Document EdK63jNbM3e5kYqV317EmLNwx

American Society of Heating and Ventilating Engineers Guide, 1935 , MEASUREMENT OF AIR MOVEMENT The quantity, velocity and pressure of air moved by a fan or flowing^ through a duct or grille may be determined by various methods. The instruments in common use are the Pitot tube, anemometer, direct reading velocity meter, and Kata-thermometer, the latter being'suitable for low air velocities and being commonly used for measurements at points where the air is not confined in a duct. The use of calibrated nozzles, orifice plates, and Venturi meters are recognized methods, which, however, have little application in connection with ventilation practice. Pitot Tube This usually consists of two tubes, one within the other, which when, properly held in the air stream will register the- total or impact pressure and the static pressure, respectively. If these tubes are connected to opposite sides of a water column, or other type of manometer, the recorded pressure will be the differential or velocity pressure. Volume measure-* ments may thus be made in a duct of known', area. Pitot tube measure ments are preferably used for air velocities exceeding 20 fps. Volumetric determinations from Pitot tube readings should take into account the barometric pressure and the temperature and humidity of the air measured. In general no accurate velocity pressure readings can be taken when the flow of air in ducts is turbulent. To insure accuracy a straight section of duct from 5 to 10 times its own diameter is desirable in order to straighten out the air currents. If it is necessary to take Pitot tube readings in shorter sections of straight duct, the results must be considered subject to some doubt and checked accordingly. For accurate work it is neces sary to make a traverse of the duct, dividing its cross section into a number of imaginary equal areas qnd taking a reading in the center of each, the average of the velocities corresponding to these pressures giving the true velocity in the duct. . Anemometer This instrument is delicate, and requires frequent calibration when accuracy is desired. The vanes of the instrument should, never be touched and it should never be held in air having a velocity greater than that for which it is calibrated. Readings taken directly in a fan inlet or discharge are likely to harm the instrument because of excessive velocities. In duct measurements the same procedure is followed as for the Pitot tube. The anemometer usually reads directly in linear feet. To obtain the velocity in feet per minute, the reading must be divided by the elapsed time in minutes. The following procedure for obtaining anemometer readings is based on research conducted at Armour Institute of Technology in cooperation with the A.S.H.V.E. Research Laboratory2. Supply Grilles. The surface of the grille should be marked off into a number of equal areas approximately 6 in. square. A 4-in. anemometer Measurement of Flow of Air through Registers and Grilles, by L. E. Davies (A.S.H.V.E. Transactions, Vol, 36. 1930, Vol. 37. 1931, and A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, Sep tember, 1933). 678 Chapter 40--Test Methods and Instruments should be used and should be held at the center of each section in contact with the grille (or as close as possible) for a period of time sufficient to insure an average reading. In the case of supply grilles, the instrument should always be held with the dial facing the operator. The average of the corrected readings should then be used in the following formula to obtain the flow in cubic feet per minute: cfm -- CV a Qr CVA (1 + p) a) where V = average of corrected anemometer readings, feet per minute. A = gross area of grille, square feet. a = net free area of grille, square feet. p = percentage of free area of grille expressed as a decimal. C = a coefficient that varies with the velocity from grille and may vary slightly with type of grUle. For average use, with supply grilles, C can be taken as 0.97 at velocities from 150 to 600 fpm, and. as 1.00 at higher velocities. Particular care should be exercised in the case of long, , narrow grilles. The nature of the approach sometimes results in there being a narrow strip along the top or bottom of the grille through which no air will be flowing. This may be detected by holding the anemometer completely out of the air stream and then moving it slowly inward over the grille until the vanes just start to move. The distance which the vanes extend over the grille opening at this moment will indicate the width of the dead strip. Only the remaining portion of the grille should be considered in making the calculations for gross and free area. Exhaust Grilles. The surface of the grille should be marked off and readings taken in the same manner as with supply grilles, except that the instrument should be held with the dial facing the grille, and in contact with it. The traverse should be' taken at a uniform rate, allowing suf ficient time in each space to minimize the percentage of error. In the case of exhaust grilles it is found that the formula in which cfm = KVA (2) " .' " V = average indicated velocity obtained by the anemometer traverse. A = gross area of grille, square feet. K = coefficient determined by experiment. For average use, with exhaust grilles-, K may be taken as 0.8 for all usual velocities. This formula is of advantage, especially with ornamental grilles, in that the free area need not be measured. The flow of air through registers and grilles is of considerable impor tance, being frequently the only convenient method of measuring the volume of supply air to a room. While duct measurements, if available, are more dependable, grille measurements provide a fairly accurate method, if care is taken in the technique of using the anemometer. Kata-Thermometer The Kata-thermometer can be used to determine air velocities pro vided the walls and surrounding objects are at or near the room tern- 679