Document KGL18n7GXyZmKGX8K4jqv2Lrw

American Society of Heating and Ventilating Engineers Guide, 1934 a square-entrance orifice (flow coefficient 0.60). In other words, the grille should be oversize as compared with the ventilator, and they should be connected by a tapering collar. If the ventilator head construction produces changes in the direction of air flow, the area of the flow passages should be increased accordingly. Air inlet openings at lower levels in the building are of course necessary for the economical use of ventilator capacity. The inlet openings should be at least equal to, and preferably twice as great as the combined throat areas of all roof ventilators. The air discharged by a roof ventilator depends on wind velocity and temperature difference, but due to the four capacity factors already mentioned, no simple formula can be devised for expressing ventilator capacity. Several types of roof ventilators are shown in Figs. 2 to 11. These may be classified as stationary, Figs. 2 to 6, pivoted or oscillating, Figs. 7 to 9, or rotating. Figs. 10 and 11. When selecting unit ventilators, some attention should be paid to ruggedness of construction, storm-proofing features, dampers and damper operating mechanisms, possibilities of noise from dampers or other moving parts, and possible maintenance costs. It should be kept in mind that a suitable combination of roof venti lators with mechanical ventilation frequently offers the best solution of a ventilating problem. The natural ventilation units may be used to sup plement power driven supply fans, and under favorable weatherconditions it may be possible to shut down the power driven units. Where low operating costs are very important, such a combination has great advantages. Roof ventilators with built-in electric fans are attracting increased attention because they combine the advantages of low instal lation and operating cost with those of continuous service. Controls In connection with any combination between natural and fan venti lation, the controls are of importance. Both the fans and the ventilator dampers may be controlled by some combination of three methods: (1) hand operation, (2) thermostat operation, and (3) control by wind velocity. 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 miniifium volume can be supplied by natural forces. ; 1 i * Stacks Stacks are really chimneys and utilize both the inductive effect of the wind and the force of temperature difference (the so-called.gravity action). While their openings projecting above the roof are not provided with any special construction for developing suction by the action of the wind, the plain vertical opening is also effective in this respect. Like the roof ventilator, the stack outlet should be. located so that the wind may act upon it from any direction. 62 Chapter 4--Natural Ventilation Stacks are applicable particularly in the case of schools, apartments, residences and small office buildings. Partitions interfere with general air circulation, and some type of outlet from each room is necessary. If the building is not too tall, and the requirements of occupancy are moder ate a system of stacks with registers in each room may be more eco-, nomical then a system of mechanical ventilation employing fans. In making the comparison, however, the building space occupied by the stacks should be considered. With little or no wind, chimney effect or temperature difference will oroduce outflow through the stacks and an equal inflow through windows m all sides of the building. With wind, the inductive force at the top of ventilating shafts is more powerful than that on the leeward side of the Fig. 10. Rotating Ventilator building, so that air is drawn in through leeward openings by a combina tion of the forces of wind and temperature difference. On the windward side, the direct forcing pressure of the wind is of course added to the temperature difference effect. Thus forces are available for. causing in flow at practically every window of such a building. Adequacy of stack size must, of course, be provided. Principles of air flow control The air flow through a .Ventilation opening'depends on the two factors already discussed, namely, (1) the natural forces available, (2) the open ings available, and the resistance !tp flow offered by these openings. The design problem includes, of course, "a determination of the desired air quantity and distribution in order that the openings may be properly placed. The purpose of ventilation is to carry off either excess heat or air impurities, and the desired air quantities depend upon the amount of heat or of impurities present. The amount of heat can be determined, in the case of forge shops for example, from the amount of fuel burned, which in turn is based upon the production capacity- for which the building is being designed. In the case of foundries, the heat given off by the metal in cooling from the molten state can be used. In some instances, not all 63