Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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~ EYING SCREW FO~ FURNACE
Ths instant invention relates to furnace design
in general and more particularly to a countercurrent fluid
cooled discharge screw disposed immediately above the
hearth in a rotary hearth furnace.
BACKGROUND
Direct reduction o~ iron oxide and other metallic
oxides may be conducted in rotary hearth furnaces ("RHF")
using pelletized or briquetted ~eed deposited upon the
rotating hearth.
Briefly, an RHF i~ continuous reheating ~urnace
generally having a aircular inner wall circum~cribed by a
spaced circular inner outerwall. The void ~ormed therebet-
ween includes a circular rotating hearth. In order to
retain the heat generated within the furnace the walls are
relatively low so as to enable the roof to be close to the
hearth. Burners may be installed in the inner and outer
walls and in the roof.
Material is usually loaded (dropped) onto the
rotating hearth by a conveyor or chute. After ~he material
is carried on the hearth, it is usually removed by a
discharge screw. Due to high temperatures (1300-2300 F
[704 - 1260 C]) involved, the screw if frequen~ly water
cooled. See U.S. patent 3,443,931. Ga~es are permitted to
vent through a flue located in the roof.
A conventional conveying or discharge screw
consists o~ a central shaft with a ~olid helical ~light
welded thereto. A cooling fluid is passed through a bore
disposed within the shaft. Other s~rew designs utilize a
plurality of spac~d solid flights disposed about the shaft.
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Due ~o corrosive nature of the gases and
materials present within the RHF, coupled with the high
temperatures therein, the discharge screw is subject to
frequent failure. The flights generally deteriorate.
Corrosion and erosion caused by high temper~tures and bad
actors (sodium, sulfides, chlorides, fluorides, potassium
lead, zinc, tin, iron, nickel and chromium) within the RHF
oftentimes chew up the screws and render them useless after
only about three months. Expensive material~ such as HH
alloy ~20% nickel, 20% chromium, remainder iron) as well as
IN 659 were not satisfactory.
In addition, the spaces b~tween the flights
accumulate fluffy fines which tend to cake together. The
fines act as a sponge which serve to collect and concen-
trate the corrosive gases present within the furnace.
As can be imagined, fre~uent screw replacement
necessitates frequant downtime, high maintenance and labor
costs, and inefficient use of the furnace which in turn
lead to higher unit costs. Clearly a ~etter screw design
is necessary.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved
discharqe screw capable of better withstanding the rigors
of an RHF.
The screw includes a central shaft and a plural-
ity of helical water cooled flights arranged thereon. The
coolant flows through the screw in two stages: once
through the flights and then in a countercurrent fashion
back through the screw before being reversed for exit.
Moreover, the ~lights may be faced with a corrosion resis-
tant overlay.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a plan view of a rotary hearth
furnace.
Figure 2 is a sidelels~ation, of an embodiment of
the inventlon.
Figure 3 is a cross sectional view takerl along
linP 3-3 in Figure 2.
Figure 4 is a cross sectional view o~ an embodi-
ment of the invention.
PR~FERRED MODE FOR CARR~ING OUT T~E INV~TIO~
Referring to Figure 1, there is shown a greatly
simplified view of a rotary hearth furnace (RHF? 10. The
RHF 10 includes an insulated outer wall 12 and an insulated
- inner wall 14. A hearth 16 rotates within the RHF 10 in
the direction shown by arrow 18. A plurality of burners 20
are situated about the RHF 10. Curtains 22 divide the RHF
10 into distinct sections. Matsrial is introduced onto the
hearth 16 by a feeder 24 mounted in the roof ~not shown~ of
the RHF 10.
After material processing is complete; tha~ i5,
after almost one complete rotation o~ the hearth 16, the
material is remov~d by discharge screw 26 and is deposited
into a bin (not shown) for subsequent treatme~t. The
discharge screw 26 is driven by motor and mechanical
linkage 28. Water is supplied to the screw 26 through
coupling 30.
Figures 2 and 3 depict the ~crew 26 in greater
detail. Th~ screw 26 includes shaft 32 affixed to two
pipes 62 each ha~ing an internal bore 34 for water passage
therethrough. A plurality of spac~d hollow helical fligh~s
36 circumscribe the shaft 32. It is preferred to utilize
six or seven ~lights 36 si~ce more flights will tend to
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cause particulate matter to cake between the flights 36.
The flights 36 are shown in a clockwise righthand spiral. Accordingly, the screw 26 rotates in direc-
tion 38. However, the invention is not limited to thisparticular embodiment.
Water is 7 ntroduced into the screw 26 at water
coupled end 40 and exit~ through drive end g2.
The flights 36 are hollow to permit water flow
therethrough. Slots 4~, formed in two sections of the
shaft 32 (see Figure 4), allow the water to pass from the
bore 34 to the flight 36 and vice-versa. A corrosion
resistant overlay ~4 may be affixed to the leading edge of
the flight 36. ST~LLITE (a trademark) 6 has been utilized
as a overlay 44 but it sometimes has craeked after a period
of time. The crac~s then propagate into the mild steel
flight 36 causing small water leaks. Although the experi-
ence with the S~LLITE alloy has been sometimes disapp~int-
ing, it is still preferred tQ use an overlay 44.
Figure 4 is a detailed vi~w of the screw 26. The
ends 40 and 42 are affixed to appropriate affixing means
(not shown) to ensure water-tight integrity and allow for
~he rotation of the screw 26. Wa~er flow is shown by
arrows 48. Caps 74 and 76 prevent the water from leaking
out of the screw 26.
The shaft 32, by a series of internal baffles,
causes the water to flow in a serpentine or countercurrent
flow before exiting.
The water flow 48 ~irst courses through the pipe
62 from the water coupling end 40 whereupon it enters
chamber 50. The chamber 50 includ~s apertures 52 which
cause the water to flow into second chamber 54 and then to
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the flights 36 via the slots 46. Although only one 810t 46
is depicted at the water coupled (or proximal) end 40, it
should be understood that the number o~ slots 46 match the
number of flights 3Ç.
The water ~lows through the entire le~gth o the
flights ~6 towards the distal end 42 where it reenters the
sha~t 32 through the slot 46 into chamber 72. The water
continues to flow through ape.rture 68 formed in bulkhead 66
into annular space 56 fo~med between the sha~t 32 and inner
tube 58. Spacers 64 secure the physical relationship
between the shaft 32 and the inner tube 58.
The water flows, in a countercurrent fashion,
towards bulkhead 60 where it i6 reversed again and forced
into tube 58. ~he water, now flowing through tube 58
passes through aperture 70 in the ~ulkhead 66 towards the
distal end 42 and then out of the screw 26.
By *orcefully routing the wat~r back toward~ the
proximal end 40 in a counter~low ~ashion, three cooling
operations are conducted simultaneously. Firstly, water
~lowing through the flights 36 cools the Plights 36 and
reduces the possibility of ~light 36 deterioration.
Secondly, the watPr ~lowing back through the annular space
56 cools the shaft 32 area between the flights 36. This
area becomes caked with hot material which if not cooled
will hasten the demise of the screw 26. ~hirdly, the water
~lowing through the tube 58 and the pipe 62 kseps these
components relatively cool.
In experimental tests, the screw 26 has lasted
approximately two to three times longer than a c~nventional
water cooled solid flight discharge screw. Such screws, on
average, lasted only two to three months whereas the
instant sarew 26 has lasted from four to nine months~
~oreover, b~y utillzing the instant design, it ie possible
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to fabricate the screw 26 out of mild steel rather than
expensive exotic alloys.
The pitch, lead angle, length and number of the
flights are, of course, a function of the size of the RHF,
the environment and materia:L to treated within the RHF.
Under particula~ conditions, the temperature within the RHF
was about 1800F (982~C) and the flight~ were about 16.25
feet [4.9 meters) long. The outer shaft 32 was about 1.5
1~) feet ~.45 meter) in diameter with the entire shaft 32 about
17.2 feet (5.2 meters) long. The lead angle 68 was about
35 15' and the pitch 70 was about 13.3 inches (33.8
centimetres). See Figure 2. Due to the cooling capability
of the screw 26, the water temperature entered the screw 26
at about 90 F (32.2C) and exited the screw 26 at about
120 F (49 C) at a flow rate of about 300 yallons per minute
(1136 Jmin.) at about 10-15 pounds per square inch (69-103
KPa).
While in accordance with the provisions of the
statute, tnere ls illustrated and de~cribed herein specific
embodiments o~ the invention~ Those skilled in the art
will understand that changes may be made in the form of the
invention covered by the claims and the ~ertain features o~
the invention may sometimes be used to advantage without a
corresponding use o~ the other features.
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