Basic Documentation

Chapter 7–Room Ventilation, Makeup Air, and Pressurization Control Systems
The individual VAV fume hood controllers vary the total amount of air being
exhausted from their respective fume hood as needed to maintain the required fume
hood face velocity. Since variations in fume hood exhaust affect the total room
exhaust, the room controller constantly adds all of the fume hood exhaust cfm values
together along with the room general exhaust airflow cfm to determine the total room
exhaust cfm
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. The room controller modulates the room general exhaust damper to
ensure that there is sufficient total room exhaust to meet the required minimum room
air change per hour rate.
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The laboratory room controller also controls the total
room supply airflow cfm to maintain the constant airflow tracking differential between
the total room supply cfm and total room exhaust cf m.
As the fume hoods begin to exhaust more air and thus increase the total room
exhaust, the room controller reduces the room general exhaust airflow. If the fume
hoods begin to exhaust less air, the room controller increases the amount of the
room general exhaust airflow, as required, to maintain sufficient total room exhaust
that helps maintain the required room ventilation rate (ACH rate) and also room
ambient conditions. As the room total exhaust airflow varies, the room controller will
adjust the room supply airflow as required to always maintain the airflow tracking
differential.
There may be times, particularly during the cooling season, when the room will
require more ventilation airflow to maintain the desired room temperature than the
amount needed to just maintain the minimum room ACH rate. In instances where the
room temperature begins to exceed its setpoint, the room controller may need to
increase the amount of supply airflow to maintain the desired room temperature. In
such situations, the room controller must also increase the room general exhaust so
that a corresponding amount room air is exhausted to maintain the airflow tracking
differential.
The difference between the room supply and room exhaust airflows is dependent
upon individual room conditions. Room size, tightness of construction, and other
factors affect the magnitude of the airflow tracking differential that is required to
maintain a specific positive or negative room static pressure. Experience has shown
that approximately 150 cfm per doorway is a nominal airflow differential that may be
used for the initial setup of a room’s ventilation system
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.
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Since digital controllers share information, the fume hood controllers each provide the room controller with the current
exhaust airflow rate of their respective fume hood. This allows the room controller to account for the total exhaust from
the room.
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Room ACH is customarily determined by the amount of the total room exhaust airflow in negatively pressurized rooms.
In positively pressurized rooms the ACH is determined by the total room supply airflow.
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The final airflow tracking value is normally established when the ventilation system is in the test and balance phase. A
measurement of the room static pressure is made and the airflow differential is adjusted until the desired static
pressure value is determined. Once determined, this value becomes the airflow tracking setpoint that is maintained by
the room controller.
68 Siemens Building Technologies, Inc.