Abstract:
An emissivity probe for measuring reflected energy from interior wall surfaces of large scale boilers. The emissivity probe uses a pair of light guides which respectively receive light energy from the interior of the boiler and that reflected off a wall surface. Using appropriate photo detectors sensitive to a desired light wavelength range, a ratio of the reflected and incident radiation is provided. This provides a measure of the reflectivity of the wall surface. The reflectivity values are used to control boiler cleaning systems.

Description:
FIELD OF THE INVENTION 
   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. 
   BACKGROUND OF THE INVENTION 
   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. 
   SUMMARY OF THE INVENTION 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a pictorial view of an emissivity probe in accordance with this invention; 
       FIG. 2  is a side elevational view of the emissivity probe shown in  FIG. 1  with the sensor housing assembly removed; 
       FIG. 3  is a cross-sectional view through the emissivity probe component shown in  FIG. 2  taken along line  3 — 3  from  FIG. 2 ; 
       FIG. 4  is a cross-sectional view taken through the proximal end of the tube assembly; 
       FIG. 5  is a cross-sectional view through the distal end of the tube assembly of the emissivity probe; 
       FIG. 6  is a side elevational view of the incident optical guide used in the emissivity probe as shown in  FIG. 1 ; 
       FIG. 7  is a elevational view of a reflective optical guide used in the emissivity probe of  FIG. 1 ; 
       FIG. 8  is a pictorial view of the distal end of the tube assembly of the emissivity probe as shown in  FIG. 1 ; 
       FIG. 9  is a cross-sectional view through an emissivity probe in accordance with a second embodiment of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   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 ( T   f   4   −T   w   4 )
 
where
         A=wall area   σ=Stefan-Boltzman constant   ε=wall emissivity   T f =flame temperature   T w =wall temperature       

   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&#39;s characteristics. This relationship can be expressed as follows:
 
ε+ρ+ T =1.0
 
where
         ρ=reflectance   T=transmissance
 
For boiler tube walls T=0 (i.e. it is opaque to incident radiation) and therefore:
 
ε+ρ=1.0
 
Therefore by quantifying reflectance (ρ), emissivity (ε) can be indirectly measured.
 
The emissivity probe in accordance with this invention utilizes this concept.
   And since ε+ρ=1.0, and
 
ρ=(Q wall /Q incident )
 
ε=1.0−(Q wall /Q incident )
 
where
   Q wall =Reflected radiation from the wall   Q incident =Incident radiation from the flame
 
Thus by measuring the reflected radiation from the boiler wall (Q wall ) and the incident radiation from the flame (Q incident ) a measure of emissivity and therefore absorbance can be made.
       

   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. 
     FIG. 1  illustrates an emissivity probe in accordance with a first embodiment of this invention. Emissivity probe  10  principally comprises tube assembly  12  which is attached to sensor housing assembly  14  by flange assembly  16 . Bulkhead  18  is provided for mounting the device to an associated boiler inner wall  19  through a port  21 (shown in  FIG. 2 ) which enables tube assembly  12  to project into the interior  23  of the boiler, while sensor housing assembly  14  is external to the interior of the boiler and thus protected from the severe environment of the boiler interior. The steam tubes  25  are shown as part of inner wall  19 . To facilitate installation, port  21  is preferably lined with port tube  26 , which is attached at one end to outer wall  27  and at the other end to inner wall  19 . 
     FIG. 2  shows tube assembly  12  apart from sensor housing assembly  14 . Tube assembly  12  defines distal end  20  and proximal end  22  which is supported by flange assembly  16 . Further references in this description to “proximal end” are used to describe components or features at or near flange assembly  16 , whereas references to “distal end” are used to describe components or features at or near the free end ( 20 ) of the tube assembly  12 . 
     FIG. 3  shows in cross-section internal components of tube assembly  12  and flange assembly  16 . As shown, tube assembly  12  includes outer tube  24  and inner tube  26 . Outer tube  24  is a hollow cylinder and open at both its proximal and distal ends. Outer tube  24  is mounted to flange  28  through bore  29 , such that its proximal end communicates with cooling fluid supply passageway  30 . Outer tube  24  further includes an aperture or window  32  positioned adjacent to the tube&#39;s distal end. Outer tube  24  is permanently fastened to flange  28  by welding, brazing, through interference fit, or using other attachment approaches. 
   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). 
     FIG. 4  shows the details of the proximal end of emissivity probe  10  when it is fully assembled with incident optical guide  36  and reflective optical guide  38  installed in position within inner tube  26 . Additional details of the incident and reflective optical guides  36  and  38  are provided with reference to  FIGS. 6 and 7 . Incident guide  36  features a pair of bends  40  and  42  such that its proximal end is parallel but displaced from the main axis of the guide. The distal end of incident guide  36  is straight and receives light inputs from light rays within an incident “cone” and along its longitudinal axis or “field of view” (FOV) as will be described in further detail in the following description. Reflective guide  38  is generally straight but has a hooked distal end  44 . Incident guide  36  can be inserted into inner tube  26  from the proximal end of the inner tube whereas reflective guide  38  can be inserted into the opposite distal end of the inner tube. 
   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  FIGS. 6 and 7 , and their heat resistance, flexibility and durability make them ideally suited for the present application. In an application of this invention, guides  36  and  38  were comprised of image conduits each having bundles of fibers of a 25 micron diameter with three thousand individual optical fibers. These image conduits have a maximum operating temperature up to 850° F. and have a melting temperature of 1,200° F. The polished ends of the image conduits comprising optical guides  36  and  38  provide a field of view angle of about 64°, as shown in  FIGS. 6 and 7 . 
     FIG. 4  illustrates that when incident and reflective guides  36  and  38  are disposed within inner tube  26 , their proximal ends terminate in a pair of photodiode adapters  46  and  48 . Sensor elements shown as photodiodes  50  and  52  are disposed within adapters  46  and  48  and receive light signal transmissions through guides  36  and  38 . 
   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  FIGS. 5 and 8 , the distal end of tube assembly  12  is described in more detail. As illustrated, the distal end of incident guide  36  protrudes slightly from inner tube  26  and is oriented in a forward looking direction. In other words, the incident cone defining the field of view (FOV) of guide  36  is oriented concentrically with the longitudinal axis of tube assembly  12 . The distal end of incident guide  36  is also recessed slightly from the distal end of outer tube  24 . This orientation is provided to reduce the fouling of the sensitive distal end of incident guide  36 . Reflective guide hooked end  44  extends through inner tube cut-out  34 . Outer tube window  32  is provided to give the distal end of reflective guide  38  a “view” of the boiler wall surface. The distal end is also recessed within the cylinder defined by outer tube  24  for fouling protection. In other words, the cone defining the field of view of reflective guide  38  is not obstructed by outer tube  24 , due to the provision of window  32 . Thus light along a direction from the boiler wall is received by reflective guide  38 . The direction of the field of view of reflective guide  38  is at a reverse angle from the direction of incident guide  36 , i.e. they form an angle of greater than 90°. 
   As best shown in  FIG. 4 , light rays entering the distal ends of incident and reflective guides  36  and  38  are transmitted along the guides and are emitted at their proximal ends onto the associated photodiodes  50  and  52 . The electrical outputs of the photodiodes are processed by signal processor  54  such that their relative values are compared. A ratio or difference calculation allows the reflected signal transmitted through reflective guide  38  to be compared with the “source” radiation emanating from the fireball of the boiler interior, transmitted through incident guide  36 . Due to time dependent changes within the boiler interior  23 , such as flame flicker, smoke, and turbulence, it is necessary to detect both the incident and reflective radiation signals in order to measure reflectance and therefore emissivity. Preferably, the signals from photodiodes  50  and  52  are also processed by processor  54  to provide some time averaging to remove high frequency effects, such as flame flicker. The signal processing requirements of processor  54  are not complex, since the outputs from photodiodes  50  and  52  are single channel outputs, each indicating a single intensity level which varies over time. 
   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  FIG. 9 , a second embodiment of an emissivity probe in accordance with this invention is shown which is generally designated by reference number  60 . Emissivity probe  60  features a tube assembly  62  which differs from that previously described. In this instance, outer tube  64  and inner tube  66  do not feature the windows for “side looking” by a reflected optical guide. Rather, each of the guides  68  and  70  have bent ends  72  and  74  for receiving radiation in different directions. Emissivity probe  60  could be used in installations where one of the guide ends  72  or  74  can be directed to an adjacent wall surface, whereas the other guide end is oriented toward the boiler fireball. 
   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  FIG. 4  may also be provided. For this embodiment of emissivity probe  60 , cooling air flow is also provided to cool guides  68  and  70  and reduce the accumulation of deposits on their distal bent ends  72  and  74 . The distal ends of guides  68  and  70  are approximately flush with the distal end of outer tube  64 . At that distal end, outer tube  64  has a reduced diameter end  80  which increases the velocity of the cooling air flow, aiding in preventing contamination of the distal ends of guides  68  and  70 . 
   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.