Encyclopedia of Environmental Science and Engineering, Volume I and II

514 INDUSTRIAL HYGIENE ENGINEERING

rate need be. Unfortunately, enclosure obscures visibility,

thus where it is necessary to observe the process inside, pro-

vision has to be made for simple viewing panels of plate

glass or transparent plastic. Video monitors or electronic

indicators can also be used.

Redesign of processes can be as simple as fitting special

covers on containers and matching discharge ports to entry

holes or as complicated as total automatic handling through-

out. With materials handling dramatic improvements on

hygiene can often be achieved by using sealed transfer

systems, eliminating the potential for inhalation and skin

contact. Where liquids are transferred the simple use of a

discharge nozzle extending below the liquid surface can

reduce the evaporation. It is wise to think through every

stage of the process noting places where emission is highest.

An experienced outsider can often notice features which are

missed by the people who are in closest contact. This should

be done before the costly expedient of extract ventilation is

adopted.

When redesigning new plant it is important to involve

experienced health, hygiene and safety professionals in the

early stages as often the environmental aspect of produc-

tion processes are considered too late to have any effect on

the installation. Often plant is installed and commissioned

before environmental hazards are realized and then there is

a great reluctance to make modifications. It should be borne

in mind that the high apparent cost of process redesign can

often be offset against the reduced capital and running costs

of supplementary control, monitoring and maintenance

which is required of an inferior process. Also the relation

between workforce and management is greatly improved

with obviously safer processes.

Extract Ventilation If it is difficult to adopt process rede-

sign or satisfactory enclosure then pollutants can be captured

before they are generally released by means of some type

of hood, slot or enclosure inside which a negative air pres-

sure has been generated. Air will thus flow in by virtue of

the difference in pressure between the mouth of the device

and the surround. If the contamination from the process can

be moved into the hood by means of the airflow, then some

degree of control will be achieved. The air in the hood is

removed by a fan via ducting to an outlet or air cleaning

system. Any contaminant not drawn in will remain free to

disperse within the workroom where it may be inhaled by

the operator or other personnel. It is therefore important to

consider the patterns of the airstream in the vicinity of the

work where the success or failure of the extract system is

often determined.

Providing no obstructions exist around a hood, then air

will flow in from all directions in a zone of influence which is

approximately spherical in shape with the mouth of the hood

at its center (Figure 1). The air velocity at any point in that

sphere being approximately related to the hood entry velocity

by the inverse square of the distance from the hood (Dallavale,

1952).^10 By careful selection and location of screens the shape

of the airstreams can be adjusted to reduce unwanted airflow

from uncontaminated areas thus increasing the flow of air

drawn through the zone of pollution emission.

For most operations a minimum velocity (capture veloc-

ity) is defined as that velocity at which the energy of motion

of the contaminant can be overcome thus drawing it into the

hood. In the case of the release of welding fumes for example

a velocity of at least 0.5 m/s (100 ft/min) is usually required

to overcome the thermal motion of the plume (ACGIH). In

processes where the velocity of emission of the contaminants

is high as with grinding or where high cross draughts exist

then higher capture velocities may be necessary. Conversely

in certain work, such as paint finishes and gas shielded weld-

ing, too high an airflow may affect the finished product. Thus

a control envelope bounded by upper and lower velocity con-

tours may have to be defined. That is, the hood should be sited

so that the source of pollution lies between chosen velocity

contours and preferably on the centerline of the slot.

Centreline velocities can be predicted in relation to the

distance from the mouth of the opening using the expres-

sions given below (Fletcher, 1977).^12

VA

Q

V

V

0 

1

093 858..a^2

where


 



a

b

xA

W

L

12 ⎛

⎝⎜

⎞

⎠⎟

and

b
 



0.2xA−

−

()

12 13

FIGURE 1 Airflow patterns and velocity contours

at the mouth of a suction inlet.

.25 m/s

50' /min. 100' /min. 200' /min.

.5 m/s 1 m/s

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