Document yZ0LyOQkN4axj5moNv1pR6wV

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 opening and the flow per square foot obtained previously, the calculated amount for this condition will be: 612 X 163 X 1.265 = 126,200 cfm. Adding the'two computed flows: Temperature Difference = 126,200 = 42 per cent. Wind = 173,200 = 58 per cent. Total 299,400 = 100 per cent. From Fig. 3, it is determined that when the flow, due to temperature difference, is 42 per cent of the total, the actual flow, due to the combined forces, will be about 1.6 times that calculated for temperature difference alone, or 201,920 cfm. The original flow, due to temperature difference alone, was 199,286 cfm with all openings in use. The effect of the wind is to increase this to 201,920 cfm even though half of the outlets are closed. A factor of judgment is necessary in the location of the openings in a building, especially those in the roof, where heat, smoke and fumes are to be removed. Usually windward monitor openings should be closed, but if 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. . 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 hori zontally, 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 will 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 advantageous in controlling the distribution of incoming air. Deflectors are sometimes . used for the same purpose, and these devices should be considered 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 temperature 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 offers to air flotv, (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 ihe full wind without interference. If ventilators are installed within the suction region created by the wind 766 CHAPTER 42. NATURAL VENTILATION passing over the building, or in a light court, or on a low building between two high buildings, their performance will be the same there as for any other type of opening of the same area. Their normal ejector action, if any, will be of no value in such a location. 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, additional resistance is introduced, and it 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, their per formance 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, or so-called heat valve, would fall in the stationary classification. When selecting roof ventilators, some attention should be given to ruggedness of construction, 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. Controls . Gravity ventilators may have dampers controlled by (1) hand, (2) thermostat, and (3) 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, and utilize both the in ductive effect of the wind and the force of 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, chimney effect depends on temperature difference to produce a removal of air from the rooms where the inlet openings are located. GENERAL RULES . A few of the important requirements in addition to those already outlined are: 1. Inlet openings in the building should be well distributed, and should be located on 767