Apparatus for controlling the ventilation of laboratory fume hoods

Apparatus for controlling the air flow through a plurality of fume hoods to maintain a predetermined average face velocity through an uncovered portion of an opening of each fume hood, each fume hood being of the type which has at least one moveable sash door adapted to selectively cover the opening as the fume hood sash door is moved, each fume hood in communication with an exhaust duct for expelling air and fumes from each fume hood, the exhaust ducts for each fume hood being in communication with an exhaust system, said apparatus comprising:

• means associated with each fume hood for detecting the position of each moveable sash door and generating a position signal that is indicative of the position of such sash door;
• means responsive to said position signals for calculating the size of the uncovered portion of the opening of each fume hood;
• means for measuring the actual flow of air through the exhaust duct in communication with each fume hood and generating an actual flow signal that is indicative of the actual flow of air through the exhaust duct;
• modulating means associated with each fume hood for varying the flow of air through the exhaust duct that is in communication with the respective fume hood responsive to a control signal being received from a controller means;
• controller means responsive to said position signals and said actual flow signal for controlling the flow modulating means associated with each respective fume hood to generate a desired flow rate signal as a function of the calculated size of the uncovered portion, said desired flow rate signal corresponding to a flow rate that is sufficient to maintain the predetermined average face velocity through the uncovered portion of the opening of each fume hood, said controller means comparing said desired flow rate signal and said actual flow rate signal for each fume hood and generating an error signal indicative of any errors that exist by taking a plurality of successive measurement samples of said actual flow rate, determining at least two distinct factors of said error signal from said successive samples and summing the said factors to generate said error signal, said controller means outputting a control signal to said modulating means associated with each fume hood for selectively reducing said error signal to a predetermined minimum value or maintaining a predetermined minimum actual flow.

Apparatus for controlling the air flow through a fume hood to maintain a predetermined average face velocity through an uncovered portion of an 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, the fume hood being in communication with an exhaust duct for expelling air and fumes from the fume hood, said apparatus comprising:

• means for detecting the position of each moveable sash door and successively generating a position signal that is indicative of the position of each sash door;
• means responsive to said position signals for calculating the size of the uncovered portion of the opening;
• means for successively generating an actual flow signal that is indicative of the actual flow of air through the exhaust duct;
• modulating means for varying the flow of air through the exhaust duct responsive to a control signal being received from a controller means;
• controller means responsive to said position signals and said actual flow signal for controlling the flow modulating means, said controller means either generating a control signal value that maintains a predetermined minimum flow rate or generating a desired flow rate signal as a function of the calculated size of the uncovered portion, said desired flow rate signal corresponding to a flow rate that is sufficient to maintain the predetermined average face velocity through the uncovered portion of the opening, said controller means generally continuously comparing said desired flow rate signal with successive instantaneous sample values of said actual flow signal and generating an error signal having a magnitude that is directly proportional to the sum of any calculated integration error, any calculated derivative error and any calculated proportional error, said controller means successively generating and outputting a control signal to said modulating means for reducing said error signal to a predetermined minimum value or maintaining said predetermined minimum actual flow.