Abstract:
An inspection lens, including a lens tube having a central longitudinal axis and a plurality of axially aligned optical elements, including an outermost element. A sheath surrounding the lens tube and defining there between an annular air flow channel. The sheath has a diversion nozzle at a distal end, terminating in a lens tip. The diversion nozzle is configured to divert the air flow in the annular channel inward toward the longitudinal axis and at a slight reverse angle of between 0-18 degrees relative to a plane normal to the axis and back toward the outermost element creating a reverse oblique impinging jet inside said lens tip that minimizes any recirculation zone in front of the outermost element.

Description:
[0001]    This invention pertains to inspection lenses, especially to inspection lenses used in dusty environments, and to a method and apparatus for maintaining an inspection lens free from fouling by gas-borne particulates in the atmosphere where it is used. 
       BACKGROUND OF THE INVENTION 
       [0002]    Standard inspection lenses are used in installations having enclosed process spaces that require monitoring by inspection cameras. In such spaces having a heavily particle laden atmosphere, especially a high temperature atmosphere such as recovery boilers, steel manufacturing, various kilns such as cement kilns, where there is a need to protect the camera optics from heat and particulate fouling, air flow around the lens is typically provided to cool the lens and prevent fouling by the particles in the particle laden atmosphere. 
         [0003]    A typical inspection lens is illustrated here to explain the nature of the structure and the air flow that the structure produces, and to compare it to the invention exemplified below. An inspection lens  40  of the prior art, shown in  FIGS. 1A and 1B , has a lens tube  44  and an outer annular sheath  42  surrounding and radially spaced from the lens tube. A plurality of lenses  45 ,  46  are supported in fixed positions spaced along a longitudinal axis  47  in a lens mounting cylinder  48 , along with a front lens window  41  at the forward end of the lens tube  44 . An annular sheath air flow channel  50  between the outside diameter of the lens tube  44 /lens mounting cylinder  48  and the annular sheath  42  carries cooling and cleaning air around the lens tube. An annular nozzle  54  attached to the distal end of the annular sheath  42  terminates at a lens tip  56 . The nozzle  54  diverts the annular air flow to converge in front of the lens tip  56  via an annular flow deflector surface  60  in the nozzle. This standard lens sheath (purge) flow jet design has been in use in these environments for more than thirty years. 
         [0004]    When a camera installation using the prior art air flow cooling and cleaning nozzle becomes unusable because of light-blocking contamination or because of breakage of the outermost optics element, the operator is faced with difficult choices. In the case of a recovery boiler, fine dust and black liquor debris can accumulate and quickly render the camera unusable by a coating of dust and other contamination on the outermost surface of the optics. The lens is not difficult to clean, but if it can become fouled quickly (for example, every 10 minutes) in an industrial environment, the instrument would typically no longer be used until its use was essential. This greatly reduces its usefulness. The diversion of manpower to perform this frequent cleaning is not acceptable, since the operator just does not have the employees available to clean the instrument every 10 minutes. 
         [0005]    In other applications such as in steel plants like LMF and EAF (Liquid Metallurgy Furnace and Electric Arc Furnace), the process space environments not only include the possibility of high dust environments but large (millimeter sized) steel particles can impact the optics. When this happens, the front camera optics element can become covered with opaque particles or can be damaged, necessitating repairs before the system can be used again. 
         [0006]    Thus, new sheath (purge) flow jet technology that significantly reduces (or completely eliminates) the particle deposition on the lens has long needed. 
       SUMMARY OF THE INVENTION 
       [0007]    We have discovered that the flow diversion nozzle in prior art inspection lenses produces a recirculation zone  62  (shown in  FIGS. 2A and 2B ) in front of the outermost optics element of the prior art inspection lens  40 , which can trap dust and other particles present in the enclosed process spaces that the inspection lens is intended to monitor, and these particles eventually become deposited on the outer surface of the outermost optics element. The particulate size distribution in these enclosed process spaces can vary from nanoparticles to large droplets (10 nm-10 mm). The smaller particles (Stokes number smaller than 1) can get entrained into recirculation areas surrounding the sheath (purge) flow jet due to turbulent mixing. The larger particles (Stokes number greater than 1) can penetrate the jet due to their inertia. In addition the liquid droplets can break up (atomize) when subjected to large shear stress in the jet and will mix into the volume in front of the lens optics. 
         [0008]    To remedy this problem, this invention provides a reverse nozzle that diverts the sheath (purge) flow jet to produce a reverse flow that converges inside the lens tip, creating a strong axial jet that is effective in keeping the particles from penetrating into the area around the lens optics. The converging reverse flow jet creates an oblique impinging jet that is focused to impinge against and along the front of the lens optics to sweep away any particles that may have intruded into the area in front of the lens and substantially reduces or completely eliminates the recirculation area on front of the lens optics, thereby significantly reducing (or completely eliminating) the particle deposition on the lens. 
         [0009]    The inspection lens of the preferred embodiment of this invention includes a lens tube having a central longitudinal axis and a plurality of axially aligned lenses, including an outermost optics element, which can be a lens or preferably an optical window. A sheath surrounds the lens tube and defines therebetween an annular air flow channel for conveying an air flow forward. A diversion nozzle at the distal end of the sheath terminates in a lens tip. The diversion nozzle is configured to divert the air flow in the annular channel inward toward the longitudinal axis and at a slight reverse angle relative to a plane normal to the axis and back toward the outermost optics element, such that the air flow converges at the outer surface of the outermost optical element creating a reverse oblique impinging jet inside the lens tip that minimizes any recirculation zone in front of the outermost optics element and produces a strong axial jet that is effective in cleaning the lens surface by transferring air flow momentum to any particulates depositing on said lens surface. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention and its many advantages and features will become better understood upon reading the following detailed description of the preferred embodiments in conjunction with the following drawings, wherein: 
           [0011]      FIG. 1A  is a sectional elevation of a standard prior art purge air tip; 
           [0012]      FIG. 1B  is a sectional perspective view of the purge air tip shown in  FIG. 1A ; 
           [0013]      FIG. 2A  is an air flow diagram showing air flow velocity contours around the purge air tip shown in  FIG. 1A ; 
           [0014]      FIG. 2B  is diagram of velocity vectors around the purge air tip shown in  FIG. 1A ; 
           [0015]      FIG. 3A  is a sectional elevation of an inner assembly of a purge air tip in accordance with this invention; 
           [0016]      FIG. 3B  is a sectional perspective of a complete assembly of the purge air tip shown in  FIG. 3A ; 
           [0017]      FIG. 4A  is an air flow diagram showing air flow velocity contours around the purge air tip shown in  FIG. 3A ; 
           [0018]      FIG. 4B  is diagram of velocity vectors around the purge air tip shown in  FIG. 3A ; 
           [0019]      FIG. 5  is a diagram showing the range of useful angles for the reverse purge system shown in  FIGS. 3A and 3B , from −7.7 degrees to “x,” where “x” is limited by the field of view of the lens objective. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Turning now to the drawings, and more particularly to  FIGS. 1A and 1B  and  FIGS. 2 a  and 2 b    thereof, we have found that the fouling problem that the standard furnace inspection lenses shown in  FIGS. 1A and 1B  suffer from are due to entrainment in the sheath air flow of particulate laden gas from the internal furnace ambient environment  58 . The forward directed converging air flow creates a very stable recirculating zone  62  in front of the lens optics, as shown in  FIGS. 2A and 2B . Note that in  FIG. 2A  (and also  FIG. 4A ), the gas flow velocity and direction is indicated by the flow vector lengths and arrow head size, and in  FIG. 2B , (and also  FIG. 4B ) the density of the arrows in the vector plot is a consequence of how the computational simulation is setup. Areas of high arrow/vector density indicate regions of the computational domain that have more “cells” meaning that that region is more resolved. Note also that the entire jet is turbulent. The flow is turbulent even before it enters the stagnant (relative to the jet velocities) dirty furnace air. The figures showing gas flow velocities are time-averaged plots showing the average flow field. In time averaged plots, the chaotic turbulent fluctuations are averaged away and design decisions can be based on the “smoothed” time-averaged data. That being said, it is important to note that although the figures look like straight laminar pathlines in reality the randomness of turbulence is mixed in. 
         [0021]    Particle laden gas is trapped in this recirculating zone  62 , or eddy, in front of the frontmost optics element, inside and also forward of the lens tip  66 , followed by particle deposition onto the lens optics. The physics of formation of this recirculation eddy  62 , shown in  FIG. 2B , is similar to the classic backward-facing step problem and it is impossible to prevent without directing flow at the center of the lens optics itself. Many particles that get entrained in this very stable recirculation zone  62  will stay in the zone for a very long time (dynamic arrest) until they deposit onto the lens optics fouling the device. 
         [0022]    As shown in  FIGS. 3A and 3B , a preferred embodiment of an inspection lens in accordance with this invention includes an outer sheath  42 ′ spaced from and surrounding a lens tube  44 ′. An annular gas flow channel  50 ′ is bounded on the outside by the outer sheath  42 ′ and on the inside by the lens tube  44 ′. A lens mounting cylinder  48 ′ is fastened to the front end of the lens tube and is held centered in the annular sheath  42 ′ by a castellated spacer ring  43 ′ which allows passage of gas along the annular gas flow channel  50 ′. A nozzle  64  is attached to the distal end of the outer sheath  42 ′ to divert the gas flow inward toward a longitudinal axis  47 ′. 
         [0023]    A reverse sheath (purge) flow jet nozzle  64  in accordance with the invention diverts the gas flow inward toward a longitudinal axis  47 ′ to produce a converging reverse flow jet  70  focused at the front of the lens optics to substantially reduce or completely eliminate the possibility of a recirculation area forming in the region  72  immediately in front of the lens optics, thereby significantly reducing (or completely eliminating) the particle deposition on the lens.  FIGS. 3A and 3B  shows the reverse flow nozzle  64 , and  FIGS. 4A and 4B  show the reverse flow jet  70  around the new flow jet nozzle  64  in accordance with the invention, showing the minimal air recirculation in the region  72  in front of the lens optics compared to  FIGS. 2A and 2B . The reverse flow gas jet  70  passes through a reverse jet passage bounded by the inner nozzle surfaces  65  of the reverse jet nozzle and the complementary shaped front surface of the lens mounting cylinder  48 ′ to produce a high velocity annular reverse flow oblique impinging jet  70  that converges inside the lens tip directly against and long front surface of the frontmost optics element, displacing any particles that may have landed on the surface and creating a strong axial jet  74  that is effective in keeping the particles from penetrating into the area around the lens&#39; optics. In the preferred embodiment, the frontmost optics element is a sapphire window  71 , mounted between the front lens housing  76  and the lens mounting cylinder  48 ′ directly in front of the outermost lens  78 , to protect the outermost lens. 
         [0024]    The reverse flow lens makes a more efficient use of the air supply (by nature of the stronger jet vs the prior art for the same purge flowrate). More “efficient” than the prior art means that the same amount of air flow will provide a more focused and higher velocity jet and thus provide better overall purging of the optics region. It allows for camera systems to be installed in environments that were previously unattainable with the prior art. 
         [0025]    The flow angle for reversed flow depends on the desired view angle; for example  FIG. 3A  shows the design for 120 degree view angle. The impinging jet angle in this design is about 18 degrees from vertical relative to the lens optics window (108 degrees from axial velocity component). If greater view angle is desired, an opposing jet approach can be used. In this scenario the purge jet are opposing to each other creating a jet parallel to the lens optics or perpendicular to the axial jet. 
         [0026]      FIGS. 2B, 4B , show the differences in velocity direction and magnitude between the standard lens tip and the reverse purge flow lens tip. Shallow angle design can greatly improve the performance of the purge air by effectively strengthening the axial jet and reducing the recirculation area in front of the lens. The smaller the angle of the purge air jet relative to the axial jet, the greater the size of the recirculation area in front of the lens tip. Based on the typical jet expansion angle of 7.5 degrees we prefer purge air angles between −7.5 degrees to an angle limited by the field of view of the lens objective (see  FIG. 5 ). The range of useful angles for the reverse purge system is about −18 degrees to one half the field of view of the lens objective, primarily to avoid partially obstructing the field of view. 
         [0027]    Specific air flowrates and pressures are different in different industrial applications because of their different constraints, (limited air supply, limited allowable air that can be used due to the process, position of camera system, the level of dust and particles that have to be rejected, etc.) The reverse flow lens allows for installations in applications that are difficult or impossible for the prior art. For example, the new lens will reject steel splatter if enough air is supplied, but the amount of air needed by the new lens to create a strong enough axial jet to protect the optics from damage is much lower that what the prior art system can provide. The exact amount of air required changes with the specific application. Also, the reverse flow lens can be used to keep the inspection lens clean in installations where the prior art was successful, but can do so while using less air, which saves money. 
         [0028]    Benefits of using the new design include a greatly reduced recirculation area in front of the lens and is located well inside of the lens tip. Particles and aerosol droplets are less likely to be captured in a small recirculation zone and therefore are less likely to be deposited on the optics. A much stronger axial jet is formed for the same operating conditions (purge air flow rate and pressure) which helps prevent particles from entering into the area of the lens tip. The oblique impinging gas jets against the outermost optics element greatly improves cleaning of the lens optics by the jets due to the transfer of the flow momentum to any particulates depositing onto the lens surface. 
         [0029]    Obviously, numerous modifications and variations of the preferred embodiment described above are possible and will become apparent to those skilled in the art in light of this specification. For example, in high temperature process environments that could not use oxygen containing air as the cooling/cleaning gas, such as explosive or other highly reactive atmospheres, nitrogen, argon or CO 2  could be used as the cooling/cleaning purge gas. In process spaces having temperatures in warm or ambient temperature, an IR camera may be unsuitable because the temperature in the process space must be elevated at least 200F to get good images. However, a cooled MWIR camera would be able to get good images and use the new reverse flow lens in a dusty cool environment to maintain the dust-free condition of the lens. Therefore, we expressly intend that all these and other embodiments, species, modifications and variations, and the equivalents thereof, are to be considered within the spirit and scope of the invention as defined in the following claims, wherein we claim: