Document EdmrbJwDVBBYRYgb7nQB39vVg

310 CHAPTER 21 1960 Guide Table 13 .... Weights and Thicknesses of Standard Copper Sheets' RoOad to Weight Weight per Sq ft 1(vetoes Nearest Gage No. 4Ox tb Decimal Nearest equivalent Fraction ..N.*. .-4.J. 10 0.625 0.0135 X4 12 0.750 0.0162 He 14 0.875 0.0189 X* 16 1.000 0.0216 Hs 18 1.125 0.0243 X, 27 28 25 23 22 29 27 26 24 23 20 1.250 0.0270 X. 24 1.500 0.0324 Hi 28 1.750 0.0378 Xj 32 2.000 0.0432 He 36 2.250 0.0486 X. 21 20 19 17 16 22 21 20 19 IS x40 2.500 0.0540 X* 44 3.750 0.0594 48 3.000 0.0648 He 56 3.500 0.0756 Hs 64 4.000 0.0864 X, 15 15 14 13 11 17 17 16 15 14 * Variations freon tbeaa mights must be expected in practice. 29 28 26 25 24 23 22 20 19 18 17 17 16 14 13 SCAM EM* 4L J e*A E SUP rUJSH SAM countered in the field, but in general, long-radius elbows and gradual changes in shape tend to maintain uniform velocities accompanied by decreased turbulence, lower resistance, and 8 minimum oi noise. Heavy canvas connections are recommended on both the inlet and outlet to all fans. Self-vulcanizing adhesive tapes are available for this purpose and for sealing joints in ductwork. Where a fire hazard exists, the material used must satisfy the requirements of any codes or authorities having jurisdiction. The fan discharge connections shown in Hg. 16 are marked good, fair, and poor in the order of the amount of turbulence produced. An inspection of the beater connections shown in Fig. 16 will readily show that uniform velocity through the heater cannot be expected in the diagram noted poor. When obstructions cannot be avoided, the duct area should never be decreased more than 10 percent, and then a streamlined collar should be used. Larger obstructions require an increase in the duct size in order to maintain as nearly uniform velocity as possible. Branch take-offs should always be arranged to cut or slice into the air stream in order to reduce as far as possible the losses in velocity head. ' Wherever ducts pass through fire walls or connect two fire areas of a building, automatic fire dampers should be provided. For design of such dampers and other fire-protective details, see Pamphlet No. 90A of the National Board of Fire Underwriters." HEAT LOSSES FROM DUCTS In designing duct systems, the beat gains or losses of the ducts can be quite considerable, not only if the duct passes through unconditioned space, but also oh long duct runs within conditioned space. Proper insulation will remedy this situation considerably, but sometimes a redistribution of the supply air is necessary in order to compensate for the heat exchanges that occur. The heat loss from a given length of duct ean be expressed by: '[(^H (14) 1 ? t I f 1 s t 3 1 i \ i ! 1 > up cato, rami, amo waste* cdmcctoms OftCRTER TYPE CLMCM COLXM) (MOUPCPl TWt tAWCH TXKEOTFX Fig. 16.... Sheet-Metal Dud and Arrangement Details Q,, ** heat loss through duct walls, Btu per hour. U -- overall coefficient of beat transfer for the duct wall, Btu per (hour) (square foot) (Fahrenheit degree). P -- perimeter of duct, feet. I length of duct, feet. ti = temperature of air entering duct, Fahrenheit. < = temperature of air leaving duct, Fahrenheit. it -- temperature of air surrounding duct, Fahrenheit. To obtain the temperature drop in warm air for a given distance of transmission, or the temperature rise if the duct carries air cooler than the room through which it passes, the following formulas can be used: <*(y + I) - 2h (y - 1) tily - 1) + 2t, (v + 1) (15) (16) j 288,4 Vp for rectangular ducts or. UPl I Air Duct Design 311 sulated with 16-in. of a material having a conductivity of 0.35 Btu at 86 F mean temperature, carrying air at a velocity of 1200 fpm, measured at 70 F, to deliver air at 120 F with air surround ing the duct at 40 F. Solution: Referring to Fig- 17, the overall heat transmission coefficient is found to be 0.49 Btu. From Table 2, Chapter 3, the density of air at 70 F and 29921 in. Hg is found to be 1/13.348 = 0.0749 lb per cu ft. Substituting these and the other given values in Equation 16, y and tx will be as follows: 28-8 X 6 X 1200 X 0.0749 ^ ^ V " 0.49 X 10 X 70 120(45.3 + I) - 80 45.3-1 Substituting in Equation 14: Qw 0.49 X 10 X 70 vatou by the percentage* Aown below. Thidcnftu el liwdsfiofl (Indie*) X 1 m2 12- to 21-in. Duet Diameter.................. 3% 5% 7% 9% 21- to 30-in. Duct Diameter.................. 1% 2% 3% 4% Rg. 17.... Heat Loss Coefficients for Insulated Ducts* 72DV(> y -- ------ for round ducts, A cross-sectional area of duct, square feet. p =* density of air, pounds per cubic foot. V = mean velocity of fluid, feet per minute. D = diameter of round duet, feet. In .dng Equations 15 and 16, one of the duct air tempera tures will be unknown and will be obtained by substitution of the other known or assumed values. Beat loss coefficients for insulated ducts with various con ductivities are given in Fig. 17. The conductivities of various materials, which are based on mean temperatures, about 70 F, will be found in Table 4 of Chapter 9. For cases where the mean temperature is other than that at which the test was conducted, a correction should be made. However, in most cases the effect of this factor will be small and may be ne glected. Example 8: Determine the entering air temperature and heat loss for a duct 24 x 36 in. cross-eection and 70 ft in length, in- * 28400 Btu per hr. For special considerations which apply to insulation of ducts in marine installations see Chapter 48. REFERENCES *D. K. Wright, Jr.: ASHVE Research Repost No. 1280--A new friction chart for round ducts (ASHVE Transactions, Vol. 51.1945,p.303). *L. F. Moody: Friction factors for pipe flow (ASMS Trans actions, Vol. 66, 1944, p. 671). *R. D. Madison and W. R. Elliot: Friction charts for gases including correction for temperature, viscosity and pipe rough ness (ASHVE Journal Section, Healing, Piping and Air Con ditioning, October 1946, p. 107). *F. W. Hutchinson: ASHVE Research Repost No. 1469-- Friction losses in round aluminum ducts (ASHVE Trans actions, Vol. 59, 1953, p. 127). * R. G. Huebscher; Friction equivalents for round, square and rectangular ducts (ASHVE Transactions, Vol. 54, 1948, p. 101 J. *D. W. Locklin: ASHVE Research Report No. 1405-- Energy losses in 90-degree duct elbows: A survey and analysis of available information (ASHVE Transactions, Vol. 56, 1950, p. 479). * J. R. Weske: Pressure Loss in Duals with Compound Elbows (National Advisory Committee for Aeronautics, Advance Re stricted Report W-39, February 1943). R. D. Madison and J. R. Parker: Pressure losses in rec tangular elbows (Heating, Piping and Air Conditioning, July, p. 365; August, p. 427; September, p. 483; 1936). *W. H. Carrier, R. E. Cherae, and W. A. Grant: Modem Air Conditioning, Heating and Ventilating (Pitman Publishing Corp., New York, 1950, 2nd ed., p. 248). "M. C. Stuart, C. F. Warner, and W. C. Roberts: ASHVE Research Report No. 1216--Effect of vanes in reducing loss in elbows in seven-inch square ventilating duct (ASHVE Trans actions, Vol. 48, 1942, p. 409). 11S. F. Gilman: Pressure losses of divided-flow fittings (ASHAE Transactions, Vol. 61, 1955, p. 281). "G. E. McElroy: Pressure Losses Due to Bends and Area Changes in Mine Airways (V. S. Bureau of Mines Information Circular I.C. 6663, p. 4). u A. P. Krsti and J. R. Fellows: Pressure Losses Resulting from Changes in Cross-Sectional Area m Air Ducts (University of Illinois, Engineering Experiment Station Bulletin No. 300). UR. D. Madison (ed.): Fan Engineering (Buffalo Forge Company, Buffalo, New York, 1948, 5th ed., p. 124). "J. R. Henry: Design of Power Plant Installations: PressureLoss Characteristics of Duet Components (National Advisory Committee for Aeronautics, Advance Restricted Report L4F26, June 1944, L-208). * L. O. Paul: Installing ducts for higher pressures, and Why higher duct velocities? (Heating, Piping and Air Conditioning, April 1953, p. 98 and February 1954, p. 109). J