U.S. patent application number 12/192721 was filed with the patent office on 2010-02-18 for tube shields having a thermal protective layer.
This patent application is currently assigned to Wessex Incorporated. Invention is credited to John W. Olver, Jason Andrew Simmons.
Application Number | 20100038061 12/192721 |
Document ID | / |
Family ID | 41680464 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100038061 |
Kind Code |
A1 |
Olver; John W. ; et
al. |
February 18, 2010 |
TUBE SHIELDS HAVING A THERMAL PROTECTIVE LAYER
Abstract
A tube shield, and method of manufacturing the tube shield,
having a support structure with an external surface, an internal
surface, and an edge, and a thermal protective layer on at least
one surface of the shield support structure. The thermal protective
layer is composed of a filler, one or more emissivity agent, and
either an inorganic adhesive or a binder that is colloidal silica,
colloidal alumina, or combinations thereof. A colorant, a
surfactant, and/or a stabilizer may be incorporated into the
thermal protective layer.
Inventors: |
Olver; John W.; (Blacksburg,
VA) ; Simmons; Jason Andrew; (Blacksburg,
VA) |
Correspondence
Address: |
JOHNSTON, HOLROYD & ASSOCIATES
1438 MAIN STREET
PRINCETON
WV
24740
US
|
Assignee: |
Wessex Incorporated
|
Family ID: |
41680464 |
Appl. No.: |
12/192721 |
Filed: |
August 15, 2008 |
Current U.S.
Class: |
165/134.1 ;
29/890.03 |
Current CPC
Class: |
F28F 19/002 20130101;
F22B 37/107 20130101; Y10T 29/4935 20150115 |
Class at
Publication: |
165/134.1 ;
29/890.03 |
International
Class: |
F28F 9/00 20060101
F28F009/00; B21D 53/02 20060101 B21D053/02 |
Claims
1. A tube shield for protecting an external surface of a tube
having a tube wall, comprising: a support structure having an
internal surface and an external surface, said internal surface
formed to encompass the tube and be disposed adjacent the tube
wall; and a thermal protective layer disposed on at least one
surface of said shield support structure, wherein said thermal
protective layer has a. from about 5% to about 40% of an inorganic
adhesive, from about 45% to about 92% of a filler, and from about
1% to about 20% of one or more emissivity agents; or b. from about
5% to about 60% of colloidal silica, colloidal alumina, or
combinations thereof; from about 23% to about 79% of a filler; and
from about 1% to about 20% of one or more emissivity agents.
2. The tube shield of claim 1, wherein: said support structure is
an elongated half-circle shell conformed to mate with a second tube
shield to receive the tube therebetween; and said support structure
further comprises an edge surface running along a periphery of said
internal and external surfaces and extending therebetween.
3. The tube shield of claim 1, wherein: said thermal protective
layer is disposed on said external surface of said support
structure; disposed on said internal surface of said support
structure; disposed on said external surface and on said internal
surface; or disposed substantially on all surfaces of said support
structure.
4. The tube shield of claim 1, wherein: said support structure
comprises a metallic substrate or a ceramic substrate.
5. The tube shield of claim 4, wherein: said metallic substrate is
taken from the group consisting of steel, low carbon steel,
stainless steel, cast iron, iron, aluminum, and alloys, and
combinations thereof.
6. The tube shield of claim 1, wherein: said thermal protective
layer further comprises from about 1.0% to about 5.0% of a
stabilizer; from about 1.0% to about 5.0% of a stabilizer taken
from the group consisting of bentonite, kaolin, magnesium alumina
silica clay, tabular alumina, and stabilized zirconium oxide; up to
about 1.0% of a surfactant; a colorant; or combinations
thereof.
7. The tube shield of claim 1, wherein: said inorganic adhesive is
taken from the group consisting of an alkali/alkaline earth metal
silicate taken from the group consisting of sodium silicate,
potassium silicate, calcium silicate, and magnesium silicate; said
filler is taken from the group consisting of silicon dioxide,
aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide,
and boron oxide; said one or more emissivity agents are taken from
the group consisting of silicon hexaboride, boron carbide, silicon
tetraboride, silicon carbide, molybdenum disilicide, tungsten
disilicide, zirconium diboride, cupric chromite, and metallic
oxides; said emissivity agents are a metal oxide taken from the
group consisting of iron oxide, magnesium oxide, manganese oxide,
chromium oxide, and derivatives thereof; or combinations
thereof.
8. The tube shield of claim 1, wherein: said thermal protective
layer contains a. from about 5% to about 40% of an inorganic
adhesive, the inorganic adhesive is taken from the group consisting
of an alkali/alkaline earth metal silicate taken from the group
consisting of sodium silicate, potassium silicate, calcium
silicate, and magnesium silicate; from about 45% to about 92% of a
filler, the filler taken from the group consisting of silicon
dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium
oxide, and boron oxide; and from about 1% to about 20% of one or
more emissivity agents taken from the group consisting of silicon
hexaboride, boron carbide, silicon tetraboride, silicon carbide,
molybdenum disilicide, tungsten disilicide, zirconium diboride,
cupric chromite, and metallic oxides; b. from about 5% to about 60%
of colloidal silica, colloidal alumina, or combinations thereof;
from about 23% to about 79% of a filler taken from the group
consisting of silicon dioxide, aluminum oxide, titanium dioxide,
magnesium oxide, calcium oxide, and boron oxide; and from about 1%
to about 20% of one or more emissivity agents taken from the group
consisting of silicon hexaboride, boron carbide, silicon
tetraboride, silicon carbide, molybdenum disilicide, tungsten
disilicide, zirconium diboride, cupric chromite, and metallic
oxides; c. from about 5% to about 40% of an inorganic adhesive, the
inorganic adhesive taken from the group consisting of an
alkali/alkaline earth metal silicate taken from the group
consisting of sodium silicate, potassium silicate, calcium
silicate, and magnesium silicate; from about 45% to about 92% of a
filler, the filler taken from the group consisting of silicon
dioxide, aluminum oxide, titanium dioxide, magnesium oxide, calcium
oxide, and boron oxide; and from about 1% to about 20% of one or
more emissivity agents taken from the group consisting of silicon
hexaboride, boron carbide, silicon tetraboride, silicon carbide,
molybdenum disilicide, tungsten disilicide, zirconium diboride,
cupric chromite, and metallic oxides; and from about 1% to about 5%
of a stabilizer taken from the group consisting of bentonite,
kaolin, magnesium alumina silica clay, tabular alumina, and
stabilized zirconium oxide; or d. from about 5% to about 60% of
colloidal silica, colloidal alumina, or combinations thereof; from
about 23% to about 79% of a filler taken from the group consisting
of silicon dioxide, aluminum oxide, titanium dioxide, magnesium
oxide, calcium oxide, and boron oxide; and from about 1% to about
20% of one or more emissivity agents taken from the group
consisting of silicon hexaboride, boron carbide, silicon
tetraboride, silicon carbide, molybdenum disilicide, tungsten
disilicide, zirconium diboride, cupric chromite, and metallic
oxides; and from about 1% to about 5.0% of a stabilizer taken from
the group consisting of bentonite, kaolin, magnesium alumina silica
clay, tabular alumina, and stabilized zirconium oxide.
9. The tube shield of claim 1, further comprising: a brace disposed
to secure said tube shield in place, said strap having a support
structure with the thermal protective layer disposed thereon.
10. The tube shield of claim 1, wherein: said support structure
formed into an elongated half-circle shell is bent to form an inner
or an outer elbow to accommodate a turn in the tube and coupled
with a second tube shield bent to form an opposing inner or outer
elbow to encapsulate the turn in the tube.
11. The tube shield of claim 1, having from about 2% to about 20%
of a first emissivity agent taken from the group consisting of,
boron carbide, silicon carbide powder, silicon tetraboride,
molybdenum disilicide, tungsten disilicide, zirconium diboride,
cupric chromite, and metal oxides; and from about 0.5% to about
3.5% of a second emissivity agent taken from the grouped consisting
of silicon hexaboride.
12. A method of manufacturing a tube shield having a thermal
protective layer, comprising: providing a support structure having
an exposed surface; wherein the exposed surface is on interior
surface, or on an exterior surface, or on combinations thereof;
mixing a thermal protective coating containing a. from about 6% to
about 40% of an inorganic adhesive, from about 23% to about 56% of
a filler, from about 0.5% to about 15% of one or more emissivity
agents, and from about 18% to about 50% water, or b. from about 15%
to about 60% of colloidal silica, colloidal alumina, or
combinations thereof; from about 23% to about 55% of a filler, from
about 0.5% to about 15% of one or more emissivity agents, and from
about 10% to 50% water; and applying the mixed thermal protective
coating to the exposed surface using a spray gun to form a thermal
protective layer from about 2 mils (5 microns) to about 10 mils
(254 microns) thick.
13. The method of claim 12, further comprising: the thermal
protective layer further comprises from about 0.5 percent to about
2.4 percent of a stabilizer; up to about 1.0% of a surfactant; from
about 0.5 percent to about 2.4 percent of a stabilizer taken from
the group consisting of bentonite, kaolin, magnesium alumina silica
clay, tabular alumina, and stabilized zirconium oxide; a colorant;
or combinations thereof.
14. The method of claim 12, wherein: the inorganic adhesive is
taken from the group consisting of an alkali/alkaline earth metal
silicate taken from the group consisting of sodium silicate,
potassium silicate, calcium silicate, and magnesium silicate; the
filler is taken from the group consisting of silicon dioxide,
aluminum oxide, titanium dioxide, magnesium oxide, calcium oxide,
and boron oxide; the one or more emissivity agents are taken from
the group consisting of silicon hexaboride, boron carbide, silicon
tetraboride, silicon carbide, molybdenum disilicide, tungsten
disilicide, zirconium diboride, cupric chromite, and metallic
oxides; the emissivity agents are a metal oxide taken from the
group consisting of iron oxide, magnesium oxide, manganese oxide,
chromium oxide, and derivatives thereof; or combinations
thereof.
15. The method of claim 12, wherein: the spray gun is taken from
the group consisting of an high volume low pressure spray gun or an
airless spray gun.
16. The method of claim 12, further comprising: agitating the
solution of thermal protective coating prior to applying.
17. The method of claim 12, further comprising: rotating the
direction of spray to facilitate an even thickness.
18. The method of claim 12, further comprising: allowing the
thermal protective layer to air dry from about two to about four
hours.
19. The method of claim 12, wherein: the support structure
comprises a metallic substrate or a ceramic substrate.
20. The method of claim 12, further comprising: preparing the
exposed surface first by cleaning, grit blasting, or combinations
thereof.
Description
BACKGROUND OF THE INVENTION
[0001] Tube shields are found in furnaces, boilers, burners, heat
exchangers, incinerators, and the like, and function to protect
boiler/condenser tubes used in the generation of power, steam,
condensation, heat, and the like, for a variety of applications
including power generation, manufacturing and production processes,
such as found in the petrochemical industries. Boiler tubes carry
air, water, steam or other fluid. Condenser tubes also carry fluids
in the form of gasses and liquids. Boiler tubes are exposed to an
extreme heat source in a combustion chamber. Condenser tubes may be
used to convert gasses to liquids, such as steam to water for
recycling back into a boiler, and may also be exposed to extreme
temperatures. Hot air, and/or ignited fuel, is dispensed through
burner tips into the combustion chamber from external fuel sources
such as coal, natural gas, biomass, slag, and the like. Under these
general circumstances the mixture of fuels is ejected through the
burner tip and combined in the combustion chamber, possibly in the
presence of ambient air. Inside the combustion chamber, the hot
mixture of fuels comes in contact with the exterior surface of the
tube shields, encasing the tube therein, which heats the fluid
within the tube.
[0002] This is a form of heat exchange. Heat exchangers are most
broadly defined as apparatuses that transfer heat from one medium
to another, usually by conduction through a solid barrier. In this
example of thermal communication, the heat is exchanged from the
hot mixture of fuel and combustion gas in the combustion chamber to
the fluids traveling through the boiler tubes. The heated fluid in
the boiler tubes may then provide energy to turbines, or the like,
for power, heat generation, and/or a variety of other applications.
The combustion gas is frequently air, while the fuel can be coal,
natural gas, wood, solid waste, medical waste, garbage, biomass, or
the like.
[0003] The environment within the combustion chamber can be
extremely corrosive and abrasive. The boiler tubes employed in the
combustion chamber are commonly exposed to highly abrasive and
corrosive environments. Exposure of the tubes to such environments
often causes premature failure resulting in expensive maintenance
and mounting boiler/facility downtime costs. Boiler tubes must be
replaced regularly. It is also known that with boiler tubes of a
refuse or solid waste incinerator-type boiler, the heat-conductive
performance deteriorates when soot and slag attach to the tubes
requiring cleaning, and acids generated by combustion corrode the
tubes, which results in deterioration of the tubes and ultimate
failure. In any case, exposed boiler tubes are subject to failure
due to the hostile environment of the combustion chamber, and must
be replaced frequently,
[0004] A tube shield, or its components, typically made of steel,
low carbon steel, stainless steel, and/or other metals, or alloys
thereof, may be attached to the surface of the boiler tubes to
provide physical protection. These tube shields are typically used
in an effort to reduce and delay corrosion and erosion which
results in eventual failure of the boiler tubes. However, since
such tube shields are readily corroded themselves with corrosive
gases contained in exhaust gases, the tube shields themselves have
durability of less than one year in the most hostile environments,
and at worse they are corroded or eroded beyond useful life in less
than three (3) months.
[0005] The tube shields cover the external surface of the boiler
tubes, and are directly exposed to the extremely corrosive
environment within the combustion chamber. While conventional tube
shields are designed to increase the life of the boiler tubes, they
are still subject to destruction due to the corrosive environment,
and must be replaced regularly. Many shield type configurations
have been devised to protect the tubes from hostile environments.
One such configuration includes axially elongated protectors with
half-circle cross sections. The shield is sized to fit over the
boiler type tube to be protected. A strap may be used to hold the
shields in place. U-bolts, mortar, interlocking configurations,
welding, rod inserted through openings in adjacent shields,
combinations thereof, and the like are also used.
[0006] It is conventional to protect boiler tubes in the path of
the gas stream with tube shields. These shields may be
semi-circular, elongate, and composed of stainless/carbon steel
members that are each secured to the tube by U-shaped clamps that
extend around the back of the tube and are welded to the shields.
It is not uncommon for tube shields to be stacked and separately
clamped on a boiler tube with the objective of protecting the tube
for longer periods. Conventional practice requires shutting down
and opening the boiler for inspection and maintenance about every
six (6) months to one and a half (11/2) years. Upon the first
shutdown, typically several of the shields will have been so eroded
that they have fallen off or need to be replaced; after the next
shutdown, all of the shields on the tubes are usually removed and a
new shield attached.
[0007] Coal fuel power plants use burner tips to inject hot/ignited
fuel into combustion chambers, which are exposed to high
temperatures and abrasion of high velocity coal particles plus slag
movement. The exposed hard faced surfaces develop rough surfaces
which increase eddy currents to the coal laden stream, thus
reducing the velocity of flow. These eddy currents increase wear,
because the stream is not moving in laminar flow, as well as
effecting combustion dynamics, emissions, and chemical byproducts.
Boiler tubes, and tube shields, exposed to this environment are
corroded and eroded by the flow.
[0008] Also solid waste/garbage incinerators used in the generation
of energy utilize boiler tubes within a combustion chamber. The
high cost of energy has led society to extract usable heat from
high thermal value waste streams. In some incinerator operations, a
heated waste stream passes over conventional cross-flow metallic
heat exchanger tubes containing clean ambient air. The ambient air
is heated by the waste stream and then typically used as either
facility or process heat. In other applications, such as solid
waste incineration in which trash and garbage are incinerated to
form gaseous products at temperatures up to 2500.degree. F., water
is passed through metallic tubes positioned within the gaseous
product stream and converted to steam by the high temperatures. The
steam produced by the tube assembly is then used to power a
turbine-driven electrical generator, or to provide heat for
commercial industrial use, such as building heat. The heated steam
may then be condensed in a condenser tube for other work or to be
recycled.
[0009] Tube shields for boiler tubes are known, and many
configurations and designs have been developed. For example, U.S.
Pat. No. 5,511,609 issued to Tyler on Apr. 30, 1996 discloses a
tube shield with tongue and locking block assembly, wherein the
tube shields are held in place using straps. The Tyler invention is
used for power generation and recovery boilers. The disclosed tube
shields may be made of stainless and/or carbon steel.
[0010] U.S. Pat. No. 5,884,695 issued to Brownlee on Mar. 23, 1999
and assigned to American Magotteaux Corporation discloses boiler
tube shields that interlock with each other to protect the weld of
a securement strap used to secure the shields to boiler tubes in a
power plant. These tube shields have semi-cylindrical tube members
that terminate in first and second ends, the first end having a
tongue portion and a stepped portion and the second end provided
with a flanged portion and a pair of longitudinally extending
bayonets. The securement strap is wound about overlapping tube, and
interlocking tube shields, which are welded into place. Similarly,
U.S. Pat. No. 6,065,532 also issued to Brownlee on May 23, 2000 and
assigned to American Megatteaux Corporation teaches interlocking
tube shields that are configured to protect the weld of a
securement strap used to secure the shields to a boiler tube of a
power plant. The heat shields of these Brownlee patents can be made
of different materials depending upon the hostility of the
environment in which they will be exposed. Different grades of
stainless steel or nickel/chrome alloys may be used. The securement
strap is preferably made of a stainless steel.
[0011] Efforts to reduce corrosion are also known. U.S. Pat. No.
7,066,242 issued to Ranville et al. on Jun. 27, 2006 disclose a
sacrificial refractory tube shield assembly for use on a boiler
tube in an effort to protect the underlying boiler tube from
erosion by a stream of hot combustion gas containing particulates.
The refractory shield assembly comprises: a semi-circular, elongate
metal shield; a plurality of spaced apart anchors, whereby the
refractory material is held on the shield by the anchors; and
means, such as clamps, for securing the shield on a boiler tube.
The refractory shield assembly functions to protect the underlying
boiler tube from erosion by a stream of hot combustion gas
containing particulates.
[0012] U.S. Design Pat. No. D436,399 issued to Poland on Jan. 16,
2001 shows a design for a shield that appears to be one that would
be applied to the outside of a burner or condenser pipe. Similarly,
U.S. Design Pat. No. D437,044 issued to Poland on Jan. 30, 2001
also shows a design for a shield which appears to be one that would
be applied to the outside of a burner or condenser pipe.
[0013] To prevent direct attack of the tubes by the products of
combustion, while allowing the tubes to be superheated, the prior
art has used refractory ceramic shields to cloak the tubes. The
refractoriness of these shields provides for high thermal
conductivity, integrity at high temperatures, erosion resistance
and corrosion resistance. For example, U.S. Pat. No. 4,682,568
issued to Green et al. on Jul. 28, 1987 and assigned to Norton
Company teaches a refractory shield for superheater tubes which are
composed of a refractory material comprising a pair of elongated
half shields of identical inter-changeable interlocking size and
shape. Each half shield has a semi-circular sidewall portion
extending between and to diametrically opposite tongue and groove
sidewall portions that are assembled together about the burner
tubes by axially inserting the tongues into the grooves. The burner
shields of the Green et al. Patent are used to protect boiler super
heater tubes from corrosive, erosive and abrasive action by the
products of combustion during incineration of trash and garbage as
fuel for the generation of energy.
[0014] The superheater tube shields, of the Green et al. Patent,
are preferably nitride bonded silicon carbide refractory material
made of 30% 30-90 mesh green silicon carbide, 17% 100 mesh and
finer green silicon carbide, 35% 3 microns green silicon carbide,
and 18% 200 and finer mesh silicon metal powder mixed with 12%
water and 0.75% sodium silicate deflocculant solution, and
dried/fired in a mold at 1450.degree. C. in a kiln with a nitrogen
atmosphere until cured. A refractory cement disclosed in the Green
et al. Patent is a mixture of 85% by weight of 10 mesh size and
finer size particles of green silicon carbide and 15% of calcium
aluminate mixed together and with 10-15% water to form a plastic
mortar. Upon firing during operation of the incinerator the mortar
becomes a bonded silicon carbide layer between the tube shield and
the super heater burner tubes.
[0015] U.S. Pat. No. 5,724,923 ('923) issued to Green on Mar. 10,
1998 and assigned to Saint-Gobain/Norton Industrial Ceramics
Corporation teaches a refractory tube shield design for superheater
tubes, having first and second partial-tubes with C-shaped
cross-sections, wherein the ends of the partial-tubes are opposably
engaged and coupled with some anti-rotation means. The shields of
the '923 invention may be made of material typically used as a
superheater tube shield, including silicon carbide, alumina,
zirconia, magnesia, chromia, and mixtures thereof. Preferably, the
shields are made of nitride bonded silicon carbide whose silicon
carbide component is made from mixing 30% of 30-90 mesh green
silicon carbide, 17% of -100 mesh green silicon carbide, 35% of
micron silicon carbide and 18% of -200 mesh silicon metal powder
mixed with 12% water and 0.75 sodium silicate defilocculant, and
molded into shape.
[0016] U.S. Pat. No. 5,881,802 ('802) issued to Green on Mar. 16,
1999, and also assigned to Sain-Gobain Industrial Ceramics, Inc.
teaches a refractory shield design for superheater tubes to protect
the superheater tube against fluid attack comprising first and
second partial-tubes, each partial-tube having a C-shaped cross
section, the C-shaped cross section defining first and second ends;
wherein the partial-tubes comprise means for preventing radial
movement of the first partial-tube relative to the second
partial-tube.
[0017] The tube shields of the '802 patent may be made of any
refractory material typically used as a superheater tube shield,
including silicon carbide, alumina, zirconia, magnesia, chromia,
and mixtures thereof. In preferred embodiments, the shields are
made from a nitride bonded silicon carbide whose silicon carbide
component is made from mixing 30% of 30-90 mesh green silicon
carbide, 17% of -100 mesh green silicon carbide, 35% of 3 micron
silicon carbide and 18% of -200 mesh silicon metal powder. This
mixture is then mixed with 12% water and 0.75% sodium silicate
deflocculant and poured in a mold to form the desired shape. A
mortar is commonly used to bond the tube shields to the superheater
tubes having a silicon carbide-based mortar containing silica,
alumina and alkalies.
[0018] U.S. Pat. No. 6,136,117 ('117) issued to Shibata et al. on
Oct. 24, 2000 and assigned to NGK Insulators, Ltd. and Mitsubishi
Heavy Industries, Ltd. teaches a boiler tube protector and a method
for attaching such protector to a boiler tube. The boiler tube
protectors have cylindrical or semi-cylindrical shape around an
outer peripheral face of a boiler tube with mortar, which boiler
tube protectors, are a plurality of ceramic bodies closely arranged
along their parting planes, wherein the parting places include
means for restraining slippage of each of the ceramic bodies along
the parting planes. In order to minimize reduction in heat
conductivity of that portion of the boiler tube at which the boiler
tube protector is attached, it is preferable to select a ceramic
material having excellent heat conductivity.
[0019] Similarly, U.S. Pat. No. 6,152,087 ('087) issued to Shibata
et al. on Nov. 28, 2000 and assigned to NGK Insulators, Ltd.
teaches a boiler tube protector which has a plurality of ceramic
bodies closely arranged along their parting planes, wherein the
parting planes include a restraining portion for restraining
slippage of each of the ceramic bodies. As such, ceramic material
for both '117 and '087 has both corrosion resistance and heat
conductivity, SiC was recited by way of example. As mortar to
attach the boiler tube protector to the outer peripheral face of
the boiler tube, SiC based mortar, mullite based mortar, alumina
based mortar or the like may be used. A SiC based mortar was
preferred. Ceramic fibers may also be used instead of a part or an
entire part of the ceramic material. The ceramic fibers may be used
in a mixed state with mortar, or appropriate ceramic fiber-based
mortar may be used for this purpose.
[0020] Examples of tube shields that may be used with heat
exchangers, either boilers or condensers, are shown in U.S. Pat.
Nos. 5,154,648 and 5,474,123 issued to Buckshaw on Oct. 13, 1992
and Dec. 12, 1995 respectively which show tube shields designed to
encase individual tubes. U.S. Pat. No. 5,094,292 also issued to
Busckshaw on Mar. 10, 1992 shows a tube shield designed to shield
multiple tubes at the same time which is composed of a shield with
a J-shaped profile that hangs like a sheet from a top tube,
SUMMARY OF THE INVENTION
[0021] The present invention is drawn to an improved heat
exchanger, and in particular, to improved heat exchangers which use
tube shields to protect tubes within the heat exchangers.
Alternative embodiments of the present invention are drawn to tube
shields used in boilers and in condensers. Embodiments of the
present invention are further drawn to improved combustion
chambers, typically found in utility, power, heat, steam and hot
water generation, and other furnaces, for example, and improved
components thereof. Embodiments of the present invention contain
tube shields having thermal protective layers on at least one
exposed surface thereof. The tube shields are disposed over tubes
that carry water, steam or other fluid to perform some work. The
tube shields used to protect boiler tubes are directly or
indirectly exposed to hot, and/or ignited combustible fluids,
including pulverized coal, to heat the fluid contained within the
boiler tubes. These applications included energy generation from
coal, waste, biomass, black liquor, pulp, paper furnaces, and the
like. Alternatively, cooler fluids may be circulated through the
tubes to generate a cooler temperature along the outer surface of
the tubes in which condensation forms on the tube shields disposed
to protect the outside surface of the tubes, or the tubes may
deliver hot fluids, including steam, through a cooler environment
to condense the fluids within the tubes. The present invention is
described in terms of heat shields used in boiler tubes disposed
within combustion chambers, but is seen to encompass all forms of
tube shields consistent with the present disclosure whether used in
boilers, condensers, other heat exchangers, and the like.
[0022] A thermal protective layer on at least one exposed
metallic/alloy surface of a burner tube shield according to an
embodiment of the present invention may contain from about 5% to
about 40% of an inorganic adhesive, from about 45% to about 92% of
a filler, and from about 1% to about 20% of one or more emissivity
agents. An alternative thermal protective layer on at least one
exposed ceramic surface of a burner tube shield according to
another embodiment of the present invention may contain from about
5% to about 60% of colloidal silica, colloidal alumina, or
combinations thereof, from about 23% to about 79% of a filler, and
from about 1% to about 20% of one or more emissivity agents. A
thermal protective layer of the present invention may further
contain from about 1% to about 5% of a stabilizer. A surfactant or
colorant may also be present therein.
[0023] An aspect of the present invention is to extend the
effective repair and replacement cycles of the heat exchangers and
combustion chambers, especially tubes and tube shields. The overall
cost of the utility is reduced by the concomitant reduction in
maintenance costs.
[0024] The present invention extends the effective lifespan of
conventional tube shields and tubes. Furthermore, it reduces down
time and repair costs.
[0025] An aspect of the present invention is to reduce the overall
costs of operating heat exchangers or combustion chambers. Cost
savings are found in the materials for replacing the damaged tubes
and tube shields, in the reduction of employee work load needed to
replace the tubes and tube shields, and in reduced facility
downtime and economic loss.
[0026] These and other aspects of the present invention will become
readily apparent upon further review of the following drawings and
specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The novel features of the described embodiments are
specifically set forth in the appended claims; however, embodiments
relating to the structure and process of making the present
invention, may best be understood with reference to the following
description and accompanying drawings.
[0028] FIG. 1 shows a schematic perspective view of a tangently
fired combustion chamber containing tube shield encased boiler
tubes, according to an embodiment of the present invention.
[0029] FIG. 2A shows a straight tube with two opposing straight
tube shields disposed about the tube, according to an embodiment of
the present invention.
[0030] FIG. 213 shows a U-shaped part of a tube with opposing
fitted inner and outer tube shields thereon, according to an
embodiment of the present invention.
[0031] FIG. 3 shows a diagrammatical perspective view of a burner
tip extending from a plenum which might be used with an embodiment
of the present invention.
[0032] FIG. 4A is a cutaway view of a tube shield with a thermal
protective layer disposed on the exterior surface thereof according
to an embodiment of the present invention.
[0033] FIG. 4B is a cutaway view of an outer fitted tube shield
designed to go around a U-turn in the tubes wherein the thermal
protective layer is disposed on both the exterior and interior
surfaces, and along the edges of the shield according to an
embodiment of the present invention.
[0034] FIG. 5A is a cross sectional side view of a tube shield
having a thermal protective layer disposed on the exterior surface
thereof, according to an embodiment of the present invention,
[0035] FIG. 5B is a cross sectional side view of a tube shield
having a thermal protective layer disposed on the exterior and
interior surfaces thereof, according to an embodiment of the
present invention.
[0036] FIG. 5C is a cross sectional side view of a tube shield
having a thermal protective layer disposed on the entire external
surface thereof, including the edges, according to an embodiment of
the present invention.
[0037] FIG. 6A is a cross sectional side view of an alternative
embodiment of a tube shield according to the present invention in
which the thermal protective layer is disposed on the external and
internal surfaces of the tube shield.
[0038] FIG. 6B is a cross sectional side view of yet another
alternative embodiment of a tube shield according to the present
invention in which the thermal protective layer is disposed on the
external surface of the tube shield.
[0039] FIG. 7 is a schematic perspective view of a combustion
chamber containing burner tube shield encased burner tubes,
according to an exemplary alternative embodiment of the present
invention.
[0040] Similar reference characters denote corresponding features
consistently throughout the attached drawings.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0041] The present invention is described herein in light of a
tangentially fired coal-burning furnace utilizing half shell tube
shields 12, graphically represented in FIGS. 1, 2A, 2B, 4A, 4B, 5A,
5B, 5C, and 7 by example only, and is not limited to such a furnace
and tube shield 12 designs, or applications. In a tangentially
fired furnace, burner tips 16, shown in FIG. 3, may be located at
each corner of a combustion chamber 10 having a square-shaped
floor/ceiling. The axis of each burner tip 16 may be offset with
respect to a central axis of the combustion chamber 10 and extend
generally tangent to an imaginary cylinder, which defines a
combustion zone 20, as shown in FIG. 1, where a fireball is
generated during operation of the burner tips 16. Burner tips 16
are also customarily referred to simply as "burners". Tangentially
fired furnaces are used to heat many utility boilers, especially
for the generation of energy from fossil fuel sources such as coal,
propane, oil, petroleum, natural gas, peat, wood, wood chips, solid
waste, other biomass, and the like, and combinations thereof.
Alternative fuel sources include, but are not limited to, waste
incineration and biofuels.
[0042] In the tangentially fired furnace of the present example,
the tube shields 12 and 14, which are disposed on the boiler tubes
22 and 24 as shown in FIGS. 2A and 2B, are depicted at the top of
the combustion chamber 10 in FIG. 1. The burner tips 16 are shown
extending tangentially into the combustion chamber 10. In this
example, the burner tips 16 are disposed in parallel groups with
one set on top of a second set, as shown in FIG. 1. The combustion
chamber 10 is shown diagramatically shorter in the vertical
relative to the horizontal than in actual use, as is well known in
the art. The combustible fluids flow 18 into the chamber 10, and
the heat in the combustion zone 20 rises to bathe the tube shields
12 and 14 and their encased boiler tubes 22 and 24 with heat. The
fluid within the boiler tubes 22 and 24 flows through the
combustion chamber 10 and is heated before flowing out of the
combustion chamber 10. The inlet and outlet of the boiler tubes 22
and 24 are not shown but are well understood in the art. The entire
assembly of boiler tubes 22 and 24, and their tube shields 12, 14
and 15, within the combustion chamber 10 may collectively be
referred to as a boiler tube rack.
[0043] FIGS. 2A and 2B are schematic depictions of tubes 22 and 24
with tube shields 12, 14 and 15 having an high emissivity
protective layer 32 depicted thereon. The tubes 22 and 24 each have
a straight part 22 and a U-part 24 permitting tight packing of the
tube shield 12, 14 or 15 encased tubes 22 and 24. The straight tube
shields 12 fit over the straight part 22 of the tubes as shown in
FIG. 2A. The U-part 24 is encased by the fitted tube shields 14 and
15 where the outside part of the U-part 24 is encased by the first
fitted tube shield 14 and the inside part of the U-part 24 is
encased by the second fitted tube shield 15. Although, the entire
burner tube rack may be encased by these three burner tube shields
12, 14 and 15, the present invention is not limited to the precise
configuration of burner shields depicted in the figures.
[0044] Various applications and designs such as whole or 360 degree
cover tube shields, shown diagramatically in cross section in FIGS.
6A and 6B, as examples, are also known in the art, and the present
invention encompasses these variations of tube shields 12 as well.
A further example of an alternative embodiment of a tube shield is
shown in U.S. Pat. No. 5,582,212, the contents of which are
incorporated herein by reference in their entirety. The thermal
protective layer 32 may be disposed upon burner tube shields which
have a variety of configurations including wrap-style tube shields
that are in long strips which wrap around the tubes. Exemplary,
alternative designs and configurations of burner tube shields are
well known and include the designs depicted hereinbefore in the
background of the invention among others.
[0045] FIG. 3 is a schematic diagram of a burner tip 16 which is
used to eject combustible fluids into a combustion zone 20. The
burner tip 16 depicted has several fluid vents 28 and a plenum 26
extending therefrom. The plenum 26 (or duct work) delivers the
combustible fluids to the burner tip 16. The plenum 26 may deliver
more than one combustible fluids to the burner tip 16, and the
burner tip 16 may have fluid vents 28 which are in fluid
communication through the plenum 26 to several separate combustible
fluids which are mixed either within the burner tip 16 or just
outside the burner tip 16 within the combustion chamber 10, as is
well known in the industry.
[0046] Combustible fluids include the medium in which other
combustible fluids flow or need to burn. For example, air is
necessary for combustion and may be delivered to the combustion
chamber 10 through the burner tips 16. Alternatively, water or
other liquid may be used to deliver pulverized fuel to the
combustion chamber 10. Furthermore, hot fluid may also be used in
some embodiments where the boiler tubes 24 and tube shields 12 are
bathed in hot fluids, including water and steam.
[0047] FIG. 4A depicts a generalized straight tube shield 12 having
a thermal protective coating 32 disposed only on the outer surface
of the support layer 30. FIG. 4B shows a first fitted tube shield
14 having the thermal protective layer 32 covering the entire
surface thereof including the outer and inner surfaces of the
support layer 30 and also along the edges thereof, as shown. The
support layer 30 is typically comprised of metal or alloys of
metals, including steel, low and high carbon steel, stainless
steel, iron, aluminum, and alloys. Alternative embodiments may
include tube shields having ceramic support layers 30.
[0048] FIGS. 5A through 5C show alternative embodiments of the
present invention. FIG. 5A is a cross sectional side view of a
half-circle tube shield 12 having a thermal protective layer 32
disposed on the exterior surface of the support layer 30 of the
tube shield 12. FIG. 5B is a cross sectional side view of a tube
shield 12 having a thermal protective layer 32 disposed on the
exterior and interior surfaces of the tube shield 12. FIG. 5C is
also a cross sectional side view of a tube shield 12 having a
thermal protective layer 32 disposed on the entire exterior and
interior surfaces, and along the edges thereof.
[0049] Alternative tube shield designs, according to an embodiment
of the present invention, include thermal protective layered tube
shields 17 that cover the entire circumference of the tube, shown
in FIGS. 6A and 6B without the tube disposed therein. FIG. 6A is a
cross sectional side view of such an alternative embodiment of a
thermal protcted tube shield 30 according to the present invention
in which the thermal protective layer is disposed on the external
and internal surfaces of the tube shield 30. FIG. 6B is a cross
sectional side view of yet another alternative embodiment of a tube
shield 30 according to the present invention in which the thermal
protective layer 32 is disposed on the external surface of the tube
shield 30.
[0050] FIG. 7 is a schematic perspective view of a combustion
chamber 10 containing tube shields 12 encasing boiler tubes 24,
according to an exemplary alternative embodiment of the present
invention. Various combustion chambers 10 are known in the art and
the present invention is not limited to combustion chambers 10 or
furnace designs, and includes alternative heat exchangers.
[0051] The thermal protective layer 32 may be applied as a high
emissivity thermal protective coating. Suitable coatings and
methods of application are described in U.S. Pat. Nos. 7,105,047
and 6,921,431 and assigned to Wessex Incorporated, the contents of
which are incorporated herein in their entirety.
[0052] A thermal protective layer 32 on at least some of the
exposed metal/alloy surfaces of the tube shields 12 of the present
invention may contain from about 5% to about 40% of an inorganic
adhesive, from about 45% to about 92% of a filler, and from about
1% to about 20% of one or more emissivity agents. The thermal
protective layer 32 may contain from about 1% to about 5% of a
stabilizer. A surfactant and/or colorant may also be present.
[0053] An alternative thermal protective layer 32 on ceramic
surfaces of the tube shields 12 according to an embodiment of the
present invention may contain from about 5% to about 60% of
colloidal silica, colloidal alumina, or combinations thereof, from
about 23% to about 79% of a filler, from about 1% to about 20% of
one or more emissivity agents. A thermal protective layer 32 of the
present invention may also contain from about 1% to about 5% of a
stabilizer. A surfactant and/or colorant may also be present.
[0054] As used herein, all percentages (%) are percent
weight-to-weight, also expressed as weight/weight %, % (w/w), w/w,
w/w % or simply %, unless otherwise indicated. Also, as used
herein, the terms "wet admixture" refers to relative percentages of
a composition of a thermal protective coating in solution and "dry
admixture" refers to the relative percentages of the composition of
the dry thermal protective layer. In other words, the dry thermal
protective layer or a dry admixture of percentages are those
present without taking water into account. Wet admixture refers to
the admixture in solution (with water). "Wet weight percentage" is
the weight in a wet admixture, and "dry weight percentage" is the
weight in a dry admixture without regard to the wet weight
percentages. The term "total solids", as used herein, refers to the
total sum of the silica/alumina and the alkali or ammonia
(NH.sub.3), plus the fraction of all solids including impurities.
Weight of the solid component divided by the total mass of the
entire solution, times one hundred, yields the percentage of "total
solids".
[0055] Additionally, as used herein, the term "fuel" includes
pulverized solids, such as coal, natural gas, solid biofuels, other
petroleum products, solid wastes, and the like, and combinations
thereof, which are commonly used in the generation of power and
heat. The term "fluid" includes fuel, air, water, steam, and the
like, whether in gaseous or liquid state. The present invention is
described herein by way of tube shields used for boiler tubes;
however, the present invention encompasses any such shields,
including those used in other heat exchangers such as for example
condenser tubes.
[0056] Method of preparation of coating involves applying a wet
admixture of the coating to the surface to be coated. Alternative
methods may include spraying the wet admixture on the surface or
atomizing the dry admixture and coating the surface accordingly.
The dry admixture is the same as the composition of the thermal
protective layer 32 once it has dried.
[0057] In a coating solution to be applied to metal/alloy support
layer surfaces of tube shields according to alternative embodiments
of the present invention, a wet admixture of the thermal protective
coating contains from about 6% to about 40% of an inorganic
adhesive, from about 23% to about 56% of a filler, from about 0.5%
to about 15% of one or more emissivity agents, and from about 18%
to about 50% water. In order to extend the shelf life of the
coating solution, from about 0.5% to about 2.5% of a stabilizer may
be added to the wet admixture. Up to about 1.0% of a surfactant may
be added. The wet admixture coating solution may contain between
about 40% and about 60% total solids.
[0058] In a coating solution to be applied to the ceramic tube
shields according to additional alternative embodiments of the
present invention, a wet admixture of a thermal protective coating
contains from about 15% to about 60% of colloidal silica, colloidal
alumina, or combinations thereof, from about 23% to about 55% of a
filler, from about 0.5% to about 15% of one or more emissivity
agents, from about 0.5% to about 2.5% of a stabilizer and from
about 10% to about 40% water. The wet admixture coating solution
contains between about 40% and about 70% total solids.
[0059] The inorganic adhesive is preferably an alkali/alkaline
earth metal silicate taken from the group consisting of sodium
silicate, potassium silicate, calcium silicate, and magnesium
silicate. The colloidal silica is preferably a mono-dispersed
distribution of colloidal silica, and therefore, has a very narrow
range of particle sizes. The filler is preferably a metal oxide
taken from the group consisting of silicon dioxide, aluminum oxide,
titanium dioxide, magnesium oxide, calcium oxide and boron oxide.
The emissivity agent is preferably taken from the group consisting
of silicon hexaboride, carbon tetraboride, silicon tetraboride,
silicon carbide, molybdenum disilicide, tungsten disilicide,
zirconium diboride, cupric chromite, and metallic oxides such as
iron oxides, magnesium oxides, manganese oxides, copper chromium
oxides, chromium oxides, cerium oxides, terbium oxides, and
derivatives, and combinations thereof. The copper chromium oxide,
as used in the present invention, is a mixture of cupric chromite
and cupric oxide. The stabilizer may be taken from the group
consisting of bentonite, kaolin, magnesium alumina silica clay,
tabular alumina, and stabilized zirconium oxide. The stabilizer is
preferably bentonite. Other ball clay stabilizers may be
substituted herein as a stabilizer. Colloidal alumina, in addition
to or instead of colloidal silica, may also be included in the
admixture of the present invention. When colloidal alumina and
colloidal silica are mixed together one or the other requires
surface modification to facilitate mixing, as is known in the
art.
[0060] Coloring may be added to the protective coating layer of the
present invention to depart coloring to the tube shields. Inorganic
pigments may be added to the protective coating without generating
toxic fumes. In general, inorganic pigments are divided into the
subclasses: colored (salts and oxides), blacks, white and metallic.
Suitable inorganic pigments include but are not limited to yellow
cadmium, orange cadmium, red cadmium, deep orange cadmium, orange
cadmium lithopone, and red cadmium lithopone.
[0061] A preferred embodiment of the present invention contains a
dry admixture of from about 10% to about 30% sodium silicate, from
about 50% to about 79% silicon dioxide powder, and from about 2% to
about 20% of one or more emittance agent(s) taken from the group
consisting of iron oxide, boron silicide, boron carbide, silicon
tetraboride, silicon carbide, molybdenum disilicide, tungsten
disilicide, zirconium diboride. Preferred embodiments of the
thermal coating may contain from about 1.0% to about 5.0% bentonite
powder in dry admixture.
[0062] The corresponding coating in solution (wet admixture) for
this embodiment contains from about 10.0% to about 35.0% sodium
silicate, from about 25.0% to about 46.0% silicon dioxide, from
about 18.0% to about 39.0% water, and from about 1.0% to about 8.5%
one or more emittance agent(s). This wet admixture must be used
immediately. In order to provide a coating solution admixture (wet
admixture), which may be stored and used later, preferred
embodiments of the thermal coating contain from about 0.25% to
about 2.50% bentonite powder. Preferably deionized water is used.
Preferred embodiments of the wet admixture have a total solids
content ranging from about 45% to about 55%.
[0063] A preferred thermal protective coating of the present
invention contains a dry admixture from about 15.0% to about 30%
sodium silicate, from about 69.0% to about 79.0% silicon dioxide
powder, about 1.00% bentonite powder, and from about 5.00% to about
15.0% of an emittance agent. The emittance agent is taken from one
or more of the following: iron oxide, boron silicide, and boron
carbide.
[0064] A most preferred wet admixture contains about 20.0% sodium
silicate based on a sodium silicate solids content of about 37.45%,
from about 34.5% to about 39.5% silicon dioxide powder, about
0.500% bentonite powder, and from about 2.50% to about 7.5% of an
emittance agent, with the balance being water. The emittance agent
is most preferably taken from the group consisting of iron oxide,
boron silicide, and boron carbide (also known as, carbon
tetraboride). Preferred embodiments include those where the
emittance agent comprises about 2.50% iron oxide, from about 2.50%
to about 7.5% boron silicide, or from about 2.50% to about 7.50%
boron carbide.
[0065] A preferred embodiment of the present invention contains a
dry admixture of from about 10.0% to about 35.0% colloidal silica,
from about 50% to about 79% silicon dioxide powder, and from about
2% to about 15% of one or more emittance agent(s) taken from the
group consisting of cerium oxide, boron silicide, boron carbide,
silicon tetraboride, silicon carbide molybdenum disilicide,
tungsten disilicide, zirconium diboride, and from about 1.5% to
about 5.0% bentonite powder.
[0066] The corresponding coating in solution (wet admixture) for
this embodiment contains from about 20.0% to about 35.0% colloidal
silica, from about 25.0% to about 55.0% silicon dioxide, from about
18.0% to about 35.0% water, and from about 2.0% to about 7.5% one
or more emittance agent(s), and from about 0.50% to about 2.50%
bentonite powder. Preferably deionized water is used. Preferred
embodiments of the wet admixture have a total solids content
ranging from about 50% to about 65%.
[0067] A most preferred thermal protective coating of the present
invention contains a dry admixture from about 15.0% to about 35.0%
colloidal silica, from about 68.0% to about 78.0% silicon dioxide
powder, about 2.00% to about 4.00% bentonite powder, and from about
4.00% to about 6.00% of an emittance agent. The emittance agent is
taken from one or more of the following: zirconium boride, boron
silicide, and boron carbide.
[0068] A most preferred wet admixture contains about 27.0%
colloidal silica based on a colloidal silica solids content of
about 40%, from about 25% to about 50% silicon dioxide powder,
about 1.50% bentonite powder, and from about 2.50% to about 5.50%
of an emittance agent, with the balance being water. The emittance
agent is most preferably taken from the group consisting of
zirconium boride, boron silicide, and boron carbide. Preferred
embodiments include those where the emittance agent comprises about
2.50% zirconium diboride, about 2.50% boron silicide, or from about
2.50% to about 7.50% boron carbide. The specific gravity of a most
preferred wet admixture is about 1.40 to 1.50 and the total solids
content is about 50% to about 60%.
[0069] An inorganic adhesive, which may be used in the present
invention, includes N (trademark) type sodium silicate that is
available from the PQ Corporation (of Valley Forge, Pa.). Sodium
silicate, also known as waterglass, is a versatile, inorganic
chemical made by combining various ratios of sand and soda ash
(sodium carbonate) at high temperature. Sodium silicates
(Na.sub.2O.XSiO.sub.2) are metal oxides of silica. All soluble
silicates can be differentiated by their ratio, defined as the
weight proportion of silica to alkali (SiO.sub.2/Na.sub.2O). Ratio
determines the physical and chemical properties of the coating. The
glassy nature of silicates imparts strong and rigid physical
properties to dried films or coatings. Silicates air dry to a
specific moisture level, according to ambient temperature and
relative humidity. Heating is necessary to take these films to
complete dryness--a condition in which silicates become nearly
insoluble. Reaction with other materials, such as aluminum or
calcium compounds, will make the film coating completely insoluble.
The N (trademark) type sodium silicate, as used in the examples
below, has a weight ratio SiO.sub.2/Na.sub.2O is 3.22, 8.9%
Na.sub.2O, 28.7% SiO.sub.2, with a density (at room temperature of
20.degree. C.) of 41.0.degree.Be', 11.6 lb/gal or 1.38 g/cm.sup.3.
The pH is 11.3 with a viscosity of 180 centipoises. The N type
sodium silicate is in a state of a syrupy liquid.
[0070] The term "total solids" refers to the sum of the silica and
the alkali. The weight ratio is a most important silicate variable.
Ratio determines the product solubility, reactivity and physical
properties. Ratio is either the weight or molar proportion of
silica to alkali. Density is an expression of total solids and is
typically determined using a hydrometer or a pycnometer.
[0071] Ludox (trademark) .TM. 50 colloidal silica are available
from Grace Davidson (of Columbia, Md.). The particles in Ludox
(trademark) colloidal silica are discrete uniform spheres of silica
which have no internal surface area or detectable crystallinity.
Most are dispersed in an alkaline medium which reacts with the
silica surface to produce a negative charge. Because of the
negative charge, the particles repel one another resulting in
stable products. Although most grades are stable between pH
8.5-11.0, some grades are stable in the neutral pH range. Ludox
(trademark) colloidal silicas are aqueous colloidal dispersions of
very small silica particles. They are opalescent to milky white
liquids. Because of their colloidal nature, particles of Ludox
(trademark) colloidal silica have a large specific surface area
which accounts for the novel properties and wide variety of uses.
Ludox (trademark) colloidal silica is available in two primary
families: mono-dispersed, very narrow particle size distribution of
Ludox (trademark) colloidal silica and poly-dispersed, broad
particle size distribution of Ludox (trademark) P. The Ludox
(trademark) colloidal silica is converted to a dry solid, usually
by gelation. The colloidal silica can be gelled by (1) removing
water, (2) changing pH, or (3) adding a salt or water-miscible
organic solvent. During drying, the hydroxyl groups on the surface
of the particles condense by splitting out water to form siloxane
bonds (Si--O--Si) resulting in coalescence and interbonding. Dried
particles of Ludox (trademark) colloidal silica are chemically
inert and heat resistant. The particles develop strong adhesive and
cohesive bonds and are effective binders for all types of granular
and fibrous materials, especially when use at elevated temperature
is required.
[0072] Colloidal alumina is available as Nyacol (trademark)
colloidal alumina available from Nyacol Nano Technologies, Inc.
(Ashland, Mass.), and is available in deionized water to reduce the
sodium and chlorine levels to less than 10 ppm. Nyacol may contain
about 20 percent by weight of AL.sub.2O.sub.3, a particle size of
50 nm, positive particle charge, pH 4.0, specific gravity of 1.19,
and a viscosity of 10 cPs.
[0073] The filler may be a silicon dioxide powder such as Min-U-Sil
(trademark) silicon dioxide available from U.S. Silica (of Berkeley
Springs, W. Va.). This silicon dioxide is fine ground silica.
Chemical analysis of the Min-U-Sil (trademark) silicon dioxide
indicates contents of 98.5% silicon dioxide, 0.060% iron oxide,
1.1% aluminum oxide, 0.02% titanium dioxide, 0.04% calcium oxide,
0.03% magnesium oxide, 0.03% sodium dioxide, 0.03% potassium oxide
and a 0.4% loss on ignition. The typical physical properties are a
compacted bulk density of 41 lbs/ft.sup.3, an uncompacted bulk
density of 36 lbs/ft.sup.3, a hardness of 7 Mohs, hegman of 7.5,
median diameter of 1.7 microns, an oil absorption (D-1483) of 44, a
pH of 6.2, 97%-5 microns, 0.005%+325 Mesh, a reflectance of 92%, a
4.2 yellowness index and a specific gravity of 2.65.
[0074] Emittance agents are available from several sources.
Emissivity is the relative power of a surface to emit heat by
radiation, and the ratio of the radiant energy emitted by a surface
to the radiant energy emitted by a blackbody at the same
temperature. Emittance is the energy radiated by the surface of a
body per unit area.
[0075] The boron carbide (B.sub.4C), also known as carbon
tetraboride, which may be used as an emissivity agent in the
present invention, is available from Electro Abrasives (of Buffalo,
N.Y.). Boron Carbide is one of the hardest man made materials
available. Above 1300.degree. C., it is even harder than diamond
and cubic boron nitride. It has a four point flexural strength of
50,000 to 70,000 psi and a compressive strength of 414,000 psi,
depending on density. Boron Carbide also has a low thermal
conductivity (29 to 67 W/mK) and has electrical resistivity ranging
from 0.1 to 10 ohm-cm. Typical chemical analysis indicates 77.5%
boron, 21.5% carbon, iron 0.2% and total Boron plus Carbon is 98%.
The hardness is 2800 Knoop and 9.6 Mohs, the melting point is
4262.degree. F. (2350.degree. C.), the oxidation temperature is
932.degree. F. (500.degree. C.), and the specific gravity is 2.52
g/cc.
[0076] Green silicon Carbide (SiC), an optional emissivity agent,
is also available from Electro Abrasives. Green Silicon Carbide is
an extremely hard (Knoop 2600 or Mohs 9.4) man made mineral that
possesses high thermal conductivity (100 W/m-K). It also has high
strength at elevated temperatures (at 1110.degree. C., Green SiC is
7.5 times stronger than Al.sub.2O.sub.3). Green SiC has a Modulus
of Elasticity of 410 GPa, with no decrease in strength up to
1600.degree. C., and it does not melt at normal pressures but
instead dissociates at 2815.5.degree. C. Green silicon carbide is a
batch composition made from silica sand and coke, and is extremely
pure. The physical properties are as follows for green silicon
carbide: the hardness is 2600 Knoop and 9.4 Mohs, the melting point
is 4712.degree. F. (2600.degree. C.), and the specific gravity is
3.2 g/cc. The typical chemical analysis is 99.5% SiC, 0.2%
SiO.sub.2, 0.03% total Si, 0.04% total Fe, and 0.1% total C.
Commercial silicon carbide and molybdenum disilicide may need to be
cleaned, as is well known in the art, to eliminate flammable gas
generated during production.
[0077] Boron silicide (B.sub.6Si) is available from Cerac (of
Milwaukee, Wis.). The boron silicide, also known as silicon
hexaboride, available from Cerac has a -200 mesh (about 2 microns
average) and a typical purity of about 98%. Zirconium boride
(ZrB.sub.2) (Item# Z-1031) is also available from Cerac with a
typical average of 10 microns or less (-325 mesh), and a typical
purity of about 99.5%. Iron oxide available from Hoover Color (of
Hiwassee, Va.) is a synthetic black iron oxide (Fe.sub.2O.sub.3)
which has an iron oxide content of 60%, a specific gravity of 4.8
gm/cc, a tap density (also known as, bulk density) of 1.3 gm/cc,
oil absorption of 15 lbs/100 lbs, a 325 mesh residue of 0.005, and
a pH ranging from 7 to 10.
[0078] The admixture may include bentonite powder, tabular alumina,
or magnesium alumina silica clay. The bentonite powder permits the
coating to be prepared and used at a later date. Otherwise, the
coating must be applied to the support layer as soon as mixed. The
examples provided for the present invention include PolarGel
bentonite powder available from Mineral and Pigment Solutions, Inc.
(of South Plainfield, N.J.). Bentonite is generally used for the
purpose of suspending, emulsifying and binding agents, and as
Theological modifiers. The typical chemical analysis is 59.00% to
61.00% of silicon dioxide (SiO.sub.2), 20.00% to 22.00% of aluminum
oxide (Al.sub.2O.sub.3), 2.00% to 3.00% calcium oxide (CaO), 3.50%
to 4.30% magnesium oxide (MgO), 0.60% to 0.70% ferric oxide
(Fe.sub.2O.sub.3), 3.50% to 4.00% sodium oxide (Na.sub.2O), 0.02%
to 0.03% potassium oxide (K.sub.2O), and 0.10% to 0.20% titanium
dioxide and a maximum of 8.0% moisture. The pH value ranges from
9.5 to 10.5. Typical physical properties are 83.0 to 87.0 dry
brightness, 2.50 to 2.60 specific gravity, 20.82 pounds/solid
gallon, 0.0480 gallons for one pound bulk, 24 ml minimum swelling
power, maximum 2 ml gel formation, and 100.00% thru 200 mesh.
Tabular alumina and magnesium alumina silica clay are also
available from Mineral and Pigment Solutions, Inc.
[0079] Colorants, which may be added to the present invention,
include but are not limited to inorganic pigments. Suitable
inorganic pigments, such as yellow iron oxide, chromium oxide
green, red iron oxide, black iron oxide, titanium dioxide, are
available from Hoover Color Corporation. Additional suitable
inorganic pigments, such as copper chromite black spinel, chromium
green-black hematite, nickel antimony titanium yellow rutile,
manganese antimony titanium buff rutile, and cobalt chromite
blue-green spinet, are available from The Shepherd Color Company
(of Cincinnati, Ohio).
[0080] A surfactant may be added to the wet admixture prior to
applying the thermal protective layer to the tube shield or plenum.
The surfactant was Surfynol (trademark) available from Air Products
and Chemicals, Inc. (of Allentown, Pa.). The Surfyonol (trademark)
has a chemical structure of ethoxylated 2,4,7,9-tetramethyl 5
decyn-4,7-diol. Other surfactants may be used, such as STANDAPOL
(trademark) T, INCI which has a chemical structure of
triethanolamine lauryl sulfate, liquid mild primary surfactant
available from Cognis-Care Chemicals (of Cincinnati, Ohio). The
amount of surfactant present by weight in the wet admixture in from
about 0.05% to about 0.2%.
[0081] The thermal protective layer on the tube shields. The
surface may be a metallic substrate such as iron, aluminum, alloys,
steel, cast iron, stainless steel, and the like, or it may be a
ceramic surface. Ceramic and metal/alloy surfaces of tube shields
and exposed surfaces within the chamber are well known in the art.
The coating is typically applied wet, and either allowed to air
dry, heat dry, or dry upon facility start up.
[0082] The coating is typically applied directly to the support
structure 30. The preparation of the tube shield support structure
30 involves surface preparation, preparation of thermal protective
coating, and application of the thermal protective coating to the
surface of the support layer 30 of the tube shield. First,
preparation of the surface occurs. The surface is prepared first by
grit basting and then cleaning the surface. Grit blasting is
desirable to remove oxidation and other contaminants. Grit media
should be chosen depending on metal type, and may include aluminum
oxide, glass beads, black beauty, and the like.
[0083] Gun pressure will vary depending on the cut type, condition
of the metal and profile desired; very old metal requires 60-80 psi
while newer metal may only require 40-60 psi. Oil free air should
be used. The surface then cleaned after the grit blasting, the
surface should be thoroughly cleaned to remove all loose particles
with air blasts. Acetone can also be used on a clean cloth to wipe
the surface clean. Acetone should be used under proper ventilation
and exercising all necessary precautions. A cleaning compound may
be used on certain stainless steel surfaces in lieu of grit
blasting.
[0084] After the grit blast, the surface should be thoroughly
cleaned to remove all loose particles with clean oil and water free
air blasts. Avoid contaminating surface with fingerprints. Acetone
can be used (under proper ventilation and exercising all necessary
precautions when working with acetone) on a clean cloth to wipe the
surface clean. A cleaning compound may be used on certain stainless
steel in lieu of grit blasting. Durlum available from Blue Wave
Ultrasonics (of Davenport, Iowa), a powdered alkaline cleaner, may
be used in cleaning metal surface instead of, or in addition to,
acetone.
[0085] When using the wet admixture containing a stabilizer, solids
may settle during shipment or storage. Prior to use all previously
mixed coating must be thoroughly re-mixed to ensure all settled
solids and clumps are completely re-dispersed. When not using a
stabilizer, the coating may not be stored for any period of time.
In any case, the coating should be used immediately after mixing to
minimize settling.
[0086] Mixing instructions for one and five gallon containers. High
speed/high shear saw tooth dispersion blade 5'' diameter for one
gallon containers and 7'' diameter for five gallon containers may
be attached to a hand drill of sufficient power with a minimum no
load speed of 2000 rpm shear. Dispersion blades can be purchased
from numerous suppliers. Mix at high speed to ensure complete
re-dispersion for a minimum of 30 minutes.
[0087] The product should be applied directly after cleaning a
metal surface so minimal surface oxidation occurs. The product
should be applied in a properly ventilated and well lit area, or
protective equipment should be used appropriate to the environment,
for example within the combustion chamber 10. The mixed product
should not be filtered or diluted.
[0088] A high volume low pressure (HVLP) spray gun should be used
with 20-40 psi of clean, oil and water free air. Proper filters for
removal of oil and water are required. Alternatively, an airless
spray gun may be used. Other types of spray equipment may be
suitable. The applicator should practice spraying on scrap metal
prior to spraying the actual part to ensure proper coverage
density. An airless spray system is preferable for applications on
ceramic surfaces such as the refractory materials. Suitable airless
spray systems are available from Graco (of Mineapolis, Minn.).
Suitable HVLP spray systems, which are desirable for metal/alloy
process tubes, are available from G. H. Reed Inc, (of Hanover,
Pa.). A high speed agitator may be desirable. Suitable spray gun
tips may be selected to provide the proper thickness without undue
experimentation.
[0089] Controlling the coverage density may be critical to coating
performance. Dry coating thickness should be from about two (2)
mils (about 50 microns (i)) to about ten (10) mils (about 255
.mu.), depending upon typed, size and condition of substrate. One
(1) mil equals 25.4 .mu.. Proper thickness may vary. If possible,
rotate the part 90 degrees at least once to maintain even coverage.
Allow 1 to 4 hours of dry time before the part is handled,
depending upon humidity and temperature.
[0090] The tube shields 12, 14, 15, and 17 at the very least have a
thermal protective layer 32 on the external surface thereof, but
may also have the thermal protective layer 32 disposed on the
entire surface, or on both the external and internal surfaces. The
term external surfaces of the tube shields 12, 14, 15, and 17 are
not in direct contact or adjacent to the enclosed tubes. The
internal surface of the tube shields 12, 14, 15, and 17 is the part
that comes into direct contact with the boiler or condenser tubes.
The edge of the tube shields are the part of the tube shields 12,
14 and 15 which mate with or contact adjacent tube shields 12, 14
and 15. Any braces, such as straps and the like, used with tube
shields may also have a thermal protective layer disposed thereon.
Further embodiments of the tube shields include shields which
substantially cover the burner tube by wrapping around the burner
tube, and others have a flexible construction forming a sleeve
fitting over and covering most of the surface of the tube shield
for 360 degrees, as shown in cross section in FIGS. 6A and 6B. The
present invention is seen to include all types of tube shields
having a protective layer on at least one surface thereof.
[0091] Prior to application of a thermal protective coating to the
prepared surface, the thermal protective coating should be
thoroughly remixed to ensure all settled solids and clumps are
completely redispersed. Also, the remixed thermal protective
coating should be used promptly after remixing to minimize
settling. To mix, a high speed/high shear dispersion blade should
be attached to a hand drill of sufficient power with a minimum
speed of 2300 rpm. Dispersion blades can be purchased from numerous
suppliers. The thermal protective coating is prepared by mixing at
high speed while moving the blade up and down inside the coating's
container to ensure complete redispersion for a minimum of 10
minutes. Alternative equivalent mixing procedures may be used.
[0092] It is desirable to apply the thermal protective coating to
the surface directly after cleaning the surface so minimal surface
oxidation occurs. The prepared surface should be at, or near, room
temperature (60.degree. F. to 80.degree. F.) and humidity should be
below 50%, if possible.
[0093] Spray equipment which may be used include a high volume low
pressure (HPLV) spray gun, which should be used with 20-40 psi of
clean, oil free air. Other types of spray equipment may be
suitable, as well, including airless spray equipment. Controlling
the coverage density is desirable to enhance coating performance.
If possible, the support layer 30, or the spray equipment, should
be rotated 90 degrees at least once to maintain even coverage.
Never reapply after the coat has completely dried. Allow 2 to 4
hours of dry time before the shield 12, 14, 15, or 17 is handled
depending upon humidity and temperature.
[0094] Example 1 contains N grade Sodium Silicate 15.0% dry weight
and 20.0% wet weight based on sodium silicate solids content of
37.45%, Min-U-Sil SiO.sub.2 powder 79.0% dry weight and 39.5% wet
weight, B.sub.4C 5.00% dry weight and 2.50% wet weight, PolarGel
bentonite powder 1.00% dry weight and 0.500% wet weight, and 37.5%
water, based on sodium silicate solids content of 37.45%. The pH of
example 1 is 11.2..+-..1.0, the specific gravity is 1.45..+-..0.05,
and the total solids content is 50..+-..0.3%. Example 1 may be
prepared by placing the liquid ingredients in a clean, relatively
dry mixing container. While mixing, the remaining ingredients are
added slowly to the mixture to prevent the powders from clumping
and sticking to the side of the mixing container. The mixture is
then mixed at high power for at least 20 minutes depending on the
configuration of the mixer. The mixing was carried out in a high
shear mixer with a 2.5 inch Cowles Hi-Shear Impeller blade with a
0.5 horsepower motor generating 7500 rpm without load.
[0095] Example 2 contains N grade Sodium Silicate 15.0% dry weight
and 20.0% wet weight based on sodium silicate solids content of
37.45%, min-U-Sil SiO.sub.2 powder 69.0% dry weight and 34.5% wet
weight, B.sub.4C 15.0% dry weight and 7.5% wet weight, PolarGel
bentonite powder 1.00% dry weight and 0.500% wet weight, and 37.5%
water, based on sodium silicate solids content of 37.45%. The pH of
example 2 is 11.2.+-.1.0, the specific gravity is 1.45.+-.0.05, and
the total solids content is 50.+-.0.3%. Example 2 is prepared in
the same fashion as example 1. This embodiment is a preferred
embodiment for sintering applications. Example 2 may be prepared in
the same manner as Example 1.
[0096] A tube shield was coated with the composition of example 1
and observed under real-life field circumstances. Coated and
sintered tube shields were placed in the super-heater of a CE,
VU40, coal fired, tangential burners (no tilts), 65 MWe/mcr unit.
Coating still shedding slag and protecting tubes after fifteen (15)
months of service.
[0097] It is to be understood that the present invention is not
limited to the embodiments described above, but encompasses any and
all embodiments within the scope of the following claims.
* * * * *