CA1335726C - Tubing enhancement - Google Patents

Abstract

A tube modification process for surface enhancement of the conduit used in fluid handling, wherein tubing is given an increase in it's outer surface area, an increase in it's inner surface area and a non-blocking, anti-fouling, turbulent interior, in one operationa and the modification may be performed before or after fabrication of the conduit in many circumstances.

Description

1 3357~6 SPECIFICATIONS

In conduit (such as a tube) used for fluid heat transfer, it is desirable to have as much surface area, on both the inside and outside of the tube, between the fluids. This is ~ currently done in a myriad of ways from welding on external fins to swaging the interior into a finned shape. It is also desirable to have aturbulent, non-fouling interior to increase and maintain the overall efficiency of heat transfer. The various means that are currently used for the increase of external surface area of a tube, also add significantly to the turbulence generated as fluids pass over the enhanced surfaces. It can be appreciated that the addition of external surface enhancement to a tube, is considerably easier to manufacture than creating the equivalent enhancements inside of a tube. Yet the amount of surface area and turbulence lS inside the tube is often more critical since the passageway is smaller in cross sectional area. As well fluid passing through any tube develops a 'boundary layer' of stationary fluid that clings next to the tube wall, and a 'laminar layer' of slow moving fluid nexl to the boundary layer. Heat lransfer is inhibited by these unwanted layers.
Further, no suitable method exists to enhance the interior wall surface of a tube for a residential wastewater heat reclaimer such as disclosed in my Canadian Patent Application Number 591,530, that will prevent solids dispersed in a fluid such as residential waste water (e.g. fatty substances, food particles and fecal matter) or even dissolved-mineral precipitates, to pass through a heat exchanger tube without eventually causing a build up of an insulating layer and a resultant reduction in the cross sectional area, bolh of which causea drop off in over all efficiency of the heat transfer process. Scale-fouling from this cause is a problem well known in the field of boiler design, where tube bundles must be periodically cleaned at great expense, to remove built-up scale.
In this invention is a very simple means to produce four desired enhancements-l) exterior surface area increase, 2) interior surface area increase, 3) reduced fouling and 4) fluid turbulence is disclosed.
This is achieved by dimpling the ex~erior of the tube to a sufficient depth, such that the dimple appears as a protuberance or bump insidc thc tube. The effect of this dimpling, is to stretch the material from which the tube is made (i.e. copper) at each and every point along the tube's length at which such a dimple is made. A dimpling procedure in a metal tube requires a sudden indent into the wall material of the tube.
Impact is energy applied over a brief time period. In Physics, kinetic energy calculations use the formula: E=l/2 MV2, where E is total energy, M is the Mass (in this case the mass of the punch or projectile) and V is the Velocity (of the punch or projectile). Velocity has a much greater influence on the energy deposited at a dimple site than does mass. Doubling the punch velocity quadruples the total energy deposited. A high velocity punch deposits higher energy more instantly. With high velocity, the material surrounding the point F ~

of impact is unaffected because of it's inertia. Thus the present invention allows highly localized and closely spaced exterior dimples and interior protuberances .
S The tube's exterior and interior, besides having it's surface stretched, also presents a tortuous route for the fluid layer flowing near to the wall due to the protuberances. This causes highly desirable turbulence, i.e. eddy currents and vortex patterns, in the fluid passing therein.
A dimpling process for plastic conduit requires a deformation of the plastic material at some stage in said tube's manufacture, perhaps with tubing in a softened state and a chilled dimpling punch, or perhaps blow molded into a cavity with appropriate protuberances therein. The manner in which this dimpling can be done are many. It's effectiveness depends on the ability to not deform the tube except in the highly localized area where the dimple is to be created. A dimpling procedure therefore requires sudden stamping, or impact, by an object into the wall of a metal tube.
This impact-dimpling can be done in the most elementary manner by the use of high velocity shot directed at the tube. Such high velocity shot can be supplied by a firearm shotgun, where the shot pellets are available in a wide range of size, weight, loaded-quantity and material type. Furthermore the terminal velocity of the shot can be quite carefully controlled by virtue of theamount and type of explosive charge or gunpowder, loaded to propel the shot.
Moreover the distance that the shot must travel to reach the tube 'target' can be varied. This process is most appropriate for flat-wound spiral heat exchanger elements whose overall shape approximales Ihal of a paltern produced by a shotgun and for experimental investigatiolls of the process.
Overall the degree of dimpling can be controlled to a high degree such tha~ the dimple produced can be made as deep as needed without piercing the tube, and the pattern of dimples produced, very random.
A more sophisticated method of impact-dimpling enhancement achieving the same effect, would be with shot fired individually with a compressed air gun, which could be computer controlled as to aim, and as to tube orientation, to provide a more exact placement and depth of the imprint dimple.
Another dimple stamping method is a toroidally positioned set of solenoid driven punches, computer driven, wherein Ihe lube passes through a toroidal shaped 'print head' similar in principal to a dot matrix printer for computer paper-printing, the dimple stamping punches, all aiming centerwards, impact onto the tube's exterior thereby dimpling the passing tube to a depth, density and pattern determined by research, the whole operation controlled by a computer program.
In either method of impact-dimpling, the lubc lo be so cllhanced, could bc filled with a granular solid, to prevent any unwanted collapse of the tube wall between closely spaced dimples. The tube might also be liquid filled for hydraulic stiffening, said liquid maintaining an outwards pressure from the tube's interior.
Another method of achieving said dimpling is to hydraulically expand the tube, either metal or plastic into the sturdy cavity of a split die, wherein there are dimple producing elements positioned in said die such that when the tube is expanded by fluid hydraulics or explosives, the tube forms around each element leaving an imprint which appears as a protuberance on the tubes interior. The split die is then separated and the tube removed. Such an arrangement could also produce reverse dimples in the tube wall is the split die were itself machined with dimples in its interior wall into which the expanding tube wall would be made to conform to. A combination of the two;
internal and external dimpling, could be done simultaneously in the split die method.
Which ever method is used, the effect would be to increase the overall efficiency of fluid heat transfer, including maintaining said efficiency by the self-cleaning action of the passing fluid interacting with the protuberances of the tube's interior. The self-cleaning action in a heat exchanger tube would be achieved by virtue of the fact that there would not be a continuous boundary layer where fouling can easily occur, but said boundary layer would, at each dimple or protuberance, be thrust centerwards in eddy currents of turbulence and vorticity, into the faster moving main stream of fluid thus keeping solids and precipitates in suspension in the fluid, retarding the development of fouling of all kinds, including inorganic solute or scale, in water boiler tubing.
The effect of the turbulent eddy currents is a kind if miniature turbulent scrubbing action, repeated continually in the fluid's wake, as the fluid flows over and about the numerous protuberances on the tube's interior.
Furthermore in mathematical modeling of heat transfer, the overall heat transfer is directly proportional to the surface area of the separating 'tube wall'. Thus if the dimpling increases the surface area by, say, 20%, then ~he heat transfer increases by a proportionate amount. Since both the internal and external areas of the tube are increased, the over all improvement in heat transfer would be still greater. Adding the effects of internal and external turbulence, which is difficult to model mathematically, the efficiency of heat transfer will improve dramatically with this dimpling process. The reduced fouling of tubing processed according to this invention, adds to it's usefulness.
In drawings which illustrate embodiments of the invention, Figure 1 shows an end view of a plain tube with the boundary layer, Figure 2 shows an end view at any point along a tube's length with the indented dimples on the tube's exterior and the resultant protuberances on the tube's interior, Figure 3 shows a longitudinal section of plain tubing with the boundary I ayer, Figure 4 shows a dimpled pipe with the turbulence crcated in the flowing fluid and the resulting absence of boundary layer, Figure S shows how the dimple is produced by a suitable object impacting upon said tube's surface creating a protuberance inside said tube, Figure 6 shows how a tube may be expanded into a die to produce ~`

protuberances on both the inside and on the outside of the tube, Figure 7 shows a cross-section of a tube after emerging from the die in figure 6, Figure 8 shows an end view of a toroidal imprinter with radial punches aiming towards the center of the tube and where one punch has stamped into the tube wall.
Tube Outer Wall I and Tube Inner Wall 2 conduct a Fluid 7 in figure 3 and cause a Boundary Layer 6 to exist next' to 2. Interior Dimple Surface 3 (and 3a and 3b in figure 2) and Dimple Exterior Surface 4 (and 4a and 4b) in figure 2, are formed when the tube wall is impacted by Shot Ball A with sphcrical Ball End C and/or by Stamp B whose Punch End C is spherical or formed as required to shape the dimple and hence ~hc resullallt proluberance. Impact Stamp Driving Energy is shown as E in all figures. The net effect is to induce a Turbulence 5 shown in figure 4 thus destroyillg the boundary layer 6.
In figure 6 is Split Die G whose interior is machined to accept the tube and whosc interior cavity could havc eithcr protubcranccs and/ or Cavities D. Thc original tube is shown as I and 2 while the explosively or hydraulically expanded tube is shown as la and 2a. Figure 7 shows a typical formed tube in section where Outer Tube Wall 1 and Inner Tube Wall 2 have been formed wilh a combination of Concave Dimples 3,4, and Convex Dimple 3~,4d. The Convex Dimple corresponds to the Cavity D in figure 6.
In figure 8 is shown an end view of a Toroidal Imprint Head F with the tube centered with Punch ends B where each punch is driven to impact by energy E which could be provided by computer controlled electric . solenoids, to dimpletube wall 1, 2 forming dimple or protuberance wall 3, 4. The toroidal imprin head could run at very high speed processing lengths Or ~ubing very rapidly making the dimpling invention suitable for mass production of enhanced tubing for heat exchangers.

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Claims (7)

1. A process for creating continuous turbulent flow in a conduit used for fluid handling, the conduit having a wall with an interior surface and an exterior surface, the process comprising the step of impacting the exterior surface of said conduit wall by a high velocity impacting means to thereby punch a plurality of highly localized dimples into the exterior surface of said conduit wall thereby creating on the interior surface of said conduit wall a plurality of highly localized protuberances to create turbulence where pressure and speed of local fluid motion will fluctuate irregularly.

2. A process as defined in claim 1 wherein said impacting means create substantially rounded and smooth protuberances to thereby prevent hooking and holding of fouling matter which may be present in a fluid passing thereabout.

3. A process as defined in claim 1 wherein the step of impacting the exterior surface of said conduit wall comprises impacting said conduit with a plurality of punches arranged circumferentially about said conduit wall in punch holder means and operating said punches to impact said conduit with sufficient velocity to produce said highly localized dimples.

4. A process as defined in claim 1 wherein the step of impacting the exterior surface of said conduit wall comprises firing projectiles at said conduit with sufficient velocity to produce said highly localized dimples.

5. A process as defined in claims 1, 2, 3, or 4 in which the conduit is filled with a fluid or a removeable solid prior to the step of impacting the exterior surface.

6. An apparatus for forming a plurality of highly localized dimples on a conduit, the apparatus comprising a plurality of punches arranged circumferentially in a punch holder means, means for feeding a conduit between said punches, and means for operating said punches lo impact on an exterior surface of said conduit with sufficient velocity to produce highly localized dimples to thereby create on an interior surface of the conduit a likeplurality of highly localized protuberances to create turbulence where pressure and speed of local fluid motion will fluctuate irregularly.

7. The apparatus as claimed in claim 6 wherein said plurality of punches, said feeding means and said operating means are controlled by a computer.