INDUSTRIAL HYGIENE ENGINEERING 521
in disposable cells or are on a continuous roll which is auto-
matically wound on as the resistance increases.
Where high concentration particulate cleaning is involved
mechanical devices imparting centrifugal action are used to
collect the larger particles. Cyclone separators and other
dry centrifugal cleaners can be used but where particles are
sticky or in a moist atmosphere wet methods such as ven-
turi scrubbers, water cabinets, and wet centrifugal devices
are employed. These techniques are more effective for large
particles rather than small which require large area dry fabric
devices such as bag and sleeve filters with reverse jet purg-
ing. Where higher collection efficiencies are required for
smaller particles then multi-stage filtration using as the last
stage inertial filters, may also be used. Cleaning air for gas-
eous pollutants is more difficult involving a variety of tower
scrubbers, washers and absorbers (Dorman, 1974).
Energy and Cost Implications of Extract Ventilation The
energy required to provide extraction ventilation can be
considered in two parts: that which is required to move the
air through the system to overcome the friction losses and
that which is required to heat incoming air from outside to
replace that which is discarded.
Friction losses The power required to overcome the fric-
tion losses can be calculated from the simple expression:Power Watt Joule/secpQ
h( )wherep total pressure loss through the system in Pa (N/m^2 )
Q total volume flow rate flowing in m^3 /s
h overall efficiency of the fan and prime mover,
usually the fan drive and motor.Thus, if the system is run continuously for a period of
time, the total energy required can be calculated in kJ or kW
hours and converted to a monetary equivalent based upon the
fuel tariff in operation.
Replacement Air Whenever air is thrown away to outside,
replacement air enters the building. This may enter via a prop-
erly designed make-up air system or it can find its way in via
breaks in the building fabric such as doors, windows, discon-
tinuities in the building. No matter which way it enters, if the
building is to be maintained at comfort condition, then enter-
ing air requires tempering, usually in the form of heating in
the winter and in some places cooling in the summer. The the-
oretical power required can be calculated from the formulaPowerrV C t Watt·
whereV is the volume flow rate of air entering m^3 /s
r is the density of that air in kg/m^3
C is the specific heat of the air in kJ/kgC
∆ t is the difference in temperature between inside air
and outside.For most engineering applications this formula can be
approximated to power 1.2 V ∆ t Watt. Figure 8 shows a
chart for calculating power required for heating and friction
losses.
When the duration of operation is taken into account
the overall theoretical cost of heating or cooling can be cal-
culated, but it must be remembered that the type of fuel
and the efficiency of conversion and tariff in operation at
the time must be considered in order to obtain a realistic
costing.
In most situations at most times of the year the cost of
tempering the incoming air far outweighs the friction costs
(Gill, 1980)^13.Difficulties with Extract SystemsMany systems are unpopular or unsuccessful because they
allow the pollutants to escape in sufficient volumes to hazard
workers health or comfort. Faulty design, installation, main-
tenance and procedures can all lead to difficulties. Some of
the commoner faults found under these headings are listed
below.Faulty Design1) The point of release for the pollutant is too far
away from the suction inlet,
2) insufficient enclosure has been provided to pre-
vent the escape of pollutants,
3) the airflow may be restricted by a poor backup
ductwork system,
4) the wrong fan has been chosen,
5) the fan performance may be restricted due to bad
inlet conditions,
6) outside wind and weather conditions are affecting
the system,
7) if multi-branched, the system may be out of
balance,
8) no provision made for heated “make-up” air to
enter,
9) discharges adjacent to air inlets, allowing recircu-
lation of contaminant.Faulty Installation1) Fans wired wrongly affecting their direction of
rotation leading to reversed flow,
2) with two stage fans, one stage installed the wrong
way round,
3) ductwork badly jointed allowing leakages of air,
4) obstructions left in ductwork during installation,
5) controlling dampers may not be functioning cor-
rectly or properly anchored.Faulty Maintenance1) Ducts can be restricted or blocked due to a build-up
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