Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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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