Note: Descriptions are shown in the official language in which they were submitted.
2196846
METHOD AMID APPARATUS FOR SAMPLING AND ANALYSIS OF
FURNACE OFF-GASES
SCOPE OF THE INVENTION
The Irresent invention relates to a method and an apparatus for the
sampling and analysis of high temperature furnace off gases, and more
particularly,
to an apparatus which includes a gas sampling probe used to continuously
obtain gas
samples to permit cemtinuous furnace monitoring and control, and a method of
using
such a probe in conjunction with a furnace monitoring and control system.
BACKGROUND OI~ THE INVENTION
In the: recycling of scrap steel in electric arc furnaces, carbon and
oxygen are injected into the furnace to enhance combustion. As a result of
inefficient
furnace operation, however, combustible hydrogen and carbon monoxide gases are
frequently vented as components of the off gases from the furnace, resulting
in a loss
of furnace operating efficiency and increasing the costs of steel recovery.
Several attempts have been made to monitor and analyze the constituent
elements of furnace off gases with a view to controlling furnace operations,
so as to
maximize furnace efficiency. Heretofore, off gas sampling and analysis has
been
performed by periodically drawing a gas sample from a hollow tube or probe
which
is extended into the off gas stream to a gas analyzer. The probe has an
opening in
one of its ends which opens to the off gas stream, and another portion
provided in
gaseous communication with the gas analyzer by means of a gas line or conduit.
Suction is used to draw a gas sample into and along the tube and to the gas
analyzer
via the gas line. Typically, during sample gas collection, the tube is
extended
through a side of a vent duct or stack leading from the furnace along which
the off
gas moves. In this pnsition, the tube is arranged in a generally horizontal
orientation
whereby off gas samples which are drawn flow in a generally horizontal
direction
along the inside of the tube. While conventional sampling apparatus may be
effective
2~~s~~~
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in obtaining a single gas sample every few hours, heretofore sampling
apparatus have
proven ineffective in the continuous collection of samples of furnace off
gases, as
they become clogged after extended periods of use.
Dust and other particulate matter from the furnace, and which is
entrained in the off l;ases, tends to accumulate in the sampling tube and gas
lines used
to draw the gas sample to the gas analyzer. The clogging of sampling tubes is
compounded in conventional gas sampling apparatus in which the sample gas is
drawn
horizontally. Because of the horizontal orientation of the gas passage,
furnace dust
tends to accumulate along the inside of the sample tube as the dust settles on
the
lower surface of the tube. The accumulating dust in turn prevents the
effective
sampling of the furnace off gases. The difficulties of accumulating dust and
particulate matter are especially acute where the unfiltered sample gas is
drawn
through sampling tulbes which have longer horizontal sample gas passages.
To minimize clogging, it is known to provide a flow of pressurized
backflow gas along the probe or tube, to dislodge any accumulated dust and the
like.
In an attempt to maximize the effectiveness of backflow cleaning by
pressurized gas,
some conventional sample tubes are formed having a comparatively short length.
The
shorter length of conventional sampling tubes, however, results in the
collection of
sample off gases from the peripheral edges of the off gas stream, adjacent the
sides
of the exhaust duct. If air inlets are provided through the sides of the
exhaust duct
upstream of the sample tube, as for example, to facilitate off gas flow and
cool the
off gases, off gas samples collected at the edge of the off gas flow may be
non-
representative of the actual off gas as a result of the mixing of off gases
and air.
If the entrained particulate matter is drawn through the gas lines and
into the gas analyzer, the dust and particles may clog or otherwise damage the
analyzer equipment. To prevent dust and other such particulate matter from
entering
the gas analyzer, various particle filters have been proposed within or across
parts of
the sampling tubes. It has been found, however, that the exceptionally high
2196846
- 3 -
temperatures of off l;ases from high temperature furnaces, which exceed
temperatures
of 3000° F, tends to degrade the filters after prolonged use, causing
the filters to
crack and/or crumble. To prolong filter life, conventional gas sampling of
high
temperature furnace off gases is, therefore, performed only on a periodic
basis.
A fur:her difficulty with conventional sampling tubes exists in that the
filters used to filter particulate matter from the off gases are mounted to
the probe,
so as to locate within the exhaust duct during gas sampling operation. As
such, if
the filter fails, it is necessary to remove and disassemble the entire gas
sampling
probe to change the filter. This, in turn, frequently necessitates the shut
down of the
entire furnace system, leading to production losses.
The disadvantages of prior art gas sampling and analysis apparatus have
thus prevented the substantially continuous sampling of off gases and the
continuous
monitoring of furna~~e operations. The result is that with existing sampling
and
monitoring apparatus, it has not been possible to actively alter the furnace
operating
conditions in response to off gas analysis data, so as to maximize furnace
efficiency.
SUMMARY OF THE INVENTION
To at least partially overcome the disadvantages of prior art devices,
the present invention provides an apparatus for monitoring and analyzing
furnace off
gases, which includes a gas sampling tube or probe which has a vertically
extending
gas passage along which sampled gas is drawn vertically for insertion into a
furnace
off gas stream.
Another object of the invention is to provide an apparatus adapted to
continuously sample and monitor off gases from high temperature furnaces, such
as
electric arc furnaces, without clogging.
A further object of the invention is to provide a probe for use in
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sampling high temperature furnace off gases at temperatures as high as
3000°F, and
which incorporates .a filter for removing dust and other particulate matter
which is
entrained in the sampled gas, and which is adapted to withstand the high
temperature
cycling associated with the start-up and shut down of the furnace.
Another object of the invention is to provide a probe for sampling high
temperature off gasea, which defines a gas passage which is open to the vff
gas
stream at its lowermost end to facilitate backflow cleaning.
Another object of the invention is to provide an apparatus for sampling
high temperature furnace off gases which includes a probe configured to obtain
a gas
sample from a middle portion of the off gas stream, and which cools the gas
sample
prior to drawing the sample through an internally housed filter so as to
prolong filter
lifespan.
Another object of the invention is to provide a probe as part of an
apparatus for sampling and monitoring furnace off gas which incorporates a
filter
which may be quickly and easily replaced without requiring the removal or
disassembly of the probe, or the shut down of the furnace.
A further object of the invention is to provide a method of operating
a high temperature furnace system whereby during the furnace heat, furnace off
gases
are continuously sampled and analyzed, and where furnace operating conditions
are
varied having regard to various components of the off gas to maximize furnace
efficiency.
Another object of the invention is to provide a method and an apparatus
for operating a furnane by continuously sampling and analyzing furnace off
gases to
provide off gas content and/or temperature data, comparing the sample data
with
stored data representiitive of a normalized furnace heat profile and/or
previous heat
profiles and/or a preferred furnace heat profile, and controlling the furnace
operating
~19s~4s
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conditions to compensate for differences between the sample data and the
stored data.
In one embodiment, a furnace and a furnace monitoring and control
system are provided in which the monitoring and control system includes a gas
analyzer for analyzing the camposition of furnace off gases, and a sampling
tube or
probe used to obtain an off gas sample from the furnace off gas stream for
delivery
to the gas analyzer. Gas samples are drawn to the gas analyzer, via the probe,
by a
vacuum pump provided either as part of, or in gaseous communication with, the
gas
analyzer. The probe may be used to sample off gases from a number of different
furnace operations and/or reaction vessels, but is most suitably used to
sample high
temperature off gases from electric arc furnaces, wherein the off gas
temperature may
be as high as 3000°h.
The probe is formed as a generally cylindrical or p~olygonally shaped
hollow tube, which defines a generally vertically extending elongated gas
passage.
The probe is provided with upper and lower openings which extend into the
elongated
gas passage. The lower opening is provided in gaseous communication with the
furnace off gas stream and the upper opening is provided in gaseous
communication
with the gas analyzer by a gas conduit tube. In use, sample gas is drawn into
the
probe through the h~wer opening, and the sample gas flows in the passage in a
generally vertical dv-ection to the upper opening where it passes out of the
probe.
Preferably, the gas passage is open to the off gas stream at its bottom end to
define
the lower opening into which sample off gases are drawn. More preferably, the
open
bottom end further is formed to permit substantially unhindered movement of
particulate matter, such as dust, downwardly along the vertical sides of the
gas
passage and outwardly therefrom through the open bottom end.
The probe is mounted through the wall of an off gas exhaust duct used
to vent the off gases; from the furnace. The probe is positioned so that the
gas
passage remains in a generally vertical orientation and the bottom end of the
probe
locates in a central 2~rea of the off gas stream. In this position, sample off
gas is
2i~ss~s
- 6 -
drawn from the midldle area of the off gas stream through the bottom end into
and
vertically along the passage to the upper opening. At the upper end of the
probe the
drawn sample gas moves through the upper opening and to the gas analyzer via
the
conduit tube.
A filter may be positioned across the upper opening to prevent dust and
other particulate matter entrained in the sample off gas from passing through
the
upper opening and into the gas analyzer. While numerous types of filters are
possible, including natural stone and ceramic filters, the filter is
preferably formed
from either a high temperature fibre based filter or porous silicon carbide,
both of
which have been found to provide enhanced resistance to thermal degradation on
prolonged contact with high temperature off gases.
The filter may be removably secured across the upper opening of the
probe, as for example, by means of screws, a locking ring, complementary
threaded
couplings on the filter and about the opening or by other types of fastening
members.
Preferably, the filter has an elongated cylindrical shape having a diameter
less than
that of the upper opening. The filter may thus be secured in place by a
threaded
coupling which permits the removal and replacement of the filter through the
upper
opening, without requiring the removal of the probe from the off gas exhaust
duct.
The monitoring and control system may further include a bacl~low gas
source provided selecaively in gaseous communication with the gas passage to
provide
pressurized gas flow downwardly along the gas passage to dislodge any
particulate
matter which accumulates therein. By selectively providing the backflow gas
source
in gaseous communication with the gas passage, backflow gas may be directed
through a vent port leading directly into an upper portion of the gas passage
and/or
through the upper oF~ening and filter to backflow purge accumulated dust from
the
surface of the filter.
To reduce the effects of thermal degradation on the filter, and partially
~~~6~4G
arrest ongoing chemical reactions occurring within the sampled high
temperature off
gases, the probe preferably also includes cooling apparatus used to lower the
temperature of the sample gas as it moves upwardly in the gas passage towards
the
filter. In the simplest embodiment, part of the probe is provided with a
double
sidewall construction having an inner sidewall and a radially spaced outer
sidewall.
The sidewalls are joined by annular lower and upper end walls and define an
annular
chamber extending about at least part of the gas passage. Two or more
partition
members extend across the annular chamber and divide the chamber into a series
of
connecting coolant <:hannels.
Cool~u~t fluid, such as water or glycol solutions, may be pumped
through the coolant channels to cool the collected sample gas as it is drawn
upwardly
along the gas passage. Coolant fluid is pumped through the coolant channels at
a rate
of between 5 and 1C~0 gallons per minute, and more preferably, at a rate of
between
about 40 and 80 gallons per minute.
The conduit tube may be made of a number of materials, including
metals, rubber, plastics, or where cooled gases are to be analyzed, even
Teflon to
prevent chemical reactions between the sample off gas and the gas conduit tube
as the
gas sample moves trirough the tube.
Preferably, the gas analyzer is electronically linked to a furnace control
unit, such as a microprocessor or central processing unit (CPU) which controls
furnace operations. In response to data input from the gas analyzer, the
furnace
control unit may thus be used to change furnace operating conditions, such as
carbon
or oxygen content, combustible gas content (including hydrogen and carbon
monoxide
gases), and/or overall furnace temperature to achieve maximum furnace
efficiency.
More preferably, the: gas analyzer and/or CPU includes data storage capability
for
storing sample gas analysis data from a number of separate furnace heats,
and/or a
normalized furnace heat and/or data representing an optimum furnace heat. The
gas
analyzer is therefore used to continuously monitor off gas samples and prepare
an
~l9sg 4~
_8_
ongoing furnace heat profile of various off gas constituents. Off gas
constituents
which can be profilE~, include carbon monoxide content, oxygen content,
hydrogen
content, carbon dioxide content and/or furnace or off gas temperatures. During
the
furnace operation, individual heat profiles may thus be continuously compared
with
profiles representative of average or preferred furnace heats, and the furnace
operating conditions may be varied to improve furnace efficiency.
Accordingly, in one aspect the present invention resides in an apparatus
for sampling high temperature furnace off gases from an off gas stream and for
delivering sample gas substantially free of particulate matter to gas analyzer
means,
the apparatus including,
an elongated sampling probe, gas conduit means providing gaseous
communication betvreen said probe and said gas analyzer means and means for
drawing said sample gas to said gas analyzer via said conduit means,
the sampling probe including,
an inner sidewall substantially defining an elongated, vertically
extending sample gas passage, said passage open to the stream at a lowermost
open
end,
an upper opening spaced above said open end and permitting gas flow
between said passage; and said conduit means,
an outer sidewall spaced radially outwardly about at least part of said
inner sidewall and defining a generally annular chamber therebetween,
upper chamber wall means extending radially from said inner sidewall
to said outer sidewalk
lower chamber wall means extending radially from said inner sidewall
to said outer sidewalk
first aJnd second partition means, each extending across said annular
chamber between said inner sidewall and said outer sidewall, and being spaced
from
said lower wall means to divide the annular chamber into at least two fluid
communicating coolant fluid channels for cooling said sample gas in said
passage,
means for introducing coolant fluid into said coolant fluid channels,
296846
_ g _
filter means disposed across said upper opening for filtering particulate
matter from said sample gas as said sample gas is drawn through said upper
opening,
the lowermost open end substantially unobstructing downward
movement of said particulate matter filtered from said sample gas, outwardly
from
said gas passage.
Accordingly, in another aspect the present invention resides in an
apparatus for obtaining a high temperature gas sample from an electric arc
furnace
off gas stream and for delivering said sample to a gas analyzer, the apparatus
comprising,
a hollow sampling probe defining a vertically oriented elongated
cylindrical sample gas passage, said passage opening downwardly at its
lowermost
end into a bottom opening of said off gas stream, said probe including an
upper
opening through which said sample gas moves from said gas passage to the gas
analyzer,
vacuum means in gaseous communication with said upper opening for
drawing said sample from said off gas stream into said passage via said bottom
end
opening and outwardly from said passage through said upper opening to said gas
analyzer,
filter means provided across said upper opening for filtering particulate
matter from said sample as said sample is drawn through said upper opening.
In a further aspect, the present invention resides in the use of an
apparatus for sampling and analyzing high temperature furnace off gases from
an off
gas stream and for controlling furnace operations in response to the sample
analysis,
the apparatus comprising,
gas analyzer means for analyzing said sample gas, an elongated
sampling probe, and gas conduit means providing gaseous communication between
said probe and said ;;as analyzer means and means for drawing said sample gas
to
said gas analyzer via said conduit means,
the sampling probe comprising,
2~968~~
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an inner sidewall substantially defining an elongated sample gas
passage, said passage open to the stream at a lowermost open end,
an upper opening spaced above said open end and permitting
gas flow between said passage and said conduit means,
an outer sidewall spaced radially outwardly about at least part
of said inner sidewall and defining a generally annular chamber therebetween,
upper chamber wall means extending radially from said inner
sidewall to said outer sidewall,
lower chamber wall means extending radially from said inner
sidewall to said outer sidewalk
first and second partition means, each extending across said
annular chamber between said inner sidewall and said outer sidewall, and being
spaced from said lower wall means to divide the annular chamber into at least
two
fluid communicatin~; coolant fluid channels for cooling said sample gas in
said
passage,
means for introducing coolant fluid into said coolant fluid
channels,
filter means disposed across said upper opening for filtering
particulate matter from said sample gas as said sample gas is drawn through
said
upper opening,
wherein the lowermost open end substantially unobstructs
downward movement of said particulate matter filtered from said sample gas,
outwardly from said gas passage, and
bacl~how gas supply means selectively operable to supply backflow gas
flow downwardly along said gas passage,
wherein sampling of said off gas is performed by the steps of:
(a) activating said means for drawing a sample to said gas analyzer via
said g~~s passage to draw an off gas gas sample into said gas passage
via the: lowermost open end,
_ I1- 2196846
(b) simultaneously while drawing said sample, circulating coolant fluid
through said coolant channels to cool said sample as its moves along
the passage, and
(c) periodically interrupting the drawing of said off gas sample and
activating said backflow gas supply means to provide backflow gas
along said gas passage to assist in dislodging particulate matter therein.
In a further aspect the present invention resides in an apparatus for sampling
high temperature furnace off=gases from an off gas stream and for delivering
sample gas
substantially free of particulate matter to gas analyzer means, the apparatus
including:
an elongated sampling probe, gas conduit means providing gaseous communication
between said probe and said gas analyzer means and means for vacuum drawing
said sample
gas to said gas analyzer via said conduit means,
the sampling probe including,
an inner side 'wall substantially defining an elongated, vertically extending
substantially cylindrical sample gas passage, said passage open to the stream
at a lowermost
open end,
an upper opening spaced above said open end and permitting gas flow
between said passage and sand conduit means,
an outer sidewall spaced radially outwardly about at least part of said inner
sidewall and defining a generally annular chamber therebetween,
upper chambf;r wall means extending radially from said inner sidewall to said
outer sidewall,
lower chamber wall means extending radially from said inner sidewall to said
outer sidewall and closing a lowermost end of the annular chamber,
first and second partition means, each extending across said annular chamber
between said inner sidewall .and said outer sidewall, and being spaced from
said lower wall
means to divide the annular chamber into at least two fluid communicating
coolant fluid
channels for cooling said sample gas in said passage,
means for introducing coolant fluid into said coolant fluid channels,
filter means disposed in said cylindrical sample gas passage said filter means
provided across said upper opening for filtering particulate matter from said
sample gas as
said sample gas is drawn through said upper opening,
~\
- lIa- 2196846
the lowermost open end substantially unobstructing downward movement of
said particulate matter filtered from said sample gas, outwardly from said gas
passage.
In another aspect the present invention resides in an apparatus for obtaining
a
high temperature gas sample from an electric arc furnace off gas stream and
for delivering
said sample to a gas analyzer, the apparatus comprising,
a hollow sampling probe defining a vertically oriented elongated cylindrical
sample
gas passage, said passage opening downwardly at its lowermost end into a
bottom opening of
said off gas stream, said probe including an upper opening through which said
sample gas
moves from said gas passage; to the gas analyzer,
vacuum means in gaseous communication with said upper opening for drawing said
sample from said off gas stream into said passage via said bottom end opening
and outwardly
from said passage through said upper opening to said gas analyzer,
filter means disposed) in said cylindrical sample gas passage said filter
means provided
across said upper opening for filtering particulate matter from said sample as
said sample is
drawn through said upper opening,
vent port means for venting backflow gas directly into said gas passage said
vent port means spaced towards said upper opening, and
backflow gas supply means selectively operable to supply backflow gas to
said passage through said vent port means and through the filter means via the
upper opening,
whereby said backflow gas supplied to said passage flows downwardly along said
passage to
dislodge particulate matter retained on said inner sidewall.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will appear from the following
description, taken together with the accompanying drawings in which:
Figure 1 is a ,schematic view of a furnace and an ofd gas sampling and
analysis
apparatus in accordance with the present invention;
H
- llb 2196846
Figure 2 is a :schematic view of a gas sampling probe for use in the off gas
sampling and analysis apparatus of Figure I, showing its position in a furnace
off gas exhaust
duct;
Figure 3 shov~rs a cross-sectional view of the probe shown in Figure 2;
Figure 4 is a <;ross-sectional view of the probe shown in Figure 3, taken
along
lines 4-4';
Figure 5 is an enlarged partially cut-away schematic view of the lowermost
bottom end of the probe shown in Figure 3;
Figure 6 is an enlarged schematic view of the uppermost end of the probe
shown in Figure 3, showing sample gas flow during off gas sample collection
and analysis;
,~~9~$~6
- 12 -
Figure 7 is an enlarged schematic view of the uppermost end of the
probe shown in Figure 3, showing backflow gas flow during backflow cleaning of
the
probe;
Figure 8 is a cross-sectional view of a filter for use in the probe of
Figure 2 in accordance with a further embodiment of the invention;
Figure 9 is an exploded view illustrating the assembly of the filter
shown in Figure 8; sand
Figure 10 is a schematic view of an alternate embodiment of a filter
for use with the present invention.
DETAIL DESCRIPTION OF THE DRAWINGS
Reference is made first to Figure 1 which shows a system 10 used in
the reclamation of scrap steel. The system 10 includes an electric arc furnace
12,
a furnace controlling central processing unit (CPU) 14 used to control various
furnace
operating conditions., and a furnace off gas sampling and analysis assembly
16. As
will be described, off gas sampling and analysis assembly 16 is used
continuously
during the furnace heat to obtain and analyze furnace off gas constituents,
and provide
data respecting the various off gas constituents to the CPU 14.
The furnace 12 uses electricity either by itself, or in conjunction with
supplemental natural gas burners, and injected oxygen to make steel or other
ferrous
or non-ferrous products. In a conventional manner, furnace operating
conditions are
varied by the activation of the natural gas burners to increase the furnace
temperature,
as well as by the selective additional combustion enhancing additives, such as
carbon
and/or oxygen, and/or combustible hydrogen and carbon monoxide gases.
Off gases from the furnace 12 are vented outwardly through an opening
18 in the top of the 'furnace 12, and then into an exhaust duct 20 via an
elbow 22.
The elbow 22 is provided to change the direction of flow of the off gas stream
from
a generally vertical flow when the off gases move through the opening 18, to a
21968~~
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generally horizontal flow, as the off gases move into and through the exhaust
duct 20.
The exhaust duct 20 opens downwardly into a dust collection hopper portion 24
which
is used to collect particulate matter, such as furnace dust, which is carried
from the
furnace 12 entrained by the off gases.
Figure 1 shows best the exhaust duct 20 as being separated from the
elbow 22 by a gap 26 approximately 2 to 8 inches wide. The gap 26
advantageously
permits air to be drawn into the duct 20. Air drawn through the gap 26 mixes
with
the off gas stream to facilitate off gas cooling, and slows the flow of off
gas as it
moves horizontally over the hopper portion 24 and towards an exhaust fume
outlet
28. As a result of th.e slowed off gas flow, entrained particulate matter
falls from the
off gas stream into t:he hopper portion 24 where it is collected in a bin 30.
Figure 2 shows best the off gas sampling and analysis assembly 16 as
including a metal gas sampling probe 40 and an off gas analyzer 42 which,
during gas
sampling, are provided in gaseous communication by a metal gas conduit tube
44.
The gas analyzer 42 is provided with an internal vacuum pump 43 which in use,
draws the sample gas into the probe 40 and to the analyzer 42 for analysis.
The siirnpling probe 40 is formed having an elongated generally hollow
tube-like structure and is positioned in the exhaust duct 20 in a generally
vertical
orientation. The probe 40 extends from a top end 46 downwardly through the top
panel 48 of the exhaust duct 20 to a lowermost end 50. The probe 40 has an
overall
length of about 5 feed and extends downwardly so that its lowermost end 50
locates
in the middle area of the off gas stream which flows through the exhaust duct
20.
As shown best in Figures 3 to 5, the probe 40 has a double sidewall
construction and includes a stainless steel cylindrical inner sidewall 52 and
a stainless
steel cylindrical outer sidewall 54 provided concentrically about the inner
sidewall 52,
and an internally housed filter 64. The inner sidewall 52 extends the length
of the
probe 40 and definea an elongated, vertically extending cylindrical gas
passage 56
~~,96846
- 14 -
which, when the probe 40 is inserted in the duct 20, is centred on a vertical
axis A-A
(Figure 3). The gas passage 56 has smooth vertical sides, and a radial
diameter of
between about 1 and 3 inches. The passage 56 is open to the off gas stream at
its
lowermost end through a bottom opening 58 which is defined by the lowermost
extent
of the cylindrical inner sidewall 52. It is to be appreciated that because the
bottom
opening 58 is also defined by the sidewall 52, dust and other particulate
matter in the
gas passage 56 may fall from the probe 40, unobstructed by either the sidewall
52 or
the bottom opening 58.
Although not essential, the outer sidewall 54 is shown slightly shorter
than the inner sidewall 52, extending from the lowermost end 50 of the probe
40 to
a position approximately 6 inches below the top end 46. The outer sidewall 54
has
a radial diameter of between about 2 and 4 inches and is spaced radially
outwardly
from the inner sidewall 52, so as to define an annular chamber 68
therebetween. An
annular upper end wall 70 (Figure 6) extends radially outwardly from the inner
sidewall 52 to the outer sidewall 54 to close an uppermost end of the chamber
68.
The lower end of the: chamber 68 is similarly closed by an annular lower end
wall 72
(Figure 5) which extends radially outwardly from the lowermost end of inner
sidewall
52 to the outer sidevvall 54.
As seen in Figures 4 and 5, a pair of elongate dividing webs 76a,76b
extend from the inner sidewall 52 across the chamber 68 to join with the outer
sidewall 56. The uppermost edge of the dividing webs 76a,76b are provided in
sealing contact with the upper end wall 70. The webs 76a,76b extend downwardly
from the end wall 70 to a position spaced approximately 3 inches above the
lower end
wall 72 (Figure 5). l:n this manner, the webs 76a,76b divide the chamber 68
into two
communicating coolant fluid channels 68a,68b.
Figure; 6 shows best the probe 40 as further including coolant fluid
ports 80a,80b formetj through outer sidewall 54 and providing fluid
communication
therethrough with the coolant fluid channels 68a,68b, respectively. The ports
80a,80b
219684
- 15 -
are spaced toward t;he chamber upper end wall 70. Coolant fluid such as water
is
pumped from a coolant fluid supply 69 (Figure 2), via inflow hose 78a, through
the
port 80a and into fluid channel 68a. As the coolant fluid is pumped into the
chamber
68, the fluid flows first downwardly along fluid coolant channel 68a to the
bottom of
the webs 76a,76b, then into and upwardly along coolant channel 68b, and
outwardly
from the chamber 68 via the port 80b. After moving out of the chamber 68, the
coolant fluid moves via outflow hose 78b, to the supply 69, where it is
chilled prior
to recirculation back: through the coolant channels 68a,68b.
Figures 6 and 7 show best the uppermost end of the gas passage 56 as
being closed by a top wall 60. A circular upper opening 62 having a diameter
of
about 1 inch is formed through the top wall 60 centred on the axis A-A. An
internally threaded rim 63 extends upwardly from the top wall 60 about the
opening
62 for use in remov;ibly coupling the filter 64 in the probe 40.
The filter 64 includes an externally threaded steel end 65 which has a
complementary size ;and threading to the rim 63, and a filter tip 67 which is
cemented
to the end 65 by a thermally stable cement. Figures 6 and 7 show best the
filter tip
67 as comprising a ~;enerally cylindrical and elongated porous silicon carbide
filter.
The tip 64 has a 10 micron porosity, and a diameter D, of approximately 22
millimetres, so as to fit through the upper opening 62. The filter tip 67 has
a length
of about 6 inches and when the filter 64 is coupled to the rim 63, the tip 67
extends
downwardly from a threaded end 65 aligned with the axis A-A. An axially
aligned
bore 66 having a diameter of between about 0.5 and 1.5 cm, extends downwardly
through the end 65 and along the centre of the filter 64 to a position
approximately
1 cm from the lowermost end of the filter tip 67. In this manner, the filter
64 may
be positioned across the upper opening 62 by lowering the tip 67 through the
opening
62 and turning the filter 64 to couple the end 65 to the rim 63.
The filter 64 is replaceably coupled in position in the upper opening
62 by the complementary threaded engagement of the end 65 with the threaded
rim
219684
- 16 -
63. The threaded coupling of the filter 64 to the top wall 60, combined with
filter
diameter D, being less than that of the opening 62, advantageously permits the
simplified replacement of the filter 64 in the event it should fail or become
clogged
after prolonged use. Because the filter 64 is configured to pass through the
opening
62, the filter 64 may be removed or inserted through the top end 46 of the
probe 40,
without requiring thE: time consuming removal of the probe 40 from the exhaust
duct
20 or even the shut down of the furnace 12.
An inlet end of the gas conduit tube 44 is coupled over the bore 66
which extends throul;h the filter end 65. In this manner, off gas samples
drawn from
the gas passage 56, through the filter 64 move along the bore 66 through the
upper
opening 62, and into the conduit tube 44.
Figurfa 6 and 7 show the probe 40 as further including a vent duct 82
spaced towards the top end 46 of the probe 40, and a first source of
pressurized
backflow gas 84. The vent duct 82 extends through the inner sidewall 52
adjacent
the filter 64. The vent duct 82 is selectively provided in gaseous
communication with
the backflow gas source 84 by the operation of a valve 86. The valve 86 may be
operated to either prevent or permit gaseous communication between the gas
source
84 and vent duct 82. The backflow gas source 84 is at a gas pressure higher
than the
gas pressure in the gays passage 56, whereby gaseous communication between the
duct
82 and gas source 84 causes pressurized gas to bacl~low from the gas source 84
into
the passage 56, downwardly along the probe 40, and out through opening 58.
Figurea 2, 6 and 7 show best the system 10 as further including a
second pressurized bacld~low gas source 88 and a second conduit valve 89. The
second backflow gas source 88 is also at a pressure higher than the gas
pressure in
the gas passage 52. The conduit valve 89 is selectively operable to either
permit
sample gas flow along the conduit tube 44 between the upper opening 62 and the
gas
analyzer 42, or to provide gaseous communication between the upper opening 62
and
the second backflow gas source 88. By the conduit valve 89 connecting the gay
~~~~s~s
- i7 -
source 88 to the upper opening 62, pressurized gas backflows through the
opening 62
and filter 64 and downwardly along the gas passage 52.
In operation of the furnace system 10, the furnace 12 is charged with
steel (or other material) to be made, and the CPU 14 is used to initialize the
furnace
heat. Simultaneously with the furnace 12 start up, the off gas sampling and
analysis
assembly 16 is activated to begin monitoring of the furnace off gases.
Upon the activation of the system 10, the coolant fluid supply 69 is
activated to pump cooling water through the coolant channels 68a,68b of the
probe
40. To ensure sufficient cooling of high temperature off gases, it is
preferred that
cooling water is pumped through the fluid coolant channels 68a,68b at a rate
of
between approximately 5 to 100 gallons per minute, and more preferably,
between
about 40 to 80 gallons per minute.
To sample the furnace off gases, the valves 86, 89 are moved to the
positions shown in 1~igure 6, whereby valve 86 prevents gaseous communication
between the baclcflow gas source 84 and vent duct 82, and valve 89 is operated
to
permit gaseous comnnunication between the upper opening 62 and the gas
analyzer
42. With the conduit valve 89 connecting the conduit tube 44 to the gas
analyzer
42, the vacuum source 43 draws a gas sample from the off gas stream upwardly
into
the probe 40 and gas. passage 56 via the bottom opening 58. As the sample gas
is
drawn towards the analyzer 42, it moves vertically along the gas passage 56 in
the
direction of arrow SSA, and is cooled by the movement of the cooling water
through
the fluid coolant charnels 68a,68b. By the time the sample gas reaches the
filter 64
at the top end 46 of the probe 40, the sample gas has been cooled to both
arrest
ongoing chemical reactions within the sampled gas, thereby stabilizing the gas
sample
for analysis, and to rWuce the gas temperature so as to prevent thermal
degradation
of the silicon carbide filter tip 67. The vacuum pump 43 draws the gas sample
through the porous silicon carbide filter tip 67 and into the tube 44 via the
bore 66
and upper opening 62. As the sample gas passes through the filter tip 67 and
into the
X196846
bore 66, particulate matter, such as dust, which is entrained in the gas
sample, is
filtered from the sarnple and particulate-free gas moves through the tube 44
to the
analyzer 42 for analysis.
Off gas sampling is performed almost continuously during the furnace
heat. To prevent du:;t and other particulate matter from clogging the filter
64 and gas
passage 52, baclcflovr gas is periodically flushed through the filter 64 and
gas passage
52 to dislodge any accumulated particles. Because the backflow gas source 88
is
provided at pressures above the gas pressure in the passage 56, when the
conduit
valve 89 connects thc: conduit tube 44 to the gas source 88, backflow gas
flows from
the source 88 into the passage 56 via the upper opening 62 and filter 64. The
backflow of gas through the filter 64 acts to dislodge any particles or debris
which
have accumulated on the outer surface of the filter tip 67.
Similarly, by operating the valve 86 to provide gaseous communication
between the vent duct 82 with the baclcflow gas source 84, pressurized gas may
further backflow into the gas passage 56 via the duct 82 to dislodge any dust
or other
particulate matter wruch may accumulate along the inner sidewall 52. In
addition,
by positioning the vent duct 82 adjacent the filter 64, the operation of the
valve 86
to backflow gas through the vent duct 82 advantageously directs a stream of
backflow
gas directly at the filter 64 to further assist in dislodging accumulated
particulate
matter thereon.
To avoid contamination of the sample gas in the passage 56 and prevent
faulty gas analysis data, it is preferable that each of the backflow gas
sources 84, 88
comprise a source of pressurized non-reactive gases. Suitable backflow gases
would,
therefore, include gases such as nitrogen, as well as inert gases including
helium,
neon, xenon and argon, to name but a few.
The gays backflow through the vent duct 82 may occur simultaneously
or independently from gas backflow through the filter 64. Preferably, the
valves 86
219f 8~6
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and 89 are operated alternately in succession to sequentially provide
communication
with the respective backflow gas sources 84,88 to produce the flow of backflow
gas
downwardly along tlhe gas passage 54 in the direction of arrow 95 shown in
Figure
7. The provision of separate backflow gas sources 84,88 advantageously permits
gas
to backflow through the vent duct 82 and filter 64 under different pressures.
If
desired, however, a single backflow gas source may equally be used, either
with or
without gas regulator valves to provide different backflow pressures.
It is t:o be appreciated that because the inner sidewall 52 is formed
having a smooth cylindrical wall construction, any dislodged particulate
matter falls,
both together with 'the backflow gas and under gravity, downwardly through the
bottom opening 58 of the probe 40. The bottom opening 58 has the same radial
diameter as inner sidewall 52 so that there is no obstruction to the downward
movement of particles from the gas passage 56. Because the bottom opening 58
is
configured to permit substantially unhindered movement of dust and other
particulate
matter from the passage 56 during backflow cleaning, particles and dust will
not tend
to accumulate about the opening 58, where they may otherwise clog the probe 40
and
hinder gas flow.
Figure 3 shows the passage 56 having parallel sidewalls. It is to be
appreciated that the passage sidewalls could also flare outwardly towards the
bottom
opening 58 and with the diameter of the bottom opening 58 being at least as
large as
the largest part of the passage 56 to permit the unobstructed vertically
downward
movement of particles therefrom. As a result of the vertical orientation and
smooth
sidewalls of the gas passage 56, backflow cleaning of the probe 40 will be
assisted
by the dislodged particulate matter falling from the passage 56 under gravity.
The
gas passage 52 constJ-uction thus permits the probe 40 to have a comparatively
longer
length, permitting the collection of gas samples from a mid-portion of the off
gas
stream.
During the furnace heat, the probe 40 is operated to substantially
21968~~
- 20 -
continuously obtain ,gas samples from the off gas stream. Gas sampling is
interrupted
only periodically, as for example after a five to sixty minute interval of gas
sampling
or timed to coincide with inherent process pauses, to backflow gas through the
filter
64 and vent duct 82 and purge the probe 40 of any accumulated furnace dust or
other
particulate matter. Preferably, gas sampling is interrupted to coincide with
process
pauses in the furnace heat which occur on charging, tapping and the like. In
this
manner, the gas analyzer 42 continuously analyses the off gas samples during
the
furnace heat, and provides ongoing data to the CPU 14 indicative of the off
gas
constituents.
The central processing unit 14 controls the furnace environment to
improve quality of the recovered steel and reduce explosive gas content. In
this
regard, during the furnace heat the CPU 14 continuously regulates furnace
operating
conditions, such as electric: current in the furnace, the oxygen and/or carbon
monoxide concentrations, carbon content and the furnace temperature, as for
example,
by the activation and deactivation of supplemental gas burners and/or electric
current
flow.
The CPU includes storage memory for storing control data representing
off gas profiles of one or more previous furnace heats, and/or data
representative of
a preferred efficiency furnace heat. The CPU 14 also includes software for
compiling
an ongoing profile of various off gas components as the furnace heat
progresses, and
for comparing the data supplied from the gas analyzer 42 with the stored
control data.
The gas analyzer 42 i.s electronically linked to the central processing unit
14, whereby
sample gas analysis data from the gas analyzer 42 is electronically sent to
the CPU
14. Over the course of the furnace heat, the gas analyzer 42 continuously
monitors
and profiles sample off gas content with respect to one or more of
temperature,
carbon monoxide <;oncentration, hydrogen gas concentration, carbon dioxide
concentration and oxygen concentration. The data from the gas analyzer 42
which
is electronically transmitted to the CPU 14 is continuously compared against
control
data. Where variances occur between the data received from the gas analyzer 42
and
2196846
- 21 -
control data stored in, the CPU 14, the CPU 14 provides control signals to the
furnace
12, adjusting the furnace operating conditions so as to maximize combustion
efficiency.
While Figures 3, 6, and 7 describe the use of a silicon carbide filter
64 as being threade,ily coupled to the rim 63, other filter configurations are
also
possible and will become apparent. Figures 8 and 9 show an alternate filter 94
for
use in the probe 40 i.n accordance with a further embodiment of the invention.
Like tlhe silicon carbide filter 64, the filter 94 includes a threaded steel
end 96 which has a complementary size and threading to the rim 63 (Figure 6),
and
an elongated generally cylindrical filter body 98. The filter body 98 has a
maximum
diameter D1 selected so as to permit its insertion downwardly through the
probe
opening 62 in the same manner as the silicon carbide filter 64. Figure 9 shows
best
the filter body 98 as comprising three parts: a perforated stainless steel
cylindrical
support 100; a high temperature fibre sleeve 104; and a stainless steel split-
ring 108.
The cylindrical support 100 is welded to the threaded steel end 96.
The support 100 has .a hollow interior 110 in gaseous communication with a
bore 102
formed axially throu~;h the threaded end 96, and which in turn connects to gas
conduit
tube 44. A plurality of apertures 112 are formed through the support to permit
substantially unhindered gas flow into and from the support interior 110.
The high temperature fibre sleeve 104 has an open top end l lla and
closed bottom end l:llb and is configured to snugly fit over the support 100.
The
fibre sleeve 104 is made of high temperature fibre based material such as
Nomex"',
or other ceramic based fibres. The sleeve 104 has a pore size of between about
0.1
to 25 microns, and preferably about 1 micron during gas sampling operations.
The
applicant has, however, discovered that on backflow purging of filter 94, the
sleeve
104 advantageously inflates partially like a balloon. The inflation of the
sleeve 104
causes the fibre material to stretch with the result that the pore size of the
sleeve 104
2196846
- 22 -
increases by up to at~out 25 %.. The increased pore size of the sleeve 104 on
purging
advantageously facilitates the dislodging of any particles which collect on
the filter
body 98 during gas sampling.
The stainless steel split-ring 108 is formed so that the ring ends are
resiliently deformable from an undeformed position to a deformed position.
When
undeformed, the ring 108 has a diameter slightly less than diameter D, . In
the
deformed position, the ends of the split-ring 108 are spaced apart whereby the
split-
ring 108 may be slid over the sleeve 104 and support 100 to the position
adjacent the
end 96 shown in Figure 8. I3y releasing the ends of the split-ring 108, the
ring 108
resiliently returns to its undeformed configuration, firmly clamping the fibre
sleeve
104 against the suppwrt 100.
It is to be appreciated that while Figures 8 and 9 describe the use of
a split-ring 108 to clamp the sleeve 104 in place on the support 100, other
clamping
structures mechanisms are also possible including, for example, hose clamps,
solid
metal rings, and wire wrap. Figure 10 shows one modified version of the filter
94
shown in Figures 8 amd 9, wherein like reference numerals are used to identify
like
components.
The fillter 94 oP Figure 10 includes a stainless steel end 96 and a filter
body 98. The filter body 98 has an overall length of about 16 inches and
comprises
a perforated hollow cylindrical support 100, which is closed at its bottom end
by a
seal 120, and a high temperature fibre sleeve 104. As shown in Figure 10, the
sleeve
104 has a diameter larger than that of the support 100, and fits loosely
thereover
during gas sampling.
The slf~ve 104 is open at both its upper and lower ends 11 la, l l lb for
simplified manufacture. The ends 1 l la, l l lb are clamped in a sealed
arrangement
to the support 100 by upper and lower wire wraps 116,118.
219ss4s
- 23 -
While the use of a sampling probe 40, having a cylindrical inner
sidewall 52 is shown in the preferred embodiments of the invention, the
invention is
not so limited. It is to be appreciated that probe 40 having polygonal or
irregular
shaped or stepped inner sidewalk, and which define a passage open at its
bottom end
to permit substantially unhindered fall of particulate matter therefrom, are
also within
the scope of the present invention.
While Figures 3 to 7 show a probe having two fluid coolant channels
68a,68b, it is to be appreciated that a probe 40 having three, four or more
separate
coolant channels is also possible. Similarly, while the use of water is
disclosed as a
suitable coolant fluid, other coolant fluids, including glycol mixtures and
the like
could equally be use~3.
The preferred embodiments describe the invention with reference to an
electric arc furnace 12, however, the present invention is equally suitable
for use in
sampling off gases from all high temperature applications where particle
filter
degradation as a result of contact with high temperature gases remains a
problem.
Although the disclosure describes and illustrates preferred embodiments
of the invention, the invention is not so limited. Many modifications and
variations
will now occur to persons skilled in the art. For a definition of the
invention,
reference may be had to the appended claims.
