Air extract method and apparatus

Claim 1

Air extraction apparatus for extracting air from an enclosure situated solely within a room, said enclosure including an air inlet having a variable opening to provide air only from within said room, and an air outlet including a duct, a suction fan fluidly connected to said duct, and bypass means in said duct fluidly connected to said suction fan, said bypass means communicating with the environment outside said room to supply only atmospheric outside air directly to said fan, said bypass means including adjustable baffle means, said baffle means being pivotable between a first position in which it substantially closes off the suction side of the fan from the enclosure and a second position in which it substantially closes off the suction side of the fan from the outside atmosphere monitoring means for directly monitoring the extent of said variable opening, and control means responsive to said monitoring means for automatically adjusting said baffle means in a manner continuously proportional to the monitored extent of said opening, so that the volume of atmospheric bypass air increases as the volume of air entering the enclosure through said variable opening from said room decreases, and vice versa, such that a predetermined velocity of air is maintained through said variable opening and such that the velocity of air through said suction fan is maintained constant while the volume of air extracted from said enclosure is varied.