Document 2ja4d48xzw2g2jVan2JxavXpb

American Society of Heating and Ventilating Engineers Guide, 1930 C--All Ventilators. 1. Freest possible outlet for the air from the building, with large areas and smallest possible change of direction of the air flow. 2. Freedom from down drafts and from entrance of rain or snow. 3. Freedom from being rendered inoperative by collection of snow or formation of ice on ventilator. 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 of ventilators of various types have been made by reliable investigators, and the reader is referred to their work for information on capacities (See Transactions, 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, Engineering Experiment Station, Kansas State Agricultural College). Conservative figures for the best types of ventilators now on the market, under conditions of unrestricted flow,of air to the ventilator, are given by the equation: where 36 X Q=A X 6+ V *0 + go 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 h inside tQ outside. 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 of 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 of cross-sectional area is reduced, on account of the frictional resistance and, in the rotary types, on account of reduction of free area 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. . 118 Chapter 4--Systems of Ventilation 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 per hour is given in Chapter 3, and the number of the renewals of air contents per hour is given in Chapter 2. 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 which obstruct air currents, it is desirable to extend the ventilators above the buildings by mounting them on stacks. Example.--A foundry building i9 40 ft. wide, 200 ft. long, with an average height of 40 ft.; the ventilators are to be mounted at the ridge of the roof, at a height of 55 ft. above the floor. What number and size of ventilators are required? Answer.--In this case, ventilation is especially necessary in summer. The air in the building should not be over 10 deg. fahr. warmer than the outside air. The wind velocity may be as low as 4 miles per hour. Spacing the ventilators, tentatively, 25 ft. apart, 8 ventilators would be required. Under average conditions, 10 air renewals per hour are sufficient. If the foundry is small and cramped, and pouring takes place over a large section of the floor space, 15 or more air renewals per hour may be needed. On the basis of 10 renewals per hour, the capacity of each ventilator must be: ?' Q = 10 X (200 ft. X^40 ft. X 40 ft. ) _ 4(X)>000 cu per hour The discharge per square inch of throat area under these conditions is; xV36 55 X 10 deg. + 20 X 4 mi. per hr. 6 + 4 mi. per hr. = 165 cu. ft. of air per hour. The required throat area per ventilator is 400,000 165 = 2420 sq. in. if there is no resistance and no wind pressure. The diameter is -/ 2430 _ .55 5 ;n Standard sizes are 54 in. and 60 in. \ 0.7854 Either eight--54-in. or else seven--60-in. ventilators could be used, spaced respectively 25 ft. or 28 ft. apart. The foregoing is based on the use of high class ventilators. If ventila tors of lower efficiency are used, or if the air flow into the building is restricted (as in winter) larger ventilators may be required. GRAVITY VENTILATING SYSTEMS Gravity ventilating systems are those which use openings especially provided for air flowi with ducts and flues, and which depend.on the difference in weight caused by heat to compel the movement of air. 119