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HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 offices, hospitals, and, most of all, radio broadcasting and movie sound studios, present a real problem which must be intelligently attacked if a satisfactory installation is to be made. In this chapter the noise of the air supply openings (and returns) only is considered, it being assumed that the noise or sound level of the room without the supply opening noise includes that which may be contributed by fans, motors, duct work, and other items of conditioning equipment. The control of noise from these sources is another problem (see Chapter 33). Where sound control is important, the actual room sound level without conditioning equipment should be known. If feasible, the contribution of the conditioning equipment, less supply openings, should be estimated to secure the working sound level. If this correction is not made, the use of the first value errs in the direction of safety. It is evident that the point within the room which should concern the designer in this problem is that at which the supply opening noise is greatest. A tentative standard listening point relative to the supply opening is suggested later in this discussion, and it is assumed that the supply opening noise data are taken with reference to this point. If it is desired that the supply opening noise result in an inaudible addition to the existing noise level, it is safe to assume the total supply opening noise to be 5 db below room level. This results in an increase in total noise of slightly over 1 db, which is unnoticeable. If an increase of 3 db is per missible, the supply opening noise level may be equal to the room noise level alone. All supply openings in the room must be considered, as will appear later; the returns may be ignored only if they are so sized that the velocity of air through them is much less than through the supply opening. DISTRIBUTION FACTORS IN ROOM COOLING In attempting to design a satisfactory air distributing system, it is first necessary to properly locate the grilles in accordance with the recom mendations already stated. Assuming that the best locations have been selected, it then becomes necessary to choose the proper grille for that location. The considerations involved are the amount of air to be handled, the velocity permissible from the standpoint of noise, and the distance the air should carry. The distance it will carry, assuming no obstructions, is affected by face, velocity, core area, aspect ratio and included angle of effluent stream as determined by vanes. For low aspect ratios, the major variables of velocity, area and effluent angle are related1 approximately as given in Equation 1 when the air stream is unaffected by obstructions of any kind. kQ: Xa = y a0b0 (1) where Xa -- throw, feet. Q -- air volume flow rate, cubic feet per minute. Oo and &o ~ grille width and height, inches. `The Rationale of Air Distribution and Grille Performance, by C. O. Mackey (Refrigerating Engineering, Vol. 35. No. 6, June. 1938. p. 417). 600 CHAPTER 31. AIR DISTRIBUTION k -- dimensionless constant with the following approximate empirical values: Vanes set straight ahead................................................... ** 0.77 Vanes causing a spread on each horizontal side of 15 deg = 0.66 30 deg *= 0.45 45 deg = 0.34 The temperature difference between incoming air and room does not appreciably effect distance of throw, but does modify the point in the room at which the stream enters the occupied zone, due to rise or drop of the entire stream2. In consideration of what constitutes the possible throw of a supply opening under a given set of conditions, it is important to remember that the throw may be unsatisfactory for any one of several reasons: 1. It may be so long that it will strike the far side of the room and come down the wall with velocities higher than are permissible, 2. It may be so short that it will fail to carry the full length of the room, and shortcircuit to the return air supply opening, or 3. It may spill into the center of the room. In the first case, the system fails for lack of uniform distribution and the presence of cold areas. In the second case,, the standards as to velocity and temperature difference in the zone of occupancy may be satisfactorily met, but air distribution and circulation throughout the entire room is not accomplished, with the result that the end of the room away from the outlet would not be satisfactorily conditioned. In the third case, the shortcomings of both case one and case two are present. It is evident, therefore, that for a given supply opening discharging air at a given velocity, there is a maximum and a minimum length of room which can be satisfactorily handled. In the latter, the velocity of the air down the far wall is just within the maximum permissible, while in the former, satisfactory circulation is barely accomplished. In general, the higher the supply opening is above the floor, the greater may be the difference between room air and incoming air temperatures. Assuming that proper supply openings for a given installation have beenselected, unsatisfactory performance may still result due to the con struction of the duct work immediately back of the supply openings. Performance data on the grilles and registers of various manufacturers are based upon results obtained with the air approaching the grille perpen dicularly and at uniform velocity over the entire duct cross-section. Where this condition does not exist in practice, performance predictions based on published data cannot be expected to be realized. Every precaution should be taken to secure as nearly ideal conditions in the approaching air stream as are possible. In addition to disturbances due to the construction of the duct work itself are those which may be created by dampers immediately behind the grille. Where either multiple louver or single blade dampers are used, considerable deflection of the air stream may result, if it is throttled appreciably by these means. This is particularly true when the fins of the register core are perpendicular to the damper blades. If the core has suf ficient depth and the fins are parallel to the blades, there is a marked ten dency to straighten theairstream, although somedeflection may still result. *A.S.H.V.E. Research Report No. 1076--Air Distribution from Side Wall Outlets, by D. W. Nelsor* and D. J. Stewart (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 77). 601