Control system for exhaust gas fan system

A method of controlling a flow of exhaust gas from a building, the building comprising ducting, a mixing plenum in fluid communication with the ducting, at least one by-pass damper supported by the mixing plenum, and a first variable speed exhaust fan supported by the mixing plenum configured to draw the exhaust gas through the mixing plenum and out the building, the method comprising the steps of:

• sensing, upstream of the first variable speed exhaust fan, a static pressure of the exhaust gas in the ducting using a static pressure probe;
• operating the first variable speed exhaust fan at a determined minimum fan speed while the at least one by-pass damper is in an open position so as to cause the exhaust gas to exit the building at least at a minimum discharge velocity;
• in response to an increase in system demand, causing the at least one by-pass damper to move from the open position toward a closed position to maintain a substantially constant system pressure set point;
• after the closing step, in response to a further increase in system demand, increasing a speed of the first variable speed exhaust fan as the sensed static pressure further increases, in order to maintain the substantially constant system pressure set point;
• decreasing the speed of the first variable speed exhaust fan in response to a decrease in system demand;
• and
• causing the at least one by-pass damper to move toward the open position when the first variable speed exhaust fan is operating at the determined minimum fan speed in response to a further decrease in system demand, thereby providing supplemental air to the first variable speed exhaust fan.

A control system configured to control a fan system that exhausts gas from a building, the building comprising ducting, a mixing plenum in fluid communication with the ducting, at least one by-pass damper supported by the mixing plenum, an exhaust fan supported by the mixing plenum and configured to draw gas through the mixing plenum so as to discharge the gas as exhaust gas through an outlet nozzle, and a variable speed fan drive that operates the exhaust fan at variable speeds;

• the control system comprising:
• a static pressure probe disposed upstream of the exhaust fan in the mixing plenum, the static pressure probe operable to measure a static pressure of the exhaust gas,
• wherein the control system adjusts the variable speed fan drive and modulates the at least one by-pass damper in order to maintain a substantially constant static pressure within the mixing plenum,
• wherein the control system closes the at least one by-pass damper when the discharge velocity of the exhaust gas is above a minimum and the control system opens the at least one by-pass damper when the discharge velocity of the exhaust gas is below the minimum,
• wherein the control system is configured to cause the at least one by-pass damper to move from an open position toward a closed position as the measured static pressure increases while the exhaust fan is operating at a determined minimum speed, and the control system is configured to cause the variable speed drive to increase a speed of the exhaust fan when the at least one by-pass damper approaches the closed position or is at the closed position, in response to a further increase in the measured static pressure, and wherein the control system is configured to 1) decrease the speed of the exhaust fan in response to a decrease in system demand, and 2) cause the at least one by-pass damper to move toward the open position when the exhaust fan is operating at the determined minimum fan speed in response to a further decrease in system demand, thereby providing supplemental air to the exhaust fan.

A method of controlling a flow of exhaust gas from a building, the building comprising ducting, a mixing plenum in fluid communication with the ducting, at least one by-pass damper supported by the mixing plenum, and a first variable speed exhaust fan supported by the mixing plenum configured to draw the exhaust gas through the mixing plenum and out the building, the method comprising the steps of:

• sensing, upstream of the first variable speed exhaust fan, a static pressure of the exhaust gas in the mixing plenum using a static pressure probe;
• operating the first variable speed exhaust fan at a determined minimum fan speed while the at least one by-pass damper is in an open position so as to cause the exhaust gas to exit the building at least at a minimum discharge velocity;
• causing the at least one by-pass damper to move from the open position toward a closed position in response to an increase in system demand;
• after the closing step, increasing a speed of the first variable speed exhaust fan in response to a further increase in system demand;
• decreasing the speed of the first variable speed exhaust fan in response to a decrease in system demand;
• and
• causing the at least one by-pass damper to move toward the open position when the first variable speed exhaust fan is operating at the determined minimum fan speed in response to a further decrease in system demand, thereby providing supplemental air to the first variable speed exhaust fan.