AFA4000/E VAV Airflow Controller / p.132
8.3 Control Setup Guide
The Prop Band and Integral Time parameters must be adjusted in order to achieve good VAV
control. Complete the calibration procedure first (section 7), then use this guide to help find the
best values for each individual installation.
Note: The information in this section describes how to set up and calibrate a single fume
cupboard incorporating the AFA4000/E VAV controller. Most VAV fume cupboard
installations use a common extract fan, complete with a fresh air bleed system
connected to multiple fume cupboards. Always consider the commissioning and setting
up of the system as a whole before you attempt to set up the individual fume cupboard
controls. This is particularly important when the extract system has a diversity factor (i.e
the extract fan is only capable of operating a percentage of the fume cupboards at full
volume).
1. Set Integral Time to 0 seconds to give proportional control only.
2. Set Prop Band to 30%.
3. Set the Econ to Automatic and select Run.
4. At the design sash height, for example, 500 mm, the face velocity will be close to the set
point, for example, 0.5 m/sec.
5. When the sash is lowered to 100 mm, the face velocity increases.
6. Adjust the Prop Band value in steps up or down until the face velocity at 100 mm is
approximately 0.75 m/sec and check that the face velocity is stable (i.e. it is not oscillating).
The Prop Band value should be approximately 15 to 35 %.
If the Prop Band is set to a low value, e.g. 10%, the sensitivity will be high and the output may
oscillate when the sash is lowered.
If the Prop Band is set to a high value, e.g. 50%, the sensitivity will be low and the face
velocity at 100 mm will be too high.
At this stage the measured face velocity will probably not be at the set point with the sash at
500 mm. The function of this first step is to get stable control at 500 mm and at 100 mm with
no ‘hunting’.
7. When the correct Prop Band is determined, set up the Integral Time to correct the measured
face velocity to the set point value.
Initially set the Integral Time to 10 seconds.
8. Now when the sash is lowered, the Integral will reduce the face velocity back to the set point
over a specified time. If the ‘correction time’ is too slow, increase the Integral Time
To Next Page
Loading...