Page 244 - IGC BOOK

The position of the extraction fans. The important factor which controls the positioning of
the extraction fan unit is the density of the contaminant. The density of air is taken as a value
of one. The density of many common solvents is greater than one, therefore they tend to
layer over the lowest floor area in the workplace. For such conditions, fans should be
positioned in the walls at a low level. Where the density is less than one then the contaminant
will rise; for this situation the fan must be positioned high on the workplace walls or in the
roof.

A major problem in setting up an efficient dilution ventilation system is the formation of dead
areas. These are areas in the workplace which, owing to the airflow pattern produced by the
extraction fan and the inlet of make-up air, remain motionless and so the air is not changed.
Dead areas can be detected by the use of smoke tracer tubes. A high density of smoke will
remain in the unventilated areas.

A second problem with dead areas is that they can move from one position in the workplace
to another. Such moves can be produced by changing the inlet for the make-up air; for
example, in cold weather the inlet may be spread over the workplace via the cracks in
windows and doors. In hot weather, opening doors and windows will produce a quite
different flow pattern. Moving the position of machinery or workbenches can also cause the
same problem. To help reduce the problem, controlled air make-up inlets can be constructed.

Where large quantities of air are being used to carry out the dilution process then
consideration must be given to recycling heat losses from the workplace. It can be achieved
by using heat exchange systems whereby make-up air is heated by the exhausted air.

Respiratory Protective Equipment

Personal protection involves the use of systems designed to be worn by individuals to help
reduce the possibility of harm from the hostile environment in which they are working. This
is called a safe person strategy. Ideally, the safe person strategy is a second line of defence
against a potential hazard, because control at source or a safe place strategy should be the
first aim.
However, there are situations where personal protection is the only, or the most appropriate,
method to deal with a particular hazard; for example, when the cost of controlling the hazard
at source is high and the time required for protection is low. Classic situations which typify
these conditions are:

• Work involving planned maintenance; for example, during plant shutdowns or deluging
(damping with water) asbestos-covered boilers.
• Failure of primary safety systems or emergency situations; for example, a chemical leak
from a totally enclosed plant, or exposure to a smoke-filled building during a fire.

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 39
Agents