Fume Ignition - Techne IFB-121 Operator's Manual

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 minimum level to which satisfactory results can be achieved.
Increased working temperatures result in increased fume concentration. By way of example, an
extraction rate of 2m 3 /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 450°C.
To safeguard the fluidised bath from damage, two safety features are included in the control console: an
adjustable over temperature cut-out which guards against controller failure, and an air pressure switch
which inhibits operation if the compressed air supply to the fluidised bath fails. Both of these devices
isolate the electrical supply to the heater elements and illuminate the cut-out indicator in a fault
condition. The over temperature cut-out is factory preset at 630°C. Under no circumstances should this
setting be increased, but lower limits may be set to safeguard particular processes in the fluidised bath.
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