Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
288
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.
Fig. 44. Variable Exhaust, Constant Face
Velocity Fume Hood.
The subject of the “correct” face velocity is still debated.
However, most research now indicates that 0.4 to 0.5 meters per
second (m/s) 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 0.6 m/s 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. 45. Comparative Fume Hood Face Velocities.
LABORATORY PRESSURIZATION
Constant supply airflow often is not capable of constant space
pressurization in research laboratories because of the use of
constant face velocity fume hoods and the use of other variable
exhausts. To accomplish containment and prevent excessive
pressurization requires some form of volumetric air flow control
(air flow tracking) or control of differential pressure within the
lab space (direct pressure control).
Airflow tracking (Fig. 46) measures all exhaust and supply
airflows and maintains a relationship between the total exhaust
and total supply. For space pressurization to be negative relative
to adjacent spaces, the total exhaust must exceed the total supply.
The difference between exhaust and supply airflows (offset)
should be a fixed quantity for an particular space to keep the
pressurization constant. A constant percentage offset value is
sometimes used.
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 0.1 0.2 0.3 0.4 0.5 0.6 0.75 1.5 2.5
M15318
2.0 1.0
SASH POSITION (% OPEN)
FACE VELOCITY (METERS PER SECOND)
CONSTANT
VOLUME HOOD
WITHOUT BYPASS
CONSTANT
VOLUME
BYPASS HOOD
VARIABLE VOLUME
HOOD WITH FACE
VELOCITY CONTROLLER
Courtesy of Dale T. Hitchings,PE, CIH.