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252. CHAPTER 11 1957 Guide the wind is low enough for the temperature head to overcome it, all windows may be opened. TYPES OF OPENINGS Types of openings may be classified as: (1) windows, doors, monitor openings and skylights; (2) roof ventilators; (3) stacks connecting to registers; and (4/ specially designed inlet or outlet openings. The various types and principles of-operation are discussed in following paragraphs. Windows, Doors and Skylights Windows have the advantage of transmitting light, as well as providing ventilating area, when open. Their movable parts are arranged to open in various ways; they may open by sliding either vertically or horizontally, by tilting on horizontal pivots at or near the center, or by swinging on pivots at the top, bottom or side. Regardless of their design, the air flow per square foot of opening may be considered to be the same under the same conditions. The type of pivoting should receive consideration from the standpoint of weather protection, and certain types may be ad vantageous in controlling the distribution of incoming air. Deflectors are sometimes used for the same purpose, and these devices should be con sidered a part of the ventilation system. Roof Ventilators The function of a roof ventilator is to provide a storm and weather proof air outlet. These are actuated by the same forces of wind and tem perature head which create flow through other types of openings. The capacity of a ventilator depends upon four things: (1) its location on the roof; (2) the resistance it and the duct work offer to air flow; (3) the height of draft; and (4) the efficiency of the ventilator in utilizing the kinetic energy of the wind for inducing flow by centrifugal or ejector action. For maximum flow induction, a ventilator should be located on that part of the roof where it will receive the full wind without interference. If ventilators are installed within the suction region created by the wind passing over the building, or in a light court, or on a low building between two high buildings, their performance will be seriously influenced. Their normal ejector action, if any, may be completely lost. The base of the ventilator should be of a taper-cone design to produce the effect of a bell-mouth nozzle whose coefficient of flow is considerably higher than that of a square-entrance orifice. If a grille is provided at the base, or if the base or structural members present obstructions, addi tional resistance is introduced, and the base opening should be increased in size accordingly. Air inlet openings located at lower levels in the building, should be at least equal to, and preferably larger than, the combined throat areas of all roof ventilators. The air discharged by a roof ventilator depends on wind velocity and temperature difference, and, in general, its performance will be the same as any monitor opening located in the same place but, due to the four capacity factors already mentioned, no simple formula can be devised for expressing ventilator capacity. Roof ventilators may be classified as stationary, pivoting or oscillating, and rotating. Generally, these have a round throat, but the continuousridge ventilator would fall in the stationary classification. When selecting roof ventilators, some attention should be given to ruggedness of construe- Tnfiltration and Ventilation 253: tion, storm-proofing features, dampers and damper operating mechanisms, possibility of noise, original cost, and maintenance. Natural ventilation units may be used to supplement power-driven supply fans, and under favorable weather conditions it may be possible to stop the power-driven units. Units are not subject to code tests for ratings. Generally, they must be selected from manufacturers' tables. It is, therefore, very important to consider the reliability of the ratings used. Controls Gravity ventilators may have dampers controlled by hand, thermostat, or wind velocity, in combination with a fan. The thermostat station may be located anywhere in the building, or it may be located within the ventilator itself. The purpose of wind velocity control is to obtain a definite volume of exhaust regardless of the natural forces, the fan motor being energized when the natural exhaust capacity falls below a certain minimum, and again shut off when the wind velocity rises to the point where this minimum volume can be supplied by natural forces. Stacks Stacks or vertical flues are really chimneys which function through the effects of the wind and temperature difference. Like the roof ventilator, the stack outlet should be located so that the wind may act upon it from any direction. With little or no wind, the chimney effect depends entirely on temperature difference to produce a removal of air from the rooms where the inlet openings are located. GENERAL VENTILATION RULES lined, are: \\ Ilt openings in the building should be well distributed, and should be located tne windward side near the bottom, while outlet openings are located on the leeard side near the top. Outside air will then be supplied to the zone to be ventilated. Jn,et openings should not be obstructed by buildings, trees, sign boards, etc., outside, nor by partitions inside. l,,,^" Greatest flow per square foot of total opening is obtained by using inlet and outlet openings of nearly equal areas. auit'o*11 th,e de" of window ventilated buildings, where the direction of the wind is and cons"ant Jnd dependable, the orientation of the building, together with amount of .W^ofventilation openings, can be readily arranged to take full advantage shoulH K Ce ` wind. Where the wind's direction is quite variable, the openings annrn- arr.aned in sidewalls and monitors so that, as far as possible, there will be there nJ'n*1! ec*ual areas on all sides. Thus, no matter what the wind's direction, to a siVet- ,aya be Bome openings directly exposed to the pressure force, and others suction force, and effective movement through the building will be assured. air at tk6* tiShirt c.ircuits between openings on two sides at a high level may clear the cupancy 1 6Ve without producing any appreciable ventilation at the level of oc- vertiM? H!'dler temPerature difference may produce a motive force, there must be able in k between openings. That is, if there are a number of openings avail- by temn DU,ldlnS> but all are at the same level, there will be no motive head produced j . Perature difference, no matter how great that difference might be. yantaee <?i?er tbo, Irce of temperature difference may operate to maximum adPossible' n verTca^rbstane(: between inlet and outlet openings should be as great as tion Openings in the vicinity of the neutral zone are less effective for ventila- ^losed^ai^6 **re P* Inondors> windows on the windward side should usually be kept nee. if they are open, the inflow tendency of the wind counteracts the outflow