Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 336258
GAS DISTRIBUTING & INFRARED
RADIATING BLOCK ASSEMBLY
This invention relates to gas-fired infrared burners
and in particular to how the gas is distributed to the
combustion zone and allowed to burn so as to efficiently
emit radiation energy.
In prior art burners, the gas is distributed to the
combustion zone through specially designed orifices or
parts which are formed within a unitary block or plate of
ceramic material. However, it is important to note that
not only does this single block/plate of material serve
to transport and distribute the gas to the burning zone,
but also that the top layer of that same material serves
as the combustion zone, which, on being heated to
incandescence, also serves to produce the infrared
radiation or radiant heat flux. Thus, it is clear that
the unitary material of prior art burner blocks serves at
least four functions: namely transportation,
distribution, combustion and radiation.
In other prior art burners involving multi-layered
porous ceramic material, the coarse, granular nature of
the material that may be used does not give the required
precision in pore size/uniformity, or the wrong materials
are specified for maximum heat transfer and reticular
integrity at very high temperatures (and low temperature
water shock), or the combustion takes place in a layer
where ~ m use cannot be made of the three modes of
heat transfer, namely conduction, convection and
radiation during the combustion and thus be unable to
enhance the final radiation.
Since these functions require different material
requirements in order to operate efficiently, it is an
object of this invention to provide different materials
and/or materials having different properties for these
functions and thus, to provide a composite rather than a
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unitary material for these functions. In a special case,
which involves the function of reverberation/enhancement,
which in prior art burners is effected by a separate
layer of material lying above the main unitary block of
material, it is also an object of this invention to
combine this separate special layer into the composite
block assembly of this invention. In a preferred
embodiment of the present invention, all three modes of
heat transfer, i.e. conduction, convection and radiation,
are able to function to their maximum in this special
layer, which takes the form of a reverberation porous
cellular/reticular ceramic layer. It is understood that
this special layer is not granular in nature, but is a
network of connected open spaces which are separated by a
wall/film-like structure of relatively large pore size
and high apparent porosity. (Such a non-granular
structure is preferably used in the other layers). Where
possible, depending on the application, essentially all
of the combustion will take place in this special
reverberatory layer, so that the composite assembly will
in such a case consist of only two layers/blocks, each
having specifications within a specific range, i.e.
thickness, pore size and apparent porosity and/or channel
size.
By providing the proper material for these
functions, it is a further object of this invention to
maximize the performance of these functions so as to
increase the radiation efficiency of infrared burners and
to make them safer to use.
Thus, in a first aspect, the invention a method for
producing infrared radiation, which comprises the steps
of (a) forcing a pressurized mixture of combustion gas
and air through a multitude of distinct channels in a
first block of material, each channel is perpendicular to
the radiation surface and consists of two sections, the
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first section having a cross-sectional area smaller than
that of the second section such that the velocity of the
mixture through said first section is greater than the
velocity of the flame propagation in the mixture, the
cross-sectional area of the second section being a
varying one commencing with that of the first then
expanding in bowl-shaped fashion until the section at
least substantially makes contact with a second block of
material, consisting of a multitude of spaces connected
together, into which the mixture is forced to flow, and
which combined with said first block forms a burner block
assembly; (b) allowing said mixture to expand and form a
turbulent mixture in said second section of said first
block and in said spaces of said second block; (c)
allowing said turbulent mixture to ignite and burn in
said second block of material thereby heating the top
surface of said second block to a very high incandescence
temperature causing it to produce very efficient infrared
radiation.
The invention also provides a method for producing
infrared radiation, which comprises steps of (a) forcing
a pressurized mixture of combustion gas and air through a
multitude of first small distinct channels in a first
block of material, each channel is perpendicular to the
radiation surface and has a cross-sectional area such
that the velocity of the mixture through said channel is
greater than the velocity of the flame propagation in the
mixture and being extended until it meets with a second
small channel in a second block of material containing a
multitude of said second channels which are in direct
alignment with said first small channels the cross-
sectional area of the second channels being a varying one
commencing with that of the first then expanding in bowl-
shaped fashion until the second channel makes contact
with the top surface of the second block of material,
which combined with said first block forms a burner block
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assembly; (b) allowing said mixture to e~p~n~ and form a
turbulent mixture in the bowl-shaped section of said
second block; (c) allowing said turbulent mixture to
ignite and burn in said second block of material thereby
heating the top surface of said second block to a very
high incandescence temperature causing it to produce very
efficient infrared radiation.
The invention also provides a method for producing
infrared radiation, which comprises the steps of (a)
forcing a pressurized mixture of combustion gas and air
through a multitude of small first spaces connected
together in a first block of material at a velocity which
is greater than the velocity of the flame propagation in
the mixture, into a second block of material, which,
while cont~ining a multitude of spaces connected together
which are larger than those of the first spaces, is
combined with said first block to form a composite burner
block assembly; (b) allowing said mixture to expand and
form a turbulent mixture in said second block; (c)
allowing said turbulent mixture to ignite and burn,
thereby heating the top surface of said second block to a
very high incandescence temperature and causing it to
produce very efficient infrared radiation, wherein at
least the top surface of said material is coated with at
least one material selected from the group consisting of:
cobalt oxide, nickel oxide, chromium oxide, thorium
oxide, silicon carbide and a metal silicate.
The invention also provides a method for producing
infrared radiation, which comprises the steps of (a)
forcing a pressurized mixture of combustion gas and air
through a multitude of small first spaces connected
together in a first block of material at a velocity which
is greater than the velocity of the flame propagation in
the mixture, into a second block of material, which,
while cont~;n;ng a multitude of spaces connected together
_ 5 _ l 3 3 6 2 5 8
which are larger than those of the first spaces, is
combined with said first block to form a composite burner
block assembly; (b) allowing said mixture to expand and
form a turbulent mixture in said second block; (c)
allowing said turbulent mixture to ignite and burn,
thereby heating the top surface of said second block to a
very high ;nc~n~escence temperature and causing it to
produce very efficient infrared radiation, wherein at
least the outer surface of the second block is coated
with a very thin layer of at least one material selected
from the group consisting of: silicon carbide and silicon
nitride.
The invention also provides a method for producing
infrared radiation, which comprises the steps of (a)
forcing a pressurized mixture of combustion gas and air
through a multitude of small first spaces connected
together in a first block of material at a velocity which
is greater than the velocity of the flame propagation in
the mixture, into a second block of material, which,
while cont~in;ng a multitude of spaces connected together
which are larger than those of the first spaces, is
combined with said first block to form a composite burner
block assembly; (b) allowing said mixture to expand and
form a turbulent mixture in said second block; (c)
allowing said turbulent mixture to ignite and burn,
thereby heating the top surface of said second block to a
very high incandescence temperature and causing it to
produce very efficient infrared radiation, wherein said
first block has a porosity in the range of 60-85 ppi, and
has a thickness in the range of 10-20 mm; and said second
block has a porosity less than 15 ppi, and has a
thickness in the range of 2-6 mm.
The invention also provides a burner assembly for
gas-fired infrared burners, which comprises (a) means
comprising a first block of material having a multitude
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1 336258
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of small first spaces for transporting and distributing a
mixture of combustion gas and air; (b) means comprising a
second block of material having a multitude of second
spaces which are larger than said first spaces for
completing said transportation and distribution of said
mixture and providing a combustion zone, wherein said
mixture can burn and heat the top surface of said second
block of material to incandescence such that it will
produce very efficient infrared radiation; and (c) said
first and second blocks of material being combined to
form a burner assembly, wherein to effectively serve the
functions of transportation and distribution of said
mixture on the one hand, and the functions of combustion
and resulting radiation on the other hand, in said first
and said second blocks respectively, the porosities and
thickness of the said material in the first block and in
the second block are different, namely, the material in
the first block comprises said first spaces expressed as
a number of pores per linear inch in the range of 40-70,
a percentage apparent porosity in the range of 75-95%,
and a thickness commensurate with said number of pores
per linear inch and said percentage apparent porosity so
that the velocity of said mixture through said first
block is greater than the velocity of flame propagation
of the mixture through the first block when the mixture
is ignited, and the said material in the second block
comprises said second spaces expressed as pores per
linear inch of less than 15, a percentage apparent
porosity in the range of 75-95%, and a thickness less
than about 2 pores, so that most of the combustion of
said mixture takes place in said second block to thereby
concentrate, reverberate and enhance the energy level,
maximize the gas temperature and attain a very high level
of radiation.
The invention also provides a burner assembly for
gas-fired infrared burners, which comprises (a) means
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comprising a first block of material having a multitude
of small first spaces for transporting and distributing a
mixture of combustion gas and air; (b) means comprising a
second block of material having a multitude of second
spaces which are larger than said first spaces for
completing said transportation and distribution of said
mixture and providing a combustion zone, wherein said
mixture can burn and heat the top surface of said second
block of material to incandescence such that it will
produce very efficient infrared radiation; and (c) said
first and second blocks of material being combined to
form a burner assembly, wherein at least said top surface
of said second block is coated with at least one material
selected from the group consisting of cobalt oxide,
nickel oxide, chromium oxide, thorium oxide, silicon
carbide and a metal silicate.
The invention also provides a burner assembly for
gas-fired infrared burners, which comprises (a) means
comprising a first block of material having a multitude
of small first spaces for transporting and distributing a
mixture of combustion gas and air; (b) means comprising a
second block of material having a multitude of second
spaces which are larger than said first spaces for
completing said transportation and distribution of said
mixture and providing a combustion zone, wherein said
mixture can burn and heat the top surface of said second
block of material to incandescence such that it will
produce very efficient infrared radiation; and (c) said
first and second blocks of material being combined to
form a burner assembly, wherein the second block is
coated with a very thin layer of at least one material
selected from the group consisting of silicon carbide and
silicon nitride.
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The invention also provides a burner assembly for
gas-fired infrared burners, which comprises (a) means
comprising a first block of material having a multitude
of small first spaces for transporting and distributing a
mixture of combustion gas and air; (b) means comprising a
second block of material having a multitude of second
spaces which are larger than said first spaces for
completing said transportation and distribution of said
mixture and providing a combustion zone, wherein said
mixture can burn and heat the top surface of said second
block of material to incandescence such that it will
produce very efficient infrared radiation; and (c) said
first and second blocks of material being combined to
form a burner assembly, and a reverberation layer of
material on said second block material, consisting of a
multitude of small spaces connected together, which has a
low heat capacity and a radiant surface area of
relatively high emissivity.
The first block of material, which is also referred
to hereafter as the "distribution block", should have low
coefficients of both thermal expansion and thermal
conductivity, as well as high temperature resistance.
Various ceramic materials can meet such needs, for
example, bonded aluminum oxide fibers, lithium aluminum
silicate, and materials sold under various trade names.
The above mentioned second block of material, which is
referred to hereafter as the "radiation block", should,
in addition to having high temperature resistance and a
low coefficient of thermal expansion, have a high
emissivity and/or the ability to receive a surface oxide
deposit or coating which exhibits a high infrared
emissivity in the wavelength region of 1.5 to 2.0
microns. Silicon carbide-is one such material, and there
are various metal oxides coatings, which will meet such
needs. Preferably, the radiation block should have a
high coefficient of thermal conductivity.
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When the first and second blocks are "combined" to
form a burner assembly, this may be accomplished in a
number of ways, e.g. they may be laminated or held
together by a chemical bonding/sealing means, or held
together mechanically. The overall thickness of the
assembly is typically less than 2.5 cm and the second
block is thinner than the first block.
As an optional arrangement in any of the above
embodiments, a surface screen may be used to increase the
overall radiation of the assembly. In prior art burners,
a high temperature metal screen is used which has a rela-
tively high heat capacity and takes time to "cool down";
it also has a relatively low radiant surface area. This
invention thus provides a reverberation/enhancement
screen/layer of material, which will have a very low heat
capacity and a high radiant surface area of high
emissivity.
In line with the burner assembly concept of this
invention, the function of reverberation, when provided
in the form of a porous reticulated structure, may be
combined with or bonded to the main burner assembly as a
special layer of material to form an overall composite
assembly of three layers of material. Thus, the first
layer would continue to perform the functions of
transporting and distributing the gas mixture (and flame
arresting), and the second layer would generate by
combustion the primary infra-red radiation and finally
the third layer would enhance this.
Finally, returning again to the basic two
block/layered assembly, the preferred embodiment is to
perform the functions of transporting and distributing
the gas mixture (and flame arresting) in the first block
of material and to perform the functions of combustion,
radiation and reverberation/enhancement in the second
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block, in order to maximize the use of the three modes of
heat transfer, conduction, convection and radiation,
within the burning mixture in that one block, thereby
maximizing the final mode, that of radiant energy from
that second block.
This invention will now be described in further
detail having reference to the accompanying drawings,
wherein:
Figure l illustrates, in cross-section, a type of
infrared burner unit in which the present invention,
involving a composite burner plate/block assembly, may be
used;
Figure 2 illustrates a portion of a cross-sectional
view of an embodiment of such a composite block,
involving separate blocks for distribution and for
radiation;
Figure 3 illustrates a similar cross-section of
another embodiment involving separate distribution and
radiation blocks, combined in one assembly;
Figure 4 illustrates still another embodiment of
such a composite assembly;
Figure 5 is a graph showing the relationship between
the radiant output and the temperature of the emitter;
and
Figure 6 illustrates a cross-section of another
embodiment involving separate distribution
(transportation), primary radiation (combustion) and
reverberation (P~h~ncement) layers of material, combined
all in one assembly.
Referring to Figure 1, reference numeral 2
illustrates a type of infrared burner unit in which the
present invention, involving a composite burner
plate/block assembly 3, may be used. Burner block
assembly 3 has a first block of material or distribution
block 4, to transport and distribute a mixture of
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combustion gas and air to a second block of material or
radiation block 5, which is different from the material
in distribution block 4. The block acts as a gross gas
distributor to aid in spreading the gas flow evenly
through the assembly. Radiation block 5 will complete
the transportation and distribution of the mixture and
provide a combustion zone, wherein the gas can burn and
heat the top surface of the second block of material 5 to
incandescence (generally in the range of 1100 - 1400C)
such that it will produce very efficient infrared
radiation. The mixture is initially ignited adjacent the
upper surface of block 5, e.g. by a conventional
piezoelectric igniter or pilot flame (not shown). Means
are provided to combine the first and second blocks of
material, i.e. distribution block 4 and radiation block
5, to form the burner block assembly 3. Such means to
hold blocks 4 and 5 together may include chemical
bonding, such as molecular bonding, sealing, gluing, etc.
and/or mechanical bonding, such as molecular attraction,
clamping, etc. Since chemical bonding will depend on the
type of block material used, for purposes of illustration
only, a more general type of mechanical bonding will be
used, i.e. clip-like clamps 11.
Various embo~ ts of block assembly 2 are
illustrated in Figures 2, 3 and 4. Block assembly 3
forms a gas-air outlet surface or side of an enclosed
plenum chamber 8. The mixture of gas and air enters
chamber 6 through tube 7 from a source 8. While source 8
preferably supplies pressurized gas and air sufficient to
provide the required mass flow rate, in certain cases, a
conventional venturi aspirator may be used. The air and
combustion gas mixture supplied from source 8 will
support complete combustion without the need of any
auxiliary air.
1 3362~8
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A special metal screen or mesh 9 is provided at a
short distance from the top of radiation block 5. Screen
9 is heated to incandescence by the combustion of the
gas-air mixture, thereby producing radiant heat in
addition to that being produced by radiation block 5.
To further reduce flashback, the inlet side to
distribution block 4 is provided with a thin metal screen
or membrane 10, contA;n;ng a large number of small holes
or orifices, the size of which is small enough to serve
as a flame arrester during low gas-air flow rates.
However the screens 9 and 10 may, if desired, be omitted.
The length and width of each block assembly will
depend on the use to which the assembly is put;
consequently, details involving cross-sectional views
only are shown. As mentioned above, the overall
thickness of the assembly is generally not greater than
2.5 cm. and the radiation block is generally thinner than
the distribution block.
Referring to Figure 2, which illustrates in greater
detail a portion of a cross-sectional view of an
embodiment of the block assembly 3 of Figure 1, reference
numeral reference 13 indicates such a portion, consisting
of a portion of a first block of material or distribution
block 14, comprising a multitude of small first spaces
(not shown) connected together, and a second block of
material or radiation block 15, comprising a multitude a
second small spaces connected together, which spaces are
larger than those of the first spaces in distribution
block 14. The size of the first spaces are such that, on
forcing a pressurized mixture of combustion gas and air
through the small first spaces in the first or
distribution block of material 14, the velocity of flow
will be greater than the velocity of the flame
propagation in the mixture. The sizes of the second
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spaces in the second or radiation block of material 15
are such as to allow the mixture to expand and form a
turbulent mixture and to ignite and burn, thereby heating
the top surface of the radiation block 15 to a very high
incandescence temperature and causing it to produce very
efficient infrared radiation. The material in each block
may have a reticulated structure, involving a precise and
uniformly distributed cellular pore structure, which may
be expressed in terms of porosity, radiation block 15
having a greater porosity than the distribution block 14.
As also mentioned, the thermal conductivity and expansion
of the distribution block 14 should be low, e.g. the
thermal conductivity should be low enough so as to
present a cool surface to the gas plenum, i.e. approx.
150C, to prevent flash-back. Various porous ceramic
materials provide such properties. While the thermal
expansion of radiation block 15 should also be low, its
thermal conductivity, temperature resistance, and
emissivity should be as high as possible, silicon carbide
being one such material, or alternatively, it must be
able to accept a surface coating of a high emissivity
material, e.g. metal oxide coatings, such as those of
cobalt, nickel, chromium, and thorium, as well as metal
silicates and siliceous carbide. Some of these materials
may also be impregnated into the top layer. Optional
screens 9 and 10 mentioned in connection with Figure 1
may be provided here to advantage: this could extend the
choice of porous materials. Depending on the type of
reticulated material chosen, the radiation block could be
very much thinner than the distribution block, e.g.
2-6 mm compared to 10-20 mm for the distribution block,
which should be thick enough to provide back pressure for
the gas-air mixture to allow uniform combustion across a
large number of burner surfaces connected to the same
manifold. The pore size of block 14 should also be small
enough so as to prevent flashback.
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Referring to Figure 3, which illustrates in greater
detail a further embodiment of the block assembly 3 of
Figure 1, reference numeral 23 indicates a portion of a
cross-sectional view of a block assembly, consisting of a
first block of material or distribution block 24,
comprising a multitude of small distinct channels 26,
each channel being perpendicular to the radiation surface
and consisting of a first section 27, and a second
section 28, the first section having a cross-sectional
area smaller than that of a second section 28, such that
when a pressurized mixture of combustion gas and air is
forced through section 27, the velocity of the mixture
through the first section 27 is greater than the velocity
of the flame propagation in the mixture. The cross-
sectional area of the second section 28 is a varying onecommencing with that of the first section and then
expanding in bowl-shaped fashion until section 28 makes
contact with a second block of material or radiation
block 25, consisting of a multitude of spaces connected
together, into which the mixture is forced to flow. The
sizes of the spaces in the second or radiation block 25
are such as to allow the mixture to expand and form a
turbulent mixture and to ignite and burn, thereby heating
the top surface of the radiation block 25 to a very high
incandescence temperature and causing it to produce very
efficient infrared radiation.
The materials and design of the channels for
distribution block 24 are well known in the prior art.
The thermal conductivity and expansion of distribution
block 24 should be low, as provided by various ceramic
materials, such as aluminum oxide fibers; lithium
aluminum silicate; and those sold under various trade
names, e.g. Cordiorite(TM), Mullite(TM), etc. The design of
the channels is disclosed in e.g. U.S. Patent Nos.
3,885,907 and 3,635,644. Details for radiation block 25
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are the same as those for radiation block 15 discussed in
connection with Figure 2.
It will be noted that since distribution and
combustion can take place in section 28 of distribution
block 24, an even thinner radiation block 25 can be used
in this embodiment than in that shown in Figure 2. It
may be noted that radiation block 25 can serve to retard
"lift-off-- of the flame and thereby allow for a wider
range of gas-air flow rates/energy inputs. Whether or
not combustion takes place in the expanded section of the
distribution block 24 will depend on the flow rate, the
thickness and porosity of radiation block 25, as well as
the design of that particular section.
Referring to Figure 4, which illustrates in greater
detail a still further embodiment of the block assembly 3
of Figure 1, reference numeral 33 indicates a portion of
a cross-sectional view of the block assembly, consisting
a multitude of first small distinct channels 36 in the
distribution block 34. Each channel 36 is perpendicular
to the radiation surface and has a cross-sectional area
such that when a pressurized mixture of combustion gas
and air is forced through distribution block 34, the
velocity of the mixture through channels 36 is greater
than the velocity of the flame propagation in the
mixture, each channel 36 being extended until it meets
with at least one second, small channel 37 in a second
block of material or radiation block 35, contA;ning a
multitude of second channels 37, which are in direct
alignment with the first small channels 36. The cross-
sectional area of second channels 37 is a varying onecommencing with that of the first channels, and then
exp~n~;ng in bowl-shaped fashion until the second channel
37 makes contact with the top surface of the radiation
block 36. The size and shape of channels 37 in the
radiation block 35 are such as to allow the mixture to
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exp~n~ and form a turbulent mixture and to ignite and
burn, thereby heating the top surface of the radiation
block 35 to a very high incandescence temperature,
causing it to produce very efficient infrared radiation.
The materials and design of the channels for distribution
block 34 would be the same as for the first meeting of
the channels described in distribution block 24 in
connection with Figure 3. The design of the channels for
the radiation block 35 is the same as that of those shown
in Figure 3. The design of the channels for the
radiation block 35 is the same as for the second section
of the channels in the distribution block 24 and
described in connection with Figure 3, i.e. as disclosed
in the aforesaid United States patents. The materials
for radiation block 35, however, should be carefully
chosen, and as mentioned above, in addition to having
high temperature resistance and a low coefficient of
thermal expansion, they should have a high emissivity
and/or the ability to receive a surface oxide deposit or
coating which exhibits a high infrared emissivity in the
wavelength region of 1.5 to 2.0 microns, e.g. silicon
carbide or various metal oxides coatings, as mentioned
above in connection with Figure 2. Preferably, the
radiation block should have a high coefficient of thermal
conductivity. The thicknesses of the distribution and
radiation blocks will depend on the type of material and
prior art design for the channels that might be selected.
While Figures 3 and 4 show a gradual ex~nsion of
the sections or channels, i.e. sections 28 in Figure 3
and channel 37 in Figure 4, the expansion could also be
fairly abrupt at first so as to form a bowl with nearly
perpendicular sides, rather than a gradual cone-shaped
bowl.
The use of the above optional screen should be given
consideration, as it will increase the radiation
1 3362~8
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efficiency of the overall assembly. This arises from the
following: while the total emissivity is a function of
the temperature and radiating surface area, the radiation
surface will reach a point of ~i~ini shing returns with
higher energy inputs; however, a proper screen mounted
above the radiating surface will increase the radiation
output, because the screen captures the flue gases and
converts this exhaust energy to radiant energy, and also
by trapping this cushion of gases, it provides an
extension of the effective radiant surface by
reverberation, and the same time prevents ambient air
from reaching the emitting surface. Such a screen may be
made from a high temperature metal or from a reticulated
open ceramic structure, as already mentioned above.
While the above discloses a general embodiment,
involving separate materials having different properties
for the various functions, the preferred embodiments
involve the use of reticulated materials having specific
porosities. This preference arises from the following:
The three critical parameters for an infrared
emitter are: surface area, temperature and emissivity.
The emissivity varies with temperature and the nature of
the material, so by choosing a material which inherently
already has a high emissivity, the fact that it has a
reticular/porous structure will further increase its
emissivity. Various materials are disclosed above, with
porous silicon carbide being an excellent example.
For a given radiating material, the radiant
flux/energy will increase in proportion to the total
surface area of the radiating body which is seen by the
absorbing body. As can be seen by comparing the
radiating surface of Figure 4 with those of Figures 2, 3
and 6, the surfaces of the porous body 15 in Figure 2,
body 25 in Figure 3, and body 45 in Figure 6 are each
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substantially greater than that of the upper surface 35
in Figure 4 (surface 35 being a typical surface for a
conventional emitter). Thus, while the radiant surface
of a conventional emitter is a relatively small fraction
of the total surface, the radiant surface of the emitter
of the present invention is nearly 100% of the total
surface.
Nevertheless, of the three parameters, temperature
can be the most important as the radiant output varies as
the fourth power of the absolute temperature of the
emitter. However, in practice as one tries to increase
the temperature of a given emitter, the output levels off
because of the nature of the surface and the method of
producing the temperature. This is illustrated in Figure
6, where curve (a) is that for a typical conventional
emitter and where by increasing the temperature from TO
to Tl, the output remains essentially the same. Factors
causing this saturation were touched on in the above and
include: insufficient contact area between the flame and
the emitting material and conventional emitters depend on
flame impingement on the emitter surface for heat
transfer; further energy input by increasing gas flow
merely results in "flame lift-off".
Curve (b) on the hand, is typical for ~ho~; ments of
Figures 2, 3 and 6, which involve a porous/reticulated
structure.
As can be seen, because the emitter of curve (b) has
more surface area and a higher emissivity, its radiant
output at temperature TO will be greater than that of the
conventional emitters of curve (a) at the same
temperature TO. However in addition because of the
nature of the emitter, the manner in which the combustion
is taking place (and the conversion of energy from
convection to radiant) within the emitter, and its
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greater resistance to "lift-off", the curve does not
level off as quickly, but continues to rise, making
possible a further increase in the output by an increase
in temperature of the emitter (through higher gas flows).
This invention, therefore, allows one to take advantage
of the benefits of the higher temperatures. Thus, when
operating at the recommended temperatures for the emitter
of the present invention, its emissivity is in the range
of 0.6-0.95.
The above aspects have led the inventor to provide a
further embodiment in which reverberation/enhancement is
preferably carried out through the use of a highly
porous/reticulated layer of material, rather than a
conventional metal screen. This was mentioned above. In
such a case, while the highly porous reverberation
material can be located at a very short distance above
the primary porous emitter, it is preferable to combine
or bond it to the top surface of the primary emitter.
This is illustrated in Figure 6, which is a cross-
sectional view of the burner assembly of Figure 1(without the use of clips), indicated by reference
numeral 43, i.e. of the various individual assembly units
that might make up the overall burner unit. This
assembly consists of a first block of material or
distribution block 44, comprising a multitude of small
first spaces (not shown) connected together, a second
block of material or radiation block 45, comprising a
multitude of second small spaces connected together,
which spaces are larger than those of the first spaces in
the distribution block 44, and a third block/layer or
reverberation block 46, comprising a multitude of third
small spaces connected together, which spaces are still
larger than those of the second block. Details of the
first and second blocks are given in reference to that
illustrated in Figure 2 above.
'f~
1 336~8
- 20 -
While the overall assembly can be physically hold
together as illustrated in Figure 1, it preferable that
the various layers/blocks be bonded together for reasons
that will be given below. A typical example for such an
assembly is: the first or distribution block may have a
porosity in the range of 60-85 ppi and be made from LAS
(lithium alumina silicate) or "petalite"; the second or
primary radiation (combustion) block may have a porosity
in the range of 25-50 ppi and be made from LAS or silicon
carbide (coated or impregnated with a higher emissivity
material); the third or reverberation (enhancement) layer
may have a porosity in the range of 5-10 ppi and be made
from silicon carbide. Thickness of the layers will
depend on various factors, but typical ranges are: first
block, 10-20 mm; second block 26 mm; and third block 2-6
mm.
The advantages given above for a porous emitter
(when used without reverberation), will also apply to the
above porous reverberator when it is used with an
emitter, and thereby make it a more efficient enhancer
than conventional screens. However, another important
feature for such a reverberation layer of very high
porosity is that it can be made from a high temperature
ceramic material such as silicon carbide. This material
does not degrade easily at the very high temperatures
used for emitters and this raises the following further
advantages: it has a long operating life and advantage
can be taken of the use of still higher temperatures,
which in turn increase the radiant output substantially
(see curve (b), Figure 5). In contrast, conventional
high temperature screens operating at a temperature of
1150C have an operating life of only 2000-3000 hours.
Since the above porous layer would properly operate in
the range 1100-1400C, not only would the radiant output
be much higher at this temperature level, but the life of
the porous layer would be very much greater than that of
C
- 21 - l 3 3 6 2 5 8
a conventional screen operating at the lower safer level.
Should attempts be made to operate this conventional
screen at the higher temperature levels that this
invention can operate at, then its life would drop even
substantially lower. Recent improvements in the
manufacture of ceramic materials have made the attainment
of the above-mentioned embo~ ts, especially that
involving combining the functions of combustion radiation
and reverberation all within one block/layer, somewhat
easier. Thus, the base material may be silicon carbide
(SiC) and/or silicon nitride (Si3N4), which may be coated
with a very thin layer of silicon carbide/silicon
nitride, which makes the structure very strong and shock
resistant. To lower the thermal conductivity of the base
material when used in the lower first block of material
in certain applications, it may be diluted with lower
conductivity material such as LAS.
Thus, looking at this two block/layered assembly in
more detail, as it is illustrated in Figure 2, the
transportation and distribution of the gas mixture takes
place in layer 14, whose pore size, expressed as pores
per inch (ppi) and apparent porosity (ratio of the volume
of open pore space to the bulk/overall volume of the
material), is such that the velocity of the gas flow in
this layer is above the velocity of flame propagation and
little if any combustion takes place in that layer, the
pore size and apparent porosity of the second layer 15,
being such that most of the combustion takes place in
this layer in order to make maximum use of the three
modes of heat transfer (conduction, convection and
radiation) during the combustion process, to thereby
concentrate, reverberate and enhance the energy level and
maximize the gas temperature and its rapid development
and hereby attain a very high level of final radiation.
However, the pore size and apparent porosity in layer 15
must not be too great such that the structure would
f~
V
- 22 - l 3 3 6 2 5 8
collapse under the higher temperatures that are generated
by this new type of layer. Preferred ranges for these
layers are as follows:
(1) in the first or main block/layer 14: a ppi in
the range of 40-70 and an apparent porosity in the range
of 75-95%. The thickness will depend on the pore size
and is discussed above in connection with other
embodiment. At this pore size and porosity or low mass
(and even though the material may have high conductivity)
little preheating of the gas mixture occurs in this
layer.
(2) in the special reverberation layer 15, a ppi of
less than 15 and an apparent density the same as the main
layer or within the same range. The thickness should be
less than about two pores, so that as the combustion
heats the top surface of the main layer 14 it can make
use of its high emissivity (in some applications the hot
top layer of the first block can radiate over 70~ of the
total radiation).
As implied in the above, the preferred materials for
all embodiments are silicon carbide/silicon nitride, very
thinly coated by the same material(s), as they are very
resistant to temperature and corrosion, have a high
emissivity (greater than .9) and a high thermal
conductivity (both for use during the combustion) and the
pores appear to offer a special resistance to gas flow so
that larger pores and/or thinner layers can be used.
One such burner assembly embodying the present
invention has the following features: two porous cellular
layers bonded together and made from SiC coated with SiC
(emissivity about .95), both layers having about the same
apparent density in the range of 80-85%, the main layer
pore size being about 65 ppi and was approx. 5/8 inch
thick; and the thin other layer having a pore size of
~ 3362~8
- 23 -
about 10 ppi and being approx. l/8 inch thick (approx.
1.2 pores).
A similar thin outer layer may also be applied to
assembly 23 of Figure 3, where it is represented as layer
25, However, the dimensions of section 28 are then such
that a mi n i~llm of combustion takes place in that section.
It should be noted that, while the apparent porosity
in each block/layer is about the same, the actual size of
each pore in each layer is substantial different. The
pore size in ppi, taken together with the apparent
density, will determine the actual pore size or diameter
of the open area. Similarly, while the specific thermal
conductivity of the material in each layer can be about
the same, the mass conductivity may not be very high due
to the high pore size and apparent porosity, i.e. its low
mass.
Conventional burners use metal parts in various
areas, as well as for the reverberation screen, and in
addition use dense ceramic for the burner itself; the
relatively high heat capacity of these materials has the
result that when the burner is turned off, the "cool-down
period" is relatively long, e.g. 180-360 seconds. While
the use of metal parts to hold the assembly of the
present invention together is not forbidden, in its
preferred form, the various layer/blocks are bonded
together chemically, thereby eliminating the high heat
capacity of these metal parts. As mentioned above, the
very low heat capacity of the various porous layers makes
the overall heat capacity of the assembly extremely low,
with the result that the "cool-down period" can be less
than 5-10 secs.
Besides resulting in a very short "cool-down", the
highly porous materials can also have a very low heat
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- 24 - l 3 3 6 2 5 8
conductivity, so by choosing such a material for the
distribution block, all surfaces, other than those
involved in combustion and reverberation, remain
relatively cool to the touch, compared to prior art
assembly surfaces, which are so hot that they can ignite
flammable material.
These features of very short "cool-downs" and cool
outer surfaces are very important in applications
involving such flammable materials as paper and textiles.
These are important safety features both from a fire
hazard point of view as well as for those persons who
have to operate the burners and the associated
paper/textile manufacturing equipment.
The high shock resistance of the preferred
materials, i.e. SiC/Si3N4 thinly coated with the same
material, also offer advantages in those applications
where cold water may be accidentally splashed on these
burner assemblies. Prior art ceramics made from weaker
materials would be hazardous in such cases.
Although illustrated embodiments of the present
invention have been described herein with reference to
the accompanying drawings, it is to be understood that
the invention is not limited to those precise
embodiments, and that various changes and modifications
may be made by those skilled in the art without departing
from the spirit and scope of this invention.
