Fume Ignition; Fume Cleaning - Techne IFB-201 Operator's Manual

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If the fluidised bath is to be used to process items that may emit toxic or inflammable
fumes, it is essential that an adequate fume extraction system be installed. Details of
fume cleaning equipment are contained in the following section of this manual.
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The extraction system must be adjusted during initial operation for optimum extraction in
line with the following instructions:
a) Heat the fluidised bath to the operating temperature as recommended later in this
manual, ensuring that the fluidising air flow rate is adjusted in accordance with
the above table. Remember that the lid must be fitted.
b) Switch on the fume extraction system.
c) Immerse a typical component that is to be treated into the fluidised bath; do not
exceed the started loading capacity, see Technical Specification for details.
d) Adjust the overall fume extraction rate by adjusting the setting of the damper
valves built into the fume extraction system so that adequate fume removal
occurs over the surface of the fluidised bath. The velocity of the extraction
system should be just sufficient to entrain the maximum quantity of fumes
produced above the bath. If a high extraction velocity is used, an excessive
amount of fluidising medium will be entrained by the extraction system.
e) Measure the fume concentration within the fume extraction system using an
explosimeter and ensure that the Lower Explosimeter Limit (LEL) is not
exceeded. The fume concentration may be diluted by fitting a dilution tee into the
extraction ductwork directly after the fluidised bath. Air may then be induced into
the system to dilute the effluent gases. It is recommended that the maximum
fume concentration be less than 25% of the LEL.
f) Measure the fume concentration in the working environment adjacent to the
fluidised bath using suitable gas analysis equipment such as 'Drager' tubes, to
ensure that any toxic fumes generated within the fluidised bath have been diluted
so that the fume concentration is below the threshold limit value. Recommended
Threshold Limit Values (TLVs) are available in publications such as the Health
and Safety Executives Guidance Note EH15/76 in the UK.

Fume Ignition

Spontaneous ignition may occur above the surface of the fluidised bath or within the exhaust
ductwork if the concentration exceeds the lower explosive limit. The fume concentration is
dependent upon the rate of fume production in the fluidised bath and the level of dilution achieved
by entraining air from above the bath or through the dilution tee. The rate of fume production is
dependent upon the type and amount of material being treated and the temperature of the bath.
To ensure that the concentration of inflammable fumes is below the lower explosive limit, ensure
that the amount of combustible material immersed in the bath at any one time is within the design
capacity of the extraction system. Ensure that the temperature of the bath is at the minimum level
at which satisfactory results can be achieved. Increased working temperatures result in increased
fume concentration. By way of example, an extraction rate of 2mý/min is required to ensure that
the fume concentration is below 25% of the lower explosive limit when thermally decomposing
1kg/hr of polythene at 450ºC.

Fume Cleaning

When used for processing items which may emit toxic or inflammable fumes, it is essential that
an adequate fume extraction system be installed. A typical schematic diagram consists of the
following components:
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