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BACKGROUND
1. Field of the Invention
This invention relates to heat recuperators for high temperature combustion
furnaces.
2. The Prior Art
High temperature combustion furnace operations particularly those operating
above about 1000.degree. C consume enormous quantities of energy. Such
high temperature furnace operations include, for example, copper smelters,
glass furnaces, steel furnaces, and the like. Customarily, the combustion
furnace is constructed as an enclosed, heat-resistant vessel containing a
pool of material being heated. A combustion flame is directed into the
enclosure over the top of the material being heated. Customarily, only
about 15-25 percent of the thermal energy of the flame is absorbed by the
material, the remaining 75-85 percent of the thermal energy is lost. About
30 percent of the thermal energy lost is lost through the furnace walls,
roof, floor and exhaust duct work and is carried out with the exhaust.
This thermal energy loss represents a significant quantity of energy that
must be supplied by consumption of additional fuel unless some form of
energy recuperation is practiced.
The portion of thermal energy discharged to the atmosphere as exhaust
represents a significant quantity of potentially recoverable thermal
energy. However, as a result of the very high temperatures and,
occasionally, the corrosive environment encountered in the exhaust gases,
very few structural materials can successfully withstand prolonged
exposure to the hot exhaust gases.
One conventional heat recovery technique involves directing the hot exhaust
gases through a chamber containing a grid work of refractory bricks known
as a checker system. The checker system is formed as two, separate systems
so that hot exhaust gases can be diverted through the bricks in one system
until they are heated to an optimum temperature. The exhaust stream is
then switched into the second system while incoming air is drawn through
the first system and heated by absorbing thermal energy from the heated
bricks in the first system. The exhaust and airstreams are alternately
switched between the two checker systems at set intervals of about 20-30
minutes. The result is that the incoming airstream to the combustion
furnace does not have a constant temperature but a cyclically varying
temperature. This results in loss of furnace efficiency and difficulty in
accurately controlling the thermal energy input to the furnace and also
results in loss of efficiency because of the difficulty in controlling the
air/fuel ratios.
Additionally, checker systems occupy a large space and involve relatively
elaborate duct work and valving systems thereby requiring relatively high
initial construction cost and ongoing maintenance costs.
Attempts to avoid the problem associated with the checker system of heat
recuperation has led to the use of lower temperature heat recuperation
systems. For example, a typical glass furnace operates at a relatively
high temperature (approximately 1000.degree.-1650.degree. C) which means
that the exhaust gases therefrom would be far in excess of the maximum
temperature capabilities of most metals. Accordingly, it is conventional
to dilute the hot exhaust with outside air and, thereby, lower the exhaust
temperature so that a standard metal recuperator can be used. However,
dilution causes a tremendous loss in the enthalpy of the exhaust stream
and, consequently, a tremendous loss in the recuperator efficiency.
Furthermore, many exhaust systems carry fumes that are extremely corrosive
to most metals. It has also been found that fumes carried over with the
exhaust stream tend to condense on the cooler recuperator surfaces. At
high temperatures, the condensate tends to be a corrosive fluid while at
lower temperatures the fumes crystallize as a dust having a fuzzy,
crystalline characteristic which tends to form an insulative layer in the
exhaust duct work. This layer must be removed periodically so as to
enhance heat transfer and lower the resistance to flow of the hot exhaust
gases.
Cleaning of large heat recuperators is difficult, time consuming and,
therefore, expensive unless the recuperator may be readily disassembled
and reassembled from elements which are easily handled and cleaned.
An energy balance between the hot exhaust gases and the incoming airstream
shows that for maximum efficiency a greater quantity of air can be heated
than is used for supporting combustion. Accordingly, it would be
advantageous to divert a portion of the heated air as a heat source for a
lower temperature process such as an annealing furnace or the like.
However, the cyclically varying temperatures resulting from an airstream
passed through a conventional checker system would render the heated
airstream unfit for use in any annealing furnace requiring a reasonably
controlled temperature.
It is also desirable to divert a portion of the heated air as an auxiliary
heated airstream for use in structure heating as comfort conditioning.
However, when used for comfort conditioning, great care must be exercised
to insure that combustion products are specifically precluded from
entering the comfort conditioning system. Accordingly, it is usually
conventional to make no attempt to use any of the recovered heat for
comfort conditioning.
It would, therefore, be a significant advancement in the art to provide a
high temperature heat recuperator apparatus and method whereby the heat
recuperator is readily fabricated from a plurality of standardized,
interchangeable, modular elements. It would also be an advancement in the
art to provide a recuperator wherein the modular elements in contact with
the most destructive portion of the exhaust gases are fabricated from a
highly refractory ceramic material while the remaining portion of the
recuperator modules may be, selectively, inexpensively fabricated from
conventional metallic materials. Another advancement in the art would be
to provide a heat recuperator wherein the incoming airstream is
pressurized so as to inhibit the infiltration of exhaust gases into the
airstream and thereby accommodate diverting at least a portion of the
airstream for use in auxiliary heat systems and, more particularly, for
use in comfort conditioning. Such an invention is disclosed herein.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
The present invention is a countercurrent-flow heat recuperator apparatus
which is, preferentially, constructed from a plurality of interchangeable,
modular elements. Modular element construction readily accommodates
handling, replacement of broken elements, disassembly and cleaning. A heat
recuperator assembled from modular elements also readily adapts to
dimensional variations resulting from temperature fluctuations within the
recuperator. The modules in the high temperature portion of the
recuperator may also be preferentially, fabricated from a highly
refractory ceramic material whereas the modules in the low temperature
portion may be more inexpensively fabricated from conventional metallic
materials. Preferably, the materials from which the modules are made will
change from highly refractory (high temperature) ceramic, to lower
refractory (lower temperature) ceramic, progressively as the modules are
more removed from the heat source. Modular construction, whether ceramic
or metallic, readily accommodates the interchangeability of the modular
elements and the assembly of various heat recuperator designs according to
the requirements of the environment to be encountered. Thus, if a module
becomes damaged or obstructed, it can be replaced without replacement of
the entire assembly.
It is, therefore, a primary object of this invention to provide
improvements in heat recuperators for high temperature combustion
furnaces.
Another object of this invention is to provide an improved method for
recovering thermal energy from the exhaust stream of a high temperature
combustion furnace.
Another object of this invention is to provide standardized modular
recuperator elements which may be interconnected to provide a heat
recuperator having the desired capabilities.
Another object of this invention is to provide means for interlocking the
modular elements so as to assure alignment of the flow channels
therethrough.
These and other objects and features of the present invention will become
more fully apparent from the following description and appended claims
taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic side elevation of a first preferred embodiment of the
heat recuperator of this invention shown in the environment of a
combustion furnace with portions broken away to reveal internal features;
FIG. 2 is an enlarged elevational view of a fragment of the heat
recuperator of FIG. 1 with portions broken away to reveal internal
features;
FIG. 3 is a side elevation of a second preferred embodiment of the heat
recuperator of this invention in a horizontal orientation with portions
broken away to reveal internal features;
FIG. 4 is a perspective view of one presently preferred embodiment for the
modular heat recuperator element of this invention with portions broken
away to reveal internal features;
FIG. 5 is a perspective view of a gasket for placement between the modular
elements of FIG. 4;
FIG. 6 is a perspective view of a second preferred embodiment of a modular
element for the heat recuperator of this invention;
FIG. 7 is a perspective view of a gasket for placement between the modular
elements illustrated in FIG. 6; and
FIG. 8 is a perspective view of a third preferred embodiment of the heat
recuperator of this invention with portions broken away to reveal internal
features.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is best understood by reference to the drawing wherein like
parts are designated with like numerals throughout.
Referring now more particularly to FIG. 1, a first preferred embodiment of
the heat recuperator of this invention is shown generally at 10 as a
countercurrent-flow heat exchanger and supported in a vertical orientation
above an exhaust outlet 27 of a high temperature combustion furnace 12.
High temperature combustion furnace 12 may be any suitable furnace
including, for example, a glass furnace having a pool of molten glass 26
in the base thereof and over which a flame 24 is directed. A resulting
high temperature exhaust stream 28 passes upwardly into heat recuperator
10 and is discharged therefrom as a cooled exhaust stream 30. Heat
recuperator 10 is separated longitudinally into at least two flow channels
60 and 62 (FIGS. 2 and 4) in this first preferred embodiment by a spirally
formed septum 46 shown in broken lines. Septum 46 is also more clearly
shown in FIGS. 2 and 4. A first flow channel 60 (FIGS. 2 and 4) serves as
a first path for the hot exhaust gases while the second flow channel 62
(FIGS. 2 and 4) serves as a second path for an incoming airstream 15 with
heat exchange occurring across septum 46. The spiral configuration
provided by the twist in septum 46 is believed to impart sufficient
turbulence to hot exhaust stream 28 for improved heat transfer across
septum 46. The spiral configuration also increases the heat transfer area
over the length of the module. Clearly, other countercurrent flow systems
(not shown) could readily be adapted to the heat recuperator 10 of this
invention including, for example, concentric ducts, straight, or
semi-cylindrical channels.
Airstream 15 is introduced into heat recuperator 10 by a blower 14 which
provides the necessary volume and pressure to airstream 15 to
substantially preclude infiltration of combustion products from hot
exhaust stream 28 into airstream 15. If desired, airstream 15 may be
supplemented with oxygen or other combustion-supporting gas. Accordingly,
a portion of the heated airstream 15 may be readily diverted through an
auxiliary air outlet 18 to be suitably used for a number of lower
temperature operations including, for example, an annealing furnace 32,
structure heating 34 and/or other industrial processes 36.
The remaining heated air of airstream 15 is directed through a duct 20 as a
stream of high-temperature combustion air. This high-temperature
combustion air is mixed with fuel introduced through a fuel inlet 22 and
ignited to provide flame 24. Preheating of the combustion air to flame 24
provides substantial economies for the operation of furnace 12 since a
portion of the thermal energy derived from flame 24 is not wasted in
preheating the incoming airstream 15.
Referring now more particularly to FIG. 2, a section of heat recuperator 10
is shown with portions broken away to reveal internal structure. In
particular, heat recuperator 10 is fabricated from a plurality of
corresponding recuperator modules 40 and 41 each of which provides at
least two separated flow channels 60 and 62 in alignment therethrough and
in heat exchange relationship across dividing septum 46. The structure of
recuperator module 40 will be discussed more fully hereinafter with
respect to FIG. 4.
Recuperator modules 40 and 41 are placed in axial alignment by means of a
tongue and groove relationship between corresponding male and female ends.
in particular, a female end 58 of recuperator module 40 includes an
annular groove 50 which mates with the male end 56 of the corresponding
recuperator module 41 having an annular ridge 48 formed thereon.
Recuperator module 40 also has a similar male end 56 (FIG. 4) but broken
away in FIG. 2 for simplicity in illustration. A similar ridge 52 and
mating groove 54 system (FIG. 4) are also contained in each end of septum
46 to assure alignment of the septa 46 when a plurality of recuperator
modules 40 and 41 are assembled in axial alignment.
A gasket 42 (shown more fully in FIG. 5) is inserted between recuperator
modules 40 and 41. Gasket 42 (FIG. 5) is formed as an annular ring 43
having a dividing strip 45, both of which dimensionally correspond to the
end of shell 44 and septum 46, respectively, to form a seal between
corresponding recuperator modules 40 and 41. Cut out segments 61 and 63
(FIG. 5) provide continuity to flow channels 60 and 62, respectively.
Gasket 42 is desirably fabricated from a ceramic wool felt material
capable of withstanding the high temperatures encountered in the operation
of recuperator 10. The ceramic wool felt material of gasket 42 provides a
seal between recuperator modules 40 and 41 and also imparts cushioning
effect to compensate for stresses which may otherwise fracture the
recuperator modules 40 and 41 when temperature fluctuations cause
dimensional variations in heat recuperator 10.
The assembled recuperator 10 is surrounded by a jacket of insulation 64 of
suitable high temperature insulative material such as KAOWOOL the
trademark for a mineral-wool insulation manufactured by BABCOCK-WILCOX,
Augusta, Georgia. Insulation 64 is enveloped in a suitable protective
covering 66 with the insulation 64 and covering 66 being secured around
recuperator 10 by a plurality of encircling bands similar to band 68.
Referring now more particularly to FIG. 4, heat recuperator module 40 is
more clearly illustrated particularly with respect to the flow channels 60
and 62 which are separated by the longitudinal septum 46. In this
presently preferred embodiment of recuperator module 40, septum 46 twists
along the axis of recuperator module 40 through 90.degree. from the male
end 56 to the female end 58. However, septum 46 may be twisted through any
number of degrees between the male end 56 and the female end 58. In this
event, the prime consideration is that the degree of twist of septum 46 be
standardized between recuperator modules 40 and 41 (FIG. 2) so that the
respective male and female ends of septum 46 mate with corresponding
septum 46 to provide continuity through flow channels 60 and 62. Clearly,
septum 46 could be planar along the axis of recuperator module 40 thereby
dividing recuperator module into two semi-cylindrical flow channels.
Referring now to FIG. 3, a second preferred embodiment for a heat
recuperator apparatus of this invention is shown generally at 100 and
includes a heat recuperator 102 superimposed over an exhaust outlet 127 of
a high temperature combustion furnace 126. In this particular embodiment
for recuperator 102, a plurality of modular recuperator elements such as
recuperator element 40 (FIG. 4) or recuperator element 70 (FIG. 6) are
placed in axial alignment with respective flow channels aligned and along
a generally horizontal axis.
Elbow modules 104 and 106 at each end of heat recuperator 102 are adapted
to receive an exhaust stream 128 and to discharge the cooled exhaust 130,
respectively. The assembled modules of heat recuperator 102 are held
together in one presently preferred embodiment of the invention by a
transverse bracket 108 having hooks at each end which nest within notches
110 and 112 of the modules. Preferentially, bracket 108 is fabricated from
a ceramic material having a coefficient of thermal expansion comparable to
the material of construction of the recuperator modules in heat
recuperator 102 so as to provide a corresponding expansion and contraction
during thermal changes in the heat recuperator 102.
The assembled heat recuperator 102 is wrapped in an insulative blanket 114
over which a protective layer 116 is placed with the entire insulative
assembly being held in position by a plurality of circumferential bands
118-120.
Heat recuperator 102 is supported in its horizontal position by a plurality
of conventional means such as saddles 123 and 125 mounted on the ends of
stanchions 122 and 124, respectively. Clearly, any other suitable,
conventional means could be used to support the horizontal portion of heat
recuperator 102 and may include a horizontal track or the like.
An airstream 124 is blown through an inlet 136 by means of a blower 132 and
passed in countercurrent heat exchange relationship with the hot exhaust
128 so as to provide a heated airstream 138 from an outlet 140. The heated
air 138 may, thereafter, be used for supporting combustion in combustion
furnace 126 and/or auxiliary heating purposes as set forth hereinabove
with respect to the first preferred embodiment shown in FIG. 1.
Referring now more particularly to FIG. 8, a third preferred embodiment for
the heat recuperator development of this invention is shown generally at
200. Heat recuperator 200 is assembled from a plurality of heat
recuperator assemblies 162-165, each of which are substantially similar to
heat recuperator 10 (FIGS. 1 and 2) and heat recuperator 100 (FIG. 3).
Each of heat recuperator assemblies 162-165 are assembled from a plurality
of heat recuperator modules and shown herein as modules 166-169 of heat
recuperator assembly 165. Clearly, of course, each of heat recuperator
assemblies 162-165 could be fabricated with a single modular element such
as recuperator module 40 (FIG. 4) or recuperator module 70 (FIG. 6),
depending upon the particular requirements of recuperator 200.
Heat recuperator 200 in this third preferred embodiment is particularly
adapted to be assembled on top of the enclosure forming a high-temperature
combustion furnace 150. Combustion furnace 150 is schematically
illustrated as a conventional glass furnace for a glass-blowing operation
and includes an opening 152 which is substantially occluded by a door 154.
Door 154 is suspended by rods 156 and 157 from an overhead trolley (not
shown) which allows lateral movement of door 154 away from its occluding
position in front of opening 152.
The upper portion of opening 152 remains unobstructed by door 154 and,
therefore, serves as a vent for hot exhaust gases 174. A hood 158 collects
the hot exhaust gases 174 emerging from opening 152 and distributes the
same to the appropriate flow channels of heat recuperator assemblies
162-165. In particular, hot exhaust gases 174 are deflected by hood 158
into exhaust flow channels of recuperator assemblies 162-165 where they
pass in heat-exchange relationship with the incoming airstream 172. The
spent exhaust gases 175 are gathered in a plenum 188 and are, thereafter,
discharged through conventional exhaust duct work 189 as a discharge
stream 182.
Incoming air is directed countercurrently through recuperator assemblies
162-165 as a forced airstream 171 by a blower 184. Blower 184 forces
airstream 171 into a header 170 to distribute airstream 171 to the
recuperator assemblies 162-165. Blower 184, advantageously, provides
sufficient over-pressure in the airstream 171 to substantially inhibit
leakage of exhaust 174 into the heated airstream 172. A septum 186
separates incoming airstream 171 from exhaust gases 174 and provides a
partition through which heat exchange occurs. Assembly of heat recuperator
200 on the top of furnace 150 accommodates absorption of thermal energy
emitted by furnace 150 thereby increasing the amount of thermal energy
absorbed by airstream 172.
The heated airstream 172 exiting from recuperator assemblies 162-165 is
gathered by a plenum chamber 160 and directed to an inlet 178 into furnace
150. Fuel 177 from a fuel inlet 176 is mixed with airstream 172 and
ignited to form a combustion flame 180.
Importantly, each of recuperator assemblies 162 and 165 are assembled from
a plurality of recuperator modules 166-169 and thereby readily accommodate
(1) assembly of a plurality of various size recuperator assemblies 162-165
and, therefore, heat recuperators 200; (2) replacement of individual
recuperator modules 166-169 are required due to wear, breakage, and the
like; and (3) disassembly for cleaning and reassembly for additional use.
Furthermore, standardization between recuperator modules 166-169 readily
accommodates interchangeability as between recuperator modules 166-169
particularly by reason of the standardization as between length, diameter,
and the location and degree of twist of septum 186 in each of recuperator
modules 166-169 as set forth hereinbefore with respect to the recuperator
modules described in FIGS. 4 and 6.
The parallel configuration of heat recuperator 200 readily adapts itself to
handling a large volume of exhaust gases 174 by reason of the parallel
flow channels provided by heat recuperator assemblies 162-165.
Correspondingly, a large volume for airstream 172 may also be readily
accommodated, where desired, for various heating purposes. This same
parallel configuration may also be adapted in heat recuperator 10 (FIG. 1)
and heat recuperator 100 (FIG. 3).
Heat recuperator 200 may be, selectively, covered with an insulative
blanket (not shown) such as insulative blanket 64 (FIG. 2) or blanket 144
(FIG. 3). The partially cooled exhaust gases 182 may be discharged to the
atmosphere or, preferably, used as a further heat source by being directed
through a second recuperator such as recuperator 10 (FIG. 10) or
recuperator 100 (FIG. 3). Additionally, heated air 172 may be used
directly in the combustion process as set forth hereinbefore or a portion
thereof may be diverted for auxiliary heating purposes as set forth
hereinbefore with respect to FIG. 1. As a further feature, heat
recuperator 200 may also be used to provide initial warm-up heat for an
adjacent combustion furnace (not shown) particularly since the volume of
heated airstream 172 could desirably be substantially in excess of that
required to support combustion of fuel 177.
Referring now more particularly to FIG. 6, a second preferred recuperator
module 70 is shown and is segregated into three flow channels, flow
channels 94, 96 and 98. Flow channels 94 and 96 are separated by a first
septum, septum 80, while flow channels 96 and 98 are separated by a second
septum, septum 82. Septa 80 and 82, preferentially, form parallel, spiral
flow channels along the axis of recuperator module 70 thereby separating
the interior of shell 84 into three spiral, parallel flow channels 94, 96
and 98. Clearly, flow channels 94, 96 and 98 could also be formed as
straight channels although the improved heat transfer obtained by spiral
channels is preferred.
Flow channels 94, 96 and 98 may be used in any suitable combination for the
various gaseous streams including, for example, hot exhaust gas stream 28
(FIG. 1), hot exhaust gas stream 128 (FIG. 3), hot exhaust gas stream 174
(FIG. 8), incoming airstream 15 and auxiliary heated airstream 19 (FIG.
1), incoming airstream 124 and auxiliary heated airstream 138 (FIG. 3),
and incoming airstream 171 (FIG. 8). Desirably, flow channel 96 could be
used for the particular hot exhaust gas stream while each of flow channels
94 and 98 could be used for an incoming airstream and auxiliary heated
air, respectively. Having the heat-receiving airstreams on the outside
periphery of recuperator module 70 would also increase the opportunity for
absorption of thermal energy from surrounding ambient particularly when
configurated on top of a furnace similar to heat recuperator 200 on
furnace 150 (FIG. 8).
The upper end of recuperator module 70 is formed as a male connector 86
having an annular ridge 88 adapted to mate with a corresponding groove
(not shown) at the opposite end of a corresponding recuperator module 70.
Each of septa 80 and 82 include ridges 90 and 92, respectively, which are
also adapted to be received in corresponding grooves (not shown) in the
opposite ends of the respective septa of a corresponding recuperator
module 70.
Referring now more particularly to FIG. 7, a gasket 72 is illustrated and
is dimensionally formed to be placed between abutting recuperator module
70 (FIG. 6). In particular, gasket 72 includes an annular ring 74 and
transverse bands 76 and 78. Ring 74 is dimensionally configurated to be
superimposed over the male end 86 of recuperator module 70 (FIG. 6) while
bands 76 and 78 are superimposed over the ends of septa 80 and 82 (FIG.
6), respectively. Accordingly, openings 95, 97 and 99 in gasket 72
correspond with flow channels 94, 96 and 98 (FIG. 6), respectively.
The heat recuperator embodiments of this invention, advantageously,
accommodate diverting at least a portion of the heated airstream for use
as an auxiliary heated air source. This is particularly useful in
annealing furnaces where objects are held under a controlled temperature
until suitably annealed to relieve intenal stresses developed during
manufacture. The auxiliary heated airstream has the advantages of being
relatively controllable with respect to temperature and substantially free
of contaminants carried over as fume with the exhaust stream.
Additionally, where feasible, hot exhaust gas streams from more than one
high temperature combustion furnace may be utilized for the purpose of
suitably heating and annealing furnace through the use of the heat
recuperators of this invention.
Importantly, the heat recuperator elements of this invention in proximity
to the high temperature combustion furnace are fabricated from a
high-refractory ceramic material of construction having a capability of
withstanding high temperatures and destructive environment of the furnace.
Such materials of construction include, for example, alumina (Al.sub.2
O.sub.3), silicon carbide (SiC), silicon nitride (Si.sub.3 N.sub.4), and
the like. Each of these materials of construction have the capability of
being formed by casting or extrusion and, more importantly, are very
resistant to extremely high temperatures and corrosive environments that
are frequently encountered in high temperature combustion furnace exhaust
streams.
The invention may be embodied in other specific forms without departing
from its spirit or essential characteristics. The described embodiments
are to be considered in all respects only as illustrative and not
restrictive and the scope of the invention is, therefore, indicated by the
appended claims rather than by the foregoing description. All changes
which come within the meaning and range of equivalency of the claims are
to be embraced within their scope.
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