An investigation of the performance of a stationary siphoning type of roof ventilator

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Virginia Agricultural and Mechanical College and Polytechnic Institute

Tests were performed to determine and compare the aspirating effectiveness of 10- to 24-inch sizes of a stationary siphoning type of automatic roof Ventilator.

The basic principles of Ventilator design were studied critically, and suitable apparatus and methods of carrying out the tests were designed accordingly. One series of tests consisted of mounting a Ventilator on a ten-foot stack at the mouth of a calibrated wind tunnel, and by measuring with a Velometer the Velocity of the air traveling up the stack, determining the volume of air drawn through the stack while winds of from two to seven miles per hour were blowing across the ventilator. The second series of tests was identical with the first, except that the Ventilator was removed and the wind was directed across the open end of the stack. Stacks and ventilators of sizes 10, 12, 14, 16, 18, 20, and 24 inches were tested. All test results are based on zero difference in temperature between the wind tunnel air and the stack air and on no friction loss in the stack. The capacities listed are for standard air at 70°F and a barometric pressure of 29.92 inches of mercury.

The tests revealed several interesting facts. The open stacks were found to be more effective than the Ventilators when the wind Velocity across the Ventilators was less than about 4.5 miles per hour; but the Ventilators proved better than the open stacks at wind Velocities above 4.5 miles per hour. For the series of Ventilators tested, the relationship between discharge capacity and wind Velocity across any given ventilator was found to be approximately linear. It was observed that small disturbances in the air currents around the Ventilator head seriously affect the discharge and that for maximum effectiveness, it is necessary to have ample window opening in the room to be ventilated.