Active Pressure Control

[SAK9]

Guys,

In most pharmaceutical facilities I have seen ,zone pressure regimes are maintained with an active pressure control system which is done by regulating exhaust volume through a VAV box in the exhaust duct based on room differential pressure reading.Further the exhaust fan is on speed control through a VSD based on duct static pressure at some location.So when the VAV boxes close to maintain room pressures,the duct static presure goes up which in turn would reduce fan speed.When this happens,the staic pressure behind some boxes may drop causing them to open more.This may result in another static pressure change.So my question is:Are pressure control system and fan speed control system at loggerheads?

 

[abcmex]

as you describe your system both controls are probably working against each other: one tries to control the pressure in the room, the other tries to control the presure at the duct. Try a little PID tuning on the controls, you may get better results if one of the controls is a slower than the other.

 

[grim900]

Another option is to use modulating dampers within the extract duct rather than a VAV box. The dampers position is moved solely in relation to the room pressure. As above, the main extract volume would still be modulated by means of a frequency inverter in relation to static pressure. These damper movements would only be very slight to allow for small changes in the room's airflow patterns, and there would normally be a delay incorporated to allow for doors opening and closing.