general DeSCrIptIOn
The fluidised bath employs the principle of fluidisation of a mass of finely divided inert particles by means
of an upward flow of gas. A state of fluidisation is achieved when the individual particles become
microscopically separated from each other by the moving gas. This “fluidised bed” of particles has
unusual properties which differ markedly from either those of the gas or of the solid particles. Instead,
the fluidised bed behaves remarkably like a liquid, exhibiting characteristics which generally attribute to
a liquid state. For example, the fluidised bed can be agitated and bubbled; it always seeks a common
level; materials of less density will float while those with densities greater than the equivalent fluidised
bed density will sink; and, most important, the heat transfer characteristics between the fluidised bed
and a solid interface can have an efficiency approaching that of an agitated liquid.
In addition, the fluidised solid phase has a most unusual physical behaviour, in that its basic characteristics
change only slightly over very large temperature ranges; it has no melting point and no boiling point.
The lowest temperature available is the liquefaction point of the gas used for fluidisation, while the high
temperature level is usable temperature of the inert solid. Various metal oxides with allowable
temperatures of over 1700°C are readily available. The metal oxide beds commonly used, (eg aluminium
oxide) are non-flammable, non-explosive and non-toxic.
The most commonly used fluidised gas is ordinary compressed air obtained from a blower or compressor.
For situations where a non - oxidising atmosphere is required, nitrogen can be utilised and if a reducing
atmosphere is required, cracked gas can be employed with a silicon carbide bed.
The unique characteristics of gas fluidised particles is the relatively high rate of heat transfer which yields
highly isothermal conditions, as well as excellent heat transfer to solid surfaces. This characteristic is due
to the turbulent motion and rapid circulation rate of the solid particles in conjunction with the extremely
high solid gas interface area. Therefore, despite the fact that gas solid interfaces normally yield low heat
transfer coefficients and the solids normally have low thermal conductivities, the overall heat transfer
characteristics of fluidised particles approach those of a liquid.
The combination of excellent heat transfer characteristics and high heat capacity are ideal for attaining
rapid stabilisation at an isothermal condition.
Techne industrial fluidised baths use aluminium oxide as the fluidising medium. They have been designed
to remove plastic residue from extruder and moulding machine tools, paint build up from paint fixtures,
and carry out various heat treatment processes.
The fluidised bed is housed in a circular stainless steel container which has an effluent gas extract duct
and fluidised medium drain chute. The container is surrounded by electrical heating elements which are
enclosed within thermal insulation. The inner assembly is housed in a square outer case which also
supports the electrical and pneumatic control console.
the following control options are available:
A manual control system which consists of a temperature controller, an independent over-temperature
cut-out device, a fluidising air pressure interlock, and a manually adjusting fluidising air control valve.
Supplied as standard with this, and all other options, is an air pressure regulator and filter assembly.
During operation the correct fluidising air flow must be determined by visually inspecting the surface of
the fluidised bath. When the fluidised bath is heated the air within the bath expands; it is therefore
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