Encyclopedia of Environmental Science and Engineering, Volume I and II

INDUSTRIAL HYGIENE ENGINEERING 515

and

Vx

V

centreline velocity at distance from the hood

0 mean vvelocity at face of hood

length of hood

width of hood

L

W

A



LW

Q required volume flow rate.

The solution of these expressions can be made simpler by

the use of the nomogram in Figure 2.

In the use of the above expressions the dimensions L

and W will depend upon the size and shape of the point or

points of emission of the pollutant, V should be the capture

velocity and Q will be the calculated volume flow rate of air

to provide the required capture velocity at distance x from

the hood.

The relative location of the operator, hood and source

of contaminant can also have considerable bearing on the

effectiveness of control. Whilst a system utilising the flow

energy of the plume to assist the airflow may require less

air volume, this will be ineffective if the operator is located

between the source and the hood. It should be borne in mind

that the operator is acting as a bluff body in a streamline

airflow, creating turbulence which can bring back contami-

nants into the breathing zone. Exposure levels will also be

contributed to by secondary emission from contaminants

that are not captured. The shape of the workpiece can also

influence airflow patterns at the point of emission direct-

ing contaminants to remote areas away from the mouth of

the hood. In welding, for example, whilst stance and the

use of a headshield can help to minimize these effects

(Akbarckhanzadeh, 1979) there will still be some residual

fume and gases in the breathing zone when using simple

extraction hoods or slots.

When designing a hood for ventilation control, it is

better to start with an idea of providing maximum enclosure

of the source of contaminant. Thus it is prudent to design a

booth or enclosure first then progressively remove sections

of it to make a practical design. Unfortunately as the size

of the openings increase so does the required volume flow

rate of air and effectiveness of the extraction may decrease.

The use of cardboard and a portable fan to produce a full

scale “mock up” model can be an invaluable aid to empirical

design, giving an opportunity to judge control effectiveness

and ergonomic performance.

Difficulties arise where work processes require access

on all sides to facilitate production techniques or where the

source of contamination is moving. In the case of benchwork

requiring all-round access, narrow slots pulling over a short

distance or assisted by supply jets (Hughes, 1988)^16 of air

can be used, as can perforated or slotted bench tops through

which air is drawn, thus the ventilation hardware occupies

the minimum of space. Where moving sources of pollution

occur it is possible to use hoods with flexible ducting sup-

ported on cantilever arms or crane type supports allowing

flexibility over a wide area. It may be necessary to provide

clamps or other devices to ensure that the hood is located to

achieve good capture and will move with the source. Care

must be taken to ensure that the use of the extract system

does not hinder the operation or it may fall into disuse to

operator resistance. Operationally portable hoods often fail

to achieve the degree of control required due to the practi-

cal imposition of regularly having to accurately reposition

the device, thus it is important to ensure that such devices

are lightweight, well balanced and quick and simple for the

operator to use.

The hood may also be designed and built into the tool

as with portable sanders, grinders and saws. This technique

known as low volume high velocity (LVHV) exhaust uses

small volumes of air at relatively high velocities. The hood

can be an annular slot or multi-hole type operating on a

vacuum of 20 kPa (150 mm Hg) at a volume of 16 liter/sec

(35 ft^3 /min). When these are accurately positioned much of

the contaminant can be controlled. However, although this

system has the advantage of low volume requirements and

portability there are several disadvantages which often pre-

clude its use. The LVHV head has a limited envelope of con-

trol due to the low volume of air extracted and workpiece

shape, misalignment of exhaust orifice or lack of mainte-

nance can seriously disrupt the efficiency of collection. The

weight of the tool plus its extract also presents problems of

manoeverability and acceptability to workers.

Transmission

Once the pollutant has escaped from its source and is air-

borne or is broadcast many workers in the vicinity can be at

risk. With wide area sources capture may be impossible.

Segregation of the worker from the source of pollution

by carefully planned production routines can often be used

to reduce the potential for exposure. For example processes

which give rise to airborne materials can be carried out in

separate areas or at times when the building is otherwise

unoccupied, operators remaining in the areas can be provided

with protective clothing. Local extraction, as described in

FIGURE 2 Nomogram for the solution of hood entry

equations.

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