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HEATING VENTILATING AIR CONDITIONING CUIDE 1942 BALANCING SYSTEM In designing an air conditioning system, it should be the aim of the engineer to so proportion the duct system that proper distribution of air to every supply opening will be obtained. Since this is almost impossible to accomplish in practice, it becomes necessary to have means of balancing the system to secure the desired amount of air in each space. There are a number of ways in which this may be accomplished, some of which are: 1. Dampers on the supply and return grilles. 2. Dampers in the supply and return ducts. 3. Reducing the effective area of some supply openings by blank-offs. 4. Combinations of dampers in both supply and return air. Dampers on the supply grilles themselves are objectionable because of their effect on the air stream. Dampers on the return grilles are frequently helpful in building up a static pressure in the room to prevent infiltration of outside air, and at the same time reduce the volume of incoming air. However, it is frequently impossible to sufficiently reduce the incoming air by this method alone. A damper in the supply duct some distance back of the supply opening forms a very satisfactory means of regulating the flow without disturbing distribution across the supply opening face. A damper in the return air duct has the advantage over one immediately behind the grille in that it does not tend to create high localized velocities through the grille as the latter might do if nearly closed. Blank-offs consisting of pieces of sheet metal covering a portion of the supply opening face can frequently be used satisfactorily, although determination of just what is required is a matter of experiment, and the balancing of the system is not nearly so conveniently accomplished as with dampers. Dampers in both supply and return air form the most flexible means of controlling the supply to the room and the static pressure within the room. When feasible, these dampers, particularly those in the supply ducts, should be a substantial distance from the supply opening, and ahead of the acoustic duct lining if used. Due consideration should also be given to the use of the several volume control and uniform distribution devices now available. See Catalog Data Section. REFERENCES A.S.H.V.E. Research Report No. 1140--The Use of Air Velocity Meters, by Prof. G. L. Tuve, D. K. Wright, Jr. and L. J. Seigel (A.S.H.V.E. Transactions, Vol. 45, 1939, p. 645). A.S.H.V.E. Research Report No. 1156--The Performance of Stack Heads, by D. W. Nelson, D. H. Krans and A. F. Tuthill (A.S.H.V.E. Transactions, Vol. 46,. 1940). A.S.H.V.E. Research Report No. 1163--Air Flow Measurements at Intake and. Discharge Openings and Grilles, by G. L. Tuve and D. K. Wright, Jr. (A.S.H.V.E. Transactions, Vol. 46, 1940). A.S.H.V.E. Research Report No. 1166--Development of Instruments for the Study of Air Distribution in Rooms, by A. P. Kratz, A. E. Hershey and R. B. Engdaht (A.S.H.V.E. Transactions, Vol. 46, 1940). A.S.H.V.E. Research Paper--Entrainment and Jet-Pump Action of Air Streams, by G. L. Tuve, G. B. Priester and D. K. Wright, Jr. (A.S.H.V.E. Journal Section,. Heating, Piping and Air Conditioning, November, 1941, p. 708). 608 Chapter 32 AIR DUCT DESIGN Pressure Losses, Friction Losses, Friction Loss Chart, Elbow Friction Losses, Proportioning the Losses, Duct Sizes, Pro cedure for Duct Design, Velocities, Main Trunk Ducts, Pro portioning the Size for Friction, Velocity Method, Equal Friction Method, Duct Construction Details THE resistance of an air handling system can be computed from the methods and data given in this chapter. The actual resistance for any given installation, however, may vary considerably from the calcu lated resistance because of variation in the smoothness of materials, the type of joints used and the ability of the mechanics to fabricate in accor dance with the design. It is best to select fans and motors of sufficient size to allow a factor of safety. Volume dampers should be installed in each branch outlet to balance the system. It is improbable that the required quantities of air will be delivered at each outlet without adjust ment of the dampers, which usually results in a total pressure exceeding that of the design, unless a liberal factor of safety is allowed. The flow of air due to large pressure differences is most accurately stated by thermodynamic formulae for air discharge under conditions of adiabatic flow, but such formulae are complicated, and the error occasioned by the assumption that the gas density remains constant throughout the flow may be considered negligible when only such pressure differences are involved as occur in ordinary heating and ventilating practice. In the development of the formulae, diagrams, and tables for the flow of air, use is made of the following basic equation for the flow of'fluids: If Hv be the velocity head in feet of a fluid, and the velocity, V, be expressed in feet per minute, the fundamental equation is V = 60 ^2g Hv The factor g is the acceleration due to gravity, or 32.16 ft per second per second. It is usual to express the head in inches of water for ventilating work and, since the heads are inversely proportional to the densities of the fluids, H, = 62.4 hy d 12 or Hv = 5.2 609