Document x1OeQoBQ9np2XNr2ngDYbr6RE

94 CHAPTER 4 1950 Guide Fluid Flow 95 FLOW MEASUREMENT BY PITOT TUBE There remains one other head type meter useful in ventilating work, , Pitot tube named for the Frenchman who discovered the principle. The Pitot tube is essentially a bent tube with its open end pointed up stream combined with another tube with its end pointed crosswise to the flnw or'downstream, or connected to openings crosswise to the flow. Used ^-ith flowing liquid, the liquid will rise in each tube, but higher m the one nninted upstream. Used with a flowing gas, and the two tubes connected bv a U-tube containing water, the liquid level in the U-tube will be disDlaced with the lower level on the side connected to the tube pointed up stream The tube directed upstream receives the impact pressure, which is the sum of the static and kinetic pressures, while the tube directed Fig. 16. Relation op Nozzle Discharge Coefficient, C, for 2-Inch Pipe, to Diameter Ratio and Reynolds Number <p'-- i- l - pj/p. i -P \ / v - FWpiW (71) This formula is plotted for k -- 1.40 (air and other diatomic gases) and 1.30 (steam, carbon dioxide, natural gas) in Figs. 19 and 20, respectively. Fig. 18. Relation op Nozzle Discharge Coefficient, C, for 10-inch Pipe, to Diameter Ratio and Reynolds Number crosswise receives only the static pressure. The difference between the two, as read on the separate tubes or on the U-tube is, of course, the kinetic pressure. The velocity is expressed as V = Vzght (72) Application of Equation 60 serves to make the formula general, assum ing that water is used in the manometer connecting the two tubes. Using this equation, and multiplying by 60 to convert feet per second to feet per minute, __ Vm = 1096.5 (73) in which Fm is the rate of flow in feet per minute. It is often difficult to obtain the exact static pressure. In the usual con struction of Pitot tubes, the static pressure openings are downstream from the impact pressure opening, and turbulence induced by the nose may affect the static pressure reading. If the static pressure openings point