Document b5jQmYB2EoLnZLk59KgmB1rLy

American Society of Heating and Ventilating Engineers Guide, 1930 ings. Often such Ventilators must depend upon infiltration only for inflow. Inlet openings of about twice the area of the roof ventilators should be provided where natural ventilation is used. Dampers should be installed in ventilators with adequate and accessible operating mechanism. In considering roof ventilators, the general action of the wind upon the building, as described in the earlier part of this chapter, should be kept in mind. A ventilator located within the region indicated by A in Fig. 1, will not contribute any inductive effect because in this region there is little or no Various* Styles of Roof Ventilators wind. The ventilator opening will function, but will function in the same way as would a window in that locality, because of the resultant effect of the suction produced by the. wind acting upon the building, and by whatever temperature differences prevail. Circular roof ventilators are effective if placed in the wind-flow;, no matter what the direction of the wind. On the other hand, if ventilators do not stand in the path of the wind, and depend only upon the same forces for producing flow as do windows or any other plain opening, they are at a disadvantage because of the greater resistance of their more complex passages. 114 Chapter 4--Systems of Ventilation Location of Discharge Openings The discharge openings for all systems of ventilation must be arranged so that they shall not be influenced by wind or other atmospheric con ditions. If a ventilating outlet looks out horizontally through the side wall of a building, sooner or later a pressure from outside will oppose the air flow and the occupants of the building will suffer. Even when the outlets for ventilation are well above the roof, they must be as distant as possible from higher buildings or obstructions, since the jumps and the eddies illustrated in Fig. 1 are ever-present to the leeward and may cause difficulty. Types of Roof Ventilators The simplest form of unit ventilator is the stationary type shown in Fig. 2 and consists of an outlet pipe with a conical hood above it. The addition of a storm band, as shown in Figs. 3, 4, and 5, gives an increased protection against the entrance of rain or snow. The storm band, if placed so close to the cones as to restrict the outflow of air, interferes with ventilation. On the other hand, if the openings are made large enough to permit free egress of the inside air, the storm band increases the venti lation by better utilizing the wind velocity to produce suction. A'further development of the latter principle is the siphon ventilator, as illustrated in Fig. 6, in which siphons or ducts are introduced for the particular purpose of producing suction. In the swiveling or rotary ventilators, typified by Figs. 7 and 8, a freely rotating cowl is used. A wind vane is provided for keeping the opening facing away from the direction of the wind. This type allows free egress of the inside air (unless the outlet is made unduly small). Its action depends chiefly upon the suction produced by the wind on the leeward side of the ventilator. In the induction or ejector type of ventilator, which is also of the swivel ing or rotary type, Fig. 9, the kinetic energy of the wind is intended to create a suction inside the cowl as well as outside. Resistance to flow of air in the ventilator is caused by: (1) restricted outlet openings, or (2) many turns or changes of the direction of the air flow. As regards the first item, this depends entirely upon the proportions, and not upon the type; some of the stationary ventilators have smaller, and others have larger outlet area than some cowl ventilators of the same nominal size. Regarding the second item, the swiveling cowl ventilators offer less resistance than the stationary type, in that the direction of air flow is changed as little as possible. Unless swiveling ventilators move very freely, the opening, at times, faces towards the wind so that ventilation produced by temperature difference is much reduced, or wholly counteracted. In that case, snow and rain may blow in. The rumbling or creaking noise caused by a hard turning swivel ventilator is also very unpleasant. These troubles are, of course, eliminated in well-designed ventilators, but must be kept in mind. In Fig. 10 is shown a rotary or air-turbine ventilator, which rotates continuously under the action of the wind, the motion being produced by the difference of wind pressure on the convex and concave sides of the 115