Document EmYJ4rRa2qnV3Q9p5ZY6V3Oqj

684 CHAPTER 31 1953 Guide Air Streams from_Perforated Panels, by A: Koestel. Philip Hermann and G. L. Tuve (A.S.H.V.E. transactions, Vol. 55, 1949, p. 283). Instrumentation A.S.H.V.E. Research Reports No. 857, 911 and 966--Measurement of the Flow of Air Through Registers and Grilles, by L. E. Davies (A.S.H.V.E. Transactions Vol. 36, 1930, p. 201; Vol. 37, 1939, p, 619 and Vol. 39, 1933, p. 373). Measuring Air Distribution and Grille Performance in Air Conditioning, by G. L. Tuve (Heating, Piping and Air Conditioning, November 1937, p. 700). A.S.H.V.E. Research Report No. 1140--The Use of Air Velocity Meters, by 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. 1162--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, p. 313). .-A.S.H.V.E. Research Report No.'1165--Development of Instruments for the Study of Air Distribution in Rooms, by A. P. Kratz, A. E. Hershey and R. B. Enedahl (A.S.H.V.E. Transactions, Vol. 46, 1940, p. 351). Miscellaneous A.S.H.V.E. Research Report No. 936 -- Investigation of Air Outlets in Class Room Ventilation, by G. L. Larson, D. W. Nelson and R. W. Kubasta (A S.H.V E Transactions, Vol. 38, 1932, p. 463). A.S.H.V.E. Research Report No. 959--Indices of Air Change and Air Distri bution, by F. C. Houghten and J. L. Blackshaw (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 261). . A.S.H.V.E. Research Report No. 1092--The Flow of Air Through Exhaust Grilles, by A. M. Greene, Jr., and M. H. Dean (A.S.H.V.E. Transactions, Vol. 44, 1938, p.387). ' Vertical Air Distribution in Tall Buildings, by Wm. Sturm (Heating Piping and Air Conditioning, June 1947, p. 69 and September 1947, p. 93). Balancing Air Delivery of a System of Manifold Air Diffusers, by G. S. Dauphinee and P. Argentien (A.S.H.V.E. Transactions, Vol: 55, 1949, p. 213). . CHAPTER 32 AIR DUCT DESIGN Pressure Losses, Friction Losses, Circular Equivalents of Rectangular Ducts, Dynamic Losses, Pressure Loss in Elbows, Losses Due to Area Changes, Pressure Changes, Duct Design Methods and Examples, Duct Construc tion Details, Heat Losses from Ducts, Maintenance AIR ducts for the transmission of the air in forced air heating, ventilatXA. mg, cooling, or air conditioning systems must be carefully designed for functional as well as economical reasons. The design should be based upon the.fundamental laws of fluid flow in pipes, and should take into ac count recent analytical and' experimental studies which complement and substantiate the fundamental laws.. The basic equations of the flow of fluids will be: found in Chapter 4, Fluid Flow. PRESSURE LOSSES Air ducts impose resistances to air flow which must be overcome by. pressure differences resulting from the expenditure of energy in maintaining the flow. Since the flow of air, in ventilating and air conditioning work, takes place under very small pressure differences, the assumption that the gas density remains constant throughout the flow will cause only a'negligi ble error. It is therefore possible to use the equation for incompressible fluids (liquids) for the flow of air in a duct, instead of the complicated thermodynamic formulas for air discharge under conditions of adiabatic flow, which would be necessary if pressure differences were large. A reasonably precise estimate of the flow resistances offered by the system is essential for satisfactory duct design. The theoretical 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 calculated resistance because of variation in the smoothness of materials, the type of joints used and the ability of the workmen to manufacture the system in accordance with the design. It is best to select fans and motors of sufficient size to provide a factor of safety. Dampers should be installed in each branch outlet to balance the system. The drop in pressure in air transmission systems is due to friction losses and dynamic losses. Pressure increases and decreases may also be caused by changes in duct areas, with resulting conversion of velocity pressure to static pressure, and vice versa. The friction losses for turbulent flow (which occur in all practical air flow problems) are due to the friction of air against the sides of the duct, and to internal friction between the air molecules. The dynamic losses are caused by changes in the direction or in the velocity of air flow, and may be caused by changes in size and shape of the crosssection of the duct, by bends (elbows), and by obstructions to flow offered by dampers. FRICTION LOSSES Pressure drop in a straight duct is caused by surface friction, and this friction loss is most readily calculated by means of the Air Friction Charts, 685