Encyclopedia of Environmental Science and Engineering, Volume I and II

494 INDOOR AIR POLLUTION

enough to be completely eliminated by a scrubber of some

design. For materials that are acidic, a simple caustic scrubber

is all that is necessary to assure essentially complete control.

Similarly, for materials of a basic nature, an acid scrubber may

be used to advantage. For those materials that do no react rap-

idly with either caustic or acidic solutions, a column fi lled with

activated charcoal will always provide the desired control.

Perchloric acid is highly soluble in water and hoods

have been developed with packed sections built into the

hood superstructure and provided with water wash rings

in the ductwork downstream of the scrubber to prevent

buildup of perchlorates which are explosive on contact.

Fume hoods handling highly radioactive materials should

have HEPA fi lters upstream and downstream of the hood.

For highly hazardous bacteriological experiments safety can

be achieved only by incineration of the exhaust air stream,

which is heated to about 650°F to destroy the bacteria.

SPECIAL SYSTEMS

Lowered Sash Operation

A hood exhaust fan maintains proper capture velocity when

the sash is wide open, but the exhaust’s hood’s vertically slid-

ing sashes are sometimes lowered to within a few inches of

the work surface when the hood is in operation. A method in

use to reduce waste of conditioned air and also to achieve a

more constant face velocity over the range of sash positions

is the use of a 2-speed fan for each hood. When the sash is

pushed up the fan runs at high speed. A micro-switch mounted

in the hood is tripped by the sash when it is lowered below a

predetermined position. The volume of air the fan will pull

on low speed is adequate to maintain desired face velocity for

the smaller cross-sectional area. The proper placement of the

switch setting can be 50 to 60% of the vertical face opening,

i.e., the fan would go on low speed when the sash is lowered to

50 to 60% of opening. This holds for all exhaust hoods despite

differences in hood dimensions and other variations in exhaust

systems. It has been found to apply equally as well to hoods

with minimum face velocities of 80, 100, and 125 fpm. The

volume of conditioned air that is normally lost is reduced by

about one-third when the sash is below the set point.

In a conventional hood with a single speed fan, the

excessively high face velocities experienced at low sash set-

tings and the cooling effect on the backs of lab personnel

using the hood has an overall adverse effect. Further, still,

when the laboratory technician stands in front of a hood in

operation his body presents an obstruction to the fl ow of

air into the hood. Thus, a low pressure area develops in the

space between the man and the hood. Under certain condi-

tions, the resulting low pressure area can cause fumes to be

aspirated from the hood and out into the room. The reduc-

tion in face velocity using the 2-speed fan reduces the prob-

ability of such a hazardous condition developing.

Type “S” hoods should be provided with fan speeds

so that at no point across the hood face should a velocity

greater than 250 fpm exist. Another way to control this

velocity is to provide by-pass dampers in the exhaust duct

just downstream of the hood itself. By-pass hoods are

made to accomplish this effect by providing this feature in

the hood structure itself.

By-Pass Hoods

These provide for a constant rate of room exhaust and uni-

form face velocities at any door position. They stabilize the

room exhaust and the room they supply. The by-pass may be

an integral part of the hood itself. As the hood door begins to

close, the damper starts to open. Another important aspect

and advantage of the by-pass hood is that the hood interior

is continuously being purged of fumes even while the door

is closed tight. For the by-pass hood see Figure 3.

Supply Air Hoods

Two types are commercially available. The fi rst has auxil-

iary air introduced outside and in front of the sash, normally

from the overhead position. In this design the auxiliary air

supply is drawn into the sash opening as a part of the room

air. Relative cost of this type compared to the conventional is

high. Relative cost of air conditioning is low because amount

of room air exhausted is reduced. Air conditioning effective-

ness, fume removal effectiveness, and convenience to lab

personnel are good. However, acceptability to local authori-

ties should be investigated. See Figure 4(a).

In the second type, auxiliary air is fed directly into the hood

on the inside. Relative cost is high, cost of air conditioning is

low, air conditioning effectiveness is good, but fume removal

effectiveness is poor. Because effective face velocities can drop

TYPICAL BY-PASS HOOD

Safety

Shelf

By-pass

Damper

FIGURE 3

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