Document 91vOBw2MOyVYRmLBpjmOd5xnR

188 CHAPTER 11 1948 Guide i' ,, Otherinformation onorificesandtheir use-is contained.inChapter4, FluidFlow. Shaped orifices, or nozzles have the.advantage. if .well made that their discharge coefficients are close to unity so that the .probability of large errors .is less.. Orifices of-this type .haveJbeen adopted-for several specific, purposes and designs are described, in' the A.S.H.V.K. Unit Heater17 and UnitVentilator Codes18and inA.S.R.E. Circular 1319entitled ."Standard Methods of Rating and Testing Air Conditioning Equipment". In some .instances nozzles are used in multiple so that the capacity of the testing equipment can.be changed by shutting off the flow through one or more nozzles. An apparatus designed for testing the air flowand capacity of air conditioning equipment is described by Wile in an article in which pertinent information on nozzle discharge coefficients, Reynolds numbers, and the resistance of perforated plates is also presented. Such apparatus in some laboratories is commonly referred to as a code tester. The Venturi meter is like the nozzle except for the addition of a down stream transition section that reduces the pressure drop through the measuring apparatus. The thin-plate square-edged orifice has a coefficient of approximately' 0.60. The exact value depends on the location of the connections, the pressure drop, the diameter ratio of orifice to pipe, and the sharpness of the edge ** Another method.of air measurement uses the thermal electric principle where, by. means of a measured amount of current, heat is put into the air stream. The weight of air flowing is calculated from the heat equiva lent of the electrical input and the temperature rise of the air. Heat should be applied uniformly to the mass of air passing/and the small temperature rise must be determined accurately. - In certain application's the air velocity through a duct, heater coil, or heating,unit may be most conveniently obtained by computation from the heat given up by the coil and the temperature rise (measured by thermocouples) of the air passing through. It is essential to have a uniform flow'over the entire inlet and outlet of the. heater at the, plane of temperature measurement. " Air Currentsin Free Spaces The measurement of air current velocities within, enclosed spaces such as. the rooms of a house is usually a tedious undertaking:. However, useful data can be . obtained by using the. instruments described in following paragraphs if they are maintained in calibration, and the user understands the operation and limitations of the instruments 23. - One of the instruments useful for determining the velocity of air'currents in free spaces is the. Kata-thermometer.. It is essentially,an alcohol thermometer with a. large bulb. The stem has two marks, ,one .corre sponding. to:95 F, and the other 100 F. The instrument is heated above 100 F and then the time in seconds required for it to cool from, 100 to 95 when located in the air current gives a measure of the non-directional velocity! It is important to have the Kata-thermometer dry before taking the reading. Each Kata has its own factor etched on the stem, and this factor must be used with its cooling formula or chart for obtaining the velocity.-. The Kata-thermometer is useful in exploring ventilated spaces to determine whether the proper air movement and distribution are being maintained. It is also used in determining, the cooling power of the atmosphere, since it loses heat by radiation and .convection when 'Instruments and Measurements - . ., _-- 189 dry, and- by radiation, convection','and. evaporation* when-the bulb is equipped with a- Wetted- cloth covering 24:-- : - - ' ; - *-: ^Another instrument for measuring'low velocity air' currents is the heated. thermometer anemometer **.:; This; consists of a mercurial glass thermometer with a: resistance winding onthe'bulb. Current is supplied from an external source in a measured amount! The difference between the temperature of this heated thermometer and that of an ordinary , thermometer-at the same location;'together with the current supplied, makes it possible to calculate the non-directional velocity of the air stream. The heated thermocouple anemometer employs' a- thermocouple instead of a thermometer 26. Another instrument is the hot wire anemometer which has been made in several patterns. In general; a measured current is supplied to raise'the temperature of,a fine bare wire above the temperature of the surrounding air. With the use of a very fine wire, miriiite fluctuations in velocity may be measured,.and the area exposed to radiant exchange with heated or cooled surfaces is at a minimum. This instrument is easily adapted to remote reading or recording. A group of them may be connected together to give the average velocity in a space, or the velocity at individual points within a test space, by suitable switching arrangements-27, 28. Deflecting Vane Anemometer The deflecting vane anemometer consists of. a pivoted vane enclosed in a case, against which-air exerts a pressure as it passes through the instrument from an uprstream to'a down-stream opening. The move ment of the vane is resisted by a hair spring and a damping maghet. The instrument gives instantaneous readings'hf directional velocities on an in dicating scale.. When used in fluctuating velocities, it is necessary to average visually the swings of the needle to obtain average velocities: This instrument,is very useful' for studying mixing of air in a room 29 and in locating and measuring peak velocities that may be objectionable. Various attachments are available, such as the double tube arrangement for obtaining velocities in ducts, and a device for measuring static pres sures. Each'instrument and the attachments for it must receive individual calibration. '' ': Propeller or Revolving Vane Anemometer. The propeller or revolving vane anemometer consists of a light revolving \vheel connected through a gear train to a set of recording dials that read the linear feet of air passing in a measured length of time. It is made in various sizes, 3 in., 4 in., and 6 in. beingmost common. Each instrument' requires individual,calibration.' At low velocities the friction drag of the mechanism is considerable. In order to compensate for this; a gear train that over-speeds is commonly used: "For this reason the borrection is often additive at the lower range and' subtractive at the upper range with the least correction in the middle range of velocities. Most of these are not sensitive enough for use below,200 fpm. .. Measurement of Velocities at Inlets and Outlets of Ducts In the field it is often desirable to make volume measurements at the face of the supply openings. It is rare to have access to the interior of duct sections where the flow is sufficiently uniform for measurement. ' For accuracy the instrument and its application should be checked on asimilar approach and grille in'the laboratory before use in the field.