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American Society of Heating and Ventilating Engineers Guide, 1937~~~ should be used with Fig. 11. Although this method may place an nn. warranted limit on velocity when used in a large room, it is seldom that such a room has a noise level low enough to make this penalty serious or to justify a more complicated though more exact procedure. In general, return grilles are selected for velocities about half the supply, velocity, and when this is done, they may be neglected in sound computations. However, if supply and return grilles are the same size, resulting in the same face velocity, they must be treated as two supply outlets. That is, if 1000 cfm is supplied and exhausted through grilles of the same area, 2000 cfm must be used in the solution with Fig. 11. SELECTION OF SUPPLY OUTLETS After the heating and cooling load calculations have been made (Chapters 7 and 8), and, or a suitable supply air temperature selected, the Fig. 13. Plan View Typical General Office volume of air required for each space can be determined. The next step is to determine the velocity at which the air may be introduced into the space quietly and without creating objectionable drafts. Present-day grille design coupled with the introduction of effective acoustical treatment for minimizing fan and duct noises have made grille face velocities in excess of 1500 fpm feasible, and 600 to 1200 fpm is now used in practice. This range of velocities is approximately three times higher than common practice values of a few. years ago. Since high velocities make for smaller ducts and outlets, and therefore savings in space as well as greater flexibility in locating the duct work to the best advantage, selection of design velocities is a very important step. The selection of proper velocity requires that the designer have before him reliable data applicable to the particular make of outlet he proposes to use. Even under these circumstances, the problem is one of cut and try because permissible velocity may be determined by either noise or throw. A method fpr selecting supply outlets is outlined below in the form of a sample cooling problem, using numerical values which have no reference to any particular make of outlet. 358 Chapter 19--Air Distribution 1 The load calculations have been made; a suitable temperature differential has been Jected (it is to be understood that the data referred to from this point on are based on >h s temperature differential), and the volume of air required determined. Assume that Fie 13 represents a small general office having a noise level of 40 db and that 2500 cfm nust be supplied for proper conditioning. 2 Select a tentative location for the outlet or outlets, having in mind the type of erille most likely to effect proper distribution. In this particular case, two outlets having a wide spread appears to be a logical choice. 3. Data from which to determine velocity which corresponds to 2500 cfm and a noise rating at *east 5 db below the noise level of the office may be presented in a number of forms, one of which is shown in Fig. 11. (Fig. 11 represents assumed values only. In practice similar data should be obtained from the manufacturer whose outlets are being considered. Several similar charts or tables may be necessary to cover any one manu facturer's complete line.) From Fig. 11 it will be noted that for 2500 cfm the type of grille selected may be used at velocities up to 700 fpm without exceeding 35 db; that is, 5 db below the noise level of office. 4. Having determined the velocity, the core area becomes fixed at 3.57 sq ft-or 257 sq in. per outlet. In this problem, the two grilles in question are so close together that consideration of their combined area in determining the permissible velocity from the standpoint of noise introduces little error. 5. The type grille selected has thus far been found satisfactory from a noise stand point, provided the face velocity does not exceed 700 fpm. The next consideration is throw, which may be assumed to be 16 ft, and by reference to a manufacturer's catalogue the proper correlative test data may be checked with the throw assumed. It is of course evident that one or more types of grilles may satisfy the requirements, and that in any one type there will be a choice of outlet proportions. It will also be evident that the tentative selection of an outlet having a wide spread may be unsatisfactory from the standpoint of throw, in which event a second choice should be made and the procedure repeated. In the case of a heating problem, the method of solution is the same, but the manufacturer's data must, of course, be based on tests with air above room temperature. . TYPES OF SUPPLY OUTLETS Grille, registers or outlet design for attaining uniform distribution and minimum air resistance consists of various fixed and adjustable arrange ments. Some types are designed with directing air blades, fins, bars, louvres, or thin metal strips shaped into a series of grooves or tubes, all of which may be set into a suitable round, square or rectangular frame. In order to attain desired long or short air throws, the emergence of air from the outlet may be directed to straight,' deflecting, converging or jet air streams depending upon the outlet design. Designs which direct the air stream to produce an ejector effect within the enclosed space tend to mix the room air with the conditioned air to provide uniform distribution. Centrally located ceiling or wall type outlets arranged for completely diffusing the air consist of several round, hollow, cone-shaped flaring members placed in the proper relationship to each other. The velocity of emergence of the air from the unit can be made practically uniform over the entire surface of the outlet, and the velocity in any direction may be varied to any desired value by adjusting the position of the cones. One or more of the smaller flaring members act as ejectors and injectors which draw a small proportion of the room air into the air spreader where it mixes with the conditioned air before it is discharged.