Combustion – Porous – capillary – particulate or sievelike flame holder,...
Reexamination Certificate
2000-08-14
2003-05-13
Clarke, Sara (Department: 3743)
Combustion
Porous, capillary, particulate or sievelike flame holder,...
C501S081000, C501S088000, C501S095100
Reexamination Certificate
active
06561793
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to heating apparatus for treating a web of material, and, more particularly to an improved matrix and an infrared (IR) heater incorporating the matrix for heating a paper web.
BACKGROUND OF THE INVENTION
Conventional papermaking machinery for producing a continuous sheet of paper includes equipment to set the sheet properties of the paper as it is being manufactured. One of the more basic operations on a paper machine is control of the cross-direction moisture profile by drying with gas infrared heaters. Such heaters are also useful for drying coatings onto a paper web.
Typical infrared heating systems designed specifically for papermaking machinery comprise a series of independently controllable heater units or emitters that are positioned over the paper web in the cross-machine direction CD. Each heater unit consists of a porous refractory ceramic matrix that is fitted into a metallic housing. A plurality of housings are positioned side by side to extend across the web. The porous ceramic matrix is bonded to an aluminum housing with silicone to define a plenum chamber. The plenum chamber of the housing is supplied with an air/fuel mixture via an inlet that connects to a fuel supply. Gaseous fuel in the form of natural gas or propane is mixed with air in a 1:10 ratio to create the air/fuel mixture. Combustion occurs only at the outer 3 mm of the ceramic matrix surface adjacent the paper web to provide fast heat up times of about 5 seconds and fast cool down times of about one second.
During normal operation, the temperature of the heater will be about 40° C. at the inner surface of the ceramic matrix to between 575° C. to 950° C. at the exterior surface of the matrix adjacent where combustion occurs.
U.S. Pat. No. 4,654,000 issued to Smith on Mar. 31, 1987 is an example of a prior art infrared heating unit.
Applicants' U.S. patent application Ser. No. 09/557,093 filed Apr. 21, 2000 entitled INFRARED HEATER is a further example of infrared heating unit. This unit has a novel mounting scheme for connecting the matrix to the metallic housing.
The matrix of a typical infrared unit uses ceramic fibres and organic binders to create a porous material that acts as an infrared emitter when heated to the above described temperatures. Often high emissivity materials such as silicon carbide are incorporated into the matrix to enhance the infra-red output.
Generally, an aqueous molding process is used to form the ceramic matrix. This tends to result in the ceramic material forming clumps which are not well wetted by the aqueous medium. As a consequence, the infrared emitters such as silicon carbide particles are deposited in a cellular configuration comprising geometrical regions surrounding the ceramic clumps.
SUMMARY OF THE INVENTION
Applicant has discovered that it is possible to better distribute the infrared emitter particles throughout the matrix by employing ionic wetting agents during the aqueous molding process. The presence of more uniformly distributed infra-red emitting particles in the matrix, particularly in the outer working surface, results in increased infrared output from the emitters at the same gas consumption. Therefore, it is possible to consume less gas to lower the exhaust gas volume and temperature to achieve the same infrared output. Lower exhaust gas temperatures lead to decreased wear on the equipment and improved durability.
The organic binder is also better distributed throughout the ceramic matrix due to the ionic wetting agent, resulting in improved strength and strength uniformity of the matrix.
Accordingly, the present invention provides a method of forming a porous ceramic matrix for use in an infrared heating unit comprising the steps of:
mixing ceramic fibers, organic binders and particulate material capable of infrared emissivity with an ionic wetting agent to form a moldable ceramic mixture; and
molding the mixture to a desired shape.
The present invention also provides a ceramic matrix for use in an infrared heating unit comprising a mixture of ceramic fibers, organic binders and particulate material capable of infrared emissivity formed with an ionic wetting agent such that the particulate material capable of infrared emissivity is mixed substantially uniformly throughout the matrix.
In a preferred embodiment, formation of the ceramic matrix involves the additional step of mixing ceramic fibers of a different composition having a different melting temperature into the mixture. This results in a ceramic matrix with improved resistance to erosion and an enhanced service life. During operation of the matrix in a heater unit over normal operating temperatures, melting and re-crystallization of the ceramic fiber composition with the lower melting temperature occurs resulting in the creation of crystalline bond regions between the two fiber compositions. This hardens the matrix at the heating surface.
In a further aspect, the present invention provides an infrared heating unit comprising:
a metallic housing;
a ceramic matrix comprising a mixture of ceramic fibers, organic binders and particulate material capable of infrared emissivity formed with an ionic wetting agent such that the particulate material capable of infrared emissivity is mixed substantially uniformly throughout the matrix;
the matrix being mounted in the housing and having an inner surface, side walls and an external surface, the inner surface of the ceramic matrix and the housing cooperating to define a chamber;
an inlet to the housing to admit a gas/air combustion mixture to the chamber whereby the gas/air mixture passes through the ceramic matrix to burn adjacent the external surface of the ceramic matrix to heat the external surface.
REFERENCES:
patent: 3833338 (1974-09-01), Badrock
patent: 4504218 (1985-03-01), Mihara et al.
patent: 4654000 (1987-03-01), Smith
patent: 5053362 (1991-10-01), Chi et al.
patent: 5108964 (1992-04-01), Corbett et al.
Christie Parker & Hale LLP
Clarke Sara
Honeywell ASCa Inc.
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