AFA4000/E VAV Airflow Controller / p.129
As the sash is closed further, the damper will not move, and
the displayed face velocity will increase. The increased
velocity represents a volume through the fume cupboard in
the sash closed position equal to approximately 15 – 25% of
the design volume with the sash at the maximum safe
working opening.
Note: Typical minimum volume requirement is set to
achieve 6 ach/hr room ventilation when all sashes
are closed.
Used to set up the maximum extract rate from the fume
cupboard in automatic VAV operation. As the sash is raised,
the damper progressively opens, or the extract fan speeds
up, to increase the extract volume and maintain the set
point face velocity.
The High Limit sets a maximum damper position to prevent
the volume from increasing too far.
Note: In most installations, the High Limit is set to 100%.
Used to select the correct output voltage range for damper
control for the VAV or inverter speed control of the extract
fan.
• Damper: 2 – 10 V
• Valve: 2 – 10 V
• Inverter: 0 – 10 V
The Prop Band (Proportional Band) is the main control
parameter for the automatic VAV control. It acts as the
sensitivity of the control system.
If it is set too low, a very small change in the measured face
velocity will result in a large change in the output, and
damper or inverter will become unstable.
If it is set too high, a large change in the measured face
velocity is required to give a small change in the output, and
the damper or inverter will react very slowly.
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