Encyclopedia of Environmental Science and Engineering, Volume I and II

492 INDOOR AIR POLLUTION

this. Cost of hood runs medium to high but unless

carefully designed and balanced fume removal

effectiveness can be poor.

5) Externally supplied hood (Figure 2e)

Because of the additional duct system required

such a system is relatively more expensive, rela-

tively low cost effect on air conditioning, and

because air is being exhausted across the hood

face, fume removal effectiveness is good.

6) Perforated ceiling supply hood (Figure 2f)

This allows ample opportunity for the conditioned

air to mix with room air and it becomes often

necessary to sensibly cool but not dehumidify this

auxiliary supply. Because air is exhausted across

the hood face, fume removal effectiveness is good.

7) Horizontal sliding sash hood (Figure 2g)

Compared to the conventional hood with its

vertical sliding door, the horizontal sliding ash

unit presents much less area to be exhausted and

total exhaust is thereby reduced. Relative cost of

the hood is low and since less air is exhausted, air

conditioning costs are low. Air conditioning and

fume removal effectiveness are good.

To keep hood face disturbances to a minimum high veloc-

ity streams from the air conditioning system should not be

permitted to disturb the even, smooth fl ow of air across the

hood face.

MAKEUP AIR SOURCE

Makeup air to balance that exhausted is the most essential

design feature of any hood exhaust system. When a fume

hood is operating poorly, closer analysis will most often show

inadequate makeup air supply. There is no air for the hood to

“breathe” and an improperly sized makeup system will starve

the fume hood and restrict its intended operation.

Some designs depend on air drawn from adjoining

corridors and offi ce spaces. Introduction of makeup air

by indirect means is an economical approach. However,

such a system can lead to balancing problems and cross-

contamination between laboratory spaces. Positive introduc-

tion of air from corridors and offi ce spaces by use of trans-

fer fans can improve this. It has been found that the most

reliable, fl exible, and easily maintained system arrangement

is that in which an adequate supply of outside conditioned

Conventional hood: All air taken from

the room.

Conventional hood

with reduced face velocity.

Externally

supplied hood.

Conventional hood with use

or diversity factor.

Perforated

ceiling supply

hood.

Horizontal sliding sash door hood.

(all room air make-up)

Internally-supplied hood.

(d) (g)

(c) (f)

(b)

(a)

(e)

FIGURE 2 Hood designs.

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