Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION I
1 The present invention relates to -Fluidized bed
furnaces for use in the treating of materials at temperatures
in the range of about 1100C to 1300C and is suitable for
use in treating of high speed steels. In particular, the
invention relates to a fluidized bed furnace wherein the bed is
contained in a retort having quartz walls and the be is heater
by infrared radiation lamps disposed exterior to the quartz
walls. The radiation emitted by the lamps is transmitted
through the quartz walls to heat a boundary layer of the fluid
lo bed particulate. The thermal transfer characteristics of the
fluidized bed results in the rapid dissipation of -this energy
throughout the bed resulting in a uniform bed temperature.
Heat treating at high temperatures, and particularly
heat treating of high speed steel, has been generally confined
to molten salt baths which are hazardous materials to the
environment and are dangerous in the work place. Some attempts
have been made to use fluidized beds for high temperature heat
treating of high speed steel, however, these have not gained
wide acceptance. One proposal with respect to fluidized beds
is to use electrically conductive fluid bed particles and to
heat the fluidized bed electrically using the electrical
resistance of the particles. This method is not suitable with
materials to be treated which are conductive.
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1 High temperature fluidized bed are possible using
externally electrically heated retorts. electrical resistance
heaters are disposed about the exterior of -the retort, heating
the walls thereof which in turn heat the fluid bed
particulate. Such beds have long startup times and the life
expectancy of the electrical heaters is short at these high
temperatures. If metal retorts are used, problems with
breakdown can occur.
The proposed system which uses infrared radiation
lamps, and preferably high intensity infrared radiation lamps,
is capable of rapidly raising the temperature of the fluidized
bed to within the desired operating range and once the be has
been raised to the operating range, the tamps are closely
controlled to maintain this temperature. Therefore, the lamps
initially provide a very high initial input energy to the bed,
by the transmission of high intensity radiation to a boundary
layer of fluidized particles to rapidly raise the temperature
of the bed to the operating temperature and thereafter, the
output is reduced to a level of radiation sufficient to
maintain the desired fluid bed temperature. With this system,
accurate temperature control of the fluidised bed is achieved
in combination with reduced start up time.
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1 BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the invention are shown in
the drawings wherein;
Figure 1 is a top view of the heat treating apparatus;
and
Figure 2 is a vertical section through the apparatus
DETAILED DESCRIPTION OF THE PREFERRED Embodiment
-
The furnace generally shown as 2 in the figures, has a
steel furnace shell 4, surrounding refractory insulation 6
which in turn surrounds an interior chamber 8 in which high
intensity shortwave infrared radiation lamps 10 are vertically
supported. These lamps 10 are spaced about the circumference
of a quartz retort 12 having a bed 14 of particles capable of
being fluidized . The particles are preferably refractory
grade aluminum or zirconium oxide particles and the like,
although not limited thereto as any fluidizable particle may be
used which would remain inert to tune steel when heated to a
temperature in the range of 1100C to 1300C. In order to
2û fluids the particles, a flow of nitrogen gas or other
suitable inert gas indicated by arrows 16, is introduced
through inlet 27 adjacent the lower portion of the retort I
The rate of the nitrogen gas flow into the fluidized bed 14
will be determined in accordance with -the characteristics of
the bed as in any fluidizing operation. Preferably, the gas
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1 used to fluids the bed is recycled. Q gas flow is also use
to cool the lamp tips 18 which have been isolated exterior to
the chamber 8 by end plates 20. The lamp tips 18 should be
cooled in order to increase -the life expectancy of the lamps 10.
The high intensity infrared radiation lamps 10 are
capable of emitting various levels of infrared radiation in
accordance with electrical power supplied thereto. The
radiation emitted by the lamps is transmitted through the
quartz walls 23 of the retort 12 used to maintain the fluid bed
14. Quartz is essentially transparent to the infrared
radiation and will allow the radiation to pass there through an
strike a boundary layer of the fluidized boll 14 adjacent -tune
walls 23 of the retort 12. Preferably the retort 12 is
circular in cross section and the axes of the lamps 10 are
disposed in a circular envelop about the retort 12. The heat
energy resulting from the radiation absorber by toe particles
is quickly dissipated throughout the fluid bed due to the
action of the bed resulting in even temperature distribution.
The quartz walls 23 of the retort 12 are relatively stable with
respect to thermal expansion rendering them suitable for -this
high -temperature application.
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1 Various methods for fluidizing of the bed can be used
in the present application and the application is not limited
to any particular method of introducing the nitrogen gas or
other inert fluidizing gas to the bed. For example, the base
25 of the retort 22 could be provided with distribution plates
or porous tile to allow the fluidizing gas to pass there through
into the bed I or the gas could be introduce through
distribution tubes located in the bed 14 above the base 25 of
the retort 12.
The furnace, as shown in the figures is suitable for
batch treating of high speed steels and the like where the
product is introduced into the fluidized bed and retained
therein the required treating time. In order to increase the
effectiveness of the infrared radiation lamps 10, reflectors
may be associated therewith for redirecting radiation emitted
by the lamps back towards the quartz walls 23 of retort 12.
Some cooling of these reflectors may be desired and if
necessary, an isolated air flow can be provided to remove heat
from the back of the reflectors. The quartz lamps are not in
direct gas contact with the fluidized bed 14 and, therefore, if
cooling of the lamps is required air may be used.
The high intensity infrared radiation lamps may be
placed at approximately every one to four inches about the
circumference of the retort 12 and are particularly responsive
to changes in input energy. This allows the radiation emitter
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1 by the lamps and absorbed by -the boundary layer of the fluid
bed 14 to be controlled in an accurate manner whereby the final
temperature of the fluids bed is limited to a desire
temperature range. For example, the bed temperature can be
sensed and the power input to the lamps automatically adjusted
in accordance with this sensed temperature. In addition, the
infrared radiation lamps are capable of rapidly raising the
temperature of the particles of the fluidized bed 14 as the
radiation is being directly transmitted to the particle or the
bed adjacent the side walls I of -one retort and the resulting
heat energy is rapidly transmitted throughout the bed. The
temperature of the bed is uniform for even heating of the
products being treated and the furnace is capable of being
raised to the operating temperature within a short time
relative to existing technology of salt baths. The capability
to rapidly raise the temperature of the bed to operating levels
allows the fluidized bed furnace to be shut down when not in
use thereby reducing operating costs. The accurate control
possible by varying the electrical power to the lamps is
important as the product being treated is approaching the
melting temperature and a temperature overshoot could render
the materials being treated useless.
The quartz retort in addition to being essentially
transparent to the infrared radiation is stable at these high
temperatures and thermal expansion is not a problem.
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1 The quartz retort isolates the lamps from the
fluidizing gas and fluidizing particulate which will be at -the
bed operating temperature. The lamp arrangement an quartz
retort also simplifies recycling of the fluidizing gas as -the
top surface of the bed is clear.
Although various preferred embodiments of tune present
invention have been described herein in detail, it will ye
appreciated by those skilled in the art, that variations may be
made thereto without departing from the spirit of the invention
or the scope of the appended claims.
