Note: Descriptions are shown in the official language in which they were submitted.
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FURNACE DISCHARGE
SYSTEM AND METHOD OF OPERATION
FIELD OF THE INVENTION
The present invention relates to a furnace discharge system and
method of operation for a rotary hearth furnace. More particularly, the
present invention relates to a furnace discharge system for a rotary hearth
furnace including at least two helical augers that cooperatively transition
material from a continuous process within the rotary hearth furnace to a non-
continuous process exterior of the rotary hearth furnace.
BACKGROUND OF THE INVENTION
Briefly, a rotary hearth furnace (RHF) is a continuous reheating
furnace generally having an annular inner wall circumscribed by a spaced
annular outer wall. The space between the inner wall and the outer wall
includes a circular rotating hearth. Burners may be installed in the inner and
outer walls and in the roof. Combustion and process effluent gases are
permitted to vent through a flue located in the roof or in a side wall.
Rotary hearth furnaces have been typically used for heating metallic
sections for downstream mechanical forming; i.e., forging or rolling. A
further application is found for heating solid material with an internal
gaseous furnace atmosphere suitable for prevention of metal oxidation
and/or reduction of metal oxide present in the solid material. The rotary
hearth furnace is equipped with inner and outer water seals which makes it
ideal for containment of the internal gases required for high temperature
oxidation resistance and/or reduction.
Generally material, such as pellets, briquettes, loose mixtures and
agglomerates is uniformly distributed and loaded (dropped) onto the rotating
hearth by a conveyor or chute for processing within the furnace. After the
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material is conveyed along the hearth path and processed within the furnace,
the material may be in the form of pellets, briquettes, loose mixtures,
agglomerates or a fused mass, and is discharged from the furnace using a
single material discharge auger. The discharge auger typically consists of a
central shaft with solid helical metal flights welded or bolted thereto
projecting outwardly from the central shaft. The discharge auger extends
across the width of the circular hearth and is connected to an external motor
for rotation. The discharge auger continuously conveys the material from
the hearth down a refractory chute and into a transport canister.
In applications where the material from the RHF is used downstream
in a non-continuous batch process, the single auger arrangement requires an
elaborate design starting with a discharge chute, discharge chute valving
and/or multiple canisters and arc gate (dump valve) to control delivery of the
material into a transport canister. It will be appreciated that the canisters
for
receiving the material from the furnace must be arranged on a turntable and
indexed into and out of the load positions beneath the canister and arc gate.
During this indexing, the continuous discharge of material is delivered to the
canister while the filled canister is being indexed to make room for a fresh
transport canister.
Alternatively, the single discharge auger can be arranged to deliver
the hot product into a bifurcated chute suitable for alternatively discharging
into multiple transport canisters. This arrangement requires a two-way
diversion valve or arc-gate for directing the flow of material to the empty
transport canister. Each leg of the of the bifurcated chute requires that a
gate
valve be located at the bottom terminus. It will be appreciated that
considerable height is necessary to accommodate the bifurcated chute
arrangement.
This single material discharge auger arrangement, in addition to
equipment complexity, can contribute to operational problems and unwanted
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down time due to potential bridging of material in the hopper and/or failure
of the dump valve (arc gate) to open or close properly. It will be appreciated
that closure of the gate can be impeded by material interfering with the full
swing of the gate. It will be further appreciated that the incomplete closure
of the gate valve may permit extremely hot material to continuously
discharge from the furnace and permit air (oxygen) to be in-drafted into the
furnace thereby providing extremely hazardous conditions for auto
combustion when a highly reducing atmosphere (CO and H,) is present in
the RHF.
The present invention relates to an improved RHF discharge svstem
that addresses the problems associated with a single material auger
discharge. The present invention also facilitates the transition from the
continuous process of the RHF to a non-continuous process downstream of
the RHF. The dual material auger arrangement of the present invention
enables the material transport canister transfer to proceed without the need
for an intermediate retention hopper and material supporting arc gate. The
simplicity of the discharge assembly of the present invention also simplifies
the RHF process plant arrangements.
SUMMARY OF THE INVENTION
Briefly, the present invention relates to a furnace discharge system
for removing material from a hearth of a rotary hearth furnace. The system
includes at least two material discharge augers positioned above the hearth
of the rotary hearth furnace. Each discharge auger is operatively mounted to
allow for height adjustment of the discharge auger in alternating sequence
from a material removal position to an elevated material by-pass position.
The material is discharged from the rotary hearth furnace by lowering
at least one discharge auger to a material removal position and elevating at
least one discharge auger to an elevated material by-pass position. A
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material transport canister is operatively attached to the discharge chute and
the valve of the discharge chute of the elevated discharge auger is closed and
the valve of the lowered discharge auger is opened. Material from the rotary
hearth furnace is then conveyed into the material transport canister.
BRIEF DESCRIPTION OF THE DRAWING
Further features and other objects and advantages of this invention
will become clear from the following detailed description made with
reference to the drawings in which:
Figure 1 is a plan view of a furnace discharge system constructed in
accordance with the present invention;
Figure 2 is a cross-sectional view of furnace discharge system
including material canister taken along line A-A of Figure 1; and
Figure 3 is a side elevation view of an embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following description, like reference characters designate like
or corresponding parts. Referring to Figure 1, a simplified sectional plan
view of a rotary hearth furnace (RHF) 10 in accordance with the present
invention is shown. In considering the RHF shown in the figures, it will be
appreciated that the structure is schematic only and that certain details of
construction are not provided for purposes of clarity and simplicity, such
details being considered well within the skill in the art once the invention
is
disclosed and explained.
As shown in Fig. 1, the RHF 10 includes an annular refractory
insulated outer wall 12, a spaced annular refractory insulated inner wall 14
and a roof 16 interconnecting the top of the outer wall and inner wall. A
circular hearth 18 rotates within the RHF 10 in the space between the outer
wall 12 and the inner wall 14. A plurality of burners (not shown) are
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positioned about the perimeter of the RHF 10 and fire into the enclosed
space between the outer wall 12, the inner wall 14, roof 16 and hearth 18.
Materia120 from feed conveyor 13 is introduced onto the hearth 18 by a
feeder 19 mounted in the lowered roof area 17 beneath feeder hood 15 or
through the outer wall (not shown) of the RHF 10 as is well known in the
art. Removable auger hoods 37 may be provided for ease of removal of the
augers during maintenance periods.
The depth of material 20 on the hearth 18 may be controlled to allow
maximum heating and exposure to the furnace environment. The hearth 18
is conventionally sealed to the hearth enclosure by a water seal (not shown),
for example as described in U.S. Pat. No. 3,452,972_
The hearth 18 is supported on wheeled members 24 which can be
driven by ar}y conventional driving means, for example, as shown in U.S.
Pat. No. 3,452,972 or in U.S. Pat. No. 4,597,564..
After processing of the material 20 is complete, typically after almost
one complete rotation of the hearth 18, the material 20 is removed by the
furnace discharge system 26 in accordance with the present invention for
subsequent treatment or use downstream of the RHF 10.
In a preferred embodiment, the furnace discharge system 26 includes
at least two discharge augers 28 and 30 positioned sequentially around the
circumference of the RHF 10. Each discharge auger 28 and 30 is
operatively mounted on a trunnion (not shown) to allow for height
adjustment of the discharge auger above the surface of the hearth 18 from a
material removal position to a retracted position. In a preferred
embodiment, the at least two discharge augers 28 and 30 may be operatively
mounted transverse to the direction of travel of the rotating hearth 18. In an
alternate embodiment, the at least two discharge augers 28 and 30 may be
operatively mounted at an angle skewed from radial to serve as a
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combination auger and plow for easier material removal from the rotating
hearth 18.
Each discharge auger 28 and 30 includes a central cylindrical shaft 32
having attached thereto at least one helical flight 34. The helical flight 34
circumscribes the central shaft 32 and projects radially outwardly from the
central shaft. Each flight 34 may extend from about 1 -12 inches, preferably
6 inches, -radially outwardly from the shaft 32 and is about'/< = 1'/~ inches
thick. The shaft 32 may be insulated between flights 34 to reduce heat loss
and may be fluid cooled as well known in the art. For a more detailed
description of a discharge auger reference is made to U.S. Patent
Application No. 08/919,399, filed August 28, 1997.
Referring to FIGS. 2 and 3, the first discharge auger 28 is shown in
its lowered, material removal position. The material 20 is removed from the
hearth 18 by the rotating helical auger and transferred to an enclosed, inert
material discharge chute 36. The material 20 flows downward through the
discharge chute 36 and is loaded into a movable material canister 38. The
flow of material 20 and access to the furnace through chute 36 is controlled
by a valve 40 such as a seal valve of a type well known in the art. It will be
appreciated that valve 40 primarily functions to seal the furnace atmosphere
from the external atmosphere. The valve 40 is operatively controlled to be
open only when a material canister 38 is operatively attached to the
discharge chute 36.
As used herein the term "inert" refers to a non-reactive atmosphere
within a sealed enclosure. As shown in the figures, the transport canister 38
is an enclosed insulated inert container having a removable top adapted to
receive the bottom end of the discharge chute 36. The transport canister 38
is mounted on wheels 42 for easy transport via a transfer dolly along a
transfer path 44 located in loading pit 48 between the RHF 10 and
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downstream processing operations, e.g., iron and steel making operations.
The furnace discharge assembly arrangement allows for lower head room
requirements between the RHF 10 supported on piers 46 and material
receiving canister 38.
Upon completion of the loading of the material transport canister 38,
the discharge auger 28 is raised off of the hearth 18 and valve 40 is closed.
As the material transport canister 38 approaches being filled, material
discharge auger 30 is lowered onto the hearth into its material removal
position to commence removing material from hearth 18 as soon as
discharge auger 28 is raised from its material removal position. Prior to
lowering auger 30 into its material removal position, material transport
canister 38 is placed in the loading position and valve 40 is opened to allow
material to be discharged from hearth 18 to transport canister 38 via chute
36. As transport canister 3 8 is being filled, the filled transport canister
is
removed from the RHF 10 area via a transfer dolly along a desired transfer
path 44 to a downstream processing operation. The filled transport canister
3 8 is then taken off of the transfer dolly at the downstream processing
operation and an empty transport canister 38 is placed on the vacant transfer
dolly and moved back to the RHF 10 below the appropriate chute 36 of the
discharge auger 28 or 30.
The procedure discussed above for the transport canister loading
cycle and unloading cycle is repeated in an alternating manner between the
first discharge auger 28 and the second discharge auger 30 as the RHF
processes material 20. It will be appreciated that the present invention
facilitates the continuity for the transition from a continuous process within
the furnace to a non-continuous process wherein material is collected
batchwise in canisters for periodic movement for batch consumption in a
further downstream process. A material discharge assembly design which
increases the life expectancy of the material discharge assembly discharge
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augers and reduces furnace downtime and maintenance requirements for
periodic auger removal required by single auger discharge systems: In
addition, the present furnace discharge system reduces maintenance
requirements and simplifies RHF design by using a direct material discharge
chute without interim material hopper storage and support dump valves
Although the detailed description provided above makes reference to
two discharge augers 28 and 30 it will be appreciated that the present
invention may include more than two discharge augers depending upon the
size and/or circumference of the RHF.
_
Having described presently preferred embodiments of the invention,
the inventiori may be otherwise embodied within the scope of the appended
claims.
