Techne IFB-201 Operator's Manual page 27

air - renew if heavily contaminated. Fit new main element gasket/ 'O' ring and
replace main element and pre-filter element. The main element 6009247 may
need renewing after two years service or if pressure drop becomes excessive. It
is not designed to be cleaned. In addition, oil vapours in the air supply which
reach the porous plate are carbonised within the pores of the plate, quickly
causing blockage and consequent poor fluidisation, local overheating, premature
heater failure and possible distortion of the inner container.
4. The fume extraction system (if used) should be regularly maintained. The cyclone trap
collection bin should be emptied at the end of each days use; the collected fluidising
medium being returned to the fluidised bath. It is important to ensure that the air tight seal
between the cyclone and collector bin is remade.
For maintenance of other fume treatment equipment see the associated maintenance
manuals.
5. Any media trapped in the extraction hood should be cleared to ensure that all fumes are
removed from the working area through the extraction system.
The top cover/hood can be removed and any media should be returned to the bath.
SPECIAL MAINTENANCE PROCEDURES TO BE ADOPTED WHEN BURNING OFF
PVC OR OTHER HALOGENATED POLYMERS.
Burning off PVC (poly vinyl chloride) in a fluidised bath offers one of the most severe
conditions of operation. Hydrogen chloride (HCL), liberated on the breakdown of PVC, is
absorbed by the bed medium, creating an acidic environment within the bed, especially
when the bed also absorbs water from the atmosphere or fluidising air when cold. HCL is
extremely corrosive, especially when aerated and wet. In addition, in water it produces
water ions which, even in neutral or alkaline solutions, promote corrosion and rusting in
steel; witness for example, the corrosive nature of sea water and calcium chloride road
de-icer. Consequently, fluidised beds used for burning off PVC require strict supervision
to minimize corrosion of the bed its self and of the parts cleaned in it, especially if these
are of uncoated steel. The purpose of most of the recommended maintenance procedure
is aimed at keeping the bed medium clean and free flowing, and free of gums, acids,
agglomerates, partly decomposed plastic, char and larger particles. These cleaning
processes have the additional benefit of ensuring good fluidisation and this, in turn
reduces burn off time, reduces uneven heating of parts and thus distortion, increases
heater life by eliminating localised hot-spots, and makes cleaning easier on a regular
basis.
The following instructions apply generally to halogenated polymers and specifically to
PVC. They are aimed at minimising corrosion of the bath and of immersed metals.
A. DAILY INTERVALS - at the end of each working day
a. Maintain the working temperature (about 400°C) for half an hour after the last
processed batch, to assist removal of corrosive acids from the bath.
b. If possible, maintain the temperature of the bath at about 100°C overnight and
over weekends and holidays to reduce absorption of water vapour into the batch.
Overnight, the residual heat in the bath will usually ensure this.
c. Scoop charred plastic residues, clods of media and articles being processed.
B. WEEKLY INTERVALS - at the end of each working week and before prolonged shut-
downs
a. Ensure compliance with instruction A, above.
b. Add about 50g (2oz) of powdered mild alkali to the bath and mix in well by
maintaining fluidisation for 5 minutes. Suitable alkalis are limestone, dolomite,
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