518 INDUSTRIAL HYGIENE ENGINEERING
Thus a balance has to be made between Q and ∆ t and this
will be influenced by draughts as illustrated in Figure 3.
Personal protection Often considered to be the last resort
in reducing worker dose it has been shown (Crockford, 1976)^8
that personal protection can play a major role in an “integrated
control programme”. It can be used as a single control item as
a tool to assist in the smooth operation of a process or it can
be used as a “fall back” feature as part of a system with built
in redundancy. It has most application where the area of emis-
sion is too large to be effectively controlled by other means or
in operations where the hazard only occurs for periods of short
duration and in a predictable manner and where the alternative
control measures are too costly to be a reasonable alternative.
Hitherto most respiratory protective equipment has been
uncomfortable to wear for long periods and workers have
been reluctant to make full use of it but with the introduction
of the self-contained ventilated helmets much of the worker
resistance has disappeared. The development of such hel-
mets has put a new light on protection in hitherto tradition-
ally dusty industries.
Finally, aspects of personal hygiene can be included in
this section. Absorption by routes other than inhalation can
be minimized by frequent washing, the provision of clean
work clothing daily, the prevention of eating, drinking and
smoking in the workplace.Ventilation System DesignFrom the formulae and diagrams already given, having deter-
mined the volume flow rate of air to undertake a specific
ventilation task and, in the case of extraction, having estab-
lished the dimensions of the inlet it is necessary to design
the remainder of the hardware that make up the ventilation
system. Textbooks abound in this topic (Hemeon, 1963;
Alden and Kane, 1970; McDermot, 1976) so only an outline
of the procedures involved will be given.
Hoods etc. In the case of the extract hood many con-
figurations are given (ACGIH, 1995) to suit a variety of
workplace situations. But as every workplace and building
varies it is necessary to adapt the designs to fit. This should
be done in conjunction with a production engineer so that
the flow of the work is not hindered by the presence of the
ventilation hardware which tends to impede the transport of
raw materials, finished products and can restrict the mobility
and visibility of the workforce.
Ducts With regard to ducting to transport air from one
part of the workplace to another it is necessary to sketch on
a plan and elevation of the workshop the best position for it
to be sited. It is usually more convenient to site in the roof,
ceiling or spaces above the workplace but the accessibility of
overhead cranage must be considered in the layout.
Fittings such as bends, changes of section, branch pieces
and dampers are necessary to accommodate multiple intakes
and to avoid obstructions forming part of the building fabric
or production equipment. Thus the installation will consist
of lengths of straight duct plus fittings. As more fittings are
added and the duct runs become longer and more complex so
the energy required to transport the air increases.Having sketched the layout it is then necessary to
determine the dimensions of each item. For identification
purposes it is useful to label each section where there is a
change of dimension or direction of airflow. Thus each
component can be labelled as AB, BC, etc. and, as such,
is suitable for entering into a computer ducting program
(Malchaire, 1981; Clapp et al., 1982; Betts, 1984).
With regard to straight ducts the cross-sectional shape
can be either rectangular or circular but cross-sectional area
is important.
For a given airflow rate the narrower the duct the higher
the average air velocity as given by the simple expression:Volume flow rate, Q VAwhere A is the cross-sectional area of the duct at the point
where V is known. Where airborne particles are to be con-
veyed via the ducting then a minimum air velocity must be
maintained that will prevent the particles from settling out
inside. This velocity is known as the Transport Velocity and
will vary with the type of particle being transported but as
a guide a velocity of 20 m/s (4000 ft/min) will ensure that
most particles remain airborne.
Thus having established a volume flow rate and a trans-
port velocity, the cross-sectional area is determined from the
above formula. In this situation a circular cross-section is
more suitable.
Where transport velocities are not important then a
com promise has to be made between the higher energy
consump tion of narrow ducting and the higher capital cost
of wide ducting. Also the passage of air through ducts cre-
ates noise which can become obtrusive in quiet situations.
As a general rule air-speeds in excess of 5 m/s (1000 ft/min)
can create notice able noise. The cross-sectional shape of the
duct is influenced by the size of the space into which it is
to be fitted. A circular section will always use less material
than a rectangular for a given area but often it is easier to
fit rectangular ducts into areas where space is limited as in
false ceilings of buildings.
The energy required to move air through the ducting
is expressed as a pressure which is necessary to overcome
fractional resistance and turbulence caused by the materi-
als of manufacture. Ducting is commonly made of galva-
nized sheet steel but other materials such as brick, concrete
and various plastics can be used each of which provides
a different frictional resistance. Charts are published
(ACGIH, 1988; CIBS, 1970) giving pressure losses for
various diameters of duct for a wide range of airflow rates.
If other materials are used the ducting can be assumed to
be of sheet steel as above but the resulting pressure losses
must be multiplied by a factor depending upon the nature
of the material. These factors are published for the com-
moner materials (CIBS, 1970) or are available from the
manufacturer.
Pressure losses in the fittings can either be calculated as
an equivalent length of straight duct (ACGIH, 1988) or by a
factor which must be multiplied by the velocity pressure in
the ductwork (CIBS, 1970; Daley, 1978).C009_003_r03.indd 518C009_003_r03.indd 518 11/18/2005 10:31:25 AM11/18/2005 10:31:25 AM