Document mbv2qDEJV7EDoQ85XpoRqJ6Zk

338 W. N. WITHERIDGE and relative roughness in Table 18 to find the friction factor, /; (5) use f-ifi equation above to find the air-flow friction loss in inches of water per hundfntj of pipe. * TABLE 17 Roughness Measure () for Various Kinds of Pipe" Kind and condition of pipe e (ft.) Perfectly smooth drown tubing Commercial steel or wrought iron, new Asphalted cast iron Galvanized iron (used for Figure 22) Cast iron Concrete, average roughness Riveted steel, very rough 0.0000015 0.00015 0.0004 0.0005 0.00085 0.003 0.0J TABLE 18 Friction Factors (/) Based on Reynolds Number and Pipe Roughness". - for Use with Friction Formula Above (page 337) y Relative roughness," /D 0.00001 0.0001 0.0005 0.001 0.005 0.01 0.030.05 10* 10,000 0.030 0.031 0.032 0.032 0.038 0.043 0.080 0.074 Reynolds Number 10* 100,000 10* 1,000,000 -0,0.18...- 0.0185 0.0205 0.022 0.031 0.038 0.057 0.072 --------- 0..012___ 0.0135 0.0175 0.020 0.030 0.038 0.057 0.072 --IF-oorn'of?i = roughness measure of pipe surface in feet and D = inside diameter of j For kinds of pipe not listed in Table 17 the roughness measure canJSe||p for the material most likely to be similar in surface characteristics, iji e.-it in .the.inter-nal-pipe-ro,ughness7-consideration'-should "be_giveHtoTi5||an^m] of the pipe for corrosion resistance, and the possibility of surfacjl-hipdifiAHt by the accumulation of particulate matter, especially if hygroscopic or electrostatic. E. REYNOLDS NUMBER :raMgK The Reynolds Number used in fluid mechanics and required i%AM|ujg Table 18 is a "dimensionless" figure that may be computed from equation: "L. F. Moody, Trans. Am. Soc. Mech. EngrS., 66, 671 (1944). .v. >S?PV . -1 I & VENTILATION 339 (velocity in f.p.s.) (diam. in ft.) gpi(|ynolds Number = (kinematic viscosity in sq. ft./sec.) ^^^^Capproximate form that applies to most air-flow conditions in ''~''a'^j|||tems, and is adjusted to the conventional units, is the following: ` {Reynolds Number = 8.5 (l.f.m.) (diam. in in.) qn'Sn?'8.5. in this equation is based on a kinematic viscosity for air at ?^m<K29:92. inches Hg 'barometer of 0.000163 sq. ft. per second. For values cosity at high and low temperatures and high pressures, see the presented by L. G. Miller.81 1%:,FLEXIBLE METAL, CANVAS, OR RUBBER DUCTS i* ` Sjpfctiqn of air flow through flexible ducts is considerably higher than for S^l^rmetal pipes. If the inside surface is deeply corrugated, the friction mytjlcijt'WO to four times the loss for smooth pipe of equal diameter. Further- Veeessary to anticipate increased friction loss to a degree depending J'',1^^ounli! of curvature to which the duct can or will be subjected. $jill||pifie ducts should be treated as unusually high-resistance material. synthetic rubber ducts are so constructed that the inside is re.^^mo^ffi, while the corrugations due to metal reinforcement appear on iSs|^|||e. Also, some metal flexible ducts are so rigidly constructed that 1 ts impossible. Field tests of friction loss should be made if its effect ffiMphwer c_o_n_s_u_m__p,t4ion is of serious consequence............... G1ADJUSTABLE DAMPERS OR FLOW REGULATORS . still exists to install in each 'branch of an exhaust or ventilating gmw^g of damper to make possible an initial or periodic variation in ^^^^^totribution of air flow.. This practice has reached the ultimate Jjg^^^mWmnection with the handling of highly explosive dusts that may TSlAmit in the vicinity of dampers as a result of eddies or sudden iSSm^^gdijeotion. A great many objections to dampers have been ex- ^ai0ng'-desi'gners of exhaust systems, but the fact remains that in many possible the correction of air-flowjiistribution when the .fflnPTsuooeed in anticipating the best proportion of ventilation PpPjjpr process in the system. It is true that tremendous power has Jgj^jtopgliput the years by the work of dragging air through the con'li^^^&lyRampers, and efforts to improve the accuracy of ventilation 'be encouraged. Likewise, some attention must be i|||||pf better flow-regulating devices to give the user the flexibility ^^^^;demand. Designs also can be, and have been, made to permit fclh.;cannot be disturbed by unauthorized persons without obvious tion. W&^rarw. ASHVE, 43, 71 (1937).