Fluidized-Bed Heat-Treating Equipment,Fluidized-Bed Heat-Treating Equipment,Fluidized-Bed Heat-Treating Equipment
FLUIDIZED-BED TECHNIQUES are not new to the metalworking industry. A 19th century American patent describes
the roasting of minerals under fluidized-bed conditions. Other established applications include potter's clay and miner's
hydraulic slurries. Systems of fluidized solid particles, such as quicksand, occur in nature.
Early attempts to use fluidized beds in the heat treatment of metals were limited in the temperatures that could be
employed. Electrically heated furnaces capable of maintaining fluidized beds at temperatures up to 500 °C (930 °F) could
be produced commercially, but difficulties were encountered when attempts were made to attain higher temperatures. A
principal problem was the high rate at which refractory distributors, which distribute the hot fluidizing gases, were
consumed.
In early gas-fired fluidized-bed furnace design, gas entered the base of the container after being mixed with air to make it
ignitable at the point of entry. With newer designs, the mixtures are introduced separately and thus cannot be ignited
accidentally. This design eliminates the danger of explosion at the point of entry. The surface of the bed is heated first,
and the heating of surface particles causes progressive ignition downward through the container until the entire contents
of the bed achieves uniform heat-treating temperature. Newer furnace designs extend fluidized-bed technology into the
higher temperature ranges (540 to 1040 °C, or 1000 to 1900 °F) required for most common heat treatments.
Principles of Fluidized-Bed Heat Treating
In fluidization, a bed of dry, finely divided particles, typically aluminum oxide in the heat-treating context, is made to
behave like a liquid by a moving gas fed upward through a diffusor or distributor into the bed. A gas-fluidized bed is
considered a dense-phase fluidized bed when it exhibits a clearly defined upper limit or surface. At a sufficiently high
fluid-flow rate, however, the terminal velocity of the solids is exceeded, the bed goes into motion, and the upper surface
of the bed disappears. This state constitutes a disperse, dilute, or lean-phase fluidized bed with pneumatic transport of
solids. The general phases or stages of fluidization are shown in Fig. 1. Usually the aggregative or bubbling-type stage is
used for heat-treatment processes.

Although the properties of solid and fluid alone determine the quality of fluidization (that is, whether smooth or bubbling
fluidization occurs), many factors influence the rate of solid mixing, the side of the bubbles, and the extent of
heterogeneity in the bed. These factors include bed geometry, gas-flow rate, type of gas distributor, and internal-vessel
features such as screens, baffles, and heat exchangers.
Determination of Fluidization Velocity. In determining the quality of fluidization, a diagram of pressure drop (Δp)
versus velocity (μ0) is useful as a rough indication when visual observation is not possible. A well-fluidized bed will
behave as shown in the diagram in Fig. 2, which has two distinct zones. In the first, at relatively low flow rates in a
packed bed, the pressure drop is approximately proportional to the gas velocity and usually reaches a maximum value
(Δpmax) slightly higher than the static pressure of the bed. With an increase in gas velocity, the packed bed suddenly
"unlocks" and becomes fluidlike.

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