Abstract:
A method for cleaning tubes and heat exchangers provides a cavitation enhancement unit between a source of pressurized fluid an a lance. The pressurized fluid flows through jets, which impart a high speed rotation to a set of propellers, preferably square in profile canted at a 15° angle. Generation of cavitation develops a cleaning vibration in the fluid discharged from the lance.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to the field of devices for cleaning clogged heat exchanger tubes and, more particularly to a system for cleaning tubes within a vessel using a cavitation enhancement unit, thereby creating an intense scrubbing action with pressure variations for cleaning such tubes. The present invention is equally applicable to cleaning corroded surfaces using fluid under pressure with the cavitation enhancement unit of this invention. 
   BACKGROUND OF THE INVENTION 
   A heat exchanger is normally formed of a plurality of tubes oriented generally parallel to one another. In normal operation, a fluid to be heated or cooled is delivered through the inside of the tubes of such a heat exchanger. The outside surface of the tubes are contacted with a fluid which adds heat or removes heat as required. The plurality of generally parallel tubes forms a bundle. A set of end plates, known as heads, commonly support the bundle of tubes at each end. 
   Heat exchangers usually operate in a continuous fashion, often for months at a time. However, such continuous operation may be periodically interrupted to clean the tubes. The cleaning process is necessary to remove residue which collects on the inside surface of the tubes which reduces their heat transfer capability. The tubes are normally formed of metal which has a relatively high thermal conductivity. The material which may coat the interior of the tubes, however, has a much lower thermal efficiency for heat transfer. Therefore, the coating formed on the interior of the tubes is detrimental to the efficiency of the operation of the heat exchanger. 
   As residue builds up on the inside surface of a heat exchanger tube, the tube becomes less and less efficient. One way to counteract this effect is to raise the temperature differential across the tube. However, there are limits to this solution. For instance, the metal used in the tubes of the heat exchanger has a limited capacity for heat as a result of metallurgical considerations. Exceeding the design temperature differential across through the tubes increases fatigue and therefore reduces the useful lifetime of the heat exchanger. 
   In a well known U-tube design, the bundle of tubes takes a 180° bend or elbow at more or less the mid-point of the respective tubes. Fluid enters an inlet box which is separated by a divider plate from an outlet box. The fluid then flows through the head, through the tubes in first one direction then the reverse direction, back through the head and finally into the outlet box on the other side of the divider plate. Cleaning the tubes involves removing the accumulated coating material on the inside of the tubes and the difficulty of cleaning the inside surfaces of the tubes is exacerbated by the bend in the tubes. Also, as exchanger designs have improved, the effective length of the tubes has increased. This makes the task of cleaning the tubes more difficult because the long and relatively narrow tubes do not permit easy access to the tubes. 
   One way that the tubes can be cleaned is by pumping water or perhaps chemically active solvents into the tubes. That is successful but it has limitations. Moreover, since a typical heat exchanger includes a large number of tubes, it is necessary to undertake the cleaning in a repetitive fashion so that a large number of tubes can be cleaned. 
   In my U.S. Pat. No. 5,423,917, I described a system and a method for cleaning heat exchanger tubes. The system described therein has proven very successful. However, the system includes a control panel with a ganged set of valves to set up a shock wave to be injected into a tube. For particularly stubborn and tenacious fowling, especially involving hundreds of tubes, this manual alignment of the control panel valves can become tiresome and tedious. In related application titled Pump Valve Mechanism, I describe a solution to this problem. The cleaning action of that system, however, can be enhance by creating vapor bubbles in the stream of fluid. The cavitation creates a high intensity vibration, particularly at the harmonics of the region to be cleaned, thereby enhancing the cleaning operation of the system. 
   SUMMARY OF THE INVENTION 
   The present invention uses the system described in the related application and further includes a cavitation enhancement unit. A pump takes a suction from a sump and the pump discharges to an output and then through a valve which is switched to deliver water under pressure through a controllable orifice. The orifice delivers the water under pressure to a lance. Up stream of the lance, an enclosure includes a propeller turning at high rpm to develop cavitation at the tips of the blades of the propeller. The collapsing of the bubbles creates a shock wave of a different frequency than that created by the supply system, thereby improving the cleaning capability of the system. 
   While the system is described in detail from the point of view of cleaning heat exchanger tubes, it is to be understood that this invention is equally applicable to cleaning exterior surfaces, particularly corroded surfaces with uneven areas, which are resistant to cleaning by other means. 
   These and other features of the present invention will be readily apparent to those skilled in the art from a review of the following description with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a schematic flow diagram of the system wherein the pump valve mechanism of the present invention finds application; 
       FIG. 2  is a section view of the pump/valve mechanism which may be used in connection with this invention; 
       FIG. 3A  is a detail section view of a cavitation enhancement unit of this invention; 
       FIG. 3B  is an exploded view of another embodiment of a cavitation enhancement unit of the invention; 
       FIG. 3C  is an exploded view of a housing adapted to contain the cavitation enhancement unit of  FIG. 3B ; 
       FIG. 4  is a side view of a lance mounting mechanism showing a lance which extends to seat against a tube to enable tube cleaning; 
       FIG. 5  is a sectional view along the line  3 - 3  of  FIG. 4  and shows details of construction of the mechanism which aligns the lance with a particular tube for cleaning; and 
       FIG. 6  is a sectional view along the line  4 - 4  of  FIG. 4  showing details of construction of the lance insertion mechanism. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Attention is now directed to  FIG. 1  of the drawings which illustrates a schematic of a system  10  for cleaning tubes and the like. The system includes a pump  11  driven by a suitable motor  12  of substantial power. The pump  11  takes a suction through a feed line  13  from a water sump or reservoir  14 . Water level is maintained in the sump by occasional replenishment. Moreover, the water is typically pure but it can be used with additives. For instance, certain types of acids or bases can be added to accomplish chemical attack on the material to be removed. 
   The pump  11  has a pump output  15  which is provided to a control valve  16 . The control valve  16  is a two position valve. In the illustrated position, water under pressure is delivered from the pump through an adjustable orifice  18 . Alternatively, the valve  16  connects with a line  17  which provides a return to the sump. The orifice  18  provides a control signal to manifold  20  of a pump valve mechanism represented in phantom in  FIG. 1  and described in greater detail below. 
   The manifold operates in conjunction with an air pressure manifold  21 . Pressurized air is provided on an air line  22  into a regulator valve  23  in the air pressure manifold. The regulator valve  23  provides a regulated air pressure output through a pair of control valves  24 . The control valves  24  are each of the same construction and connect in parallel at the output of the regulator  23 . The manifold  21  may be replaced with other actuation means, including a hydraulic actuator, an oscillating electric switch, a gas pilot valve, or other means to control a pump/valve mechanism in the manifold  20 . 
   The control valves  24  in the manifold are input to the manifold  10  which includes the pump/valve mechanism. Specifically, the control valves  24  provide air inlet lines  25  and  25 ′, respectively, to either side of an actuator  26 .  FIG. 2  provides greater detail of the pump/valve mechanism. As previously stated, the pump valve mechanism may be operated by any appropriate and convenient actuation means, but the pneumatic actuator is the preferred means and is illustrated. 
   As shown in  FIG. 2 , the air inlet lines  25  and  25 ′ provide air pressure into the actuator  26 . In the preferred embodiment, the actuator comprises a piston  70  within a cylinder  72 . Air pressure ported to the air inlet line  25  moves the piston to the right as seen in  FIG. 2 , and air pressure ported to the air inlet line  25 ′ moves the piston to the left. The piston  72  is coupled to a piston rod  74  which terminates at a linkage  76 . The linkage  76  pivots about a fulcrum  78  and links to a valve rod  80 . The valve rod  80  is mounted for movement within a pump/valve block  82 . Together, the actuator  26 , the block  82  and the parts associated therewith form the pump/valve mechanism of the invention. 
   A packing  84  seals around the valve rod  80  where it then enters a manifold  86 . The valve rod  80  terminates in a valve disc  88  which is configured to seat against a valve seat  90 . When the disc  88  is off the seat  90 , fluid under pressure from the manifold  86  is free to flow out an outlet fitting  92 . 
   Referring again to  FIG. 1 , the pump  11  provides fluid under pressure through the orifice  18  to the block  82  where it pressurizes the manifold  86 . Depending on the position of the valve rod  80 , fluid flows from the block  82  to the outlet fitting  92  or a waste discharge  94 . 
   The pump/valve mechanism has appropriate fittings on it to enable connection of a lance feed line  32 . The line  32  extends some distance, typically from 10 to 50 feet. Preferably the length of the line is kept relatively short so that pressure surges are not damped in the flow line. 
   The line  32  feeds fluid into a cavitation enhancement unit  100 . Fluid flowing into the unit at high pressure, such as for example 10K psi, provides the energy to rotate a propeller  102 . The propeller  102  is held within a water box  104  which provides an outlet to a lance  36 . The cavitation enhancement unit is shown in greater detail in  FIGS. 3A ,  3 B, and  3 C. 
   Referring first to  FIG. 3A , water flows from the line  32  into an axial channel  106 . Water then flows through at least two jets (for stability) such as a jet  108 . The jet creates rotation movement of a propeller shaft  110  on which the propeller  102  is mounted or integrally formed. The propeller turns at a speed adequate to create cavitation with the water box  104 , preferably about 100K rpm for example. The propeller is preferably of a square silhouette, as shown in  FIGS. 3A and 3B , and pitched at a 15° angle for the most productive cavitation, although other shapes and pitch angles may be used within the scope and spirit of the invention. The water, with vapor bubble entrained therein, flow through an outlet  112  to the lance  36 . 
   Referring now to  FIGS. 3B and 3C , a presently preferred embodiment of the cavitation enhancement unit is illustrated. Water flows into an inlet channel  120  formed in a stator element  122 . The stator element  122  includes a preferred screw connection  124  to couple the cavitation enhancement unit  100  to the line  32  ( FIG. 3A ). The water then exits the inlet channel  120  through an outlet orifice  126 . This action pressurizes the annulus between the stator element  122  and a rotor element  128 . The annulus is further defined by a pair of bearing surfaces  130  and  132  which ride along the inside surface of the rotor element  128 . The water under pressure, for example at about 10K psi, exits the annulus at a plurality of outlet jets  134  at a defined angle. The rotor element is joined to a propeller  136  which has a plurality of blades  138 , preferably one blade for each outlet jet  134 . The propeller  136  is mounted to the rotor element  128  at a drive spur  140 . The entire assembly is held together as a unit with a retainer nut  142 , which threads onto a male end  144 . 
     FIG. 3C  illustrates the present preferred embodiment of a housing  150  which surrounds and encloses the cavitation enhancement unit. The housing preferably comprises a left hand enclosure  152  and a right hand enclosure  154 , which screw together with threads  156  for ease of assembling the unit. The blades  138  ( FIG. 3B ) turn within a cyclone chamber  158  in order to develop cavitation bubbles  160 , which are directed out of the unit to the lance. 
   Returning to  FIG. 1 , the lance  36  is coaxial with an elongate cylinder  37  which encloses a piston  38 . The piston  38  is moved under hydraulic pressure in a double acting construction. This enables positive insertion and retraction of the lance. The hydraulic system preferably uses air from a suitable air pressure source delivered through a control valve  39  which connects to an air pressure regulator  40 . The air pressure is regulated and provided to an air motor  41 . The motor in turn is driven by the air to operate a hydraulic pump  42 . 
   An inlet line  43  connects to hydraulic oil sump  44 . Hydraulic oil is delivered to a control valve  45  to control the movement of the lance. 
   Specifically, the lance is extended when the valve is in the illustrated position. The lance is retracted when the valve moves to the opposite position. A return line  46  returns the low pressure oil to the sump. The valve is connected so that power is applied for extension of the lance and for retraction of the lance on operation of the valve. There is also additional equipment for positioning of the cylinder  37  as described below. 
   As shown in  FIG. 1 , the lance has an elongate rod portion which terminates at a tip  48 . The lance tip is sized to nest in the end of a tube  50 . A seal is made when the tube and tip make contact. The seal enables fluid to be introduced under pressure into the tube  50 . 
   An air inlet line  51  introduces pressurized air into the block  82  and into the manifold  86 . This permits the system to blow air through a tube to be cleaned prior the introduction of a shock wave of fluid from the system, thereby providing a water hammer to enhance the clearing effect of particularly stubborn blockages in tubes. 
   The lance  50  is moved with respect to a set of tubes in a fashion shown in  FIGS. 4 ,  5 , and  6 . It should be recalled that the present invention, although described in detail regarding the cleaning of tubes, is applicable as well to the cleaning of flat or curved surfaces.  FIG. 4  shows the lance  36  which is supported and aligned by cylinder  37 . It is mounted so that it travels on a pair of parallel rails  52  and  53  shown in  FIG. 4  of the drawings. These permit movement in the X direction. The rails are parallel steel beams supported on rollers. A bracket is comprised of left and right frame members  54  and  55  which move as a unit. They enable vertical movement of the cylinder  37 . 
   More specifically, the frame members  54  and  55  define a gap where the lance extends through the gap. The cylinder  37  is anchored to the spaced plates  56  and  57  which capture the cylinder. The cylinder extends into a pair of guide surfaces and is supported against these guide services for controlled movement. The guide surfaces are formed along the edges of the frame members  54  and  55  and thus define the channel  58  shown in  FIG. 5  of the drawings. Rollers at  60  are located in this channel. There are typically four rollers, two at each corner as shown in  FIG. 5 , and a corresponding duplicate pair on the opposite side. 
   The several rollers guide the cylinder  37  for movement as illustrated. When it moves up or down, it is guided by the rollers  60  which clamp on the outside of the parallel frame members  54  and  55 . As previously mentioned, the frame members are able to move as a unit to the left or right as viewed in  FIG. 4 . While this provides one dimension of movement, the movement in the vertical direction in  FIG. 4  is the second dimension of movement. When the cylinder  37  is extended, the lance is moved in the Z direction toward the tubes  50 . 
   Attention is now directed to  FIG. 4  of the drawings where it shows the nozzle  48  at a particular tube  50 . The tube  50  is one of many. In fact, hundreds of tubes can be constructed in the heat exchanger. The heat exchanger is defined by a head  64  better shown in  FIG. 4  of the drawings. The extendible lance is forced against one of the tubes. The heat exchanger tube  50  is temporarily plugged by a plug  66  shown in  FIG. 1  to perform the method of this invention. 
   In the practice of this method, the first step is to temporarily plug the tube  50  with the plug. The plug can leak somewhat. It is not important that it maintain a perfect seal; in fact, it is desirable that it provides some leakage so that the plug restricts flow but does not totally block fluid flow. The plug serves as a liquid flow barrier. Preferably it has a length equal to the diameter of the tube plus a friction of an inch greater length. If it were longer, it would work equally well, but it would also cause more frictional drag while the plug moves along the tube  50 . In cleaning the tubes, the plug  66  is first placed in a tube and the lance is moved in an X and Y coordinate system until it is aligned with that particular tube. Then, the lance is extended and seats against the tube that has been plugged and the lance seats against the tube with a water-tight seal. As previously described, the tube is then blown free with pressurized air using air from the line  51 . 
   The next step is to fill the tube with water. This is accomplished by pressurizing the manifold  86  from the pump  11  and holding the disc  88  off the seat  90 . Fluid then flows through the lance to fill up the selected tube  50 . At this point, the system is set up to deliver a series of repeated shock waves from oscillating action of the pump/valve mechanism. Movement of the actuator piston  70  back and forth moves the valve rod back and forth at the same rate. In the action, the disk and rod act as a pump, forcing fluid under pressure with a pressure surge out through the lance. This has the form of a fluid shock which is administered through the solid column of water. When that occurs, there is a tube impact which jars the coating materials on the inside of the tube. 
   When this shock loading is formed in the tube, the plug  66  may leak or may be forced downstream. No particular problem arises from that because water is always being added through the pump output. The incorporation of the orifice  18  coupled with the standing column of water downstream assures that the system transmits into the dirty tube the cleaning shock wave. The shock wave has the form of a change in pressure propagated through the standing column of water. This forms a shock wave which is experienced in the tube but it is not a pressure wave which is built up behind the plug  66 . In fact, it is not normal to use a plug to hold against high pump pressure. The plug is only a sufficient retardant to prevent complete escape of the water. The plug  66  will chatter and skid, moving finally to the far end of the tube  50 . The system utilizes a positive displacement pump  11  which enables the system to provide a relatively constant fluid output. As the pressure buildup is formed and is switched by the pumping action of the pump/valve mechanism, the water in the tube serves to break up the coating of material on the inside of the tube. 
   As a generalization, a representative pressure at the discharge of the pump  11  may exceed 10,000 psi. The pressure at the tip of the lance  48  is preferably also in that range. 
   The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention.