Encyclopedia of Environmental Science and Engineering, Volume I and II

INDOOR AIR POLLUTION 495

below the safe value needed to prevent leakage of fumes, its use

is discouraged by many health authorities. See Figure 4(b).

Induction Venturi

For many fume exhaust applications such as those involv-

ing hazardous fumes or gases, the conventional exhaust

method of passing gases through the fan casing could be

potentially hazardous. With exhaust from perchloric acid

fume hoods in particular, a build-up of crystals can occur

on duct walls and fan. This crystalline growth is explosive

under normal conditions and special treatment of such a

system is mandatory.

To overcome this, there are commercially available

induction venturi systems with water wash facilities. Since

perchloric acid crystals are highly soluble this system is

provided with spray rings or nozzles and are washed down

internally at regular intervals. Drainage is provided to a

trough attached to the back of the hood table (see Figure 5).

System operation is accomplished by introducing a high

velocity air stream jet inside a specially designed venturi.

This in turn induces a fl ow of gas into the venturi inlet. This

induced fl ow can then be used to exhaust the hood without

any of the gas having to pass through the fan. Venturi is usu-

ally of stainless steel (316L). Blower is mild steel. Such a

system used to exhaust 12000 cfm against ½
 w.g. required a

primary fl ow of clean air of 500 cfm and ¾ hp fan motor.

Other perchloric acid fume exhaust systems use fans of

PVC construction but wash the gas stream upstream of the

fan. Its construction is also PVC. Each hood should be pro-

vided with its own exhaust system; no combinations should

be manifolded. Organic compounds must be avoided in the

construction of the system as well as the chemical used in

testing inside the hood.

Multihood Single Fan System

Should each fume hood be provided with its own exhaust

fan or should several hoods be serviced by one fan common

to all? A common exhaust duct and fan system may be

used if the facility handles similar and compatible chemi-

cal reagents. In the consideration of exhaust systems for

a chemical research facility, where the chemical nature of

the reagents to be used cannot be predicted in advance, or

cannot be controlled, safest procedure is to use separate and

individual exhaust fans and ducts.

DESIGN PROCEDURE

For a most economical design and the use of the various cri-

teria outlined herein the following procedure is suggested:

1) Set inside conditions of dry bulb temperature

and relative humidity in the upper range of the

comfort zone. Since relative humidity is criti-

cal to operating costs, place greater emphasis on

this aspect.

2) Select a hood face velocity sufficiently high to

control the type hazard, using the recommen-

dations outlined in reference 1. Review hood

(a) (b)

FIGURE 4 Auxiliary air supply schematic.

Perchloric Acid Hood

Drain

Flushing

Water

35 psi

Flushing rings every 10 to 12 ft. in

vertical as well as horizontal runs

of duct

Flushing Ring

Venturi

Roof

Eductor

Nozzle

FIGURE 5 Induction venturi system.

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