Honeywell AUTOMATIC CONTROL, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATION CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
289
The bypass hood limits face velocity to about twice the full
sash open face velocity which may be acceptable. However,
conditioned air is always exhausted making energy savings
improbable.
The auxiliary air hood is a bypass type with a supply air
diffuser located in front of and above the sash. If the make-up
air through the diffuser is not conditioned as well as room air,
some minimal energy savings result by employing this type of
equipment. However, the performance of this hood is
controversial regarding containment of materials in the hood,
operator discomfort, and thermal loading of the laboratory. Its
use is usually discouraged.
The standard hood can be controlled either by adding a face
velocity sensor at the sash opening or by installing devices to
measure the sash opening to calculate face velocity. See Figure
44. This information is then used to modulate a motorized damper,
air valve, or variable speed motor to vary exhaust airflow and
to maintain a near constant face velocity regardless of sash
position. Since the hood removes non-recirculated, conditioned
air from the space, significant energy savings can be realized
by adding these controls to vary air volume and minimize the
rate of exhaust. The other common method used to moderate
energy usage is to provide two-position controls which control
all hoods at one constant volume rate during occupied periods
and a reduced constant volume when the lab is unoccupied.
New technologies now available allow the air flow in individual
fumehoods to be reduced when sensors determine no one is
present at the face of the hood.
The subject of the “correct” face velocity is still debated.
However, most research now indicates that 80-100 feet per
minute (fpm) at the sash opening provides a zone of maximum
containment and operating efficiency provided that the supply
air delivery system is designed to minimize cross drafts.
Velocities lower than this challenge the containment properties
of the hood, and without specialized lab design and training in
lab safety protocols, can create unsafe working conditions.
Velocities higher than 120 fpm can cause excessive turbulence
within the hood which not only compromise its containment
properties but contributes to excessive energy usage.
Figure 45 illustrates a face velocity chart showing the
comparative face velocities which may be experienced with
different types of hoods using either constant volume or variable
volume control strategies. The variable volume hood with face
velocity controls in this example shows increased air velocities
at the low aspect of sash closure because, in most cases, a
minimum air volume is required to be continuously exhausted
from the fume hood.
Fig. 44. Variable Exhaust, Constant
Face Velocity Fume Hood.
DAMPER AND
ACTUATOR OR AIR VALVE
CONSTANT FACE
VELOCITY CONTROL
OR SASH SENSOR
FUME HOOD
C2636
CLOSED OPEN
POSITION
EXHAUST AIRFLOW
MAX
MIN
VERTICAL SASH
INTERIOR
BAFFLE
100
90
80
70
60
50
40
30
20
10
0
0 20 40 60 80 100 120 150 300 500
M12213
400 200
SASH POSITION (% OPEN)
FACE VELOCITY (FEET PER MINUTE - FPM)
CONSTANT
VOLUME HOOD
WITHOUT BYPASS
CONSTANT
VOLUME
BYPASS HOOD
VARIABLE VOLUME
HOOD WITH FACE
VELOCITY CONTROLLER
Courtesy of Dale T. Hitchings,PE , CIH.
Fig. 45. Comparative Fume Hood Face Velocities.