Document ZLyyLbONapwY0vYGN8q50GJ0

American Society of Heating and Ventilating Engineers Guide, 1937 The proper distribution of air in occupied spaces is an element almost as important as that of sufficient air quantity. Advantageous pivoting of sash is very useful for securing good air distribution. Deflectors are some times used for the same purpose, and these devices should be considered a part of the ventilation system. Door openings are seldom included in the ventilation calculations, though they may be of great value for extreme summer conditions, and should be considered in this connection as. well as in garage design. Skylight and monitor openings are of importance as these and the roof ventilators are outlets, while the lower windows are usually inlets on the windward side and outlets on the leeward side. In general the areas of inlets and outlets should be about equal. It is important to make a check on this ratio in any installation, as any great excess of area of one set of openings over another means waste opening area. The operating devices used for sash, monitors, skylights and roof ventilators should be well selected as poor operating devices may defeat the entire design. Roof Ventilators The function of a roof' ventilator is to provide a storm and weather proof air outlet, which is sensitive to wind action for producing additional flow capacity, and at the same time is subject to manual or automatic control by suitable dampers. The capacity of a ventilator at a constant wind velocity and temperature difference, depends upon four things: (1) its location on the roof, (2) the resistance it offers to air flow, (3) the area and location of openings provided for air inflow at a lower level, and' (4) the ability of the ventilator head to utilize the kinetic energy of the wind for inducing flow by centrifugal or ejector action. Frequently one or more of these capacity factors is overlooked in a ventilator installation. For maximum flow induction, a ventilator should be located on that part of the roof which receives the full wind without interferences (See Fig. 1.) This does not mean that no ventilators are to be installed within the suction region created by the wind jumping over the building, or in a light court, or on a low building between two high buildings. Ventilators are highly effective in such low-pressure areas, but their ejector action, caused by wind velocity, is of little importance in these locations', and hence their size should be increased proportionally. Ventilator resistance depends on (1) type of inlet, (2) area of openings and passages, and (3) number of turns or changes of direction of the air flow. The inlet grille, if any, should have ample free area, and the venti lator should always be provided with a taper-cone inlet in order to produce the effect of a bell-mouth nozzle (flow coefficient 0.97) rather than that of a square-entrance orifice (flow coefficient 0.60). In other .words, the grilles should be oversize as compared with the ventilator, and they should be connected by tapering collars. 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 88 Chapter 4--Natural Ventilation nds on wind velocity and temperature difference, but due to the four dePe -t 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, rotating, Figs. 10 and 11. When selecting roof 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 CItSshould 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 weather con ditions 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 minimum volume can be supplied by natural forces. 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. 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 than 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 produce outflow through the stacks and an equal inflow through windows 89