Principles Of Operation; Operator Safety - Bibby Scientific Techne SBL-1 Manual Del Operador

OperatOr SaFety

It is important that only suitably instructed personnel operate this equipment. It must also be used in accordance
with the instructions contained in this manual and with proper safety standards and procedures.
It is imperative that all personnel who may come into contact with our equipment have available such of
our literature as they require to ensure their safety.

principles of operation

Small solid particles can be readily 'fluidised' by means of a suitable gas (air) stream. Clean dry air at a
constant pressure of about 21kN/m (3 lb/in ) from a pump, or from an air line, is supplied via a control
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valve to a chamber beneath the diffuser (porous plate). This diffuser ensures a uniform flow of air across
the full section of the container and acts also as a support plate for the sand.
As the control valve is slowly opened the solid sand bed remains undisturbed and the air finds its way
between the particles; under such conditions the pressure drop is proportional to the rate of flow of air.
As the valve is opened further the air drag on the particles will cause them to separate and the whole mass
of the bed can be seen to have expanded. The bed now behaves as a fluid and is said to be 'fluidised'.
Further opening of the valve is not accompanied by an increase in pressure drop, which remains constant
at a value corresponding to the head of the column of particles, but the bed becomes more turbulent and
will have the appearance of boiling liquid. The heat transfer and most uniform temperatures are obtained
when the bath is in this 'boiling' state.
Characteristic of a fluidised bath
A bath of fluidised solid particles behaves as a bath filled with an insulating non-volatile liquid and it shares
with liquid baths the desirable characteristics of accessibility, uniformity of temperature and good heat
transference. It is not, however, possible to obtain such a close temperature control as in a true liquid as
the effective specific heat of a fluidised solid is much less than that of a liquid. On the other hand this
small thermal capacity gives a rapid heat up from room temperature.
A typical heat transfer figure is 600 W/m /°C. Mr H. Sutcliffe* has a pointed out the advantages of a
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fluidised bath for testing electrical components. The only electrical leakage that takes place is by physical
transfer of charged particles so that the leakage currents are very small, being of the order of 10-9 amperes
for a potential difference of 200 volts between electrodes 10 mm square at a spacing of 5 mm.
A fluidised bath is, of course, much cleaner than an oil bath and objects do not have to be cleansed on
removal from the bath. A light dusting of the object with a brush is enough to return particles to the bath.
It is necessary to clamp vessels as the fluidised bath produces appreciable buoyancy.
* Electronic Engineering, February 1961. pp. 94-95.
Cleaning your unit
Before cleaning your unit ALWAYS disconnect from the power suppply and allow to cool below 50°C.
The outside of your fluidised bath can be cleaned by wiping with a damp soapy cloth. Care should be
exercised to prevent water from running inside the unit. Do not use abrasive cleaners.
Before using any cleaning or decontamination method, except those recommended here, the customer
should check with Techne that the proposed method will not damage the fluidised bath.
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