FIELD OF THE INVENTION
This invention refers to the design of an internally roughened tube using a three
dimensional roughness and its manufacturing for the heat transfer applications
involving single-phase flow, two-phase flow, boiling and condensation.
BACKGROUND OF INVENTION
Three-dimensional roughness is used for enhancing the heat transfer in both
single and two-phase flow applications. Number of element designs are
catalogued by Webb (1994) and for a specific roughness type, a family of
geometrically similar roughnesses is possible, which provides virtually thousands
of possible specific roughness geometries and sizes to choose.
US patent number 3734140 discloses a cross-rifled vapour generating tube with
many projections of a rhombic or parallelogramic shape made on the inside wall
of the tube and provided uniformly. This tube is mostly used as a wall tube for a
high temperature, high pressure boiler using a fossil fuel for remarkable

improvement in the critical heat flux than the smooth tube by promoting the
nucleate boiling of the fluid passing through the tube. Moreover, the heat
transfer is also promoted in single phase flow and thus this tube when used for
general heat exchangers having no boiling or vapour generation under super
critical pressure, a remarkable improvement is obtained. This tube is made by
using a cold-drawing process with a die and plug. The disadvantage of these
roughness element shapes is that they are not very useful in condensation
applications. The performance of cross-rifled tube of this invention can be
improved, for condensation for example, by appropriately modifying the element
shapes. More importantly, this tube's usefulness can be improved by using flat
topped trapezoidal roughness shapes, which show better liquid film drainage
behaviour.
US patent number 4476704 discloses German invention (Auslegeschrift No.
2032891), which uses two step cold drawing approach to manufacture raised
projections on the inner surface of the tube. These raised projections essentially
have the shape of truncated pyramids and this roughened tube shows improved
heat transfer performance in comparison to smooth tube for the applications

such as an evaporator for example. The disadvantage of this tube is the shape
of its elements, which restricts the broad usefulness of this tube. This was
overcome as disclosed in US patent number 4476704 by manufacturing raised
portions provided on the inner side of a heat exchanger tube, where these raised
portions were arranged in rows extending in a longitudinal direction of the tube
and which possessed lateral surfaces that also extend in the longitudinal
direction of the tube. The disadvantage being, these raised portions were
arranged at irregular intervals within a row. The heat transfer performance of
this tube is therefore not guaranteed regarding the uniformity in performance.
This limits application of this tube.
US patent number 5070937 discloses plurality of roughness elements having flat
topped pyramid shapes on the inside surface of an evaporator / condenser tube.
This tube provides excellent material savings thus reducing cost. The roughness
elements are formed by deforming material from the internal surface of the heat
transfer tube in such a manner as to leave only roughness elements projecting
above the internal surface. This was mainly done using embossing the strip and
then forming it into a seam welded tube. This seem welded tube however is not

very much suitable for medium to high pressure applications as experienced in
boilers. In fact, as Babcock and Wilcox points out, there is still some resistance
to the use of ERW tubing boilers because of possibilities of failures at the weld
seam as experienced at the time of its introduction (Babcock and Wilcox, 1992)
and boiler regulations typically recommend thick walled seamless enhanced
(enhanced surface) boiler tubes to avoid any failures at the seam because of
high pressure.
Improvement in three dimensional roughness element shapes is disclosed in US
patent numbers 5975196 and 6182743 Bl. In the invention disclosed in US
patent number 5975196, pattern of parallel notches is provided on the ribs of the
tube internal surface, both of which increase the total internal surface area of the
tube and also promote conditions for the flow of refrigerant within the tube that
Increase heat transfer performance. This pattern is impressed at an angle into
the inner wall of the tube so that the tube inner wall between the ribs presents
recessed area. This gives improved heat transfer performance in both
condensing and evaporating applications. In the invention disclosed in US
6182743 Bl the internal enhancement has a plurality of polyhedrons extending

from the inner wall of the tubing. A recessed area similar to previous invention
is provided adjacent to polyhedrons. This recessed area may also present itself
as a notch if the recessed area is above the base of the polyhedrons. The
resulting surface increases the internal surface area of the tube and thus
increase the heat transfer performance of the tube. The disadvantages of these
inventions are that these tubes are adaptable to manufacturing from a copper or
copper alloy strip by roll embossing the enhancement pattern on one surface on
the strip before roll forming and seam welding the strip into tubing, which is not
very much suitable for high pressure applications such as boilers where steel and
not copper is tube material in addition to possibilities of failures of these tubes at
seam welded joints.
The present invention overcomes the limitations as posed by above inventions
for heat transfer applications of both boiler and low pressure evaporators and
condensers including single and two-phase flows. More importantly, the present
invention provides row-wise regularly spaced three dimensional roughness
patterns, which are useful for medium and high pressure water boiling inside
tubes and low pressure intube refrigerant and water evaporation and

condensation. The three dimensional roughness patterns are flat topped
trapezoidal projections for heat transfer applications of these internally
roughened tubes for single-phase flow, two-phase flow, boilers, evaporators and
condensers. Additionally, the roughness patterns also include element shapes
disclosed in US patent numbers 5070937 and 6182743 Bl but in staggered
arrangement for applications in medium and high pressure boiler. Additionally,
with respect to US patent number 6182743 Bl, where recessed area is provided
adjacent to the polyhedrons, the present invention provides this recessed area in
parallel with the base of polyhedrons and thus avoids the enhancement pattern
showing recesses or notches.
To achieve the objectives of this invention, the flat topped trapezoidal element
shapes shown in Figures l(a) to (d) have following characteristics:



where
wt = top width of the element
wb = base width of the element
Pe = pitch of the element
H = Height of the element
d = internal diameter of the tube
θ1 and θ2 = are lead angles of spiral with respect to the tube axes.
The internally roughened tube (inside diameter d) of present invention is
manufactured using a cold drawing process with a die and plug.
Following points summarize this invention:
Manufacturing of internally roughened tube for heat transfer applications in
single-phase flow, two-phase flow, boiling and condensation.

Internal roughness is provided by three-dimensional roughness having flat
topped trapezoidal element shapes.
The flat topped trapezoidal element shapes are row-wise regularly spaced and
have following characteristics:

where
wt = top width of the element
wb = base width of the element
Pe = pitch of the element
H = Height of the element
d = internal diameter of the tube
θ1 and θ2 = are lead angles of spiral with respect to the tube axes.

The internally roughened tube (inside diameter d) of present invention is
manufactured using a cold drawing process with a die and plug.
In the first step of cold drawing, spiral lands are formed on the inside of a tube
using a plug having plurality of spiral grooves on its surface.
In the second step of cold drawing, another plug having plurality of spiral
grooves with the lead angle reversed on its surface is used inside the drawn tube
of the first cold drawing step and a part of spiral lands of this drawn tube are
plastically pressed down by the second cold drawing operation.
The spirally grooved plug in the second step of cold drawing can also be replaced
by a plug having straight grooves on its surface (θ2 = 0°).
The three dimensional roughness also include element shapes of flat topped
pyramids and polyhedrons in staggered arrangement with characteristics above
for applications in medium and high pressure boilers.

DETAILED DESCRIPTION
Figures l(a) to (d) show schematically three dimensional flat topped trapezoidal
elements used inside the internally roughened tube of this invention along with
the cross-sectional view and different arrangements.
This invention refers to the design of an internally roughened tube using a three
dimensional roughness having flat topped trapezoidal elements and the
manufacturing of these internally roughened tubes for heat transfer applications
involving single-phase flows, two-phase flows, boiling and condensation. The
present invention overcomes the limitations as posed by applications of prior art
to both boiler and low pressure evaporators and condensers. More importantly,
the present invention provides row-wise regularly spaced three dimensional
roughness patterns, which are useful for medium and high pressure water boiling
inside tubes and low pressure intube refrigerant and water evaporation and
condensation, including heat transfer applications in single and two phase flows.

The flat topped trapezoidal element shapes (refers Figures l(a) to (d)) are row-
wise regularly spaced and have following characteristics:

where
wt = top width of the element
wb = base width of the element
Pe = pitch of the element
H = Height of the element
d = internal diameter of the tube
θ1 and θ2 = are lead angles of spiral with respect to the tube axes.

The three dimensional roughness patterns also include flat topped pyramids and
polyhedrons in staggered arrangement with above characteristics for applications
in medium and high pressure boilers.
The internally roughened tube (inside diameter d) of present invention is
manufactured using a cold drawing process with a die and plug. In the first step
of cold drawing, spiral lands are formed on the inside of a tube using a plug
having plurality of spiral grooves on Its surface. In the second step of cold
drawing, another plug having plurality of spiral grooves with the lead angle
reversed on its surface is used inside the drawn tube of the first cold drawing
step and a part of spiral lands of this drawn tube are plastically pressed down by
the second cold drawing operation. The spirally grooved plug in the second step
of cold drawing can also be replaced by a plug having straight grooves on its
surface (θ2 = 0°).

Referred Patents
1. Nakamura, H. and Tanaka, M., 1973, Cross-rifled vapor generating tube,
US Patent No. 3734140.
2. Hage, M. and Schinkoth, G., 1984, Method for producing finned tubes, US
Patent No. 4476704.
3. Mougin, LJ. and Hayes, F.C., 1991, Internally enhanced heat transfer
tube, US Patent No. 5070937.
4. Gaffaney, D.P., Spencer, S.J., Bennett, D.L, Heiskanen, H.T., Riggs, G.L,
Rottmann, E.G. and Satterly, J.M., 1999, Heat transfer tube, US Patent
No. 5975196.
5. Bennett, D.L. and Tang, L, 2001, Polyhedral array heat transfer tube, US
Patent No. 6182743 Bl.

Reference
1. Webb, R.L, 1994, Principles of enhanced heat transfer, John Wiley and
Sons, Inc., New York.
2. Babcock and Wilcox, 1992, STEAM its generation and use, S.C. Stultz and
J.B. Kitto (Eds.), 40th Ed., Babcock and Wilcox a McDermott Company, pp.
6-20.

WE CLAIM
1. Internally roughened tube with an inside diameter d, for heat transfer
application comprising row-wise regularly spaced three dimensional
internal roughness elements represented by flat topped trapezoidal
element shapes in aligned or staggered arrangement (schematically
shown in figures l(a) to (d)) for efficient heat transfer application, in
single phase flow, two phase flow, boiling and condensation, said
internal roughness element satisfying following characteristics:

where
wt = top width of the element
wb = base width of the element
Pe = pitch of the element
H = Height of the element

d = internal diameter of the tube
θ1 and θ2 = are lead angles of spiral with respect to the tube axes.
2. Internally roughened tube as claimed in claim 1, wherein said three
dimensional internal roughness elements are flat topped trapezoidal in
shape.
3. Internally roughened tube as claimed in claim 1, wherein said three
dimensional internal roughness elements are provided in an aligned
arrangement of flat topped trapezoidal shaped elements.
4. Internally roughened tube as claimed in claim 1, wherein said three
dimensional internal roughness elements are provided in staggered
arrangement of flat topped trapezoidal in shape.
5. Internally roughened tube (inside diameter d) of claim (1) for heat
transfer applications in single-phase flows, more specifically for flows of
water, calcium chloride brine solutions and ethylene glycol brine
solutions, among others.

6. Internally roughened tube (inside diameter d) of claim (1) for heat
transfer applications of medium and high pressure water boiling inside
tubes and low pressure intube refrigerant and water evaporation and
condensation
7. Internally roughened tube (inside diameter d) for heat transfer
application having row-wise regularly spaced internal roughness provided
by three dimensional roughnessess represented by flat topped pyramids
and polyhedrons in staggered arrangement for heat transfer applications
in medium and high pressure boilers and having following characteristics:

where
wt = top width of the element
wb = base width of the element

Pe = pitch of the element
H = Height of the element
d = internal diameter of the tube
θ1 and θ2 = are lead angles of spiral with respect to the tube axes.
8. A method of manufacturing internally roughened tubes as claimed in the
preceding claims using a cold drawing process with die and plug,
comprising the steps of:
a. cold drawing with a die and a plug having a plurality of spiral
grooves on its surface for forming spiral lands;
b. repeating the cold drawing step on the inside surface of the drawn
tube of the first cold drawing step with another plug having
plurality of spiral grooves with the lead angle reversed or straight
grooves on its surface; and
c. plastically pressing down a part of spiral lands of the drawn tube of
the first cold drawing operation by the second cold drawing
operation.

Internally roughened tube with an inside diameter d, for heat transfer application comprising row-wise regularly spaced three dimensional internal roughness elements represented by flat topped trapezoidal
element shapes in aligned or staggered arrangement (schematically shown in figures l(a) to (d)) for efficient heat transfer application, in single phase flow, two phase flow, boiling and condensation, said internal roughness element satisfying following characteristics: