Document 7MGXj9GE6ZeQdjD623QVO6BVR

834 CHAPTER 41 7 "1949' ^ui'dfe For special considerations which apply to insulation of duets in marine installations see Chapter 49. MAINTENANCE Ducts should be designed in such a manner as to enable easy mainte nance. They should have enough access doors, not only to enable in spection, but also to facilitate cleaning of the ducts.55 The periodic clean ing of the ducts should be part of the regular maintenance schedule. It should be done efficiently and competently to avoid difficulties or hazards in the operation of the system.54'55 LETTER SYMBOLS USED IN CHAPTER 41 a = coefficient of contraction. e = absolute roughness, feet: i_ p0 = density of air under actual (operating) conditions, any consistent units, p, = density of air under standard conditions, any consistent units. pY = density at which Vm is measured, pounds per cubic foot. A = cross-section area of duct, square feet. Ai = area of smaller duct. At = area of larger duct. ' o = length of one side of rectangular duct, feet.or inches. : (Other side is 6.) 6 = length of one side of rectangular duct, feet or inches. (Other side is a.) C = shock loss coefficient for standard air. : c = shock loss coefficient. " D = inside diameter of conduit, feet. : I)a = circular equivalent of a rectangular duct for equal friction and capacity, inches. . d = equivalent diameter, feet or inches. e = Naperian base of.logarithms = 2.718. / = non-dimensional friction coefficient. fi -- surface conductance (inside) Btu per (hour) (square foot) (Fahrenheit degree). ' ' ' ' /,, = surface conductance (outside) Btu per (hour) (square foot) (Fahrenheit degree). '' g = acceleration due to gravity, 32.17, feet per (second)' (second). //,, = pressure loss due to sudden enlargement, based on.standard air, inches of water. -i //,, = pressure loss due to sudden enlargement, based on standard air, inches of water. 11y = total shock pressure loss for standard air, inches of water,, hi and hj = static pressure head at given points (1) and (2). ho = friction loss under actual (operating) conditions, any consistent units. ho = pressure loss due to sudden contraction, inches of water. ho -- pressure loss due to sudden enlargement, inches of water. ht = friction loss, feet of fluid flowing. h, = regain in static pressure, feet of fluid flowing. h, = friction loss under standard conditions, any consistent units. hv = total shock pressure loss, inches of water. k = conductivity, Btu per (hour) (sq ft) (Fahrenheit degree per inch). I. = length of conduit, feet. ' Pperimeter of duct, feet. Qw -- heat loss through duct walls, Btu per hour, ti = temperature of air entering duct, Fahrenheit degrees, fj = temperature of air leaving duct, Fahrenheit degrees, fi = temperature of air surrounding duct, Fahrenheit degrees. ' U = thermal transmittance coefficient, Btu per (hour) (square foot), (Fahren heit degree). V -- fluid or air velocity, feet per second. ,, Vo = velocity of air in duct, feet per second. Vm = velocity oflair or fluid, feet per minute. Vt = velocity in smaller duct, feet per minute: Vt = velocity inTarger duct, feet per]minute. Air DuctDesign ; 835; v -- velocity of air, feet per second. i = mean velocity in smaller' duct section, feet per second. vs = mean velocity in larger duct section, feet per second. W = weight of air through duct, pounds per hour. x = thickness; inches. REFERENCES 1 A.S.H.V.E. Reseabch Eapek--A New Friction Chart for Round Ducts, by D. K. Wright, Jr. (A.S.H.V.E. Transactions, Vol. 51,1945, p. 303). I Friction Factors for Pipe Flow by L. F. Moody (A.S.MJ2. Transactions, Vol. 66, 1944|p. 67l). ,> See Chapter 1 for definition of standard air. ... 4 Friction Charts for Gases Including Correction for Temperature, Viscosity and Pipe Roughness, by R. D. Madison and W. R. Elliot (A.S.H.V.E. Joubnal Section, Healing, Piping and Air Conditioning, October, 1946). 4 Friction Equivalents for Round, Square and Rectangular Ducts, by R. G. Hueb- scher, (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, Decern^ ber, 1947). "V Modern Air Conditioning, Heating and Ventilating, by W. H. Carrier, R. E. Cherne and W. A. Grant, 1940, p. 234 (Pitman Publishing Corp.). , ' Heating, Ventilating and Air Conditioning, by L. A. Harding and A. C. Willard, 1932, p. 673 (John Wiley and Sons). .. Pressure Losses Due to Bends and Area Changes in Mine Airways, by G.. E. McElroy, (If.S. Bureau o/Mines, Information Circular I.C. 6663, p. 4). ) ' A.S.H.V.E. Research Report No. 1211--Pressure Loss Caused by Elbows in 8-inch Round Ventilating Duct, by M. C. Stuart, C..F. Warner and W. C. Roberts (ABH.V.E. Transactions, Vol. 48,1942, p. 335). " Loss of Pressure Due to Elbows in the Transmission of Air Through Pipes or Ducts, by F. L. Busey (A.S.H.V.E. Transactions, Vol. 19,1913, p. 366). II Pressure Losses in Rectangular Elbows, by R. D. Madison and J. R. Parker (Heating, Piping and Air Conditioning, July, p. 365,- August, p. 427, September, p. 483, 1936). " Engineering Factors in the Ventilation of Metal Mines, by G. E. McElroy, (if. >S. Bureau ojMines, Bulletin 385, pp. 55-68). u Pressure Losses Resulting from Changes in Cross-Sectional Area in Air Duets; by A..P. Kratz and J. R. Fellows, (University of Illinois Engineering Experiment Station, Bulletin No. 300). 14 Fan Engineering, (R. D. Madison, Editor) 5th ed., 1948, pp. 124-128 (Buffalo Forge Company). "Air Conditioning Principles, by C. 0! Mackey, 1941 j p. 158. (International Textbook Company). " Fluid Mechanics, by R. C. Binder, 1943, p. 152. (Prentice-Hall). 17 Modern Diffuser Design by G. N. Patterson. (.Aircraft Engineering, Sept.,T938, p. 267) and The Design of Airplane Ducts by G. N. Patterson (Aircraft Engineering, July, 1939, p. 263). " Duct Sizing with Partial Static Regain, by R. H. Anderegg and F. W. Hutchin son, (Heating.Piping and Air Conditioning, August, 1946, p. 97). f " Power Savings Through Static Pressure Regain in Air Ducts, by J. R. Fellows, (Heating, Piping and Air Conditioning, April, 1939, p. 219). " Reference 6, p. 240. " Method of Determining Rectangular Equivalents and Weights of Ducts, by P. Franck, (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning December, 1940, p. 743). " Design of Air Conditioning Systems for Low Maintenance, by M. G. Kershaw, (Refrigerating Engineering, October, 1946, p. 315). 55 Standards of the National Board of Fire Underwriters, (N.B.F.U. Pamphlet No. 90, p. 21). 54 Clean Your Ducts, by G. L. Candler, (Heating, Piping and Air Conditioning, July, 1947, p. 83). * Duct Leakage and Job Revision, by E. L. Schulz, (Heating and Ventilating, October, 1944, p. 83).