Patent Grant
6909495
This invention relates to an emissivity probe of a type adapted for evaluating, through optical inputs, the condition of internal surfaces of large scale combustion devices such as utility and processing boilers.
In the operation of large scale combustion devices such as coal burning boilers, there is a continual build up of slag deposits on their internal walls and heat transfer surfaces. As the slag builds up on these surfaces, the slag layer reflects some of the radiant heat energy produced within the boiler by combustion and thus, less steam and thermal output is produced by the boiler. Accordingly, it is important to periodically remove the slag layers to maintain efficient operation.
Various means for detecting the presence of slag development on internal surfaces of combustion devices have been developed. One approach is through the monitoring of various operating parameters of the boiler which provide an indirect indication of the development of such slag as efficiency is adversely affected. When slag deposits reach a point where cleaning is needed, various cleaning technologies are used. For example, sootblower systems are used which project a stream of fluid cleaning medium against the surfaces, such as air, steam, or water. These fluids remove the slag through a combination of heat quenching which embrittles the coating and mechanical impact energy which causes the encrustations to lose their adherence to the surfaces and fall away. Other approaches include mechanical rodding and shakers which vibrate the surfaces to remove the layers.
For efficient boiler operation, it is desirable to clean surfaces only when needed. Operating sootblowers causes an efficiency penalty for the boiler when it is not used then at a time when it is actually needed. Operating cleaning systems based strictly on time or other indirect measures can result in operating the cleaning devices on a schedule which is not optimal.
It is known that the inside surfaces of a boiler can be imaged using cameras sensitive to infrared light. These devices employ an objective lens positioned inside the boiler which images a wall surface of the boiler. Although these systems are effective in many applications, they have the shortcomings of high cost and sophistication, as well as the requirement for complex image processing. Moreover, since the wall being imaged is typically some distance from the objective lens, disturbances such as the fireball or products of combustion in the boiler can interfere with the clear visibility of the surfaces being imaged. Such cameras generally have lens tubes of a diameter of two inches or more, which pose installation difficulties in penetrating the boiler outer wall.
Another approach toward detecting the state of cleanliness of interior boiler surfaces is through the use of so-called heat flux sensors. These devices are typically thermocouple elements mounted to the steam tubes which carry the steam being produced by the boiler. When the temperature of the internal surface of the boiler and the steam carried within the pipe adjacent to that surface approach one another, it is then known that the rate of heat transfer from the combustion processes in the boiler has been reduced. This is an indirect indication of the development of slag encrustations and can be used to activate cleaning systems.
There is a need in the art to provide additional mechanisms for the detection of slag build ups on internal surfaces of combustion systems including coal fired boilers. Ideally, the device would be relatively inexpensive to manufacture, install, and use, be durable, require little maintenance, and reliably and accurately detect the presence of slag development.
In accordance with this invention, an emissivity probe for boiler wall monitoring is provided. The device uses a pair of optical guides, with one optical guide facing and receiving infrared radiation from the internal fireball within the boiler interior. The other optical guide is directed to receive radiation reflected from the wall surface. Photo detectors receive light transmitted by the optical guides, and produce an electrical signal output. By comparing the ratio or difference of these outputs, a measure of the degree to which the surface reflects radiation can be developed. Preferably, the probe is sensitive to radiation in the infrared region of the spectrum and over a band of radiation in the wavelength region of 0.4 to 4.0 microns. Peak sensitivity at around 1.45 microns is particularly suitable as this corresponds with the peak radiation from a source at about 3000° F. Slag encrustations which cause high reflectivity of the surfaces adversely affect heat transfer through the wall and thus such encrustations must be periodically removed. The emissivity probe in accordance with this invention provides a pair of single channel outputs from photodiodes which signals may be easily processed to provide control inputs for the boiler cleaning systems. The probe is further robust and durable in its construction. Due to the small diameter of the tube assembly, the emissivity probe of this invention can be easily installed through the outer wall of a boiler.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates from the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.
In order to provide more background for a description of the components and features of the emissivity probe of this invention, certain background principles are described. It is generally recognized that heat transfer occurs by three mechanisms; namely, radiation, convection, and conduction. In a coal burning furnace or boiler, heat transfer to the steam pipes primarily occurs through the mechanism of radiation. A well-known equation which describes the transfer of heat through radiation to a surface (wall) is as follows:
Q=σεA(Tf4−Tw4)
where
Therefore, a critical component in the transfer of heat to boiler walls is trough its characteristic emissivity (ε) factor. Emissivity can also be thought of as the absorbance of radiation by the wall surface. Of the total radiation incident on the wall surface, three related quantities of radiant heat energy can be identified that are related to the wall's characteristics. This relationship can be expressed as follows:
ε+ρ+T=1.0
where
As stated previously in this description, the heat transfer characteristics of a coal-fired boiler wall change over time due to the buildup of layers of fouling material on those surfaces which adversely affect absorbance and therefore heat transfer.
Inner tube 26 is disposed within outer tube 24 in a co-axial manner and is also a hollow cylinder open at both its proximal and distal ends. However, the proximal end of inner tube 26 passes through cooling fluid supply passageway 30 and through bore 31. The distal end of inner tube 26 is recessed slightly from the distal end of outer tube 24 as shown in FIG. 3. Inner tube 26 features a cut-out 34 at its distal end. Windows 32 and cut-out 34 cooperate to provide clearance for optical guides which are contained within inner tube 26, as will be described in further detail later in this description. For durability and resistance to corrosion considerations, outer and inner tubes 24 and 26 are preferably made of stainless steel. For ease of installation, outer tube 24 preferably has a small outside diameter (for example, three-quarters of an inch or less).
Various optical guide types may be used for forming incident and reflective optical guides 36 and 38. However, the optical guide system chosen must possess the features of high resistance to heat, not have excessive bend sensitivity, and must be durable in the severe operating environment of a boiler interior.
These inventors have found that so-called “image conduits” are an ideal guide type for incident and reflective optical guides 36 and 38. Image conduits are rods made of many individual optical fibers. The fibers are bundled and fused together with ground polished faces at their distal and proximal ends. Each guide made of an image conduit 36 and 38 is a bundle of several thousand individual fibers which are normally used to provide the ability to project and transmit images, with each fiber providing an individual “pixel”. In this application, however, imaging is not accomplished or intended. However, the ability for the image conduits of this type to be bent to the orientations shown in
Optical filters 51 and 53 are positioned between the ends of optical guides 36 and 38 and their respective photodiodes 50 and 52 so that probe 10 is sensitive to a limited spectral band of light. For coal burning boilers, an ideal maximum fireball temperature is about 3,300° F. which, according to well known principles of black body radiation, produces an intensely peak light output (or center wavelength) at a wavelength of 1.39 microns (micro-meters). More common temperatures are about 3,000° F., which correspond to a center wavelength of about 1.5 microns. By selecting filters 51 and 53 to have a band pass characteristic, with their maximum transmissivity occurring at around a wavelength of 1.4 to 1.5 microns, a high signal level will be available for measurement. The filters 51 and 53 should have a band pass range (defined as the difference in wavelength bounded by where a reduction of transmissivity of 50% of the maximum occurs) of around 200 nanometers. It should be noted that the characteristics of optical guides 36 and 38, and photodiodes 50 and 52 could be selected such that the combination is inherently sensitive over the previously described wavelength range without the use of filters 51 and 53.
Although, as described above, designing probe 10 to be sensitive of over a limited range corresponding to the maximum intensity of the fireball produces the highest output for measurement, it is also possible to operate over a broader wavelength range. Such a range is believed to be bounded by wavelengths of between 0.4 and 4.0 m micrometers.
Now with particular reference to
As best shown in
While tube assembly 12 is inserted within the boiler, air flow through passageway 30 enters the annular cooling fluid passageway 56 between inner and outer tubes 24 and 26. In addition to cooling the guides 36 and 38, this flow of flushing air (or other fluid) reduces the likelihood that contaminants will directly contact the distal ends of guides 36 and 38.
Now with reference to
Emissivity probe 60 also differs from probe 10 in that the proximal ends of guides 68 and 70 are bent to diverge. Photodiode adapters 76 and 78 are provided for the same function as previously described. Filters such as filters 51 and 53 shown in
Emissivity probe 60 preferably uses guides 68 and 70 of the type previously described; namely, so-called image conduits comprised of a large number of small diameter individual optical fibers which are bonded together. The advantageous attributes of such products previously described and are equally attractive for implementation with emissivity probe 60.
While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.
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