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250 CHAPTER 16 1965 Guide And Data Book i r* - Since the low peripheral velocities are not measured. th air flow calculated by this method is usually slightly high Where the velocity profile is unstable or asymmetric, a greater number of readings should be taken. The use of traverses at the inlets and outlets of ducts requires special techniques.1**-** Those contemplating such measurements should consult the references cited. Pulsating or disturbed flow will give erroneous results and therefore, if possible, the Pitot tube should be located at least 7 H diameters downstream from a disturbance such as that caused by a turn. Straightening vanes11 located duct diameters ahead of the Pitot tube will serve to im prove the precision of the measurements. The type of manometer to be used with a Pitot tube depends upon the magnitude of the velocity pressure being . measured and the accuracy desired.. At velocities greater . than 1500 feet.per minute, a draft gage of appropriate range is usually satisfactory. If the Pitot tube is being used to measure low air velocities, a precision manometer of some type is essential. Many forms of Pitot tubes, other than the one described, Cup Anemometer - have been used and calibrated.11 A double-ended tube,*5 one end pointing downstream, and one upstream, is some- The cup-type anemometer is almost universally, used for times used for low velocities, but it should be carefully measuring wind speeds. It consists of three or four - hemi calibrated for accurate results! A special form of this tube spherical cups mounted radially!from a vertical shaft!'Wind ' from any point of the compass will cause the cups and shaft to rotate. The instrument is usually so constructed that wind speeds-may be recorded or indicated electrically at some remote point. design consists of two straight. H-in. tubes soldered to gether, closed at the end, and with a 0.04 in. hole in each tube opposite the line of contact. This tube is useful in ^exploring velocities in exhaust inlets, such as hoods placed around grinding wheels. To meet special conditions, different Pitot Tube sized Pitot tubes which are geometrically similar to-'the standard tube can be used. The Pitot-tube, used in conjunction with a suitable ma nometer, provides a simple method -of- determining'.the air Impact Tube velocity in a duct. The construction of a Standard. Pitot Tube,11'and the method of connecting .it. ta'a; draft gage is shown in fig. 3. The equation for determining, the air velocity .from, the measured velocity .pressure is as follows:' For relatively high velocities and use in rnH tubes, total pressure. readings may be obtained tiring some form of im pact tube. Where the static pressure across the stream is rela tively constant, as in turbulent flow in a straight duct, a side- V - 1000.5 (3) wall tap for static pressure may be used in`conjunction with the impact tube to obtain velocity pressure. One form of im V " velocity, feet per minute. ; TM velocity pressure (Pitot tube manometer/ reading), inches of water. p ---density of air, pounds per cubic foot.-, . ' ... pact tube is a small tube placed in a position perpendicular to. the stream and having a fine hole in its upstream ride. Where a side-wall tap cannot be used because of variable static pressure across the stream a variety of pressure-sensing probes have been used. One of these is a wedge-shaped probe with static holes in the side walls. Another is the Fechheimer tube, a round'transverse tube with two holes set at exactly -Since, the velocity in a duct is' seldom, uniform across any;section, and since a Pitot .tube, reading''indicates a velocity'at only one location,' a~trouerae.is'uhially.inade''to determine the average velocity so. that'the flow "can be computed. In general, the velocity is lowest near the edges or comers,-`and`greatest at or near the'center.`Suggested Pitot ;tube locations for .traversing round and rectangular ducta.are shown in Fig..4. In round ducts not less than 20 readings' should be taken along .two. diameters at.-centers of equal areas as'shown.. In rectangular ducts .the readings shouId.be taken in the-.center .of equal areas, over the crosssection of.the.duct. The number of spaces should.noti.be less, than 16, and need not be more than;64. When less f,h*" 64 are i taken, the number, of ..equal,spaces should be such that tiie centers of the: areas are not;more than' 6 in. apart. In. determining the-average: velocity ini the duct .from the readings given, the calculated individual c velocities or .the -square roots of the velocity heads-must be.averaged.-It is incorrect to use the average velocity head for this purpose. Rg. 4----- Pitot Tube Traverse for Round and Rectangular Ducts Measurement and Instruments Table 5 ..,. Measurement of Volume or Mass Row Rate Mnoatcwoeot Mmm 1 Orifice and manometer 2 Nozzle and manometer 3 Venturi tube and ma nometer 4 Rotameters Application Rang* Flow through pipes, duets and plenums all Smds - Above Reynolds number of 5,000 Smn as 1 and 2 above but used where permissible -presure drop is limited Normally used for liquids . Any Praorion 1% 1% 1% tlOftcflOAl Coefficient and accuracy' influ enced by approach conditions * Must be calibrated for the liquid with which used 5 Timing a given weight flow 6 Displacement meter Liquids only--used for cali brating other flow means Relatively volume flow at high pressure drop Any - " 0.1% -- Aft high as 1,000 cfm 0.2-2.0% Some types require calibration depending on type depending on type - 7 Gasometer or volume dis Short duration tests; used for Total flow limited by 0.5-1.0% placement calibrating other flow means available volume of containers -- g Element of resistance to flow and manometer Used for check where there is calibrated resistance element Lower liifiif set by readable press drop in the system 1-5% Secondary reading depends on accuracy of calibration 9 Thomas meter (tempera Where elaborate setup is justi ture rise of stream due to fied by need for good accuracy electrical heating) Any 1% - Uniform velocity, usually used with gases 10 Heat input and tempera ture change with steam or water coil 5 11 Instrument for measur ing point velocity Check value in heater or cooler tests Any Primarily used in installed sys Lower Hmit.retby tems where no special provision accuracy of velocity for flow measurements have measurement been made 1-3% . 2-4% Accuracy depends upon uniform ity of .flow, and completeness of traverse determined opposite rides of the air direction. Both of these instruments must be carefully made and* calibrated. This probe technique should be limited to Mach numbers of 0.3 to as. . : : , ; VOLUME OR MASS FLOW RATE MEASUREMENT Means of measuring flow rate are listed in Table 5. Gas and liquid mass or volume flow rates are most often determined by measurement of the pressure difference across an orifice, name, or Venturi tube. The orifice is more easily.changed than is the nozzle or Venturi tube and is less affected by change of Reynolds number. The nozzle is often preferred to the orifice because of its relative .freedom from the influence of ap proach conditions and accurate predictability of its coeffi cient. The Venturi tube is in essence a nozzle followed by an expanding recovery section to reduce the net pressure drop. liquid flow may be measured with good precision by timing <* given weight flow. While thii'rnethod is commonly used'for calibrating other methods, it is particularly useful where the flw mte is tow or intermittent'and-where a high degree of precision is required. A convenient direct-reading flow-meter for liquids and gases is the variable area meter (rotameter), which comprises a transparent-t&pered tube in which the rate of flow is indicated by the position tugnimpH by a bob or plug suspended in the upward flow.. .*: For measuring total liquid.qr gas flow over a period of time, ma?5r types ofdisplacement'meters arer,availabte! The' two major types for gases ue tiie' gds'meters, which employ leather bellows commonly, and the wet test meters, which use a water displacement principle. The Thomas meter has been used in the laboratory for measurement of high gas flow rates with a small pressure drop. The gas is heated by electric heaters and the temperature rise measured by two resistance thermometer grids. Knowing the heat input and temperature rise, tiie flow is calculated as the quantity of gas that will remove the equiv alent heat at the same temperature rise. For field measurement of air flow rates or calibration of large nozzles, a frequently used method is to make a velocity traverse, naing for the purpose a Pitot tube or other velocity-measuring instrument. This method is inapplicable in many cases be cause of its. lack of precirion'on tow velocities-or the im practicability of taking traverses where many test runs are in prospect. ! . ; ' Another relatively rough field.testing method is to take the pressure drop across elements' Having known pressure-drop characteristics, such as heating, and cooling coils, or fans. If the pressure drop-flow rate relationship is calibrated previ ously, fair precision: can ,be obtained, but if dependence is placed on rating data the method should be used for check purposes only. ., Venturi Meter, .Nozzle, Orifice . . .Consideration will first be given.to three flow measuring 'devices: Venturi .meter (F1g.'5),' flow nozzle (Fig.'6), Mid orifice plate"(Fig. 7)The locatioq__6f pressure taps usaTwith flow nozzles is shown in'Fig! 8.'1'