Encyclopedia of Environmental Science and Engineering, Volume I and II

INDOOR AIR POLLUTION 493

air as makeup is supplied to the laboratory space to balance

the air being exhausted. It is good practice to supply a little

less makeup air this way than that being exhausted. A slight

negative pressure will be maintained, drawing air through

door louvers from corridors or adjacent offi ces.

Air exhausted from a hood is never recirculated so that

hood burden goes up. Operating costs can be reduced by

supplying makeup air from an auxiliary source instead from

the cooling system. The air handled is fi ltered and tempered

in winter only. Of the seven basic hood designs, numbers 4d,

4e, and 4f make use of the auxiliary system. An auxiliary

system can be either a central unit or unitary type with an

outside air inlet for each laboratory. Correct selection of the

type of makeup air system can be made only by an engineer-

ing analysis and fl ow sheet of the hood exhaust system.

One of the most important characteristics of an exhaust

system is that at some point the system must end and discharge

to atmosphere. Unfortunately, while the exhaust system has

ended at this point, the problems associated with that exhaust

system may have just begun. If too much air discharged from

an exhaust system is recirculated through the supply system

not much good has been accomplished. If by poor design the

exhaust air is not properly located with respect to the intakes

of other supply systems, potentially disastrous results can

be attained. Many poor designs are commonplace. The real

cure for this type of problem is not higher exhaust velocities,

higher stacks, better weather caps, better separation of dis-

charge and intake openings, or other, although one or more of

these can contribute to the cure. The real remedy must start

back at the source of contamination itself.

Because the pattern of natural air fl ow around buildings

is not predictable, contamination by the location of vent effl u-

ents and air intakes is diffi cult to put to practice. Halitsky^2

(1963) and Clarke^3 (1967) have advanced theoretical

knowledge and rule of thumb that aid greatly in the solution

of such problems.

AIR DISTRIBUTION

In review, air movement within each room of a laboratory

complex must be such that a defi nite fl ow pattern will be

maintained throughout the building along with fl ow from

non-contaminated to potentially contaminated areas. To

bring about this differential fl ow pattern, the nature barriers

between the various classes of rooms will assist. The pattern

will also be assisted by supplying outside clean air to the non-

contaminated and semi-contaminated areas and by exhaust-

ing air only from the moderately and extremely contaminated

areas. In general, supply fans should take suction from the

upper portions of the building. Also, the exhaust fans should

discharge to the outdoors through stacks of varying heights

depending on adjacent structures. To help, the building should

be maintained at a slight positive pressure with respect to the

outdoors. Laboratory rooms should be maintained at a nega-

tive pressure with respect to the surrounding rooms.

Only an adequate supply of makeup air to satisfy exhaust

needs will keep the building in balance. This certainly implies

there must be an excess of supply over exhaust needs. In actual

installations, experience shows that when two fans are exhaust-

ing from the same space with no provision for makeup air, the

stronger fan will take command and outside air will enter the

room through the weaker fan system. When there are multiple

exhaust hoods and no makeup air, with one hood off, outside air

can downdraft through the idle fan. When a fan must exhaust

from a room without makeup, fan capacity will be reduced

from design and will result in less control at the hood.

EXHAUST SYSTEM

The exhaust system being under negative pressure will cause

leakage fl ow to be drawn into the system and contamination

will be confi ned. Best location for an exhaust fan serving

hoods is on the roof. Then all exhaust ductwork will be on

the suction side of the fan and indoors. But this is not always

possible. If the fan location must be indoors, say just above

the hood, then careful attention must be paid to duct tight-

ness on the discharge side. When fl ammable material is han-

dled, mounting fan on roof is a distinct advantage because

explosion-proof construction may not be required of the fan

motor. However, fan wheel should be non-ferrous and inside

casing should be epoxy coated for corrosion protection.

EXHAUST DUCT MATERIALS

In many buildings ductwork is often concealed in ceilings or

inside walls, making duct inspection and replacement a major

problem. Where this condition exists it is reasonable to use

ductwork with long life expectancy. For chemicals used in lab-

oratories, galvanized iron and black iron ductwork are highly

susceptible to corrosion. Stainless steel, transite, polyvinyl

chloride-coated steel or fi berglass- reinforced plastic (FRP)

ductwork will not require early replacement for such corro-

sive service but are costly. Actually, selection of materials

will depend on the nature and concentration of contaminants

or chemical reagents, space conditions, cost, accessibility.

Whatever materials are selected, duct joints must be leaktight

and the ductwork should have ample supports. For best ser-

vice life all longitudinal duct seams should be run along the

top panel. An extensive duct system should have inspection

and cleaning facilities. Ducts that could develop condensa-

tion loading should pitch toward a pocket in the bottom of the

run and be provided with a trapped drain.

Type 316 passive stainless steel may be used for bac-

teriological, radiological, perchloric acid and other general

chemical purposes. 316 stainless steel is easy to work but

is not suitable for chemical hoods handling concentrated

hydrochloric and sulfuric acids.

EXHAUST AIR TREATMENT

Gases that are bubbled through reaction mixtures and then

discharge to the hood are generally, by their nature, reactive

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