Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
     This specification claims priority to U.S. Provisional Patent Application No. 60/261,542, filed on Jan. 12, 2001, entitled “Sootblower Nozzle Assembly With an Improved Downstream Nozzle”. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention generally relates to a sootblower device for cleaning interior surfaces of large-scale combustion devices. More specifically, this invention relates to new designs of nozzles for a sootblower lance tube providing enhanced cleaning performance. 
     BACKGROUND OF THE INVENTION 
     Sootblowers are used to project a stream of a blowing medium, such as steam, air, or water against heat exchanger surfaces of large-scale combustion devices, such as utility boilers and process recovery boilers. In operation, combustion products cause slag and ash encrustation to build on heat transfer surfaces, degrading thermal performance of the system. Sootblowers are periodically operated to clean the surfaces to restore desired operational characteristics. Generally, sootblowers include a lance tube that is connected to a pressurized source of blowing medium. The sootblowers also include at least one nozzle from which the blowing medium is discharged in a stream or jet. In a retracting sootblower, the lance tube is periodically advanced into and retracted from the interior of the boiler as the blowing medium is discharged from the nozzles. In a stationary sootblower, the lance tube is fixed in position within the boiler but may be periodically rotated while the blowing medium is discharged from the nozzles. In either type, the impact of the discharged blowing medium with the deposits accumulated on the heat exchange surfaces dislodges the deposits. U.S. Patents which generally disclose sootblowers include the following, which are hereby incorporated by reference U.S. Pat. Nos. 3,439,376; 3,585,673; 3,782,336; and 4,422,882. 
     A typical sootblower lance tube comprises at least two nozzles that are typically diametrically oriented to discharge streams in directions 180° from one another. These nozzles may be directly opposing, i.e. at the same longitudinal position along the lance tube or are longitudinally separated from each other. In the latter case, the nozzle closer to the distal end of the lance tube is typically referred to as the downstream nozzle. The nozzle longitudinally furthest from the distal end is commonly referred to as the upstream nozzle. The nozzles are generally but not always oriented with their central passage perpendicular to and intersecting the longitudinal axis of the lance tube and are positioned near the distal end of the lance tube. 
     Various cleaning mediums are used in sootblowers. Steam and air are used in many applications. Cleaning of slag and ash encrustations within the internal surfaces of a combustion device occurs through a combination of mechanical and thermal shock caused by the impact of the cleaning medium. In order to maximize this effect, lance tubes and nozzles are designed to produce a coherent stream of cleaning medium having a high peak impact pressure on the surface being cleaned. Nozzle performance is generally quantified by measuring dynamic pressure impacting a surface located at the intersection of the centerline of the nozzle at a given distance from the nozzle. In order to maximize the cleaning effect, it is desired to have the stream of compressible blowing medium fully expanded as it exits the nozzle. Full expansion refers to a condition in which the static pressure of the stream exiting the nozzle approaches that of the ambient pressure within the boiler. The degree of expansion that a jet undergoes as it passes through the nozzle is dependent, in part, on the throat diameter (D) and the length of the expansion zone within the nozzle (L), commonly expressed as an L/D ratio. Within limits, a higher L/D ratio generally provides better performance of the nozzle. 
     Classical supersonic nozzle design theory for compressible fluids such as air or steam require that the nozzle have a minimum flow cross-sectional area often referred to as the throat, followed by an expanding cross-sectional area (expansion zone) which allows the pressure of the fluid to be reduced as it passes through the nozzle and accelerates the flow to velocities higher than the speed of sound. Various nozzle designs have been developed that optimize the L/D ratio to substantially expand the stream or jet, as it exits the nozzle. Constraining the practical lengths that sootblower nozzles can have is a requirement that the lance assembly must pass through a small opening in the exterior wall of the boiler, called a wall box. For long retracting sootblowers, the lance tubes typically have a diameter on the order of three to five inches. Nozzles for such lance tubes cannot extend a significant distance beyond the exterior cylindrical surface of the lance tube. In applications in which two nozzles are diametrically opposed, severe limitations in extending the length of the nozzles are imposed to avoid direct physical interference between the nozzles or an unacceptable restriction of fluid flow into the nozzle inlets occurs. In an effort to permit longer sootblower nozzles, nozzles of sootblower lance tubes are frequently longitudinally displaced. Although this configuration generally enhances performance by facilitating the use of nozzles having a more ideal L/D ratio, it has been found that the upstream nozzle exhibits significantly better performance than the downstream nozzle. Thus, an undesirable difference in cleaning effect results between the nozzles. 
     Initially, low performance of the downstream nozzle was attributed to the loss of static pressure associated with the fluid flow passing around the bluff body presented by the upstream nozzle in the form of the cylindrical projection of the nozzle into the lance tube interior. However, experiments conducted revealed that even when the upstream nozzle is moved radially outward to present no obstruction to the flow through the lance tube, the performance of the downstream nozzle did not significantly improve. The low performance of the downstream nozzle is believed to be due, in a significant manner, to the stagnation area created in the distal end of the conventional lance tube. A typical lance tube end or “nozzle block” has a rounded, hemispherical distal end surface. Since the downstream nozzle penetrates the nozzle block before the distal end hemispherical end surface, an internal volume exists beyond the downstream nozzle. Accordingly, a significant portion of the cleaning fluid approaching the downstream nozzle is forced to flow past the nozzle inlet and come to a stagnation condition at the distal end of the lance tube, and then re-accelerate to enter the nozzle. Furthermore, the back streams returning from the distal end are colliding with the forward streams at the downstream nozzle inlet leading to greater hydraulic losses and most importantly distorting the flow distribution into the nozzle. The hydraulic losses associated with the stagnation conditions at the distal end and at the nozzle inlet coupled with the flow mal-distribution which, based on concepts developed in connection with this invention, were believed, in large part, responsible for the low performance of the downstream nozzle. Therefore, there is a need in the art to provide a new lance tube design that will substantially increase the performance of the downstream nozzle. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, improvements in nozzle design are provided which provide enhanced performance of the downstream nozzle. In each case according to this invention, the nozzle block is formed to substantially eliminate the stagnation within the lance tube area beyond the downstream nozzle found in the prior art designs. Another beneficial feature of this invention involves streamlining at the upstream nozzle which minimizes the disruption to flow of cleaning medium to the downstream nozzle. 
     Briefly, a first embodiment of the present invention includes a downstream nozzle at the distal end of the lance tube with a converging channel formed in the interior of the lance tube to direct the flow of the cleaning medium passing the upstream nozzle and directing the flow to the downstream nozzle. The converging channel substantially eliminates the stagnation volume of the distal end of the conventional lance tube. This has the benefit of reducing hydraulic losses and improving the degree of uniformity of flow velocity at the throat, which in turn enhances the flow expansion and the conversion of static energy into kinetic energy. 
     The second embodiment of the present invention has an interior surface substantially identical to the first embodiment. However, the second embodiment nozzle block has a thin wall configuration which reduces the mass of the nozzle block. 
     A third embodiment of the present invention includes an airfoil body around the outside surface of the upstream nozzle. By providing streamline design of the outer surface of the upstream nozzle, the flow disturbances associated with the upstream nozzle is minimized. 
     A fourth embodiment of the invention features an upstream nozzle with its inlet end tipped toward the flow of the cleaning medium flowing through the lance tube. 
     In a fifth embodiment, the upstream nozzle features a longitudinal axis perpendicular to the longitudinal axis of the lance tube with the nozzle inlet tipped toward the flow of the blowing medium. 
     In a sixth embodiment in accordance with the teaching of the present invention provides for the design of the upstream nozzle having its outlet end flush with the body of the lance tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the invention will become apparent from the following discussion and accompanying drawings, in which: 
     FIG. 1 is a pictorial view of a long retracting sootblower which is one type of sootblower which may incorporate the nozzle assemblies of the present invention; 
     FIG. 2 is a cross-sectional view of a sootblower nozzle block according to prior art teachings; 
     FIG. 2A is a cross section view similar to FIG. 2 but showing alternative stagnation regions for the nozzle head; 
     FIG. 3 is a perspective representation of a lance tube nozzle block incorporating the features according to a first embodiment of the invention; 
     FIG. 4 is a cross section front view of the lance tube nozzle block according to the first embodiment of the present invention as shown in FIG. 3; 
     FIG. 5A is an enlarged cross-sectional view of the upstream nozzle in accordance with the teachings of the first embodiment of the present invention; 
     FIG. 5B is an enlarged cross-sectional view of the downstream nozzle in accordance with the teachings of the first embodiment of the present invention; 
     FIG. 6 is a cross-sectional front view of the lance tube nozzle block having a thin wall configuration in accordance with the teachings of the second embodiment of the present invention; 
     FIG. 7 is a cross-sectional front view of the lance tube nozzle block incorporating the airfoil or streamlining body around the upstream nozzle in accordance with the teachings of the third embodiment of the present invention; 
     FIG. 7A is an elevated cross-section view of the lance tube nozzle block incorporating the airfoil body around the upstream nozzle in accordance with the teachings of the third embodiment of the present invention; 
     FIG. 7B is a top perceptive view of the lance tube nozzle block incorporating the airfoil body around the upstream nozzle wherein the external surface of the nozzle has a trapezoidal cross section in accordance with the teachings of the third embodiment of the present invention; 
     FIG. 8 is a cross-sectional representation of the lance tube nozzle block having a curved upstream nozzle with respect to the longitudinal axis of the lance tube in accordance with the fourth embodiment of the present invention; 
     FIG. 9 is a cross-sectional representation of the lance tube nozzle block having an upstream nozzle with a straight discharge axis and a slanted inlet opening in accordance with the fifth embodiment of the present invention; and 
     FIG. 10 is a cross-sectional representation of the lance tube nozzle block having a exit plane of the upstream nozzle flush with the outer diameter of the lance tube nozzle block and having a thin wall construction in accordance with the sixth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description of the preferred embodiment is merely exemplary in nature, and is in no way intended to limit the invention or its application or uses. 
     A representative sootblower, is shown in FIG.  1  and is generally designated there by reference number  10 . Sootblower  10  principally comprises frame assembly  12 , lance tube  14 , feed tube  16 , and carriage  18 . Sootblower  10  is shown in its normal retracted resting position. Upon actuation, lance tube  14  is extended into and retracted from a combustion system such as a boiler (not shown) and may be simultaneously rotated. 
     Frame assembly  12  includes a generally rectangularly shaped frame box  20 , which forms a housing for the entire unit. Carriage  18  is guided along two pairs of tracks located on opposite sides of frame box  20 , including a pair of lower tracks (not shown) and upper tracks  22 . A pair of toothed racks (not shown) are rigidly connected to upper tracks  22  and are provided to enable longitudinal movement of carriage  18 . Frame assembly  12  is supported at a wall box (not shown) which is affixed to the boiler wall or another mounting structure and is further supported by rear support brackets  24 . 
     Carriage  18  drives lance tube  14  into and out of the boiler and includes drive motor  26  and gear box  28  which is enclosed by housing  30 . Carriage  18  drives a pair of pinion gears  32  which engage the toothed racks to advance the carriage and lance tube  14 . Support rollers  34  engage the guide tracks to support carriage  18 . 
     Feed tube  16  is attached at one end to rear bracket  36  and conducts the flow of cleaning medium which is controlled through the action of poppet valve  38 . Poppet valve  38  is actuated through linkages  40  which are engaged by carriage  18  to begin cleaning medium discharge upon extension of lance tube  14 , and cuts off the flow once the lance tube and carriage return to their idle retracted position, as shown in FIG.  1 . Lance tube  14  over-fits feed tube  16  and a fluid seal between them is provided by packing (not shown). A sootblowing medium such as air or steam flows inside of lance tube  14  and exits through one or more nozzles  50  mounted to nozzle block  52 , which defines a distal end  51 . The distal end  51  is closed by a semispherical wall  53 . 
     Coiled electrical cable  42  conducts power to the drive motor  26 . Front support bracket  44  supports lance tube  14  during its longitudinal and rotational motion. For long lance tube lengths, an intermediate support  46  may be provided to prevent excessive bending deflection of the lance tube. 
     Now with reference to FIG. 2, a more detailed illustration of a nozzle block  52  according to prior art is provided. As shown, nozzle block  52  includes a pair of diametrically opposite positioned nozzles  50 A and  50 B. The nozzles  50 A and  50 B are displaced from the distal end  51 , with nozzle  50 B being referred to as the downstream nozzle (closer to distal end  51 ) and nozzle  50 A being the upstream nozzle (farther from distal end  51 ). 
     The cleaning medium, typically steam under a gage pressure of about 150 psi or higher, flows into nozzle block  52  in the direction as indicated by arrow  21 . A portion of the cleaning medium enters and is discharged from the upstream nozzle  50 A as designated by arrow  23 . A portion of the flow designated by arrows  25  passes the nozzle  50 A and continues to flow toward downstream nozzle  50 B. Some of that fluid directly exits nozzle  50 B, designated by arrow  27 . As explained above, the downstream nozzle  50 B typically exhibits lower performance as compared to the upstream nozzle  50 A. This is attributed to the fact that the flow of cleaning medium that passes the upstream nozzle  50 A and downstream nozzle  50 B designated by arrows  29  comes to a complete halt (stagnates) at the distal end  51  of the lance tube  14 , thereby creating a stagnation region  31  at the distal end  51  beyond downstream nozzle  50 B. Hence, the cleaning medium represented by arrow  33  has to re-accelerate, flow backward and merge with the incoming flow  27 . The merging of the forward flow represented by arrow  27  and backward flow represented by arrow  33  results in loss of energy due to hydraulic losses at the nozzle inlet, and also results in flow mal-distribution. The loss of energy associated with stagnation conditions at the distal end and hydraulic losses at the nozzle inlet, and the deformation of the inlet flow profile is believed to be responsible for the downstream nozzle&#39;s lower performance in prior art designs. 
     As mentioned previously, there are various explanations for the comparatively lower performance of downstream nozzle  50 B as compared with nozzle  50 A. These inventors have found that the performance of downstream nozzle  50 B is enhanced by eliminating the stagnation area at nozzle block distal end  51  and moving the stagnation area to the inlet of the downstream nozzle; in other words, substantially eliminating the cleaning medium flows represented by arrows  29  and  33  shown in FIG.  2 . The advantages of this design concept can be described mathematically with reference to the following description and FIG.  2 A. 
     One of the key parameters in designing an efficient convergent-divergent Laval nozzle, such as nozzles  50 A and  50 B, is the throat-to-exit area ratio (Ae/At). A nozzle with an ideal throat-to-exit area ratio would achieve uniform, fully expanded, flow at the nozzle exit plane. The amount of gas expansion in the divergent section is given by the following equation which characterizes cleaning medium flow as one-dimensional for the same of simplified calculation.                  A                 e       A                 t       =         1     M                 e            [       (     2     γ   +   1       )     ·     (     1   +           γ   -   1     2     ·   M                     e   2         )       ]           (     γ   +   1     )       2        (     γ   -   1     )                   Equation                 1                                
     Where, 
     Ae=Nozzle exit area 
     At=Throat area which is also equal to the area of the ideal sonic plane 
     The exit Mach number, Me, is related to the throat-to-exit area ratio via the continuity equation and the isentropic relations of an ideal gas (See Michael A. Saad, “Compressible Fluid Flow”, Prentice Hall, Second Edition, page 98.)                P                 e     =     P                   o   ·       (     1   +           γ   -   1     2     ·   M                     e   2         )       γ     1   -   γ                     Equation                 2                                
     Where, 
     γ=Specific heat ratio of cleaning fluid. For air γ=1.4. For steam, γ−1.329 
     Pe=Nozzle exit static pressure, psia 
     Po=Total pressure, psia 
     Me=Nozzle exit Mach number 
     In the above equation 2, the relationship between exit Mach number and the pressure ratio is based on the assumption that the flow reaches the speed of sound at the plane of the smallest cross-sectional area of the convergent-divergent nozzle, nominally the throat. However, in practice, especially in sootblower applications, the flow does not reach the speed of sound at the throat, and not even in the same plane. The actual sonic plane is usually skewered further downstream from the throat, and its shape becomes more non-uniform and three-dimensional. 
     The distortion of the sonic plane is mainly due to the flow mal-distribution into the nozzle inlet section. In sootblower applications, as shown by arrows  23  for nozzle  50 A and arrows  33  and  27  for nozzle  50 B in FIG. 2, the cleaning fluid approaches the nozzle at 90° off its center axis. With such configuration, the flow entering the nozzle favors the downstream half of the nozzle inlet section because the entry angle is less steep. 
     The distortion and dislocation of the sonic plane consequently impacts the expansion of the cleaning fluid in the divergent section, and results in non-uniformly distributed exit pressure and Mach number. These findings were consistent with the measured and predicted exit static pressure for one of the conventional sootblower nozzles. 
     To account for the shift in the sonic plane, the actual Mach number at the exit can be related to the ideal throat-to-exit area as follows:                    A                 e       A                 t       ·       A                 t       A                 t_a         =         1     M                 e_a            [       (     2     γ   +   1       )     ·     (     1   +           γ   -   1     2     ·   M                     e_a   2         )       ]           (     γ   +   1     )       2        (     γ   -   1     )                   Equation                 3                                
     Where, 
     At_a=Effective area of the actual sonic plane 
     Me_a=Average of the actual Mach number at the nozzle exit 
     The degree of mal-distribution of the exit Mach number and the static pressure varies between the upstream and downstream nozzles  50 A and  50 B respectively of a sootblower. It appears that the downstream nozzle  50 B exhibits more non-uniform exit conditions than the upstream nozzle  50 A, which is believed to be part of the cause of its relatively poor performance. 
     The location of the downstream nozzle  50 B relative to the distal end  51  not only causes greater hydraulic losses, but also causes further misalignment of the incoming flow streams with the nozzle inlet. Again, greater flow mal-distribution at the nozzle inlet would translate to greater shift and distortion in the sonic plane, and consequently poorer performance. For the prior art designs, the ratio (At/At_a) is smaller for the downstream nozzle  50 B compared to the upstream nozzle  50 A. 
     In designing more efficient sootblower nozzles, it is necessary to keep the ideal and actual area ratio (At/At_a) closer to unity. Several methods are proposed in this discovery to accomplish this goal. For the upstream nozzle, the “At/At_a” ratio is in part influenced by dimension “X” and “α” shown in FIG. 2A, (At/At_a=f(α, X). Dimension X designates the longitudinal separation between nozzles  50 A and  50 B. 
     A smaller spacing X would cause the incoming flow stream  27  to become more mis-aligned with the upstream nozzle axis. For example, a five inch space for X has a relatively better performance than a four inch spacing for X. 
     While the greater X distance is beneficial, it is at the same time desired in most sootblower applications to keep X to a minimum for mechanical reasons. In such circumstances, an optimum X distance should be used which would minimize flow disturbance and yet satisfy mechanical requirements. Also, reducing the flow streams approach angle (α) shown in FIG. 2A would reduce flow mal-distribution at the nozzle inlet, and potentially reduce inlet losses. 
     For downstream nozzle  50 B, the “At/At_a” ratio is in part influenced by dimension “Y” shown in FIG. 2A, (At/At_a=f(Y)). Dimension Y is defined as the longitudinal distance between the inside surface of distal end  51  and the inlet axis of downstream nozzle  50 B. 
     Again referring to FIG. 2A, the location of the distal plane relative to the downstream nozzle  50 B, influences the alignment of the flow stream into the nozzle and cause greater flow mal-distribution. For instance, Y1 (which typifies the prior art) is the least favorable distance between the nozzle center axis and the distal end  51  of the lance tube. With such configuration, the nozzle performance is relatively poor. Y2 is an improved distance which is based on a modified distal end surface designated as  51 ′. In the case of Y2, the cleaning fluid  25  does not flow past the downstream nozzle  50 B, therefore eliminating stagnation conditions of the flows represented by arrows  29  and  33 . Instead the flow is efficiently channeled to the nozzle inlet. Thus, if the dimension Y is assumed positive in the left hand direction along the longitudinal axis of nozzle block  52  shown in FIG. 2A, there is an absence of any substantial flow of cleaning medium in the negative Y direction. Also, if the longitudinal axis (shown as a dashed line) of nozzle  50 B defines a Z axis assumed positive in the direction of discharge from the nozzle, then it is further true that once the longitudinal point is reached along the nozzle block  52  where flow first begins to enter downstream nozzle  50 B, there is a complete absence of any flow velocity vector having a negative Z component. In this way the hydraulic and energy losses at the nozzle inlet are minimized, improving the performance of downstream nozzle  50 B. Furthermore, with this improvement the cleaning fluid enters the downstream nozzle  50 B more uniformly, therefore minimizing the distortion of the sonic plane which in turn enhances the fluid expansion and the conversion of total pressure to kinetic energy. The optimal value of Y is substantially equal to Y2 which is one-half the diameter of the inlet end of downstream nozzle  50 B. 
     On the other hand, providing a shape of the distal end inside surface to  51 ″ is not beneficial. In such a configuration, the inlet flow area is reduced and the flow streams are further mis-aligned relative to the nozzle center axis, which could lead to flow separation and shedding. 
     Now with reference to FIGS. 3 and 4, a lance tube nozzle block  102  in accordance with the teachings of the first embodiment of this invention is shown. The lance tube nozzle block  102  comprises a hollow interior body or plenum  104  having an exterior surface  105 . The distal end of the lance tube nozzle block is generally represented by reference numeral  106 . The lance tube nozzle block includes two nozzles  108  and  110  radially positioned and longitudinally spaced. Preferably, lance tube nozzle block  102  and the nozzles  108  and  110  are formed as one integral piece. Alternatively, it is also possible to weld the nozzles into the nozzle block  102 . 
     FIG. 4 illustrates in detail the nozzles  108  and  110 . As shown, the nozzle  108  is disposed at the distal end  106  of the lance tube nozzle block  102  and is commonly referred to as the downstream nozzle. The nozzle  110  disposed longitudinally away from the distal end  106  is commonly referred to as the upstream nozzle. 
     With reference to FIGS. 4 and 5A the upstream nozzle  110  is shown which is a typical converging and diverging nozzle of the well-known Laval configuration. In particular, the upstream nozzle  110  defines an inlet end  112  that is in communication with the interior body  104  of the lance tube nozzle block  102 . The nozzle  110  also defines an outlet end  114  through which the cleaning medium is discharged. The converging wall  116  and the diverging wall  118  form the throat  120 . The central axis  122  of the discharge of the nozzle  110  is substantially perpendicular to the longitudinal axis  125  of the lance tube nozzle block  102 . However, it is also possible to have the central axis of discharge  122  oriented within an angle of about seventy degrees (70°) to about an angle substantially perpendicular to the longitudinal axis. The diverging wall  118  of the nozzle  110  defines a divergence angle φ 1  as measured from the central axis of discharge  122 . The nozzle  110  further defines an expansion zone  124  having a length L 1  between the throat  120  and the outlet end  114 . 
     With reference to FIGS. 4 and 5B, the downstream nozzle  108  also comprises an inlet end  126  and outlet end  128  formed about axis  136 . A portion of the cleaning medium not entering the upstream nozzle  110 , enters the downstream nozzle  108  at the inlet end  126 . The cleaning medium enters the inlet end  126  and exits the nozzle  108 , through the outlet end  128 . The converging wall  130  and the diverging wall  132  define the throat  134  of the downstream nozzle  108 . The plane of the throat  134  is substantially parallel to the longitudinal axis  125  of the nozzle block. The diverging walls  132  of the downstream nozzle  108  are straight, i.e. conical in shape, but other shapes could be used. The central axis  136  of nozzle  108  is oriented within an angle of about seventy degrees (70°) to about an angle substantially perpendicular to the longitudinal axis  125  of the lance tube nozzle block  102 . The nozzle  108  defines a divergent angle φ 2  as measured from the central axis of discharge  136 . An expansion zone  138  having a length L 2  is defined between throat  134  and the outlet end  128 . 
     Referring to FIG. 4, since the performance of a nozzle depends, in part, on the degree of expansion of the cleaning medium jet that exits through the nozzle. Preferably, the downstream nozzle  108  and the upstream nozzle  110  have identical geometry. Alternatively, the present invention may also incorporate downstream and upstream nozzle  108  and  110 , respectively, having different geometry. In particular, the diameter of throat  134  of the downstream nozzle  108  may be larger than the diameter of throat  120  of the upstream nozzle  110 . Further, the length L 2  of the expansion chamber  138  may be greater than the length L 1  of the expansion chamber  124  of the upstream nozzle  110 . In an alternate embodiment, the diameter of the throat  134  is at least 5% larger than the diameter of throat  120  and the length L 2  is at least 10% greater than length L 1 . Hence, the L/D ratio of the downstream nozzle  108  may be larger than the L/D ratio of the upstream nozzle  110 . 
     As shown in FIG. 4, the flow of cleaning medium that passes the upstream nozzle  110  represented by arrow  152  is directed by a converging channel  142 . The converging channel  142  is formed in the interior  104  of the lance tube nozzle block  102  between the upstream nozzle  110  and the downstream nozzle  108 . The converging channel  142  is preferably formed by placing an aerodynamic converging contour body  144  around the surface of downstream nozzle throat  134 . The converging channel  142  gradually decreases the cross-section of the interior  104  of the lance tube nozzle block  102  between the inlet end  112  of the upstream nozzle  110  and the inlet end  126  of the downstream nozzle  108 . The tip  148  of the body  144  is in the same plane as the inlet end  126  of the nozzle  108 . In the preferred embodiment, the contour body  144  is an integral part of the lance tube nozzle block  102  and the downstream nozzle  108 . The contour body  144  has a sloping contour such that the flow of the cleaning medium will be directed toward the inlet end  126  of the downstream nozzle  108 . Thus, converging channel  142  presents a cross-sectional flow area for the blowing medium which smoothly reduces from just past upstream nozzle  110  to the downstream nozzle  108  and turns the flow of cleaning medium to enter the downstream nozzle with reduced hydraulic losses. 
     As shown in FIG. 4, operation of nozzle block  102  in accordance with the first embodiment of the present invention is illustrated. The cleaning medium flows in the interior  104  of the lance tube nozzle block  102  in the direction shown by arrows  150 . A portion of the cleaning medium enters the upstream nozzle  110  through the inlet end  112 . The cleaning medium then enters the throat  120  where the medium may reach the speed of sound. The medium then enters the expansion chamber  124  where it is further accelerated and exits the upstream nozzle  110  at the outlet end  114 . 
     A portion of the cleaning medium not entering the inlet end  112  of the upstream nozzle  110  flows towards the downstream nozzle  108  as indicated by arrows  152 . The cleaning medium flows into the converging channel  142  formed in the interior  104  of the lance tube nozzle block  102 . The converging channel  142  directs the cleaning medium to the inlet end  126  of the downstream nozzle  108 . Therefore, the cleaning medium does not substantially flow longitudinally beyond the inlet end  126  of the downstream nozzle  108 . In addition, once the flow reaches inlet end  126 , there is no flow velocity component in the negative “Z” direction (defined as aligned with axis  136  and positive in the direction of flow discharge). Due to the presence of the converging channel  142 , the flow of the cleaning medium is more efficiently driven to the nozzle inlet  126 . The loss of energy associated with the cleaning medium entering the throat  134  of the downstream nozzle  108  is reduced, hence increasing the performance of the downstream nozzle  108 . Unlike prior art designs, the flowing medium does not have to come to a complete halt in a region beyond the downstream nozzle and then re-accelerate to enter the inlet end  126  of the nozzle  108 . Further, since it is also possible to have different geometry for the upstream nozzle  110  and the downstream nozzle  108 , the cleaning medium entering the expansion zone  138  in the downstream nozzle  108  is expanded more than the cleaning medium in the expansion zone  124  of the upstream nozzle  110  so as to compensate for any nozzle inlet pressure difference between the nozzles  108  and  110 . The kinetic energy of the cleaning medium exiting the downstream nozzle  108  more closely approximates the kinetic energy of the cleaning medium exiting the upstream nozzle  110 . 
     With particular reference to FIG. 6, a lance tube nozzle block  202  in accordance with the second embodiment of the present invention is shown. The lance tube nozzle block  202  is similar to the lance tube nozzle block  102  defining a hollow interior  204  and exterior surface  205 . The lance tube nozzle block  202  has a downstream nozzle  208  and an upstream nozzle  210  that have identical configuration to nozzles  108  and  110  of the first embodiment. Further, the nozzle block  202  has identical internal volume and flow paths as the nozzle block  102 . 
     The second embodiment differs from the first embodiment in the wall thickness of the nozzle block  202  is reduced. The flow obstruction  244  is hollow, thereby reducing the mass of the nozzle block  202 . 
     With reference to FIGS. 7,  7 A and  7 B, a lance tube nozzle block  302  for a sootblower in accordance with the teaching of the third embodiment of the present invention is shown. The lance tube nozzle block  302  includes a hollow interior  304 . The lance tube nozzle block  302  includes a downstream nozzle  306  and an upstream nozzle  310 . The dimension and geometry of the downstream and upstream nozzles  306  and  310 , respectively, are identical to the dimension and geometry of the nozzles  108  and  110  of the first embodiment. 
     This embodiment of the lance tube nozzle block  302  differs from the previously described embodiment in that the upstream nozzle  310  includes an airfoil or streamline body  311  around the nozzle diverging surface  312  of the upstream nozzle  310 . Preferably, the upstream nozzle airfoil body  311  has a trapezoidal cross section. The divergent section  307  (as shown in FIG. 7A) of the upstream nozzle  310  is circular at each point along its axis from the inlet to the exit plane. The airfoil body  311  has a smooth upstream incline surface  314 A and a downstream incline surface  314 B. The upstream incline surface  314 A receives the cleaning medium from the proximate end of the nozzle block which flows in the direction as shown by arrows  319  in FIG.  7 . The downward incline surface  314 B allows a smooth flow of the cleaning medium past the upstream nozzle  310  to the inlet end  316  of the downstream nozzle  306  as shown by arrows  320 . The angle of incline Ψ 1  of the airfoil body  311  is measured between central axis  315  of upstream nozzle  310  and the inclining surface  314 B of the airfoil body  311  as shown in FIG.  7 . In the preferred embodiment the airfoil body  311  is made of same material as the nozzle block  302 . The airfoil body  311  provides for a smooth flow of the cleaning medium to the inlet end  316  of the downstream nozzle  306  as shown by arrows  320 . Further, the airfoil body  311  will help reduce the turbulent eddies influencing the upstream nozzle  310  and minimize pressure drop of the flow  320  that passes upstream nozzle  310  to feed the downstream nozzle  306 . FIG. 7A is a sectional view of nozzle block  302  which is tipped slightly. This perspective helps to further illustrate the contours of hollow interior  304 . FIG. 7B shows particularly a solidified form of airfoil body  311 . This view shows that airfoil body  311 ′, like airfoil body  311 , includes side surfaces  324 . Airfoil bodies  311  and  311 ′ are configured to minimize obstructions of flow area past nozzle  310 . This is, in part, provided by having side surface  324  closely approach these inside surfaces,  307 , of nozzle  310 . 
     Now referring to FIG. 8, a lance tube nozzle block  402  in accordance with the fourth embodiment of the present invention is illustrated. The lance tube nozzle block hollow interior  404  defines a longitudinal axis  407 . The lance tube nozzle block  402  has a downstream nozzle  408 , positioned at a distal end  406  of the lance tube nozzle block  402 . The upstream nozzle  410  is longitudinally spaced from the downstream nozzle  408 . In this embodiment, the downstream nozzle  408  has the same configuration as the nozzle  108  of the first embodiment. However, the geometry of the upstream nozzle  410  is different. In this embodiment, the upstream nozzle  410  has a curved interior shape such that the inlet end  412  curves towards the flow of the cleaning medium as shown by arrows  411 . The central axis of discharge end  416  as measured from the inlet end  412  to the outlet end  418  is curved and not straight. The upstream nozzle  410  has converging walls  420  and diverging wall  422  joining the converging walls. The converging walls  420  and the diverging walls  422  define a throat  424 . A central axis of throat  424  is curved such that the angle Ψ 3  defined between the throat  424  and the longitudinal axis  407  of the nozzle block  402  is in the range of 0 to 90 degrees. Preferably the angle Ψ 3  is equal to about 45 degrees. 
     FIG. 9 represents a lance tube nozzle block  502  in accordance with the fifth embodiment of the present invention. The lance tube nozzle block  502  has identical configuration as the lance tube nozzle block in the fourth embodiment. The lance tube nozzle block  502  has a downstream nozzle  508  positioned at the distal end  506  of the lance tube nozzle block  502 . The lance tube nozzle block  502  has an upstream nozzle  510  that defines an inlet end  512  and an outlet end  514 . A throat  516  is defined by converging walls  520  and diverging walls  522 . 
     The present embodiment differs from the nozzle geometry in the fourth embodiment in that the upstream nozzle  510  has a central axis  518 , which is straight and not curved as described in the previous embodiment. The present embodiment has an inlet end  512  angled towards the flow of the cleaning medium, as shown by arrows  511 . In order to have the inlet end  512  angled toward the flow of the cleaning medium, the converging and diverging walls  520  and  522 , diametrically opposite each other are of different length. Thus, the diverging wall  522 A is longer than the diverging wall  522 B. 
     FIG. 10 represents the sixth embodiment of the present invention. The lance tube nozzle block  602  defines an interior surface  604  and an exterior surface  606 . The downstream nozzle  608  is positioned at the distal end  607  of the lance tube nozzle block  602 . The downstream nozzle  608  is of the same configuration and dimension as the nozzle  108  of the first embodiment. 
     The upstream nozzle  610  is a straight nozzle having an inlet end  612  and an outlet end  614 . Like the upstream nozzle of the previous embodiments, the upstream nozzle  610  has a throat  616  defined by the converging walls  618  and diverging walls  620 . The upstream nozzle  610  defines a central axis of discharge  622  between the inlet end  612  and the outlet end  614 . In this embodiment, the plane  624  of the outlet end  614  is flush with the exterior surface  606  of the lance tube nozzle block  602 . The nozzle expansion zone  622  provided by the diverging walls  620  is located entirely inside the diameter of lance tube nozzle block  602 . Nozzle block  602  further features a “thin wall” construction in which the outer wall has a nearly uniform thickness, yet forms ramp surfaces  628  and  630 , and tip  632 . 
     The foregoing discussion discloses and describes a preferred embodiment of the invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that changes and modifications can be made to the invention without departing from the true spirit and fair scope of the invention as defined in the following claims.

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