditions involving buoyant forces have not yet been fully explored. 4. All investigations have been concerned with free jets, whereas air streams in practical room air distribution are not free streams but are influenced by walls, ceilings, floors and obstructions. v 5. The science of air distribution has not yet reached its final stage where a basic theorem permits the exact mathematical solution of all problems, and velocity pat terns in closed rooms, and of a great variety of outlets, such as radial and slotted outlets, diffusers and perforated panels, must be further studied before their per formance can be predicted with confidence. However, the air distribution research sponsored by the American Society of Heating and Air-conditioning Engineers has proved that the phenomena of room distribution are amenable to scientific research and rational interpretation, and the ventilation jet problem has reached a stage where a definite technique of experimentation in combination with semi-empirical theories can be of great help in the design of air outlets. 6. In the design of air outlets, the days of unguided and unrelated experimentation should be over. Practical room air distribution, however, is still predominantly an art, where decisions depend on isolated tests and judgments must be based on the accumulated experience of the behavior of outlets in the' laboratory and in the field. In designing a satisfactory air distribution system, the engineer must make use of such tests and draw on this experience, and weigh the different factors of the outlet performance which place considerable limitations on his proceedings. A short dis cussion follows regarding some of these factors which are: (1) jet pattern, (2) capac ity, (3) temperature differential, (4) permissible room air motion, (5) permissible noise level, and (6) smudging. Jet Pattern Refer to section Ventilating Jets in Air Distribution for a general discusr sion of the jet pattern. In selecting the throw and drop of outlets the follow ing considerations are important: (1) throw, (2) effect of vanes; (3) effect of type of outlet, and (4) drop. 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 occupied zone and thus create excessive vertical tem perature variation (stratification); in cooling operation it may cause cold air to drop [?t0 j^e occupied zone before supply ana room air are mixed sufficiently, and thereby create a condition of acute discomfort (draft). On the other hand, overmowing will result in objectionable downdrafts from any surface the primary air stream may strike. wVHaVera?e> ^ *s considered most practicable to select a throw which is three- of p**18! distance toward an exposed wall or window, as shown in diagram A ^ However, structural characteristics, mounting height of outlet, tempera load <liferentmij and resultant drop or rise, or location of greatest heating or cooling 7s strongly affect the selection of the optimum throw. In spaces with beamed ceil- ff* outlets should be located below the bottom of the lowest beam level, and shouhfh low enough so that an upward or arched blow may be employed. The blow ne be arched sufficiently to miss the beams and, at the same time, in such a man- i Prevent the primary or induced air stream from striking furniture and ob- es and producing objectionable drafts. Effect of Vanes. Vanes affect grille performance if their depth corresponds at