The following was taken from “Comparative Tests of Automatic Ventilators” published in 1925 by the Engineering Experiment Station of the Kansas State Agricultural College.
Plain Stationary
Page_9_Fig-1The plain stationary type of automatic ventilator makes no special provision for utilizing the wind velocity in producing additional draft. Examples of such ventilators are shown in figure 1. This type consists simply of a cap above the ventilator pipe which prevents the direct entrance of rain or snow and is made storm proof by means of a circular band. In some cases an additional top is provided, while in others the band is so shaped as to provide an additional top.
Plain_StationaryMy take on Type A, B and C. I know that the sheet metal isn’t thick like I show .. that is just to MAKE it visible after all. I think this is close other than the sheet-metal supports that would have held everything together.
Stationary Siphoning
Page_9_Fig-2The stationary siphoning type is shown in figure 2. Its principle of operation is based upon the breaking up of the wind currents and directing them in such a manner as to create a decreased pressure in the upper portion of the ventilator. Ventilation is thus secured by an ejector action. When no wind is blowing, the ventilation resulting is that due to natural circulation of the air. When winds prevail the siphoning action is established and additional ventilation results.
Plain Rotary
Page_10_Fig-3The plain rotary type, figure 3, consists chiefly of an elbow or its equivalent, which is supported upon a vertical shaft. The position of the elbow is regulated upon the principle of the weathervane. The opening from the ventilator, consequently, always points away from the direction of the wind. This ventilator makes use of the slight vacuum produced by the wind in the production of additional draft.
Rotary Siphoning
Page_12_Fig-5The rotary siphoning type is shown in figure 5. It has, in addition to the principle embodied in the plain rotary type, a feature similar to that used in the stationary siphoning ventilators. The air is directed by flutes or vanes so that an ejector action is established, thereby increasing the velocity through the ventilator. In some cases, the ejector is placed within the ventilator, while in others it surrounds the ventilator.