INDOOR AIR POLLUTION 491
FUME HOOD DESIGNThe function of a hood exhaust system is to protect the lab
technician from exposure. Thus, the heart of the system is
the hood and the design begins with the hood, which is,
at best, a compromise between the ideal and the practical.
Basically, a hood is a simple box, Figure 1(a). Without
the necessary indraft shown for the basic ventilated hood,
Figure 1(b), the material inside the hood can become
airborne and be emitted into the room by one or a combi-
nation of the following normal laboratory operations: ther-
mal action and convection currents, mechanical agitation,
aspirating action by cross currents of the air outside the
box. Material can escape from the basic hood only through
the door or opening in front. However, in the simple ven-
tilated hood, contaminants are kept inside by the action of
the air fl owing into the opening. To contain and keep the
material from escaping, suffi cient air must be exhausted
to create and maintain an indraft through the face of the
hood opening.
Hoods should control contaminated air so that the contam-
inant does not reach the breathing zone of the lab technician in
signifi cant quantities.
Nearly all hood designs presently in use attempt to
provide protection in three ways: a mechanical shield, direc-
tion of air movement, dilution of contaminant by mixing
with large volumes of air inside the hood. The mechani-
cal shield comprises the hood sash. When an experiment
is being set up the sash is in the raised position. In many
experiments, the sash is lowered two-thirds the way down
or even closed off entirely while an unattended experiment
is being carried out. Only the occasional visit by the techni-
cian is needed. Care should be exercised not to lower the
sash to a level that can cause too high an indraft velocity
with attendant overcooling or snuffi ng out of a burner fl ame.
Protection is provided by the direction of air fl ow across the
back of the worker and into the hood proper, past the equip-
ment within the hood and thence into the exhaust system,
Figure 1(c). Lastly, because large amounts of air are being
moved through the hood, dilution of the contaminated air
takes place readily and further reduces the hazard of breath-
ing hood air.SEVEN BASIC HOOD DESIGNSSeven basic hood designs are in use, all as shown in
Figures 2a–2g.1) Conventional hood (Figure 2a)
All exhausted air taken from the room. This is the
simplest, low in initial cost and effective. However,
high exhaust air rates place a heavy burden on air
conditioning capital cost and operation.
2) Conventional hood with reduced face velocity
(Figure 2b)
An attempt to compromise hood effectiveness
to reduce air conditioning load chargeable to the
hoods. Although low in relative cost, it does reduce
air conditioning load but its effectiveness in remov-
ing fumes generated within the hood is weakened.
3) Conventional hood with use factor (Figure 2c)
Exhaust hoods may be needed at random inter-
vals and it is not likely that they would be simul-
taneously. As with other types of air conditioning
loads, there is a usage or diversity factor that is
apparent, yet difficult to define precisely. This
factor depends upon judgment, experience, and
logic. For example, a large number of hoods in
a laboratory room does not necessarily mean all
hoods will be operating at one time since the
number of lab personnel will be limited and thus
reflect on the number of hoods in operation. On
the other hand, it is the policy of some laborato-
ries to keep all hoods in operation 24 hours a day,
even though they are used intermittently. So much
depends on the management of the facility and it
behooves the designer to explore the total opera-
tion with the ultimate user.
4) Internally supplied hood (Figure 2d)
Required makeup air is fed directly inside the
hood without affecting the overall room air con-
ditioning. This air need not be cooled in summer
but merely tempered in winter. Although an
additional air handling system is required, the
saving on the air conditioning load can offsetSash Sash Sash(a) Correct Distribution (b) Improper Distribution (c) Relevant to WorkerFIGURE 1 Flow directions through hoods.C009_001_r03.indd 491C009_001_r03.indd 491 11/18/2005 10:29:56 AM11/18/2005 10:29:56 AM