Document DGQaK7jO5r40xm774k85n8KdB

248 CHAPTER 16 1965 Guide And Data Boole, precise work. These are based on a standard gravitational?? acceleration of 32.1740 ft per (sec) (sec) and are as follows: 1 Standard Atmosphere " 14.696 ib per sq. in. 29.921 in. mercury at 32 F * 33.96 ft water column at 68 F For most ordinary purposes, the following figures are of ample accuracy: 1 Atmosphere 14.7 lb per sq in. = 29.9 in. mercury ** 34.0 ft (408 in.) water column air-conditioning engineers are called upon to measure the flow of air more often than that of other gases, and usually the air b measured at or near atmospheric pressure. Under this condition, the air can be treated substantially as an incompressible fluid which implies that simplified formulas can be used with sufficient precision for the solution of many problems.1* -; Instruments for measurement of fluid velocity are shown in Table 4, together with their rangeof application and precision. Manometer tubes should be chemically clean. The bore : Thermal Anemometers b not important, except insofar as it affects the meniscus through wetting or surface tension. Bores of at least */i< j Measurementoflow air velocities (0-100 fpm) b particularly in. for rough, and H hi. for more precise, measurements difficult. Frequently, the flow pattern b very unstable, causing are recommended. liquids other than water are sometimes' the mass turbulence level to be of the same order of magnitude used for low-pressure measurement and, when this is done, as the velocity. However, useful data can be obtained with the readings must be corrected for the density of the fluid.; any of several instruments now available, if they are main For measuring: pressure differences of a few inches of- : tained in calibration and the user understands their operation water, or -less,' U-gages are often set at an angle for scale and limitations.1* Several types of thermal anemometers, both amplification. In many gages of this type, commonly termed directional and non-directional, are applicable to thb range, draft gages or inclined manometers, only one tube of small but have questionable accuracy at the lower end. Virtually bore is used and the otfaer leg is replaced by a reservoir. all of the non-directional instruments suffer the additional The scale is calibrated to read in inches of water, and it is handicap^of being unable to distinguish between large scale necessary: to use a fluid having the same gravity as that turbulence and the mass velocity of the air. for. which the. gage was originally .calibrated, or to apply a If a suitable sensing element b heated electrically at a correction if another fluid-is -used. Such gages may be fixed - rate and exposed to an air stream, the temperature checked one against another. For more accurate calibration difference' between the element and the stream becomes a the gage may be checked against a micromanometer or . a measure of velocity by calibration. In the hotwire ane calibrating device known as a book' gage " The accuracy mometer, a very fine heated wire is employed-as a resist of a draft gage is dependent on the dope of a tube, and ance-thermometer element whose temperature may be de consequently the base of the gage must be leveled care termined accurately.n,w,u` In1 the heated-bulb thermometer fully. It is not desirable to use a slope of less than 1. in type, a heating wire b wound around the bulb of a mercury- 10. Where pressures are read under extreme conditions of in-glass thermometer, and the temperature difference be^ temperature, and calibration is possible only at normal-tem tween tins thermometer and a similar unheated one serves perature, it b necessary to correct for the change in density as an index of air - speed." The heated-thermocouple ane of the liquid in the manometer.11 For measuring low-pres mometer b calibrated to give velocity in terms of the dif sure -differences to within 0.001 in. of water, veiy sensitive ferential emj between' heated and unheated thermo-junc micromanometers are available, such as the' Illinois of tions exposed to an air stream.1'-* Combined measurements Wahlen,** and the Emswiler.** Various1 instrument manu of air temperature and velocity are particularly useful for facturers are also prepared to furnish accurate micromanom- air distribution studies, and automatic recording poten? etera. trimeters or resistance-thermometer devices facilitate spa Pressure Gages tial traverses. A correct calibration of thermal anemometers requires consideration of the effects of temperature, hu The Bourdon b the most common type of pressure gage) and its appearance probably is familiar to anyone having an acquaintance with power plants or laboratories.v The essential element of such a gage is the Bourdon tube, a metal tube of oval cross-section curved along its length to , form an almost complete circle. One end is closed and the other b connected to the vessel in which the pressure is midity, and pressure upon the: air properties which in? fluence convective heat transfer.* With sensing elements of ample shapes for which convection .data. are known, thermal anemometers may be designed, both thermally and electrically, for desired characteristics. Directional sen sitivity b controllable. Thermal anemometers are convenient and practical for low velocities. to be measured. With an-increase of pressure, the tube The Kata Thermometer tends to straighten,-and vice versa. The resulting motion '1 of the closed end b communicated by suitable linkages to Another instrument useful for studying air currents in a needle moving over a graduated dial. -If the range is., free spaces b the Kata thermometer. Thb is essentially above about 20 psi,.such gages are.usually calibrated by an alcohol thermometer with a very large bulb, and with means of a dead weight teeter, whereby known pressures only two scale divisions etched on its stem. On the Low are produced in a fluid by imposing known weights on. a <! Kata these divisions are 95 and 100 F; on the High Kata piston of known area. Suction gages and. pressure gages ' they are 125 and 130 F. In use, the instrument is heated, with, ranges below about 20 psi are ordinarily calibrated - usually in a water bath, until the alcohol has risen suf against mercury manometers. Gages are commonly set -to - ficiently above' the upper calibration on the stem. It b read accurately at or near the pressure of .probable use. ' then thoroughly dried, and placed at the point at which Gages of several different types or qualities are-available the air velocity b to be measured. The time in seconds on the market.14 required for the temperature to drop from the upper to the lower scale division b observed. From thb cooling time FLUID VaOCITY MEASUREMENTS The theory of various means for measuring the flow of fluids b discussed in Chapter 6, Fluid Flow. Heating and and the calibration factor inscribed on each Kata - ther mometer, the air velocity can be calculated or determined, from a chart. The Kata thermometer is also used in de-' termining the cooling power of the atmosphere, since it. 1 | Measurement .and.lnstniments 249 Table 4 ... ..Measurement of Velocity AppRceffon ftaofpa PimUsr (imitofioo* j Hot-wire anemometer (a) Low air velocitieej'di- rectiooal and nondirectional available 6-1,000 1-20%' Accuracy of some types not good at lower end of range 2 Kata thermometer ' ' (b) High air velocities,.. , Lowair velocities in rooms; non-directional - up to 60,000 5-300 1-0.01% 5-15% Awkward to use; affected by ra diation 3 Smoke puff or air-borne Lowair velocities in rooms; . - highly directional 5-50 10-20% Awkward to use but valuable in tracing air movement 4 gwinpng-vanetypeane- Air velocities in rooms, at monaster outlets, etc;-directional-' 30-24,000 5 Veaturi-type multiply- Low air,velocities in rooms ; 100-2,000 with micromanom 'ing pitot tube ' and ducts; directional eter; 180-2,000 with draft gages' 5% . ..1-5% Not well suited for duct readings; needs periodic check calibration Accuracy falls off at low end of range; requires calibration 6 Revolving-vane type ae- Moderate air velocities in ,. v , mometer *. ducts-and rooms; some what directional. 100-2,000 5-20% Extremely subject to error with variations in velocities with space or time; easily damaged; needs periodic calibration 7. pitot tube 8 Impact tube and sidewall or other static tap Std instrument for meas urement of duct velocities and pressures 180-10,000:- with' micromanometer; 600-10,000 with draft gages; 10,000 up with manometer. High velocities, small, ,120-10,000 - with tmiiroma^" tubes and where air direc . nometer; 600^10,000 " with tion may be variable draft'gages; 10,000 up with manometer 1;5%: Accuracy faUs'ofTat low end of range - . , >. \~s% ' Accuracy depends upon' constan cy of static pressure across stream section loses heat by radiation and convection when dry, and-,by radiation, convection, and evaporation when the bulb' js equipped with a wetted cloth covering'.* To' minimize* the effect of radiation, the Kata thermometer b also made with a silvered bulb. ' Airborne Solid Tracer Techniques i} ' Another useful'device for very low velocities b to time the rate of movement of a smoke puff, feather dr piece.of .lint: Thb technique, like the Kata thermometer, b well suited'to velocity measurement in an open space, but not particularly adaptable to measurements in ductwork.. : Smoke b a qualitative tool which is very useful in study?, in; sir movements.! Satisfactory smoke can [be obtained from titanium tetrachloride (which, however, is very ir ritating to membranes) or by miring potassium chlo rate and powdered sugar (a nonirritating smoke) and firing the mixture with-a mutdi. Thb latter process evolves con siderable heat, and it should be confined in a pan away from flammable materials.-The titanium tetrachloride,smoke lends itself to spot determinations, particularly for leakage through casings and ducts, as it can. be easily handled in a small pistol-like' ejector. The fumes of aqua ammonia and of sulfuric add, if permitted to mix, form,a. white precipitate which b useful for some purposes. Two bottles, one containing ammonia, water and the -other add, are connected to a common nozzle by. means of rubber tubing. Ah b forced over the surfaces of the liquids in.'the bottles by means of a syringe/ and the two streams, upon mixing at the nozzle, form a white : A satisfactory test smoke may! also be made by bubbling an air stream through ammonium hydroxide and then through muriatic acid:" Smoke tubes, smoke, candles,1- and! smoke bombs are available for studying airflow patterns. Deflecting'Vane Anemometer ' 'The'deflecting.vane'anemometer consists of a pivoted vane enclosed in a case. Air exerts a pressure on the vane as it', passes through the instrument from' an upstream to a downstream opening. The movement"of the vane b resbted by a hair spring and a damping magnet. The instru ment gives instantaneous readings of directional velocities on an indicating scale. With fluctuating velocities, it b necessary to average visually the swings of the needle, to obtain average velocities. Thb instrument b very useful for studying motion of the air in a room," and in locating'objec tionable-drafts. Various attachments are ayaii&ble.such' as the double tube arrangement for determining velocities in ducts, and a device for measuring static pressures.' Each instrument, and the. attachments for it, must receive in dividual calibration.' For determining average velocity 'in a duct, it b necessary to traverse the duct asisdone when using the Pitot tube.' ' PrbpeUer or Revolving Vane Anemometer y' The'propeller or revolving vane'anemometer consists'of a-light revolving wind-driven wheel connected' through::a gear train to a set of recording dials that read' the; linear feet of air'passing in a1 measured-length'of'time. It b -made in- various rises,- 3 in., 4 in., and 6in. being most common! Each instrument - requires individual calibration:: At'-low velocities the-friction'drag of the mechanism is' consider! able. In order-- to compensate for this, a gear'train: that overspeeds is commonly used: For thb reason the correc tion b often- additive at the lower range and1 subtractive at the upper range,-with theleast correction in the middle range of velocities. Most of these are not sensitive' enough for use below 200 fpm: