Document OzQjvy3XzJKbJ8XJVZLRMgzLw

224 CHAPTER 10 1951 Guide ing wind is 8 mph perpendicular to the long dimension. What is the necessary area for the inlets and outlets, and what is the rate of air flow through the building? Solution for Temperature Difference Only. The heat H =* -- - -- *8,000 ,, 34,875-Btu per min. 00 By Equation 5, the air flow required to remove this heat with an average temper ature difference of 10 deg is: " 0.0175U - (.) = 0.0175 X 10 " 199,286 cfm' This is equal to about 20 air changes per hour. From Equation 4, the inlet (or outlet) opening area should be: ,Q 199,286 ,J , A t---= - = -----y - =* 1224 sq ft. 9.4Vfc(f- O 9.4VS0 X 10 The flow per square foot of inlet or outlet would be 199,286 + 1224 163 cfm, with all windows open. Solution for Wind Only. With 1,224 sq ft of inlet openings distributed around the sidewalls, there will be about 410 sq ft in each long side ana 202 sq ft in each end. The outlet area will be equally distributed on the two sides of the monitor, or 612 sq ft on each side. With the wind perpendicular to the long side, there will be 410 sq ft of opening in its path for inflow, and 612 in the lee side of the monitor for outflow, with the windward side closed. The air flow, as calculated by Equation 3, will be: Q ~ 0.60 X 410 X 704 ~. 173,200 cfm. This gives 17.3 air changes per hour, which should be more than ample when there is no heat to be removed. Solution to Combined Heads. Since the windward side of the monitor is closed when the wind is blowing, the flow due to temperature difference must be calculated for this condition, using Fig. 3. This chart shows that, when inlets are twice the size of the outlets, in this case 1,224 sq ft in the sidewalls and 612 sq ft in the monitor, the flow will be increased 26.5 percent over that produced by equal openings. Using the smaller 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 percent. Wind = 173,200 = 68 percent. Total 299,400 = 100 percent. From Fig. 4, it is determined that, when the flow, due to temperature difference, is 42 percent 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, doom, monitor openings and skylights; (2) roof ventilators; (3) stacks connecting to Infiltration and Ventilation 225 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 construc 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