Document R2j0nkeRneVaE0Vkx5j0bxDDv

7^-.; r_ American Society of Heating and Ventilating Engineers Guide, 193^ Table 2. Values of C for Various Air Velocities AtbbaVoexloIcnidttic. ated /pm SupHt Grilles Straust Games 150 200 300 400 500 600 700 800 0.952 0.957 0.967 0.977 0.985 0.992 0.998 1.000 0.993 1.005 1.028 1.049 1.067 1.078 1.084 Factors for average use__ low velocity._________ 0.97 } high velocity 1.00 1.02 number of equal areas approximately 6 in. square. The anemometer 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 -- or CVA ^ where V = average of corrected anemometer readings in 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 grille. (See Table 2). As the velocity increases, C approaches a constant, value, and the highest value shown can be used for all higher velocities with very slight error. Exhaust Grilles. The surface of the grille should be marked off and readings taken in the same manner as ^ith supply grilles, except that the instrument should be held with the dial facing the register. The average of the corrected readings, together with the proper constant for exhaust grilles (Table 2), when used in Formula (5), will give the flow in cubic feet per minute'with a maximum error of 10 per cent, although in the majority of cases the error will be less than 3 per cent. 506 Chapter 32 FANS AND MOTIVE POWER of Fans; Characteristic Curves; Selection of Fans; Fan ft"'HE fans use<i for heating, ventilation and air conditioning are usually .1 of the disc, propeller and centrifugal type. Electric motors are 1 r - -- -- rwaurPr v TYPES OF FANS Disc and propeller fans are especially desirable for the ventilation of small theaters, public buildings, restaurants, clubs and factories, and for removing steam, gases, foul air, etc., from dye houses, laundries, kitchens, laboratories and similar places where more or less large quantities of air are to be moved against a low resistance. This type of fan is most satis factory when installed in such a manner as to have no duct work con nected to it. The centrifugal fans considered in this chapter are known as the multi blade type as distinguished from the positive pressure blower type which handle comparatively small quantities of air at high pressures. Centrifugal fans are used where more or less large quantities of air are required at pressures ranging from about }/% in. to 3 in. or more of water pressure. These fans usually are housed and are used for building and tunnel venti lation, heating, forced and induced draft, drying, dust collecting, con veying, etc. There are many types of centrifugal fans, but their principal difference is in the design of the wheel. These may be classified with respect to the - --------- 1. Forward curve. 2. Radial tip. 3. Steel plate and partial backward curve. 4. Full backward curve. CHARACTERISTIC CURVES Characteristic curves of fans are determined by tests performed in accordance with the Standard Code for the testing of Centrifugal and Disc Fans,1 as adopted by the American Society of Heating and `A.S.H.V.E. Transactions. Vol. 29. 1923. p. 407.