On this particular project the stair pressurization fans for
the smoke control system were oversized. Since the fans were direct drive and
not on variable frequency drives (VFDs), they could not be slowed down. The
only other alternative was to install sheet metal to limit the airflow going
into the stairwell.
Having oversized fans creates another interesting challenge
- fan status. In order to verify if a fan is running, stopped or is missing a
belt (trouble), either a differential pressure switch (DPS) or current
transducer switch (CT) is installed. The
DPS measures pressure created by using sensors.
The CT measures electrical current.
There are pros and cons to both, but usually CTs are preferred, mainly
because a DPS can give a false reading if there is a wind gust. In this case, CTs were used. However, since the fan motors were oversized, the difference in current between belts off and fan running was only about 0.1 amps, and it took several adjustments to get an accurate reading of fan status – when on normal building power. When the emergency generator was activated, the current readings shifted because the output voltage of the generator was slightly less than the normal building power. Further adjustments had to be made to get true fan status.
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