Encyclopedia of Environmental Science and Engineering, Volume I and II

498 INDOOR AIR POLLUTION

may be nullifi ed. And with the present concern over air pollu-

tion, failure to disperse the fumes may give rise to legal action

against the building owner.

Fume absorbers such as charcoal have been proposed

to relieve the fume disposal problem, so have air washers

and catalysts. These devices have not been used because

the kinds and amounts of fumes released are constantly

changing in research and are therefore unpredictable.

Despite the number of warnings in the literature, rain

caps, cone shaped covers or hoods fastened to the tops of ver-

tical stacks—are still being used to prevent rain from enter-

ing exhaust stacks. It is important that their use be avoided

completely. There are several simple stack arrangements that

will prevent entry of rain into exhaust stacks when fans are

not operating. One such arrangement is shown in Figure 7.

BUILDING AIR INTAKES

In high-rise research buildings, mechanical equipment is

frequently installed in the penthouse and in the basement.

Because of the possibility of recirculating fumes released from

or near the roof, outdoor air is often taken at the second fl oor

level on the prevailing wind side of the building, and away

from fume exhausts. Assistance in determining the prevailing

wind direction at the building site may be obtained from the

local weather bureau.

BASIC PERFORMANCE CRITERIA

The following may be used as a general guide for the selec-

tion of hood blower systems that will provide optimum

AIR FLOW PATTERNS AROUND A BUILDING

Wake cavity

boundary

Wake boundary

Free stream

Wind direction Building

Peripheral

flow

Cavity

Return flow

FIGURE 6

Very low toxicity level materials

Noxious odours, nuisance dusts and fumes 80 fpm

General lab use

Corrosive materials

Moderate toxicity level materials

(TLV of 10–1000 ppm)

100 fpm

Tracer quantities of radioisotopes

Higher toxicity level materials

(TLV less than 10 ppm)

125–150 fpm

Pathogenic microorganisms

High alpha or beta emitters

Very high toxicity level materials (TLV less than 0.01

ppm)

An enclosed glove box

should be used

average face velocities for various exhaust materials. Tables

listing the TLV for various chemical compounds may be

obtained from the American Conference of Governmental

Industrial Hygienists.

HOW TO CUT AIR CONDITIONING COSTS

As a rule of thumb, each 300 cfm of air exhausted through

hoods requires one ton of refrigeration. Current operating

costs are about 50 to 60 dollars per ton of air conditioning

for a four month period. Installed equipment averages about

$1,000 per ton. So, a hood exhausting at 900 cfm would

require about three tons of air conditioning at a capital expense

of $150 to $180 per season. However, if the same hood had

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