Note: Descriptions are shown in the official language in which they were submitted.
CA 02617564 2015-02-23
HIGH EFFICIENCY RADIANT BURNER WITH HEAT
EXCHANGER OPTION
BACKGROUND
Technical Field
The present disclosure relates to controlled combustion and more particularly
to
pressurized hydrocarbon gas burners and most particularly to a liquid
pressurized gas
(LPG) stove/cookware system that includes a high efficiency heat exchanger
working in
conjunction with a fully aerated radiant burner.
Description of the Prior Art
Conventional gas combustion apparatus use partially aerated burners and
require introduction of relatively large quantities of secondary air for
complete
combustion to occur. This dilution of the combustion gases reduces flame
temperatures and heat transfer efficiencies into a heat transfer surface, such
as a fluid
container in a cooking system, e.g., a pot. Generally, the volume of
introduced
secondary air is dependent on natural convection and diffusion of the
combustion
gasses, which limit the driving pressure of the gases and excess air to
pressures that
can be attained only by the buoyancy effect of the hot rising gases. Thus,
heat transfer
values for forced convection are much larger than values for free convection.
Currently
a number of companies (Cascade Designs, Inc. and JetBoil, Inc.) offer gas
combustion
apparatus with heat exchangers that boost efficiency from conventional stove
and pot
combinations (35%- 55%) to (45%- 65%). Because these apparatus are limited by
free convection heat transfer coefficients and dilution of the combustion
gases with
secondary air, higher efficiency values for apparatus of these designs are
limited.
While manufacturers of combustion-based heat transfer apparatus continually
strive for increased combustion and heat transfer efficiencies, they must also
address
environmental concerns relating to combustion byproducts. One such combustion
byproduct, nitrous oxides (N0x), is of particular concern with respect to
domestic gas
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water heaters. Initial combustion of gases in natural convection heaters
occurs at high
temperatures which are conducive to nitrous oxide formation. The combustion
gases are
diluted by freely convecting air where some additional combustion occurs but
gas
departure and velocities drop.
SUMMARY
Illustrative embodiments of the present invention utilize a radiant burner and
optional heat exchanger arrangement to achieve high heat transfer values to
containers
through forced convection and hotter undiluted combustion gases, which
increase overall
efficiency of the system from (70% to 85%), without adding excessive heat
exchanger
surface area. The burner also greatly lowers the temperature at which complete
combustion occurs, thereby greatly reducing nitrous oxide emissions. A feature
of such
embodiments is that as power output is increased, the driving pressure for
forced
convection with the optional heat exchanger is also increased, and thus heat
transfer
efficiency is generally constant over a wide range of power outputs. The
result of this
arrangement provides for a radiant burner that is highly fuel efficient, that
has increased
resistance to the deleterious effects of wind on the burner, that greatly
increases the safety
of operation of the radiant burner, and that significantly reduces the output
of nitrous
oxides. When used in combination with the optional heat exchanger, fuel
efficiency is
further increased and emissions further decreased.
In an illustrative embodiment, the radiant burner comprises a generally
enclosed
cavity defined, at least in part, by a fuel gas impermeable surrounding and a
lower surface
of a fuel gas permeable burner element, wherein the cavity has at least one
opening
exposed to an oxidizer source. Sealingly coupled to the at least one opening
is a mix tube
that defines a longitudinal axis, and has a first end and a second end wherein
the first end
occupies the at least one opening and the second end extends into and is
exposed to the
pressure cavity. As those persons skilled in the art will appreciate, any
structure capable
of mixing a gaseous fuel with a gaseous oxidizer can be used as a mix tube,
and therefore
such structures are considered as an equivalent. A fuel gas injector, which
during use of
the burner is in fluid communication with a source of fuel gas, is positioned
to introduce
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fuel gas into the mix tube, preferably at or proximate to the first end,
thereby encouraging
momentum transfer of the oxidizer into the fuel gas stream when the oxidizer
is also
introduced at or proximate to this location.
Because of the porosity of the burner element, a pressure gradient exists
between
the cavity and an upper surface of the burner element. Consequently, pre-
combustion
gasses diffuse from the lower surface of the burner element to the upper
surface. Pre-
combustion gasses at the upper surface may then be ignited, such as by an
igniter that is
associated with the burner, whereupon combustion takes place.
In one series of embodiments, a plurality of openings is present in the
pressure
cavity. A corresponding number of cylindrical mix tubes are fluidly coupled to
the
openings, and are exposed to the ambient environment at their first end and to
the cavity
at their second ends. Thus, the ambient environment provides the oxidizer
source, i.e.,
oxygen. A corresponding number of fuel gas injectors are preferably positioned
at the
first end of the mix tubes such that the fuel gas, when introduced into the
mix tubes,
entrains a volume of air and mixes the two gasses to form a pre-combustion
gas. The
pre-combustion gasses are preferably further mixed and turbulence imparted
into the
pre-combustion gas stream by a plurality of static mixing posts. The mixing
posts also
preferably serve to radiate heat that may accumulate in the burner housing
through
exposure to the cool pre-combustion gasses.
A feature of the burner is the incorporation of a thermal fuse (trip filter)
disposed
between the fuel gas source and the gas injector(s). This fuse may be
constructed from
any material that will be predictably responsive to heat such that when
exposed to heat
higher than a certain temperature for an established period of time, the
material changes
form, which operates to interrupt fuel flow to the gas injector(s). In one
series of
embodiments, the filter is a eutectic metal such as cadmium, lead tin alloy,
which is
formed into a washer that operatively keeps a check valve in the open
position. Thus, in
the event of a light back or thermally derived malfunction, the increased
temperature will
cause the washer to liquefy, and thereby permit the check valve to close and
isolate the
fuel gas from the thermal condition that caused the melting of the thermal
fuse.
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In order to increase the efficiency of systems employing the radiant burner,
containment vessels, such as pots, can be specially adapted to exploit the
quantity and
quality of heat output by the radiant burner. A primary mode of adaptation
involves the
use of heat exchanging structure at or near the bottom of the containment
vessel, which
preferably comprises a plurality of fins, either as fin elements integral with
the vessel or
as fin bodies attachable to the vessel, arranged to maximize radiant and
convective
heat transfer of combustion gasses from the burner. Each relevant containment
vessel
will have a bottom surface and a lower side surface that is linked to the
bottom surface
by a shoulder. The intention of the heat exchanging structure is to increase
the duration
of the vessel's exposure to the burner output, thereby further increasing the
efficiency of
the system employing the radiant burner.
In an illustrative embodiment, a fuel gas burner for use with a source of fuel
gas
includes a cavity defined, at least in part, by a fuel gas impermeable
surrounding and a
fuel gas permeable burner element having an interior surface exposed to the
cavity and
an exterior surface exposed to the environment, wherein the fuel gas
impermeable
surrounding has at least one opening. The burner further includes a mixing
element
having a first end and a second end, wherein the first end is exposed to the
environment by being fluidly coupled to the at least one opening and the
second end is
fluidly coupled to the cavity. The burner further includes a fuel gas injector
including at
least one gas jet exposed to the environment and operatively coupled to the
source of
fuel gas and the mixing element for injecting fuel gas into the mixing
element, whereby
introduction of pressurized fuel gas by the gas jet into the mixing element
and
entrainment of an oxidizer from the first end of the mixing element create a
volume of
pressurized pre-combustion gas within the cavity, which diffuses from the
interior
surface of the burner element to the exterior surface of the burner element.
In another illustrative embodiment, a fuel gas burner for use with a source of
fuel
gas includes a cavity defined, at least in part, by a fuel gas impermeable
surrounding
and a fuel gas permeable burner element having an interior surface exposed to
the
cavity and an exterior surface exposed to the environment, wherein the fuel
gas
impermeable
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surrounding has at least one opening. The burner includes a mixing element
located
inside the cavity and having a first end and a second end, wherein the first
end is
sealingly coupled to the at least one opening. The burner further includes a
fuel gas
injector including at least one gas jet, operatively coupled to the source of
fuel gas,
exposed to the environment and located at or proximate to the first end of the
mixing
element for injecting fuel gas thereinto whereby introduction of pressurized
fuel gas by
the injector into the mixing element and entrainment of an oxidizer from the
first end of
the mixing element create a volume of pressurized pre- combustion gas within
the
cavity, which diffuses from the interior surface of the burner element to the
exterior
surface of the burner element. The burner further includes a thermal fuse for
occluding
a fuel passage between the source of fuel gas.
The described and illustrated burners provide a user with exceptional
efficiency
and significantly decreased undesirable combustion byproducts. For example, CO
emissions are about 8 times less than a comparably sized conventional stove.
Similarly,
nitrogen oxides are significantly reduced (approximately 80-93%) when compared
to
commercially available competing stoves.
BRIEF DESCRIPTION OFTHE DRAWINGS
Fig. 1 is an elevation view of an assembled burner and heat exchanger equipped
pot system;
Fig. 2 is a cross section elevation view of a burner;
Fig. 2A is a detailed cross section of a thermal fuse/trip that can be used in
the
embodiment shown in Fig. 2;
Fig. 3 is a cross section plan view of the burner of Fig. 2;
Fig.4 is a cross section elevation view of a first heat exchanger equipped
pot;
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Fig. 5A is a perspective view of the first heat exchanger equipped pot wherein
post pot manufacture fin elements are attached to the bottom of the pot and
external
covers and rings are removed for clarity;
Fig. 5B is a perspective view of the first heat exchanger equipped pot wherein
fin bodies are integrated into the bottom of the pot during manufacture of the
pot and
external covers and rings are removed for clarity;
Fig. 6 is a cross section elevation view of a second heat exchanger equipped
pot wherein a peripheral heat exchanger ring is employed to increase the
surface
area available for heat transfer; and
Fig. 7 is a perspective view of a peripheral heat exchanger ring segment for
use with the embodiment of Fig. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following discussion is presented to enable a person skilled in the art to
make
and use the invention. Various modifications to the preferred embodiment will
be readily
apparent to those skilled in the art, and the generic principles herein may be
applied to
other embodiments and applications without departing from the scope of the
present
invention as defined by the appended claims. Thus, the present invention is
not intended
to be limited to the embodiments shown, but is to be accorded the widest scope
consistent
with the accompanying claims.
Unless otherwise noted herein, all parts of burner 10 and heat exchanger 90
are
constructed from metal. Depending upon the part's application, the metal may
be
aluminum, steel, copper, brass or similar conventional metal. The selection of
metal is
primarily driven by thermal transfer considerations, although resistances to
corrosion and
high temperatures, as well as weight considerations are also valid criteria
for material
selection. In a preferred embodiment, burner element 60 comprises a porous
metal foam
material sold under the trademark METPORE by Porvair Advanced Materials, Inc.
of
Hendersonville, North Carolina. However, those
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persons skilled in the art will appreciate that other gas porous, heat
resistant
materials can be used, such as ceramics and metal-ceramic composites.
Turning then to Figs. 2 and 3, a burner embodiment of the invention is shown
in cross section elevation and plan views, respectively. Burner 10 comprises
metallic
base 12, which provides fuel delivery infrastructure 30 (discussed below) and
which
partially defines cavity 24. Cavity 24 is further defined by metal surround 14
and
burner element 60. As will be described in more detail below, cavity 24 is
generally
sealed from the environment with two major exceptions. First, mix tubes 50a
and
50b are sealingly attached to surround 14 and are exposed to the environment
proximal ends 52a and 52b (see Fig. 3). Second, burner element 60 is porous to
gasses (see Fig. 2). As a result of this arrangement, gasses introduced at
proximal
ends 52a and 52b of mix tubes 50a and 50b travel the length of the mix tubes
until
expelled into cavity 24 at distal ends 54a and 54b. Because burner element 60
is
highly porous, a gas pressure gradient exists between cavity 24 and the
environment
at outer surface 64 such that gasses present in cavity 24 will diffuse through
burner
element 60 towards outer surface 64.
Fuel gas, such as Liquid Pressurized Gas (LPG), is delivered to burner
element 60 in the following manner. An LPG bottle (not shown) is rotationally
coupled to inlet 30, as is best shown in Figs. 2 and 2A. To permit such
coupling, inlet
includes threaded portion 32, preferably conforming to the B-188 standards to
ensure wide compatibility with gas bottle suppliers. Once securely coupled,
probe 36
opens a valve in the LPG bottle and pressurized gas travels through probe 36
and
25 into chamber 26. Chamber 26 is generally defined by inlet housing 27 and
seat 28.
Within chamber 26 are ball 29 and compression spring 25. Compression spring 25
provides an outward bias to ball 29, which is prevented from translational
movement
by seat 28 reacting against outlet housing 31 via thermal fuse body 38. LPG
occupies both chamber 26 and area 26', which is in fluid communication with
outlet
30 conduit 40 via port 39. Outlet conduit 40 then permits LPG to discharge
into
regulator 42.
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A feature of the disclosed arrangement is directed towards a thermal LPG
interrupt that functions to autonomously stop the flow of gas from the
container to the
burner. As briefly described above and as best shown in Fig. 2A, seat 28
functions
to prevent ball 29 from extending into contact with sealing surface 41. In
turn, seat
28, which is in a compression mode through the bias imparted by spring 25 to
ball 29,
reacts against outlet housing 31 via thermal fuse 38. But for the presence of
fuse 38,
seat 28 would be urged to translate away from compression spring 25, thereby
permitting ball 29 to come in sealing contact with sealing surface 41, and
thereby
occlude further gas passage into outlet conduit 40. Therefore, fuse 38 is
intentionally
constructed to lose structural cohesion at or above a general temperature to
prevent
potentially explosive conditions such as might be encountered during a "light
back" or
reverse ignition propagation event. While the ultimate determination of the
appropriate temperature is a matter of design consideration, the disclosed
embodiment contemplates thermal conditions of between about 145 C to 200 C as
being candidate temperatures for a thermal trip.
While those persons skilled in the art will appreciate the broad selection of
candidate materials, particularly satisfying results have been obtained when
Ultra-
High Molecular Weight (UHMW) plastics are chosen, or eutectic alloys. A
benefit of
using eutectic alloys concerns both the precise nature of their phase
conversion and
the very sharp transition provided by them. This second characteristic is of
importance to the operational life of the burner, because the thermal fuse is
in an
axial compression mode, mechanical creep can occur, particularly at higher
temperatures, thereby potentially decreasing the performance of the
arrangement
during normal conditions. One alloy that has yielded favorable results
comprises
cadmium -18.2% wt.; lead - 30.6% wt.; tin- 51.2% wt. This alloy has a melting
point of about 145 C 1.5 C.
Upon passing thermal fuse 38, the compressed gas is directed towards regulator
and valve assembly 42 for pressure and volume regulation. Control handle 44
provides functionality to assembly 42 as is appreciated by those persons
skilled in
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the art. Regulated gas is then directed to both gas jets 48a and 48b via
distribution
manifold 46, which in turn direct fuel gas into mix tubes 50a and 50b.
Entrainment of an
oxidizer, in this case oxygen bearing air, occurs at the injector and
throughout the length
of the mix tube by drawing air into the mix tube at openings 16a and 16b,
which
represents the only major openings within pressure cavity 24. Those persons
skilled in
the art will appreciate that other forms of oxidizer introduction could take
place via the
same or different structure. However, the present embodiment represents an
efficient and
cost-effective approach to the production of a combustible gas. Because the
described
method and related structure rely upon momentum transfer (a venturi effect is
established
at opening 16a and 16b, which creates a localized area of low pressure,
thereby drawing
in ambient air to aid in combustion), mixing of the fuel gas with an oxidizer
is
accomplished efficiently inexpensively. Moreover, because there are no moving
parts,
reliability and longevity are increased.
To optimize the introduction of airas an oxidizer and minimize the effects of
the
environment (primarily wind for portable burner operations), surrounding 14 is
coaxially
surrounded by perforated housing 18. Consequently, a generally annular space
is
created between surrounding 14 and housing 18, from which air is drawn into
openings
16a and 16b. In this manner, any wind impacting perforated housing 18 is
diffused
prior to entering opening 16a and 16b.
The fuel gas and oxidizer combination (pre-combustion gas) exits from ends 54a
and 54b of mixing tubes 50a and 50b and enters cavity 24, where upon it
impinges
static mixing and heat transfer posts 56. As intimated by its name, static
mixing and
heat transfer posts 56 perform a dual function: Because posts 56 are thermally
coupled
to base 12, heat generated by burner 10 and transferred to base 12 by
radiation,
conduction and/or convention is partially removed by contact between posts 56
and
incoming cool pre-combustion gas. Beneficially, this drawing of heat from base
12
increases the heat content of pre-combustion gas, which promotes more
efficient
combustion thereof. Posts 56 also beneficially function to increase mixing of
pre-
combustion gas prior to combustion and aid in uniform
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distribution of pre-combustion gas by decreasing the gas velocity so that
diffusion of
pre-combustion gas through burner element 60 occurs more uniformly.
As noted earlier, during operation of burner 10, a pressure gradient exists
between upper surface 64 of burner element 60, which is exposed to ambient
conditions, and lower surface 62 of burner element 60, which is exposed to
slightly
pressurized pre-combustion gas. After transport of pre-combustion gas from
cavity
24 to upper surface 64, piezoelectric igniter 66 may be operated to initiate
combustion of pre-combustion gasses, in a manner well known in the art. Upon
ignition, combustion migrates to just below upper surface 64 of burner element
60,
and is prevented from further propagation by the low bulk thermal conductivity
and
small pore size of burner element 60. At this point, burner 10 becomes a
radiant
burner with no perceptible freely convective frame.
Screen 20 is provided as a safety feature to prevent unintentional physical
contact with burner element 60 and to serve as an interface with cookware
employing
a heat exchanger as described in detail below. Both screen 20 and perforated
housing 18 are secured to burner 10 by way of screen retainer ring 22. Should
maintenance of burner 10 become necessary, a user need only remove retainer
ring
22 to expose upper surface 64 of burner element 60, or through removal of
burner
element 60, base 12.
While radiant burner 10 represents a significant advance in heating technology
with respect to efficiency, safety and reliability, further advances have been
achieved
when this technology is used in conjunction with a heat exchanger purposefully
adapted to extract the maximum amount of heat from burner 10. As best shown in
Figs. 1 and 4-7, heat exchanger 90 can be integrated into a fluid vessel, and
more
particularly vessel or pot 70. The purpose of heat exchanger 90 is to
efficiently
extract heat generated by burner 10 by taking advantage of its combustion
mode. In
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this respect, the mass flow and temperature attributes of heat generated by
burner 10
are considered in the design of heat exchanger 90.
As shown in the several drawings, the constitution of heat exchanger 90 can
take many forms. The ultimate selection of one form over another may be driven
by
design considerations such as the volume of vessel 70, the nature of the
liquid to be
heated, the fluid dynamic properties of the combustion gasses, and similar
factors.
Thus, the presently illustrated embodiments are intended to show several
variations,
but are by no means representative of an exhaustive inventory of available
heat
exchangers. However, the presently illustrated embodiments all attempt to
maximize
the surface area exposed to the radiant heat and combustion gasses of burner
10
without significantly minimizing the benefits achieved through convection
heating.
Thus, the illustrated embodiments employ a plurality of channels having
relatively
unobstructed exit paths where the channels maximize the distance the
combustion
gasses must travel from burner element 60 to the ambient environment.
Turning first to Fig. 5A, a weld-on heat exchanger arrangement is shown.
Here, a plurality of fin elements 80 are formed separately from pot 70, and
subsequently attached to pot 70 such as by spot welding, brazing or similar
heating
techniques to create a plurality of channels 86 through which combustion
gasses
may travel. Fin elements 80 are preferably constructed from aluminum by
stamping
or similar high volume creation means. Fin elements 80 are preferably formed
for
placement on bottom surface 78 of pot 70 in a spiral or involute pattern to
maximize
exposure time of the combustion gasses with the elements. Fig. 5B shows a
similar
pattern of fin bodies 82 formed on bottom surface 78 of pot 70, however, fin
bodies
82 are integral with bottom surface 78. In this embodiment, fin bodies 82 may
be
formed by machining the desired pattern in bottom surface 78 or during casting
of
bottom surface. While the thermal transfer rates from fin bodies 82 to pot 70
and
overall durability are greater than the thermal transfer rates from fin
elements 80 to
pot 70 due to the more robust association of the former with the pot,
manufacturing
costs are higher.
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In addition to machining or casting methods for creating suitable fin bodies,
a
preferred means of manufacturing integral fin bodies is by impact extrusion
processes.
These processes provide the benefits of exceptional thermal conductivity
(superior to that
of casting), desirable surface finish for the cooking surface (superior to
that of casting or
machining), low weight (superior to that of casting and machining, which also
generates
avoidable waste) and low cost (superior to that of machining and welding).
While there
are size limitations using these processes, they are not material to the form
factors
commonly used in backpacking cookware.
It should also be noted that bottom surface 78 need not be planar or flat.
Again
depending upon design parameters, bottom surface 78 can be conical or frusto-
conical
like, with the apex at the center of the vessel. Such a geometry will not only
beneficially
modify the residency of any combustion gasses during operation of a burner,
but when
used in conjunction with a burner such as burner 10 having screen 20 will
restrict properly
mate with the burner to the exclusion of other cookware. Alternatively, a
plurality of
surface features such as convex or concave features can be established in or
on bottom
surface 78 to alter the egress of combustion gasses to the environment.
The embodiment of Fig. 6 illustrates a perimeter heat exchanger arrangement
that
can be used in conjunction with the heat exchangers of Figs. 5A and 5B, or
with other
arrangements. By linking a plurality of perimeter elements 84 as shown in Fig.
7, for
example, and surrounding the perimeter of pot 70 with such elements, waste
heat exiting
from channels 86, for example, impinges upon perimeter elements 84 and is
redirected
along reduced diameter portion 74 of pot 70. In this manner, additional
surface area for
heat exchange is created at both perimeter elements 84, which are thermally
linked to
heat exchanger 90, as well as directly to pot 70. To prevent the unintentional
migration
of fluid in pot 70 from entering heat exchanger 90, drip ring 76 is provided
above
reduced diameter portion 74.
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