[0001] It is an object of the present invention to provide a thermal radiation and conduction
shield suitable for mounting on the rear side of a protective face block placed over
an opening, such as a manhole, in the refractory ceramic lining of a boiler, furnace,
or similar apparatus. The shield is employed in said openings in order to prevent
conduction of heat through the protective face block from the firebox to those frame
constructions of the apparatus that are not manufactured from heat-resistant materials.
The internal temperature in these sections typically exceeds 800 °C, while the boiler
frame construction materials may generally be subjected to a maximum temperature of
about 100 to 150 °C. Since the protective face block is typically made of a refractory
high-density ceramic material with very high thermal conductivity, the operating temperature
of the hot side of the shield is practically equal to that of the firebox. Consequently,
the backing insulation of the shield must provide effective thermal insulation as
well as good resistance to high temperatures. A third prerequisite is resistance to
mechanical stress under practical operating conditions. Namely, the shield is removed
during any shut-down period, and even this repeated displacement on its own imposes
stresses on the fragile shield. The opening covered by the shield may be round, oval
or angular, for instance.
[0002] Conventionally, the construction of the above-described shields preventing radiation
and conduction of heat is accomplished as follows: glass-fibre fabric is prepared
into the shape of a pouch that has approximately the size and shape of the opening,
and the pouch is stuffed with a ceramic fibre resistant to high temperatures. Subsequently,
the pouch is closed by stitching in order to keep the fibre within. The bag formed
in this fashion is then introduced into the opening in the lining behind the face
block, and the door of the opening is secured in place.
[0003] Another method for implementing the shields is to manufacture them from an insulating
heat-resistant castable by mould casting. The material is inelastic, and the shield
is always designed to be smaller in its dimension than the opening, in order to ensure
that it fits the opening.
[0004] A drawback of employing prior art technique is that the glass-fibre fabric and the
knittings therein withstand the effects of high temperatures only for a short period
of time. When the brittle bag is displaced during shut-downs, the knittings and the
surface fabric in the bag break down and the, by nature, more heat-resistant ceramic
fibre contained within is discharged from the bag. Therefore, this prior art solution
has a short life-cycle. The materials, manufacture, and service life of the solution
combine to bring high costs of manufacture and use, and also, malfunctions during
operation.
[0005] The drawbacks of said shield being manufactured from castable in the second embodiment
lie in the costs of manufacture and a construction that is tightly sealed. Manufacture
requires an individual mould, the fabrication of which adds further costs to materials
and work. The shield has to be dimensioned in such a way that there will be a margin
for installation. In practice, the result is relatively loose, which causes heat transfer
to the frame past the shield.
[0006] The prior art technique described above is used,
e.
g., in production at Foster Wheeler Energy Ltd. (bag) and in production as performed
by the Applicant (shield manufactured by casting).
[0007] It is an object of the present invention to provide a shield that avoids prior art
drawbacks. The characteristic features of construction according to the present invention
are presented in the characterising clause of claim 1.
[0008] The main advantage of the present invention over the prior art technique is that
the shield according to the present invention is reliable in use. In addition to being
highly insulating, all parts of the shield are very resistant to the effects of high
temperature. The shield is stable in construction, it is elastic, and it withstands
shutdown-necessitated displacement well. As a result of the above, the shield is durable
and cost-effective.
[0009] The structure according to the present invention ensures a tight seal, which also
creates an extremely important advantage. The outer periphery of the shield E is elastic,
and it is able to adapt to the shape of the opening. The tight seal thus obtained
is efficient in preventing heat transfer past the shield.
[0010] The invention will be described in greater detail with reference to the attached
drawings, wherein
Fig. 1 shows a structure according to the invention mounted on the manhole of a lined
firebox,
Fig. 2 shows a shield according to the invention as viewed from its rear side
Fig. 3 shows a shield according to the invention as viewed from its front side.
[0011] With reference to the drawings, the shield according to the invention functions as
follows:
[0012] Fig. 1 shows a manhole 1 of a firebox, the manhole having a round cross-section about
600 mm in diameter in this case. During the operation of the apparatus, the manhole
is closed by a prior art protective face block 2. The material of the protective face
block 2 is similar to the refractory ceramic cast material used for the surface layer
A of the firebox's lining, and is shaped to have a shoulder along its side facing
the firebox to prevent the protective block from falling into the firebox. The surface
layer A of the firebox wall lining has a thermal insulation layer B. The surface lining
A is to withstand the effects of thermal, chemical, and mechanical stress imposed
by the combustion process that is occurring in the firebox, while the thermal insulation
layer prevents thermal conduction to the firebox frame construction C. The protective
face block 2 of the manhole 1 serves the same function as the firebox wall surface
layer A and, has a protective shield E according to the invention, having the same
function as the thermal insulation layer B of the wall, adapted behind the face block.
The protective shield E is composed as follows: Four round sheets 3 that are stacked
on each other are cut from a mat of high-temperature-resistant, flexible ceramic fibre
with a typical thickness of approximately 25 mm. A support element 4,
e.
g., advantageously, a round steel disc having a diameter smaller than that of the fibre
mat sheets, is mounted on each end of the sheet stack. The support elements are advantageously
made from stainless steel sheet of 1 mm thickness, for instance. Each support element
4 has four holes for mounting bolts. The disc-like support elements 4 are connected
to each other via clamping elements 5, advantageously bolts and nuts, for instance,
which are passed through the holes made in support elements and sheets 3. The sheets
3 and support elements 4 are held tightly against each other by such clamping elements
5, which may be bolts and nuts. The outer support element of the thus assembled shield
E has two handles 6 mounted thereon for mounting the shield in place. During mounting
of the shield, the edges of the sheets 3, which are oversized with respect to the
dimensions of the manhole, are bent backward. After the shield E has been correctly
inserted in its place in the manhole 1, the bent edges of the sheets 3 are gently
pushed in to be level with the rear surface of the shield E. To remove the shield
E from the manhole, its handles 6 are grabbed to pull the shield out from the manhole.
[0013] The sheets 3 manufactured from the ceramic fibre mat of the shield E according to
the present invention can be shaped to fit openings of different shape. The shape
of the opening, as described above, can also be oval, be angular, or have arcuate
and/or straight surfaces. In their manufacture, the sheets 3 are mainly sized to be
somewhat larger than the opening, in order to ensure a tight seal.
[0014] The shield E according to the invention can be employed, in addition to what has
been described in the above example, for other types of openings besides manholes.
For example, maintenance and explosion openings can be provided with a solution according
to the invention.
[0015] The material for the sheets 3 cut from high-temperature-resistant fibre mat and forming
part of the present invention, is typically a substance that chemically is mainly
composed of aluminium oxide aluminium oxide (Al2O3), silicon oxide (Si02) and zirconium
oxide (ZrO3) or any of these in combination. As an example, the product named Cerablanket
1260, by Thermal Ceramics, can be mentioned. The proportions of its main substances
are 44%: Al2O3, and 56% SiO2, and it is heat-resistant to approximately 1260 °C. The
thickness of the fabric mat used is most often approximately 25 mm.
[0016] The sheets 3 can also be manufactured from another material suitable for the purpose,
and their thickness and number can be arranged individually according to the depth
of the opening 1. Thus, it is possible to achieve the total thickness of the shield
E also in such a way that it helps the protective block 2 sty in position. The closed
door D, for its part, supports the shield E.
[0017] In some cases, the periphery/rim of the stack formed by the sheets 3 can be covered
with a fabric resistant to abrasion and high temperature. The fabric is attached to
support elements or clamping elements 4, 5, and, it may be used,
e.
g., when the shield E has to be displaced relatively frequently.
[0018] Although this application presents one advantageous exemplary embodiment of the present
invention, it is not intended to limit broader use of the invention in any way, all
alternatives for implementing the invention are possible within the inventive idea
defined by the claims.
1. A thermal radiation and conduction shield suitable for mounting on the rear side of
a protective face block (2) placed on an opening, advantageously such as a manhole
(1), in the refractory ceramic lining of a boiler, furnace, or like apparatus,
characterised in that it is composed of sheets (3) that have essentially the size and the shape of the
opening and are cut from a ceramic, elastic fibre mat, the sheets being:
- at least two in number,
- stacked evenly on each other or one after another attached to each other,
and support elements (4),
- that are at least two in number,
- that are mounted on each ends of the layered construction formed by the sheets (3),
in order to support the fibre material between them to form a substantially even block
with the shape of the opening,
and clamping elements (5),
- that number at least one,
- that are positioned to connect the support elements (4) to each other and thus to
form a solid shield (E).
2. A shield according to claim 1, characterised in that the outer dimension of the sheets (3) being larger than the inner dimension of the
opening (1), advantageously in such a way that the outer edges of the elastic sheets
(3) adapt to the shape of the opening (1) to form a tight seal when the shield (E)
is being installed.
3. A shield, according to claim 1 or 2, characterised in that the material of the support elements (4) and the clamping elements (5) is stainless
steel, high-temperature-resistant steel, or another material suitable for the purpose.
4. A shield according to claim 1, 2, or 3, characterised in that it has at least one handle (6) attached to the other support element (4) for removal
of the shield (E).
5. A shield according to any of claims 1 to 4, characterised in that the thickness of the shield (E) is advantageously adapted to support the protective
face block (2) and thus help keep it in its position.
6. A shield according to any of claims 1 to 5, characterised in that the periphery/rim of the stack formed by the sheets (3) is covered with a suitable
fabric or corresponding material resistant to abrasion and high temperature.