Encyclopedia of Environmental Science and Engineering, Volume I and II

INDOOR AIR POLLUTION 497

• Extremely brittle, requiring care in handling to
  avoid breakage.


• Poor light reflectivity.


• Stains badly when exposed to many acids, etc.


• Easily modified in field with only minor tooling
  difficulties.


• Inexpensive.

Stainless Steel

• Better general chemical resistance than cold rolled
  steel.


• Not well suited to many acid applications.


• Generally provided in type 316 for specific
  applications to which it is well suited such as
  perchloric acid.


• Heavy and expensive.


• Difficult to modify in field.


• Excellent fire resistance.

Polyvinyl Chloride

• Excellent chemical resistance except for some
  solvents.


• Good fire-retardant properties.


• Particularly well suited to acid digestion applica-
  tions such as sulfuric and hydrofluoric.


• Easily modified in field.


• Generally not available in molded configurations.


• Expensive.


• Distorts when exposed to intense direct heat.

Stone

• Excellent chemical resistance.


• Excellent fire resistance.


• Difficult and extremely heavy to install.


• Extremely difficult to field modify.


• Expensive.

WALK-IN HOOD

This type of hood was not mentioned in Part I but will be

now included. The walk-in hood is a standard hood whose

walls extend to the fl oor, thus providing suffi cient space to

accommodate a more elaborate experimental setup requir-

ing additional height. Such hoods have double or triple hung

sashes, which may be raised and lowered to provide access

to any part of the setup while the remaining space is enclosed

to contain fumes. The back baffl e of such a hood extends

over the full height of the hood and is equipped with at least

three adjustable slots to regulate the amount of air passing

over various parts of the setup.

SPECIAL PURPOSE FUME HOODS

Perchloric Acid Fume Hood

Due to the potential explosion hazard of perchloric acid in

contact with organic materials, this type hood must be used

for perchloric digestion. It must be constructed of relatively

inert materials such as type 316 stainless steel, Alberene stone,

or ceramic coated material. Wash-down features are desirable

since the hood and duct system must be thoroughly rinsed after

each use to prevent the accumulation of explosive residue. Air

fl ow monitoring systems are recommended to assure 150 fpm

open face velocity operation. An additional monitoring system

for the wash-down facilities is also recommended.

Radiological Fume Hoods

Hoods used for radioactive applications should have integral

bottoms and covered interiors to facilitate decontamination.

These units should also be strong enough to support lead

shielding bricks in case they are required. They should also

be constructed to facilitate the use of HEPA fi lters.

Canopy Fume Hoods

Canopy fume hoods are a type of local exhauster which nor-

mally has limited application in a laboratory. Their main dis-

advantage is the large amount of air required to provide an

effective capture velocity. Since the contaminant is drawn

across the operator’s breathing zone, toxic materials can be

quite dangerous. A canopy hood can, however provide a

local exhaust for heat or steam.

INTEGRAL MOTOR-BLOWERS

Many hoods are available with motors and blowers built

directly into the hood superstructure. From the standpoint

of convenience, the hood is relatively portable and can be

installed easily. A built-in motor-blower should not be used

for highly toxic applications since it causes a positive pres-

sure in the exhaust system ductwork and any leaks in the

duct could spill the effl uent into the lab area. There may be

more noise associated with this type hood since the motor-

blower is closer to the operator.

Fume Discharge

Each individual exhaust fan on the roof should have its own

discharge duct to convey the fumes vertically upward at a

high velocity as far above the topmost adjacent roof as pos-

sible. Failure in this will result in potential recirculation

of fumes into building air intakes and will be particularly

hazardous to personnel who use the roof for maintenance,

research, or relaxation.

As the wind blows over the leading edge of a roof para-

pet, as shown in Figure 6, a disturbance is created that sweeps

from the edge of the parapet up over the top of the building.

Above the boundary of this disturbance, wind fl ow is undis-

turbed. Below the boundary, the infl uence of the sharp edge

of the building creates eddy currents that can pocket fumes

released at the roof. This is known as the wake cavity. Unless

fumes are discharged into the undisturbed air stream above the

boundary, where they can be carried away, they will remain

relatively undisturbed and undiluted on the roof and in the lee

of the building, where they can enter the building air intakes

either on the roof or at ground level. When this happens, all

the care taken in the design of a good fume exhaust system

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