Document Gzvmd9Kz9EVmYvLE7xkrZBjbn

American Society of Heating and Ventilating Engineers Guide, 1929 of course, eliminated in well designed ventilators, but must be kept in mind: In Fig. 10, is shown a rotary or air-turbine ventilator, which rotates continuously under the action of the wind, the motion being produced by the difference.of wind pressure on the convex and concave sides of the Rotary Ventilators 3 SECTION Fig. 10 Air-Turbine Ventilator vanes. This type of ventilator must be very carefully designed if it is to be leak-proof, and if the noises and impact forces, due to ice accumu lating on the vanes in the winter are to be eliminated. : CAPACITIES OF UNIT TYPE ROOF VENTILATORS The variety of factors affecting capacity makes it essential for the user of ventilators to exercise-great care in respect to the item of capacity. All comparisons of capacity must be referred to a given, dimension, namely the throat area, corresponding to the nominal size of the ventilator. '.Careful tests pf ventilators of various types have been made by reliable investigators, and the reader is referred to their work for information on 478 Chapter XXXIII--Natural Ventilation capacities (See Trans. A. S. H. & V. E,, Vol. 27, 1921, p. 67; Vol. 28, 1922, p. 189; and Vol. 29, 1923, p. 39. -Also Bulletin No. .14, Engi neering Experiment Station, Kansas State Agricultural College). Conservative figures for the best types of ventilators now on themarket, under conditions of'unrestricted flow of air to the ventilator, are given by the equation: where 36 X Q=A X HX 6+ V + 20 X, V Q = cubic feet of air exhausted per hour through a ventilator having a free area at the throat of A square inches, mounted on a roof at a height of H feet from the center of the ventilator outlet to the inlet opening of the building, and with a wind velocity of V miles per hour, and average tem perature t\ inside tQ outside. ' ' j'- High class ventilators, for instance those of the: ejector type, will, under favorable conditions, discharge continuously 25 per cent more air than these conservative figures indicate. Capacities are lower, on the other hand, if ventilators of lower efficiency are used, or if the flow of air into or through the building is restricted, or if the ventilator is not ex posed to the free sweep of the wind. Tests occasionally show consider ably higher discharge rates over short periods of time. These abnormally high results are produced by the action of the wind upon certain openings pf the building; they are not due to the ventilator, Itself, and cannot be depended upon for continuous ventilation if the direction of the wind changes. In the smaller sizes of ventilators (12 in. or less in throat diameter) the air discharge per square inch pf cross-sectional area is reduced," on account of the frictional' resistance and, in the rotary types, on account of reduction of free atea'by the supports, bearings, etc! `' Example.--What is the capacity of an 18 in. ventilator, located 35 ft. above the inlet openings, with 6 miles per hour wind velocity, 50 deg. fahr. outside temperature, 68 deg. fahr. inside temperature? Answer.--:A ..= 0.7854 X (18)' = 255 sq. in. 36 X. y 35 X ^68 - 5o) Q = 255 X 6+6 + 20 X 6 average capacity under these conditions. = 50,000 cu. ft. per hour, The air supply per person and pier hour, or the number of the renewals of air contents per hour is given in Chapter I. (See also Chapter I under "Infiltration," and Chapter XXVI). To obtain effective, uniform Ventilation and avoid local drafts, the ventilators should not be placed more than 30 ft. apart; 20 ft. apart is a good average. It is best, although not absolutely necessary, to locate the ventilator at the ridge of the roof, unless the building exceeds 40 ft. in width, in which case two rows of smaller ventilators should be used. Where the building, to be ventilated is surrounded by higher buildings 479