Note: Descriptions are shown in the official language in which they were submitted.
~EAT FLUX METER
The present invention relates to heat flux
meters and the measurement of radiation heat f 1UY~.
5Heat flux meters have previously been described
by Northover et al in J. Sci. Instrum., 1967, Vol. 44,
pp 371-374. ~s described in that article, a heat flux
meter intended for use in furnaces of steam generators
comprises a cylindrical body member and a disc member
joined to the cylindrical body. The assembly is mounted
to a boiler wall, typically by welding, thereby forming
an internal cavity.
~ he principle of operation of this heat flux
meter is that heat flowing into the disc distributes
l~ radially to the cylindrical body and is then conducted
to the boiler wall. The finite thermal resistance of
the disc gives rise to a temperature difference between
its centre and periphery. Upon attachment of suitable
connecting wires to the disc centre and the cylindrical
body, an e.m.f. is produced which is proportional to the
magnitude of the disc radial temperature difEerence and,
therefore, the heat flux. The connecting wires are
constructed of the same material as the cylindrical body
while the disc material is thermoelectrically-dissimilar
to the cylindrical body material.
A connector tube of the same material as the
meter body is joined to it by weldingO When the flux
meter is mounted in a boiler, both the body and the
connector tube are welded to a boiler tube to provide a
low resistance heat path through all parts of the meter
system. The connecting tube is made long enough to
protrude outside the boiler into a region at or near
room temperature. The two connecting wires are lead
through the connector tube to the interior of the flux
meter. A seal is provided between the room temperature
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end of the connecting tube and the wires, so that a
controlled atmosphere may be maintained in the meter
cavity.
In this prior art heat flux meter, the disc
member is joined to the cylindrical body by a
thermoconductive solder, which is specified to be a
silver-based solder in the Northover et al disclosure.
An oxygen-free atmosphere must be maintained within the
heat flux meter cavity to avoid destruction by oxidation
of the various joints within the meter, in particular
between the wires and the disc member and/or the body
member. Heat flux meters are required ~o have a service
life of, preferably, several years and over such
extended periods of time, even the very slow ingress of
corrosive elements may prove destructive~
It has now been found that these prior art heat
flux meters are prone to failure upon extended periods
of operation, especially when mounted in those regions
of the boiler which experience the highest heat fluxes,
namely the furnace walls in the vicinity of the burnersO
Examination of failed flux meters reveals corrosive
attack at the joint between disc and body members and
corrosive oxidative failure of the joint between the
disc and the connecting wire. In the nigh heat flux
service applications, the temperature of the disc centre
rises seve~al hundred degrees Celsius above the
temperature oE the tube walls. The disc may then be
required to operate above the melting temperature of
commercial silver-based soldexsO
Any junction betw~en two dissimilar metals is
known to be an area of enhanced corrosive attack. The
presence of any solder layer, but in particular a
silver-based solder at the interface of the disc and the
body members, amplifies the effect. Sllver, and liquid
silver even more so than solid silver, has a high
solubility for oxygen and sulfur, the two principal
corrosive elements in the boi].er atmosphere. As a
result of this property, the silver solder layer acts as
a preferred path for the diffusion of the corrosive
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atoms to reaction sites, so as to enhance the rate of
corrosive attack.
While the applicant does not wish to be bound by
any theory to explain the observed failures of the prior
art heat flux meters, they are thought to occur by
oxidation and destruction of the joint between the disc
and body members as a result of corrosion at the high
temperatures typically encountered in boilers. The
concomitant ingress of oxygen and possibly sulfur into
the flux meter cavity of the flux meter leads to
corrosive attack upon joint between connecting wire and
disc and ultimately destruction of the meter. Moreover,
before destruction, corrosion at the disc/body solder
joint increases the thermal resistance of the joint,
resulting in an unstable heat flux versus e.m.f.
calibration.
The present invention overcomes the prior art
difficulties by a unique design, which results in a heat
flux meter which is resistant to failure in long term
use. In the present invention, those portions of the
heat flux meter which are exposed to the combustion
gases in the hot environment are constructed of a single
piece of material and no j oints are exposed to the
combustion gases. By constructing a heat flux meter in
this way, the prior art problems associated with
exposure of the silver solder to the combustion gases
and ingress of deleterious gases to the internal cavity
are avoided~
In this way, a new method of measuring heat flux
in an atmosphere in a closed space is provided. The
novel method of the invention involves exposing a
surface of thermoconductive material to the heat flux
within the enclosed space and permitting the heat flux
to impinge on the surface. A
-thermoelectrically-dissimilar material is positioned
adjacent the periphery of the surface out of direct
exposure to the heat flux and out of comrnunication with
the atmosphere in the enclosure. An e.m.f. thereby is
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established between the middle of the surface and the
thermoelectrically-dissimilar material and is measured
as a determination of the heat flux.
In accordance with the present invention, there
is also provided a novel heat flux meter suitable for
measuring heat flux by the generation of an e.m.f.
proportional to the heat flux, comprising a plurality of
elements. A generally cup-shaped one piece unitary
construction body member is constructed of a material of
relatively low the~mal conductivity and comprises a
generally planar circular top portion and a cylindrical
wall portion extending therefrom to define an interior
cavity to the body member. An annular member,
constructed of a material, thermoelectrically-dissimilar
to that of the body member, is mounted within the cavity
in or near the corner between the circular top and the
cylindrical wall. A pair of electrically-conducting
wires of the same material as the annular member enters
the cavity and is connected, one to the underside of the
circular portion substantially at the centre thereof,
and the othex to the annular memher.
By providing a one-piece body member and an
internally-mounted annular member the potential problems
of connective joints exposed to the corrosive components
of combustion gases in a steam-generating furnace are
avoided. When the heat flux meter of the invention is
mounted to the furnace wall, the internal cavity is
sealed off from the ingress of deleterious gases. In a
preferred em~odiment of the heat flux meter of the
invention, the internal cavity is sealed by a plate of
3 the same material of construction as the body member,
welded to the cylindrical wall of the body member
adjacent to the opposite end from the annular member.
The annular member may be mounted in the cavity
in any convenient manner to establish a heat conducting
3S relationship with the body member remote from the centre
of the circular top portion to which one of the wires is
attached. Preferably, the annular memb r is located in
a complimentarily-shaped recess in the internal wall of
.
the body me~ber. Preferably, the annular member has a
part-circular cross-section and is mounted in the recess
by the use of high temperature stainless brazing
materials, for example, nickel-chromium based brazing
materials, such as those sold under the trademark
"Nicrobraz". Such brazing materials also preferably are
used to join the e.m.f. conducting wires to the centre
of the clrcular top portion, and to the annular member.
Preferably, an oxygen ancl/or sulfur scavenger,
for example, copper or titanium metal, is located in the
cavity to ensure removal of any traces of oxygen which
may be present therein or which may enter the cavity
while in use. ~ ceramic or other thermally-stable inert
filler material of low thermal conductivity preferably
is positioned in the cavity to minimize the gas space
and further to provide support for the annular member
and the oxygen and/or sulfur scavenger.
The invention is described further, by way of
illustration, with reference to the accompanying
drawings, in which:
Fi~ure 1 is a sectional view of a heat flux
meter constructed in accordance with the presently
preferred embodiment of the invention, taken on line 1-1
of Figure 2; and
Figure 2 is a plan view of the flux meter of
Figure 1.
Referring to the drawings, a heat flux meter 10
comprises a generally cup-shaped body member 12. The
body member 12 is of one piece unitary construction and
includes a planar circular top portion 1~ and a
cylindrical wall portion 16 depending from the planar
top portion 12. The body member 12 may be constructed
of any convenient construction material which withstands
the temperatures of the operating environment of the
heat flux meter 10 and which produces a thermal gradient
between the centre and periphery of the disc portion 1~.
Preferred materials of construction include various
stainless steels and those high temperature all~ys sold
for example, under the trademarks "Inconel",
"Hastalloy'l, "~ultimet" and "Haynes".
Located in the internal wall 18 of the
cylindrical wall portion 16 is a part-circular recess 20
in which is rece.ived the periphery of an annular member
or ring 22. Thermal contact between the ring 22 and the
cylindrical wall portion 16 is achieved by brazing,
preferably fluxless vacuum brazing using stainless
brazes or other suitable joining procedure.
The annular ring 22 ls constructed of
thermally-conductive material which is
~hermoelectrically-dissimilar to that of the cup-shaped
body member 12. Suitable materials of construction of
the annular ring 22 include high nickel bearing alloys,
for example, tho~e sold under the trademarks
"Constantan", "Alumel" and "Nickel D".
15Electrically-conducting wires 24, 26 are
connected, one (24) to the circular portion 14 adjacent
its centre and the other (26) to the annular ring 22.
The wires 24 and 26 usually are constructed of the same
material as the annular ring 22 and may be connected to
the respective elements by any suitable bonding method.
Preferably, however, the connections, especially the
connection to the circular member 14, are made with
fluxless vacuum brazing using stainless brazesO
In addition to the wires 24, 26, an additional
pair of wires (not shown) may also be connected one to
the circular portion 14 adjacent .its centre and the
other to the annular ring 22 adjacent the location of
the joint of the wire 24. This latter pair of wires is
of a thermoelectrically-dissimilar material to the wires
24 and 26 and, in this way, provide thermocouples which
may be used to measure the absolute temperature at the
t.wo locations.
A sealing cover 28 is welded to the lower end of
the cylindrical wall portion 16 to define an enclosed
cavity 30. The sealing cover 28 is preferably
constructed of the same material as the cup-shaped body
12.
The connecting wires 22 and 24 extend into the
cavity 30 through a connecting tube 32 which is intended
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to extend through the boiler wall to a room temperature
environment. A suitable seal is provided at the room
temperature end of the tube 32 to prevent the passage of
gases to the cavity 30~
The recessed location of the cover 28 ensures
that, upon mounting of the heat flux meter 10 to a
furnace wall or the like, the joint between the cover 28
and the body 12 is not exposed to the gases located in
the furnace.
1~ The cavity 30 may be evacuated to provide a
vacuum, may be filled with an inert gas, such as argon,
or may be packed with a temperature-resistant inert
filler material of low thermal conductivity~ such as a
ceramic material, which occupies substantially the whole
pace of the cavity 30. The use of a ceramic filler
material also assists in providing support to the
annular ring 22 and in spacing the wires 24 and 26
apart.
An oxygen and/or sulfur scavenger also may be
positioned in the cavity 30 to ensure removal of
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residual oxygen and/or sulfur from the cavity and to
provide long term protection against subsequent leakage
of oxygen and/or sulfur gases into the cavity 30. Any
suitable known oxygen-scavenging material may be
utilized, such as, copper or titanium metal, in the form
of fine wire mesh, powder or sponge. The oxygen
scavenger material in fine powder form is preferably
mixed with the ceramic powder in suitable proportions
and located near the under surface of the circular
member 14 t where the temp~ratures are the highest.
The heat flux meter 10 comprises a few parts
only, comprising the cup-shap~d member 12, the annular
ring 22, the cover 28, and the wires 24 and 26. ~he
unitary nature of the cup-shaped member 12 and the
preferred use of a ring 22 of circular cross-section
permit ready formation and assembly of the flux meter,
especially during mass production manufacture.
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In operation, the heat flux meter 10 is mounted
in an environment in which it is desired to measure the
heat flux, for example, the region of the water walls of
a boiler. Heat flux is received over the whole of the
outer surface of the circulax portion 14 of the
cup-shaped member 12. By reason of the temperature
difference between the centre of the circular portion 14
and the annular ring 22 and the inherent thermoelectric
dissimilarity of the materials of construction of the
-two, an e.m.f. is induced between the centre of the
circular portion 14 and the annular member 22, and is
measured through the wires 24 and 26. The e.m.f. is
proportional to the heat flux received by the meter 10.
The latter measurement may be used in systems to
monitor and control ash formation on the walls of the
boiler, such as is described in U.S. Patent No.
4,~08,568, or for any other convenient purpose.
In summary of this disclosure, the present
invention provides a novel heat flux meter of improved
construction and a novel method of measuring radiation
heat flux which do not suffer from the drawbacks of the
prior art. Modifications are possible within the scope
of the invention.
