Document GKRezovYBr0wz9NL0Yx0VG03v

American Society of Heating and Ventilating Engineers Guide, 1932 under conditions of unrestricted flow of air to the ventilator, are given by the equation: where Q=A x 36 X ^ H X (ft - <o) 6+ V + 20 X V (S) 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 <1 inside ta 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 1.--What is the capacity of an 18-in. ventilator, located 35 ft above the inlet openings, with 6 mph wind velocity, 50 F outside temperature, 68 F inside temperature? Answer.--A = 0.7854 X (18)' = 255 sq. in. Q = 255 X 36 X . 35 X ^68 - 5o) 6+6 + 20 X 6 _ = 50,000 cfh, average capacity under these conditions. The air supply required based on square feet of floor space is given in Chapter 24, and the number of the renewals of air contents per hour is given in Chapter 4. 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 S.--A foundry building is 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? 368 1 Chapter 26--Natural Ventilation Answer.--In this case, ventilation is especially necessary in summer. The air in the building should not be over 10 F 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: <2 - -!9-x (20ftxjQft x 40ft) = 400,000cfh. o The discharge per square inch of throat area under these conditions is; 36 X y 55 X 10 deg 6 + 4 mi. per hr. + 20 X 4 mi. per hr. = 165 cfh of air. The required throat area per ventilator is 400,000 165 2420 sq in. Vif there is no resistance and no wind pressure. --2--4--2--0-- = 55.5 m. 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. APPLICATION OF NATURAL VENTILATION General Rules Although it is practically impossible to predetermine accurately the pressure difference at each individual ventilating opening, from funda mental data of wind velocity, wind direction, temperature difference, building dimensions and disposition of openings, in most cases merely a general knowledge of how the forces of wind and temperature difference act, what their maximum magnitudes are, and how they are disposed in and about a building, will be helpful in planning ventilation. The following simple rules are suggested; 1. In an industrial building where furnaces, that give off heat and fumes, are to be installed, it is better to locate them in the end of the building exposed to the prevailing wind. The strong suction effect of the wind at the roof near the windward end will then cooperate with temperature difference, to provide for the most active and' satisfactory removal of the heat and gas laden air. 2. In case it is impossible to locate furnaces in the windward end, that part of the building in which they are to be located should be built higher than the rest, so that the windT in splashing therefrom will create a suction. The additional height also, increases the effect of temperature difference to cooperate with the wind. 3. In the use of monitors, windows on the windward side should usually be kept closed, since, if they are open, the inflow tendency of the wind counteracts the outflow tendency of temperature difference. Openings on the leeward side of the monitor result in cooperation of wind and temperature difference. 4. In order that the force of temperature difference may operate to maximum advan tage, the vertical distance between inlet and outlet openings should be as great as possible. Openings in the vicinity of the neutral zone are less effective for ventilation. 369