Assortment / System Description

Room pressurization and fume hood control
Fume hood
21
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Check low sash for low air volume flow – Defines whether the system checks
sash position upon a schedule change from occupied to unoccupied. If set to
Inactive, the fume hood occupancy is allowed to go to unoccupied regardless of
sash position / face area. If set to Active, sash must be lower than the
unoccupied alert area or unoccupied alert position before fume hood occupancy
can change to unoccupied. If set to Active and the sash is open during a
schedule change, fume hood occupancy goes to "Unoccupied pending".
Switch-on delay to low air volume flow – The amount of time that must
elapse after fume hood occupancy changes to Unoccupied, before fume hood
operating mode changes from Control to Low air flow. The ODP will indicate
this impending change if there are no alarms present.
Enable schedule override from ODP – (QMX3.P88 only) For a fume hood that
follows an assigned schedule, setting to 1: Yes will enable the fume hood
operator to manually override the schedule at the ODP. When the hand button
on the ODP is pressed by the hood operator, the fume hood will remain in
occupied mode until the command is released at the next scheduled occupancy
state.
21.9.2 Exhaust flow control strategy
The fume hood application regulates exhaust flow to a setpoint selected
according to several selectable strategies. The strategies determine the
required exhaust flow setpoint using sensors and indications from the fume
hood user at the ODP.
The application uses one or two sensors two indicate the position of a sash or
sashes that move vertically. The modulating exhaust flow setpoint can be
calculated directly from sash position or through face area calculations for a
constant face velocity. Configurable minimum and maximum flow limits apply to
either strategy.
The application uses a velocity sensor, installed through the wall of the fume
hood, to represent face velocity. A control loop modulates the exhaust flow
setpoint to maintain the indicated velocity at the face velocity setpoint.
Configurable minimum and maximum flow limits apply to either strategy.
Face velocity sensing may be combined with sash sensing.
21.9.2.1 Exhaust flow control
A differential pressure sensor is typically connected to the DXR2 for an
application with internal volume flow control.
The application is optimized for use with a fast damper actuator (end to end
stroke in 2 seconds) and fast flow control performance, but can also operate
more slowly. The present damper position is determined in all cases by DXR2
and provided to the central function for optimizing the supply air fan.
21.9.3 Communication to room control
The fume hood controller communicates with a room controller through group
data exchange.
The fume hood setpoint may need to be increased due to laboratory
temperature or laboratory airflow requirements. A fume hood demand request
will be sent from the laboratory room to maintain proper pressurization.
Variables collected by room:
● Air flow value for tracking (reflects flow or setpoint)
● Fume hood minimum provided air flow for exhaust
● Fume hood maximum provided air flow for exhaust
Sash sensing
Face velocity sensing