3.3. Velocity operating mode
The velocity operating mode regulates the front air velocity
(v=0.1 … 0.5m/s) so as to maintain it constant despite the
sash position.
The exhausted air is either regulated by a servomotor (in
centralised exhaust systems ) or by a fan mounted on a roof
and controlled by a frequency inverter.
AIR EXHAUSTED
AIR INTRODUCED
Figure 1 : Velocity operating mode
Figure 2 : Velocity regulation diagram
Air velocity probe :
Nowadays, 3 different kinds of sensors able to measure
velocity can be found on the market.
- Hot wire
- hot body
- propellers
- The hot-wire solution is obviously the most professional way
to measure accurately and quickly low speeds from 0 to 1 m/s
- The hot-body solution is economic and resistant. Its disadvan-
tages are its accuracy and its reaction time.
- The systems with propellers are used to measure velocities
above 1 m/s.
With the velocity probe developed by Comelec, the least
change in the sash position begets a fluctuation of air which
is translated by the probe into an electrical signal 0.5Vdc.
The technology chosen is the hot-wire type. This gives a very
fast response time to the probe in the measuring scale 0…1m/s
(Tr100ms).
This velocity range is particularly adapted to measure the front
velocity of fume hoods and laboratory hoods.
The hot-wire anemometry consists in a measurement at a fixed
point. This technology is little intrusive. Its strength lays in its
excellent spatial and temporal resolution. Thus, it makes it the
best technology to study turbulent fluctuations.
The usual hot-wire anemometers are made of a thin wire
of a few millimetres long and a diameter from 1 to 10 µm.
Most of the time, the measurements are made in the air (for
velocities from 0.1 m/s to several dozens of m/s)
The installation of the probe onto the top of the fume hood
permits to measure the air velocity.
If the place has been wisely chosen, the air flow measure
corresponds exactly to the air front entering the fume hood.
Control unit ECRO-RV
Display panel VFC11
Low sash contact
3’-sash >500mm contact
Air velocity probe
PIR presence detector
Fume hood light
Motorised damper
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