Document RJmYxqgYYVL56zB1Gg44BvVZX

American Society of Heating and Ventilating Engineers Guide, 1926-27 5. Air admission below the ventilator, (resistance to flow of air into building). 6. Resistance to air flow through the building. 7. Resistance to air flow in the ventilators themselves. 8. Location of the ventilator with respect to surrounding objects. Of the factors mentioned, only items 4 and 7 depend upon the ventila tor itself; the other items depend upon circumstances wholly outside of ventilator size and design. While ventilators may be divided into certain classes or groups and the average efficiency of one class will be higher or lower than the average efficiency of another class, this does not in any way determine the capacity of individual ventilators, as ventilators of the same class and, which from a casual observation appear to be the same, will have entirely different characteristics, due to the fact that some of the fundamentals have been overlooked or changed in one or the other. The principles which should be followed are: A--Stationary Ventilators. 1. A head sufficiently large to produce a large low-pressure area on the side opposite the wind, and to give an area of outlet for the air leaving the head large enough to obviate undue resistance to flow. At the same time the head should not be so large as to be unwieldy handling or to be structurely weak when erected. 2. A storm band on stationary non-siphoning ventilators sufficiently wide and so placed as to prevent the entrance of external air into the ventilator head. 3. If the ventilator is a siphoning type additional outlet air space must be provided in the head in order not to restrict the air pas sage from the exhaust pipe. B--Rotary Ventilators. 1. A flaring outlet from a rotary ventilator will give a better exhaust than a straight oultet. 2. Practically frictionless and noiseless turning of the ventilator head, when the wind direction changes. The head should turn at very low wind velocities. 3. Smallest possible change of direction of the air ascending from the building and least possible resistance to its egress by louvres or other obstructions at the outlet opening. C--All Ventilators. 1. Freest possible outlet for the air from the building, with large areas and smallest possible change of direction of the air flow. 2. Freedom from down drafts and from entrance of rain or snow. 3. Freedom from being rendered inoperative by collection of snow or formation of ice on ventilator. The simplest form of ventilator, shown in Fig. 87, consists of an outlet pipe with a conical hood above it. The addition of a storm band, as 290 American Society of Heating and Ventilating Engineers Guide, 1926-27 shown in Figs. 88, 89 and 90, 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 ventilation by utilizing the wind velocity to produce suction. A further development of the later principle is the siphon ventilator, as illustrated in Fig. 91, in which siphons or ducts are introduced for the particular purpose of producing suction. Various Styles of Roof Ventilators In the swiveling or rotary ventilators, typified by Figs. 92 and 93, 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). For producing suction, it depends upon the viscous drag of the wind pas sing along the outside of the cowl. In the induction or ejector type of ventilator, which is also of the swivel ing or rotary type Fig. 94, the kinetic energy of the wind is used to a large extent by creating suction, due to the viscous drag both inside and out side the cowl. This device is effective for ventilation even with very low wind velocities. In some stationary ventilators of the siphon type, or of 291