Basic Documentation

Room Pressurization Control Application Guide
SUPPLY
TERMINAL
ROOM
CONTROLLER
FUME
HOOD
CONTROLLERS
ROOM
GENERAL
EXHAUST
CFM
CFM
CFM
CFM
LAB0195R1
Figure 4. Airflow Tracking Control Arrangement for a Chemical Laboratory Room.
The specific amount of air exhausted by a VAV fume hood depends on the extent that its
respective sash is open
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. Aside from controlling the fume hood exhaust airflow, each Fume
Hood Controller also provides an output signal to the Room Controller that indicates the
exact amount of fume hood exhaust airflow. The Room Controller is apprised of the amount
of exhaust air that is being exhausted by each fume hood in the room.
In addition to fume hoods, the Room General Exhaust that is directly controlled by the room
controller may also draw air out of the laboratory room. A room general exhaust is often
required for VAV laboratories with VAV fume hoods for one of the following reasons:
• Allow more air to enter the room from the supply terminal.
• Maintain sufficient room ventilation.
• Maintain the desired room temperature when the fume hood sashes are closed.
The Room Controller maintains direct control over the Supply Terminal that provides the
supply make-up air for the room. In a CAV laboratory, the supply airflow into the room is
normally maintained at a constant value since the fume hood exhaust normally remains
constant. However, in a VAV laboratory the total room exhaust is dependent upon the fume
hood exhaust that varies in accordance with the fume hood sash position. Thus, the Room
Controller in a VAV laboratory must modulate both the incoming supply airflow via the Supply
Terminal and the Room General Exhaust to maintain a proper balance between the total
room exhaust airflow and the incoming supply airflow.
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A VAV fume hood controller uses an airflow sensor and a modulating damper in the fume hood exhaust to maintain the
required exhaust airflow in accordance with the fume hood’s total open sash area. This ensures that fume hood face velocity
remains constant for all sash positions. Each VAV fume hood controller continuously sends the fume hood exhaust airflow
value to the room controller.
18 Siemens Building Technologies, Inc.