Method and Apparatus for Controlling Fume Hood Face Velocity Using Variable By-Pass Resistance

摘要

Apparatus for controlling a flow control means for controlling the air flow through a fume hood to maintain a predetermined average face velocity through an uncovered portion of a face opening of a fume hood of the type which has at least one moveable sash door adapted to cover the opening as the fume hood sash door is moved and having a by-pass opening with a grill means overlying the by-pass opening, the by-pass opening being adapted to be at least partially blocked responsive to movement of at least one of said sash doors, the fume hood being in communication with an exhaust duct for expelling air and fumes from the fume hood, said apparatus comprising: means for determining the position of each of said sash doors and generating signals indicative thereof; processing means including memory means for determining the size of the uncovered face opening responsive to said generated signals and to stored data in said memory means relating to the physical and operational parameters of the fume hood, said processing means being adapted to determine the size of the unblocked by-pass opening responsive to said generated signals and to said stored data; said processing means modifying the size of said unblocked by-pass opening by a conductance factor to provide an effective size of said unblocked by-pass opening; said processing means adding said size of the uncovered face opening and said effective size of said unblocked by-pass opening to provide a total effective opening size, and controlling said flow control means to provide the desired average face velocity utilizing said total effective opening size, by varying the speed of a variable speed blower and/or by adjusting the position of a variable position damper means in the exhaust duct of the fume hood. IMAGE