1. Field of Application
The invention is for the manufacturing of enhanced surface heat transfer tube, which has applications in the areas of heat exchange equipments requiring enhanced heat transfer performance on tubeside. Typical applications include, single-phase convection, boiling and condensation, among others.
2. Background of the Invention
Enhanced heat transfer performance is desired for compactness, improved efficiency and overall economics, among others. The approaches typically employ finned and structured surfaces for obtaining better heat transfer performance. In general, the enhancement type is specific to the application and present enhancements are considered as 3rd generation enhancement technology. These enhancements improve heat transfer rate through increase in heat transfer surface area, promoting turbulence in the fluid flow, increasing nucleation sites, and other approaches such as secondary flows, among others. A typical enhancement type has surface pattern or enhancement device, which runs along the complete length of the tube. This leads to characteristic heat transfer behaviour/performance, which is specific to the enhancement employed.
1

Variation in enhancement types along the circumference has been presented by Bennett, et al. (2004) in their patent WO 2004/048873 Al. As disclosed, inner surface of the tube is divided into at least two regions of enhancements along the circumferential direction. This is useful in applications where heat transfer rate needs to be augmented as in the case of two-phase stratified flow, where vapour flows along the top and liquid flows along the bottom. The different regions of enhancements help in improving intermixing between the fluids. This improves the heat transfer performance of the tube. In condensation heat transfer, which requires strong vapour-liquid interfacial mixing, this tube has been found to perform better than the tubes having single type of enhancement on their inside surfaces.
A heat transfer tube as presented by Meng, et al. (2005) itself can provide enhancement through formation of secondary flows. This is alternating elliptical axis tube, which has smooth, plain inside surfaces and forms secondary flows, in the forced convection typically in the turbulent flows, which lead to the heat transfer enhancement. This enhanced tube design is more effective in the transition region of laminar to turbulent flow transition, where secondary flows are more significant.
The above enhancement types do not however provide control over the heat transfer rates and/or heat transfer coefficients along the length of the tube used, for example, in a heat exchanger, where the outside heat transfer coefficient varies and/or there is variation of the inside heat transfer coefficient, along the length of the tube. The present innovation gives good control over the heat transfer coefficient along the tube length through enhancements of different types used along the length of the tube. These enhancements are enhanced surfaces (profiled, patterned surfaces) obtained through rolling / pressing / embossing / forming approaches on the inside surface of the heat transfer tube at different axial sections along its length. The enhanced heat
2

transfer tube as disclosed in this patent helps in designing compact heat transfer (heat exchange) systems. Some of the typical examples (not limited to only these) for which this enhanced heat transfer tube finds its application are heat exchanger using heat transfer tube with isothermal wall and varying fluid temperature along its length, boiler having flow boiling in vertical tube with uniform heat flux leading to different flow patterns during evaporation along the length-of the tube, heat exchangers with counter and parallel flow arrangements and having varying mean temperature differences along the tube lengths, and heat pipes (having heat transfer regions of evaporation, adiabatic heat transfer and condensation), among others.
Compound enhancements on tubeside (inside) and doubly enhanced heat transfer tubes can also be obtained, which include porous surfaces for the tube and fins (integral fins are produced by approaches like embossing) for example, as disclosed in the patent (Patent Application No. 894/KOL/06) by the author, where galvannealed strips (or plates), which are porous, are processed to form enhanced heat transfer tubes.
3. Summary of the Invention
Following points summarise this invention:
1. Manufacturing of enhanced surface heat transfer tubes for tubeside
(inside) enhancements
2. Obtaining enhanced surfaces (profiled, patterned surfaces) using
approaches such as rolling / pressing / embossing / forming
3. Forming tube with enhanced surface of strip (or plate) on the inside of the
tube
3

4. Obtaining compound enhancement on tubeside (inside) and manufacturing doubly enhanced heat transfer tubes using porous surface strip (or plate) such as galvannealed (Wasekar, 2006, Patent Application No. 894/KOL/06)
These tubes have applications in the areas of heat exchange equipments. 4. Brief Description of the Drawings

Schematic of manufacturing process and enhanced surface strip (enhanced surface pattern section lengths need not be of equal length dimension)
5. Detailed Description
Enhanced surfaces (profiled, patterned surfaces) on metal strips (or plates) as described earlier are obtained using approaches such as rolling / pressing / embossing / forming. The embossing, for example, can be carried out using a textured roll having enhanced surface patterns (profiles) on its surface in sections (i.e. patterns of different enhancement types along the roll circumference). In other words, the roll circumference is divided into number of enhanced surface pattern sections. These enhanced surface pattern sections can be of any length dimension and need not necessarily be of same length dimension. The schematic presented above gives information about the manufacturing process and the enhanced surface strip, which has at least two
4

(more than one) enhanced surface pattern sections (more than one type of enhancement), which may also repeat themselves along the length of the tube. For tube having two sections of different enhancements, which may also repeat along the tube length, one enhanced surface pattern section can be of non-enhancement type (i.e., plain, smooth) as shown in the schematic.
Another approach to emboss these enhanced surface patterns on the metal strip is to employ embossing die arrangement moving in a circular motion (die attached on the roller surface, for example, logo marking rolls) or non-circular motion approach of two-roll arrangement with a conveyor channel that has rolls/dies for patterns and an arrangement to form these patterns on the moving metal strip. Here the distance between the rolls can be adjusted to accommodate the length of the tube dimension along with the roll sizes. Compound enhancement approaches such as use of porous surface (galvannealed) can also be employed, which may also be used to produce doubly enhanced tube.
5

Claims:
1. Enhanced surface heat transfer tube of any cross-section (not necessarily
only circular) with at least two (more than one) enhanced surface pattern
sections (more than one enhancement type), which may also repeat
themselves along the length of the tube, for tubeside enhancement
2. The enhanced surface pattern sections for enhanced heat transfer tube,
as claimed in 1
3. Enhanced surface strip (or plate) as the enhanced heat transfer surface
obtained prior to tube forming
4. Compound enhancement on inside and doubly enhanced tube using
porous surface(s) such as galvannealed (Wasekar, Patent Application No.
894/KOL/06)
5

5. Manufacturing approach for the enhanced surface heat transfer tube
claimed in 1 (this includes, rolling/pressing/embossing/forming for
enhanced strip/plate surface and tube forming from this strip/plate)
6. Manufacturing of tube with compound enhancement (tubeside) and
doubly enhanced tube
Referred Patent
1. Bennett, D.L, Tang, L., and Bryan, J.E., 2004, "POLYHEDRAL ARRAY
HEAT TRANSFER TUBE", WO 2004/048873 Al.
2. Wasekar, V.M., 2006, "MANUFACTURING OF GALVANNEALED ENHANCED
HEAT TRANSFER SURFACE(S)", Patent Application No. 894/KOL/06.
Reference
6
3. Meng, J.-A., Chen, Z.-J., Li, Z.-X. and Guo, Z.-Y., 2005, "Field-
Coordination Analysis and Numerical Study on Turbulent Convective Heat
Transfer Enhancement", Journal of Enhanced Heat Transfer, 12(1), pp.
73-83.

Following points summarise this invention;
1, Manufacturing of enhanced surface heat transfer tubes for tubeside
(inside) enhancements
2, Obtaining enhanced surfaces (profiled, patterned surfaces) using
approaches such as rolling / pressing / embossing / forming
3, Forming tube with enhanced surface of strip (or plate) on the inside of the
tube
4. Obtaining compound enhancement on tubeside (inside) and manufacturing doubly enhanced heat transfer tubes using porous surface strip (or plate) such as gaivannealed (Wasekar, 2006, Patent Application No. 894/KOL/06)
These tubes have applications in the areas of heat exchange equipments.

Field of Invention
The present invention relates to the enhanced surface heat transfer tubes or
enhanced heat transfer tubes for tubeside (tube inside surface) enhancements
having variations in the heat transfer enhancement or types of enhanced surface
patterns along their lengths to control the lengthwise variations in the heat
transfer coefficients. The invention further relates to the manufacturing of these
enhanced surface heat transfer tubes, which have applications in the areas of
heat exchange equipments requiring enhanced heat transfer performance on
tubeside for example, single-phase convection, boiling and condensation, among
others.
Background of the Invention
Enhanced heat transfer performance is desired for compactness, improved
efficiency and overall economics, among others. The approaches typically
employ finned and structured surfaces for obtaining better heat transfer
performance. In general, the enhancement type is specific to the application
and present enhancements are considered as 3rd generation enhancement
technology. These enhancements improve heat transfer rate through increase in
the heat transfer surface area, promoting turbulence in the fluid flow, increasing
nucleation sites, and other approaches such as secondary flows, among others.
A conventional enhanced heat transfer tube has enhanced surface pattern, which
runs along the complete length of the tube. This leads to a characteristic heat
transfer performance, which is specific to the type of enhancement employed.

McLain has presented heat exchange tubes in his patents US 3861462, US
3885622 and US 3902552, which have variations in heat exchange enhancement
patterns either in degree of enhancement or type of enhancement. These
variations as presented by McLain are along the length of heat exchange tubes
however McLain mentions in his patents US 3861462 and US 3885622 that the
heat exchange pattern may also vary over the width of the strip, which then
gives circumferential variations and also combinations of both longitudinal and
circumferential variations.
Variation in types of enhanced surface patterns along the circumference as
mentioned by McLain in his patents US 3861462 and 3885622 has been
presented through an example by Bennett, et al. (2004) in their patent WO
2004/048873 Al. As disclosed, inner surface of the tube is divided into at least
two regions of enhancements along the circumferential direction. This is useful
in applications where heat transfer rate needs to be augmented as in the case cf
two-phase stratified flow, where vapour flows along the top and liquid flows
along the bottom. The different regions of enhancements help in improving
intermixing between the fluids. This improves the heat transfer performance of
the tube. In condensation heat transfer, which requires strong vapour-liquid
interfacial mixing, this tube has been found to perform better than the tubes
having single type of enhanced surface pattern on their inside surfaces.
McLain in his patent US3902552, claims a welded, hollow metal heat exchanger
tubing having enhancement pattern interrupted to create intervening smooth
unembossed segments and a discontinuity in the pattern, which essentially gives
an alternating type of enhancement, as presented by the heat transfer tube of

Wadekar (1998), which is an evaporator tube with its top and bottom 1 m
internal surface coated with a porous coating. The coated sections had about a
ten-fold increase in heat transfer coefficient, as compared to the central
uncoated section. Another alternating type of enhancement is presented by heat
transfer tube of Meng, et al. (2005), which is an alternating elliptical axis tube
providing enhancement through formation of secondary flows. It has smooth,
plain inside surfaces and forms secondary flows in the forced convection typically
in the turbulent flows, which lead to the heat transfer enhancement and is more
effective in the transition region of laminar to turbulent flow transition, where
secondary flows are more significant.
In US patent number 3885622, McLain applies longitudinally extending variations
in types of heat exchange enhancement patterns, where first heat exchange
enhancement pattern provides maximum heat exchange efficiency at the
upstream portion of the heat exchanger tube and second enhancement provides
less or reduced drag, thereby improving or increasing overall heat exchange
efficiency. The second enhancement, which provides less or reduced drag, can
be either smooth surface, i.e., elimination of enhancement or heat exchange
enhancement pattern, which may not be as efficient as the first heat exchange
enhancement pattern for the desired heat exchange function. This invention of
McLain however has great disadvantage in improving the heat transfer
performance of a once-through type boiler or evaporator tube, where heat
transfer coefficient in the post-CHF (post critical heat flux) length of the tube,
which is the downstream portion, is poor compared to the pre-CHF portion of the
tube, which is an upstream portion of the boiler/evaporator tube. The poor heat
transfer performance in the downstream portion of the boiler/evaporator tube

results in drastic increase in the tube inside wall temperature as shown in Fig. 1.
As discussed by Collier and Thome (1994), reduced critical heat flux values in the
higher quality regions lead to quite extensive "liquid deficient or mist flow or
droplet flow" region. The reduction of this post-CHF liquid deficient region in
terms of boiler and evaporator tube lengths can therefore provide excellent
savings on costs and materials.
In US patent number 3902552, McLain claims a welded, hollow metal heat
exchanger tubing formed from metal strip material and having longitudinally
extending weld seam, which has alternating type of enhancement (discussed
above) and this tubing after welding is separated into discrete lengths having
alternating smooth segments and patterned segments such that these discrete
lengths have (i) patterned ends and an intermediate smooth segment and (ii)
smooth ends and an intermediate pattered segment. The separation, which is a
cutting operation carried out after the welding of the heat exchanger tubing
involves additional operational aspects with respect to the production of
enhanced heat transfer tubes of the types (i) and (ii) mentioned above in this
paragraph.
Thus, a need exits to provide enhanced heat transfer tubes with tubeside
enhancements, which provide superior heat transfer performances for once-
through type evaporators and boilers and heat exchanger applications and which
include, single-phase convection and/or boiling and/or condensation, among
others and additionally offer practical and economical features to end users.

Summary of the Invention
The present invention meets the above-described need by providing enhanced
heat transfer tubes with varying heat transfer enhancement along their lengths.
The enhanced heat transfer tube of the present invention provides better
alternative to a ribbed or rifled tube, which has ribs on all of its inside surface for
the reasons of lesser pressure drop observed with the smooth portion of the
enhanced heat transfer tube of present invention. Presently, ribbed tubes are
employed in utility boilers and other boiler/evaporator applications, where the
use of ribs can be justified on the background of additional pressure drop penalty
only for pushing the critical quality towards higher values, as extremely high heat
transfer rates during boiling till CHF in principle do not provide any significant
benefits in the heat transfer enhancements with ribbed tubes over smooth
surface tubes. In addition to this, the manufacturing of cold drawn ribbed or
rifled tubes is costlier because of the economic aspects of the ERW over cold
drawn process. Furthermore, as discussed above, reduction in tube length of
around 60% can be achieved with the heat transfer tube of present invention,
which results into material cost reduction, in addition to the compact and energy
efficient designs.
The present invention provides enhanced heat transfer tubes having on their
inside surface patterned ends and intermediate smooth segment, which gives
lengthwise varying heat transfer enhancement and has applicability for example
in boilers where water/steam undergoes three different heat transfer modes,
such as water preheating, water boiling, and steam superheating.

The present invention also provides enhanced heat transfer tubes having on their
inside surface smooth ends and an intermediate patterned segment, which gives
lengthwise varying heat transfer enhancement and has applicability for example
in boilers where water/steam undergoes three different modes of heat transfer,
such as pre-CHF boiling, post-CHF boiling and steam superheating.
In the present invention the disadvantages with respect to cost, tube material,
tube manufacturing process, flow regime operations and energy efficiency or
performance of the enhanced heat transfer tubes for enhancement of water flow
inside tubes under turbulent flow conditions for the applications in shell-and-tube
condensers, which are most common type of water-cooled condensers, have
been largely eliminated by having two enhanced heat transfer patterns or
enhanced surface patterns varying in type along the lengthwise or along the
longitudinal direction of the enhanced heat transfer tube of the present invention
in such a way that the second enhanced surface pattern provides drag or
pressure drop either equal to or greater than the first enhanced surface pattern,
however yielding overall efficiency index having a value greater than one.
In the present invention for the applications such as water-cooled condensers,
the embossed heat exchange enhancement pattern having higher heat transfer
coefficient is located towards the downstream portion of the enhanced heat
transfer tubes because of the reduced driving potential for heat transfer at these
ends of the heat transfer tubes of the present invention, which have two
enhanced heat transfer patterns along their length.

The present invention also includes enhanced heat transfer tubes that have
compound enhancements on tubeside (inside) or doubly enhanced heat transfer
tubes having porous surfaces for the tubes and fins obtained using galvannealed
strips (or plates), which are porous and are processed to manufacture enhanced
heat transfer tubes of the present invention.
The present invention also includes the variations in the heat transfer
enhancement using more than two types of enhanced heat transfer or enhanced
surface patterns along the length of the enhanced heat transfer tube of the
present invention. Application examples of this invention include flow pattern
dependent heat transfer regimes of boilers, evaporators, condensers, and/or
heat exchangers, where more than two heat transfer enhancement patterns can
be embossed on the metal strip in such a way that these enhancement patterns
yield multiple lengthwise enhancement, i.e., enhancement varying in the
longitudinal direction on the inside surface of the enhanced heat transfer tube
formed from this strip.
As discussed with the examples illustrated later in the section of "Detailed
Description", it is the intent of this invention to provide good control over the
heat transfer coefficient along the tube length through different enhancements
such as boiler/evaporator tube having smooth pre-CHF heat transfer surface and
ribbed post-CHF heat transfer surface (Refer "Detailed Description") or
enhancements of different types used along the length of the tube such as
water-cooled condenser tube (Refer "Detailed Description"). The heat transfer
enhancement patterns of these enhanced heat transfer tubes are obtained by
embossing the strip surface, which forms the inside surface of the enhanced heat

transfer tube. These enhanced heat transfer or enhanced surface patterns are
embossed at different axial locations along the length of the enhanced heat
transfer tube of interest according to the present invention or in other words,
these enhancement patterns have longitudinally extending variations in types.
The enhanced heat transfer tubes as disclosed in the present invention help in
designing energy efficient and compact heat transfer (heat exchange) systems.
Some of the typical examples (not limited to only these) for which this enhanced
heat transfer tubes find their application are boilers having flow boiling in vertical
tubes with uniform heat flux leading to different flow patterns during evaporation
along the length of the tube, heat exchangers using heat transfer tubes with
isothermal wall and varying fluid temperature along the tube lengths, heat
exchangers with counter and parallel flow arrangements and having varying
mean temperature differences along the tube lengths, and heat pipes (having
heat transfer regions of evaporation, adiabatic heat transfer and condensation),
among others.
For the manufacturing of enhanced heat transfer tubes of the present invention,
the embossing is carried out using a textured roll having enhanced surface
patterns (profiles) on its surface in sections (i.e. patterns of different
enhancement types along the roll circumference). In other words, the roll
circumference is divided into number of enhanced surface pattern sections.
These enhanced surface pattern sections can be of any length dimension and
need not necessarily be of the same length dimension. These enhanced surface
pattern sections are either binded to the textured roll surface or are formed from
the embossing dies attached to the textured roll surface (for example as in logo
marking rolls) with a mating roll having plain or smooth surface. Additionally,

bonding of these enhanced surface patterns can also be carried out to
manufacture the textured roll required for the present invention. One method of
obtaining composite roll by diffusion bonding is disclosed in the US patent
number 6565498 B2 by Nakada and Kobayashi.
Brief Description of the accompanying Drawings
The invention is illustrated in the drawings given by following figures.
Figure 1 shows schematically manufacturing process for economical, continuous
manufacturing of the welded (ERW), hollow enhanced heat transfer tube
Figure 2 shows schematically tube inside wall temperature variation along length
of the tube for once-through boiler/evaporator;
Figure 3 shows the schematic of the enhanced heat transfer tube with tubeside
enhancement of present invention having pre-CHF heat transfer surface as
smooth surface and post-CHF heat transfer surface till the quality reaches value
of one or unity as ribbed surface;
Figure 4 shows graphically two types of boiler tube inside wall temperature
increases with quality (i.e. along the tube length) in post-CHF heat transfer
region;

Figure 5 shows graphically, for critical qualities above 0.56, boiler tube inside
wall temperature increase with quality (i.e. along the tube length) in post-CHF
heat transfer till quality is one or unity;
Figure 6 shows graphically boiler tube inside wall temperature increase with
quality (i.e. along the tube length) in post-CHF heat transfer till quality is one or
unity for the data under high pressure conditions of 170 bar and having critical
quality value of 0.4 with mass flux of 800 kg/m2s and heat flux values ranging
from 392 to 565 kW/m2;
Figure 7 shows schematically boiling crisis;
Figure 8 shows schematic of dryout mechanisms from medium critical quality
values to high critical quality values;
Figure 9 shows schematically enhanced heat transfer tubes of present invention;
Figure 10 shows graphically temperature distribution along the tube for a water-
cooled shell-and-tube freon condenser with isothermal wall;
Figure 11 shows schematically (a) enhanced heat transfer tube of the present
invention and (b) fin cross-section of the enhanced heat transfer tube of the
present invention

Figure 12 shows graphically performance of the enhanced heat transfer tube of
the present invention having two types of enhanced surface patterns and
depicted schematically with cross-sectional details shown in Fig. 10
Figure 13 shows schematically enhanced heat transfer tube of the present
invention having more than two types of enhanced surface patterns
Figure 14 shows schematically embossing process to obtain enhanced heat
transfer patterns or enhanced surface patterns
Detailed Description of a preferred embodiment of the invention
The embodiments of the present invention will be described in detail in the
following with reference to the drawings.
Fig. 1 shows the schematic of manufacturing process for economical, continuous
manufacturing of the welded (ERW), hollow enhanced heat transfer tube of the
present invention. The metal strip 19 first passes through the embossing station,
where it gets embossed on one of its surface, which forms the inside surface of
the enhanced heat transfer tube of the present invention. After getting
embossed, the metal strip 19 is fed to the tube forming rolls, where the
enhanced heat transfer tube of the present invention is formed and welded.
Finishing and packaging operations follow later. Additional step of separating the
tube segments from a single hollow, welded tube as carried out by the invention
of McLain disclosed in his invention as presented in US patent number 3902552
is thus eliminated in the present invention.

Figure 2 shows schematically tube inside wall temperature variation along length
of the tube for once-through boiler/evaporator tubes. As shown in Fig. 1, the
boiler/evaporator tube 1 has inlet 2 through which subcooled liquid enters and
undergoes phase-change and exits as superheated vapour from exit 3. The bulk
fluid temperature 5 and inside wall temperature variations are as shown in Fig.
2. The location of CHF or dryout is shown by 6, which represents sharp increase
in inside wall temperature.
Figure 3 shows the schematic of the enhanced heat transfer tube of the present
invention for once-through type boiler/evaporator applications having inlet 9 and
exit 10 with tubeside enhancement of present invention having pre-CHF heat
transfer surface as smooth surface 7 and post-CHF heat transfer surface till the
quality reaches value of one or unity as ribbed surface 8. These evaporator and
boiler tubes have lengthwise varying heat transfer enhancement with a smooth
inside surface followed by a ribbed inside surface thus providing superior heat
transfer performances for once-through type evaporator/boiler applications
where the exit quality is one or unity, i.e., the vapour leaving the evaporators
and boilers is dry and saturated. The pressure drop per unit length in the ribbed
portion of these evaporator/boiler tubes is higher than the pressure drop per unit
length in the smooth surface portion (upto the location of dryout or CHF) of
these tubes. As presented by Cheng and Chen (2007), in general, heat transfer is
increased in the enhanced heat transfer tubes by accompanying an increase of
two-phase frictional pressure drop as compared with that in the smooth tubes
and two-phase frictional pressure drops in the spirally internally ribbed tubes are
1.6 to 2.7 times greater than that in smooth tubes during flow boiling of fluids
such as water and kerosene. The present invention provides enhanced heat

transfer tube having improved pressure drop per unit length than a ribbed tube,
which has ribs on all of its inside surface, thereby improving energy efficiency for
evaporator and boiler tubes. For conditions of CHF or dryout occurring at
medium to high qualities, the enhanced heat transfer tubes of the present
invention shown in Fig. 3 have improved energy efficiency because of less drag
in the smooth portion and enhanced heat transfer coefficient in the ribbed
portion of these tubes.
Figure 4 shows schematically two types of boiler tube inside wall temperature
increases with quality (i.e. along the tube length) in post-CHF heat transfer
region. These two distinct types of characteristics are observed in the post-CHF
data of Foster Wheeler Power Group, 2006 - curve 11, Bennett, et al., 1967 -
curve 12; Swenson, et al., 1961 - curve 13; Era, et al., 1966 - curve 13; Cheng
and Xia, 2002 - curve 14, curve 15, curve 16; Schmidt, 1959 - curve 17).
Figure 4(a) shows wall temperature increases from saturation temperature,
whereas the wall temperature has finite temperature difference with saturation
temperature as seen from Fig. 4(b). These differences impact the heat transfer
mechanisms in the post-CHF region till the quality reaches a value of one or
unity. Additionally, as discussed by Cumo, et al. (1972) the natures of the wall
temperature increases through the dryout points have significant influences with
respect to the operations of the once-through type heat exchange equipments
such that the sharper the temperature rise, the more restricted is the length of
the channel of temperature rise thereby indicating a quicker and worse
deterioration in post CHF or post dryout heat transfer. Furthermore, increase in
wall temperature through the dryout point is more dangerous for the reason of

mechanical stresses it induces with the relative thermal expansion of the tube
and concentrated corrosion among others (Cumo, 1972).
It is found in this invention that for either of the types described above in Fig. 4,
for critical qualities above 0.56, the wall temperatures show variations in the
post-CHF heat transfer region till quality is one or unity as presented in Fig. 5. It
is further found that for these wall temperature increases shown in Fig. 5, the
ribbed portion of the enhanced heat transfer tube of the present invention shown
in Fig. 3 has heat transfer coefficient of around 2.5 ±10% times that of
corresponding post-CHF portion of smooth tube heat transfer coefficient for the
applications of water flowing inside once-through type of boiler and evaporator
tubes operating in the pressure range of 100 bar to 150 bar. The corresponding
ranges of mass flux and heat flux values are 400 to 800 kg/m2s and 300 to 970
kW/m2, respectively.
The design correlation, which is the finding of the present invention, for the
design, development and manufacturing of enhanced heat transfer tubes of the
present invention for once-through type applications in boilers, evaporators and
reboilers, among others is given by Eq. (1) as presented below.

It is also found in the present invention that Eq. (1) is also applicable for the wall
temperature increase shown in Fig. 6 for the data under high pressure conditions
of 170 bar and having critical quality value of 0.4 with mass flux of 800 kg/m2s
and heat flux values ranging from 392 to 565 kW/m2. For this, another finding of

the present invention is that the nature of wall temperature increase as shown
schematically can have either of the characteristics, viz., increasing continuously
till quality reaches value of one (x = 1), shown by centerline curve 14 as
depicted for wall temperature increases in Fig. 5 or showing a peak before
decreasing towards x = 1 as shown by dotted curve 16. This applicability of Eq.
(1) thus can be extended further for wall temperature curve 17 schematically
shown in Fig. 4(b), which is a representative in the critical quality range from
0.4 to 0.56 of family of curves by Schmidt (1959) for operations under high
pressure condition of 167 bar, mass flux value of 700 kg/m2s and heat flux
ranging from 290 to 580 kW/m2.
The above discussed applicability of the findings of the present invention for
medium critical qualities in the range of 0.4 to 0.56 under the operating
conditions of medium mass fluxes of around 700 to 800 kg/m2s can be explained
from the basic understanding of the heat transfer mechanisms governing the
CHF and post-CHF heat transfer under these conditions. According to Stephan
(1992) boiling crisis shown in Fig. 7 can be distinguished as below.
(a) Film boiling (Refer Fig. 7 (a)), which occurs when there is a
small amount of vapour by volume. The liquid forms a
continuous phase and after CHF is reached, a vapour film is
formed at the heated wall that separates the liquid from the
wall.
(b) Dryout (Refer Fig. 7 (b) & 8(d)), which occurs when there is
large amount of vapour by volume leading to annular flow. At
the wall, there is mainly liquid and in the core the vapour forms

the continuous phase. After the CHF is reached, the liquid film at
the wall is dispersed and the wall is covered by vapour.
Furthermore, as presented by Stephon (1992), film boiling no longer occurs for
qualities above xσ = 0.5. However, in this invention it is understood from the
applicability of Eq. (1), which represents the annular flow dryout occurrences, to
operations of critical qualities with lower limit on the critical quality of 0.4 and
the wall temperature increases presented by curve 14 in Fig. 6 that the boiling
crisis has similar nature for the critical quality range of 0.4 to 0.55 to that
presented by Tong and Tang (1997) for medium quality dryout mechanism in
annular flows and represented in Fig. 8(a). According to this, medium quality
dryout is initiated by liquid layer getting disrupted due to surface wave instability.
This medium quality dryout, according to present invention changes to
intermittent patches of extremely thin liquid films shown by Fig. 8(b) at medium
mass fluxes, which later change to intermittent nature of annular flow dryout
shown by Fig. 8(c) and characterised by small liquid patches with tube surface
showing dry regions, which may get wet by either droplets primarily of larger
sizes or by liquid films or combinations of both. The comparatively slower
temperature rise and later decrease in wall temperatures in these medium mass
flux and low heat flux flow boiling situations are primarily due to the above
explained boiling crisis mechanisms in addition to higher vapour velocities and
small drops impacting at higher qualities.
Considering the basic nature of heat transfer mechanisms, which consists of
droplet evaporation and vapour superheating, in the post-CHF or liquid deficient
region to be similar or same in both the smooth and ribbed tubes till quality

reaches a value of one or unity, the ratio of heat flux associated with the droplet
evaporation to the total heat flux from the tube wall during post-CHF or post
dryout in boiler and evaporator tubes is same for both the smooth and ribbed
tubes. With this, the present invention thus provides reduction of around 60% in
the length of the post-CHF or post dryout tube portions of the once-through type
applications in boilers, evaporators, reboilers and other similar such applications.
In terms of broader scope of applicability, for a given value of total mass velocity
or mass flux, following relation, which is one more finding of the present
invention given by Eq. (2) gives the length of the ribbed portion of the enhanced
heat transfer tube of the present invention in the post-CHF or post dryout heat
transfer region till the location of quality reaching a value of one or unity.

With the consideration of constant mass flow rate, following relation, which is
another finding of the present invention given by Eq. (3) gives the length of the
ribbed portion of the enhanced heat transfer tube of the present invention in the
post-CHF heat transfer region till the location of quality reaching a value of one
or unity.

The enhanced heat transfer tube of present invention is a better alternative to a
ribbed or rifled tube, which has ribs on all of its inside surface for the reasons of
lesser pressure drop observed with the smooth portion of the enhanced heat
transfer tube of present invention. Presently, ribbed tubes are employed in utility

boilers and other boiler/evaporator applications, where the use of ribs can be
justified on the background of additional pressure drop penalty only for pushing
the critical quality towards higher values, as extremely high heat transfer rates
during boiling till CHF in principle do not provide any significant benefit in the
heat transfer enhancements with ribbed tubes over smooth surface tubes. In
addition to this, the manufacturing of cold drawn ribbed or rifled tubes is costlier
because of the economic aspects of the ERW over cold drawn process.
Furthermore, as discussed above, reduction in tube length of around 60% in the
length of the post CHF or post dryout tube portion can be achieved with the
enhanced heat transfer tube of present invention, which results into material
cost reduction, in addition to the compact and energy efficient designs.
Figure 9(a) shows enhanced heat transfer tubes of the present invention, which
have on their inside surface patterned ends and intermediate smooth segment
that gives lengthwise varying heat transfer enhancement. These enhanced heat
transfer tubes of the present invention have applicability for example in boilers
where water/steam undergoes three different heat transfer modes, such as
water preheating, water boiling, and steam superheating. These enhanced heat
transfer tubes of the present invention shown in Fig. 9(a) are manufactured
using an ERW manufacturing process, where the tubes when manufactured are
not separated by cutting into discrete lengths as carried out by the invention
disclosed by McLain in his US patent number 3902552.
Figure 9(b) shows enhanced heat transfer tubes of the present invention, which
have on their inside surface smooth ends and an intermediate patterned
segment, which gives lengthwise varying heat transfer enhancement. These

enhanced heat transfer tubes of the present invention have applicability for
example in boilers where water/steam undergoes three different modes of heat
transfer, such as pre-CHF boiling, post-CHF boiling and steam superheating.
These enhanced heat transfer tubes of the present invention shown in Fig. 9 (b)
have ribbed portion of tubes for the post-CHF boiling as illustrated in the
enhanced heat transfer tube of the present invention and shown in Fig. 3 and
smooth ends for pre-CHF boiling and steam superheating. In addition, as
discussed above, these enhanced heat transfer tubes of the present invention
are manufactured using an ERW manufacturing process, where the tubes when
manufactured are not separated by cutting into discrete lengths as carried out by
the invention disclosed by McLain in his US patent number 3902552. Micro-fin
tubes are routinely used for tubeside enhancement in heat transfer. Virtually all
air-cooled air conditioners use this tube. Additionally, these tubes have
applications in commercial water chiller evaporators. The micro-fin tubes typically
have 0.02 ≤ e/d< 0.04, 1.5 ≤ p/e ≤ 2.5 and 13 ≤ α ≤ 30° (Brognaux, et al.,
1997), where e, d, p, and a are fin height, tube inside diameter, fin pitch, and
helix angle relative to the tube axis, respectively. Wolverine Tube, Inc.
manufactures an internally finned tube, which has fin geometry similar to the
micro-fin tube having eld = 0.028 and p/e = 1.94. This tube is used for heat
transfer enhancement of water flow inside the tubes of commercial water chiller
evaporators and provides an h•a-enhancement ratio 2.3 times that of a smooth
tube in the turbulent flow regime. The disadvantages of using these tubes are
their cost and tube material because they are manufactured in non-ferrous
materials such as copper and copper alloys using either swaging process, which
is slow and relatively expensive or drawing process, which adds to the cost of
tube manufacturing, in addition to the non-compatibility of copper and copper

alloys for ammonia refrigeration systems in the industrial refrigeration and chiller
applications.
ERW tubes provide a cost-effective solution and Wieland Werke AG manufactures
welded inner-grooved copper tube with embossed rolls. However, non-
compatibility with ammonia limits its use. Use of low-carbon steel for ammonia
heat exchangers has been carried out by Rabas and Mitchell (2000) and the
internally enhanced heat transfer tubes are manufactured by embossing the strip
with spiral ribs and the strip formed into tube and welded using ERW process.
These tubes have ribs on entire inside surface of the tube. For glycol mixtures as
the tubeside fluids, their pressure drop and heat transfer performances are
available for turbulent flow regime with Re ≤ 12000, which limits their use for
high Reynolds number applications. Furthermore, the heat transfer enhancement
levels with these tubes are around 2.5 times or greater with same or slightly less
increase in pressure drop, which limits their efficiency index to values around one
or unity.
In the present invention the above-discussed disadvantages with respect to cost,
tube material, tube manufacturing process, flow regime operations and energy
efficiency or performance of the enhanced heat transfer tubes for enhancement
of water flow inside tubes under turbulent flow conditions for the applications in
shell-and-tube condensers, which are most common type of water-cooled
condensers, have been largely eliminated by having the enhanced heat transfer
patterns varying in types along the lengthwise or along the longitudinal direction
of the enhanced heat transfer tube of the present invention. This enhanced heat
transfer tube of the present invention having tubeside enhancement is superior,

with respect to the location of the embossed heat exchange enhancement
patterns having variations in their types along the longitudinally extending
portions of the heat exchange tube, to a welded hollow metal heat exchanger
tube by McLain according to his invention as disclosed in US patent number
3885622, which has enhancement patterns on its inside surface obtained by
embossing these enhancement patterns on the corresponding side of the metal
strip and these patterns have longitudinally extending variations in the type such
that first heat exchange enhancement pattern provides maximum heat exchange
efficiency for the desired heat exchange function of water flowing through the
tubes and receiving heat from the condensing refrigerant vapour entering and
condensed liquid-refrigerant exiting with flow of refrigerant over the outside
surface of tubes in shell-and-tube condensers, followed by the second
longitudinally extending portion of the tube surface by a second heat exchange
enhancement pattern, which may not be as efficient for this heat exchange
function as compared to the first enhancement type, however the second heat
exchange enhancement pattern provides improved heat exchange efficiency with
reduced drag on the heat exchange fluid, thereby improving the overall heat
exchange efficiency of the heat exchange tube. In the present invention, for the
application of the above-discussed water-cooled condensers, the embossed heat
exchange enhancement pattern having higher heat transfer coefficient is
however located towards the downstream portion of the enhanced heat transfer
tubes of the present invention because of the reduced driving potential for the
heat transfer at these ends of the heat transfer tubes. In this specific example,
the heat exchange enhancement pattern having higher heat transfer coefficient
also has maximum heat exchange efficiency. The disadvantage in the McLain's

invention is disclosed below alongwith the inventive step of the present
invention.
Temperature distribution along the tube for a water-cooled shell-and-tube freon
condenser with isothermal wall is depicted in Fig. 10. For this temperature
distribution, the total rate of heat transfer, Q between the wall and a stream with
the mass flow rate, m is

Here, Cpf, ∆Tinf, h, and a are specific heat at constant pressure, inlet temperature
difference, heat transfer coefficient, and surface area, respectively. For the
above condenser with the tube length divided into two halves viz., upstream
portion A and downstream portion B, following relation is obtained.

Here, De is equivalent diameter.
In Eq. (5), numerator and denominator on the right hand side (RHS) decrease
and increase respectively with increase in heat transfer coefficient values. For
better insight, the RHS of Eq. (5) can be given in the following form below.


For heat exchange tube of McLain, hA ≥ hBf, RHS of Eq. (6) has higher value of
fraction reduced than for the enhanced heat transfer tube of present invention
which has hB > hA. This indicates higher heat transfer rate with the enhanced
heat transfer tube of present invention compared to that of heat exchange tube
according to McLain's invention as disclosed in US patent number 3885622. Fig.
11(a) shows graphically enhanced heat transfer tube of the present invention,
which has enhanced surface pattern I having e/d = 0.024, p/e = 1.71, α =
17.5°, and ratio of fin width at its top to base = 0.3 in the upstream portion of
the enhanced heat transfer tube of the present invention and enhanced surface
pattern II having e/d = 0.024, p/e = 1.71, α = 20°, and ratio of fin width at its
top to base = 0.3 in the downstream portion of the enhanced heat transfer tube
of the present invention along with the trapezoidal fin cross-section for both the
enhanced heat transfer patterns or enhanced surface patterns as shown in Fig.
11(b). Fig. 12 shows the performance of this enhanced heat transfer tube of
the present invention including the upper and lower bounds and represented by
the efficiency index variation with Reynolds number, Re. The range of
application of this enhanced heat transfer tube of the present invention, which
has efficiency index > 1 is 20,000 ≤ Re < 60,000.
The present invention also includes enhanced heat transfer tubes that have
compound enhancements on tubeside (inside) or doubly enhanced heat transfer
tubes having porous surfaces for the tubes and fins obtained using galvannealed
strips (or plates), which are porous and are processed to manufacture enhanced
heat transfer tubes of the present invention.

The present invention additionally includes the variations in heat transfer
enhancement using more than two types of enhancement patterns along the
length of the tube. Application examples of this invention include flow pattern
dependent heat transfer regimes of boilers, evaporators, condensers, and/or
heat exchangers, where more than two heat transfer enhancement patterns can
be embossed on the metal strip in such a way that these enhancement patterns
yield multiple lengthwise enhancement, i.e., enhancement varying in the
longitudinal direction, on the inside surface of the enhanced heat transfer tube
formed from this strip. Fig. 13 illustrates this enhanced heat transfer tube.
As discussed with the examples illustrated above, it is the intent of this invention
that the present invention provides good control over the heat transfer
coefficient along the tube length through different enhancements such as
boiler/evaporator tube having smooth pre-CHF heat transfer surface and ribbed
post-CHF heat transfer surface or enhancements of different types used along
the length of the tube such as water-cooled condenser tube. The heat transfer
enhancement patterns of these enhanced heat transfer tubes are obtained by
embossing the strip surface, which forms the inside surface of the enhanced heat
transfer tube. These enhancement patterns are embossed at different axial
locations along the length of the enhanced heat transfer tube of interest
according to the present invention with the examples as discussed above, or in
other words, these enhancement patterns have longitudinally extending
variations in types. The enhanced heat transfer tubes as disclosed in the preserit
invention help in designing energy efficient and compact heat transfer (heat
exchange) systems. Some of the typical examples (not limited to only these) for
which this enhanced heat transfer tube finds its application are boilers having

flow boiling in vertical tubes with uniform heat flux leading to different flow
patterns during evaporation along the length of the tube, heat exchangers using
heat transfer tubes with isothermal wall and varying fluid temperature along the
tube lengths, heat exchangers with counter and parallel flow arrangements and
having varying mean temperature differences along the tube lengths, and heat
pipes (having heat transfer regions of evaporation, adiabatic heat transfer and
condensation), among others.
Fig. 14 shows the schematic of the embossing process to obtain enhanced heat
transfer patterns or enhanced surface patterns on the metal strip surface 18,
which forms the inside surface of the enhanced heat transfer tube of the present
invention. The metal strip 19 is fed through the textured roll 20 and plain roll 21
before it goes to the tube forming rolls. The embossing is carried out using the
textured roll 20 having enhanced surface patterns (profiles) on its surface in
sections (i.e. patterns of different enhancement types along the roll
circumference) with a mating roll having plain or smooth surface. In other words,
the roll circumference is divided into number of enhanced surface pattern
sections. These enhanced surface pattern sections can be of any length
dimension and need not necessarily be of same length dimension. These
enhanced surface pattern sections on the textured roll are either binded, welded,
or brazed to the plain roll surface to form textured roll or are formed by
attaching the embossing dies to the plain roll surface (for example as in logo
marking rolls). Additionally, bonding of these enhanced surface patterns can also
be carried out to obtain the textured roll required for the present invention. One
method of obtaining composite roll by diffusion bonding is disclosed in the US
patent number 6565498 B2 by Nakada and Kobayashi.

Referred Patent
1. McLain, CD., 1975, "HEAT EXCHANGE TUBE", US 3,861,462.
2. McLain, CD., 1975, "HEAT EXCHANGE TUBE", US 3,885,622.
3. McLain, CD., 1975, "PATTERNED TUBING", US 3,902,552.
4. Bennett, D.L., Tang, L, and Bryan, J.E., 2004, "POLYHEDRAL ARRAY
HEAT TRANSFER TUBE", WO 2004/048873 Al.
5. Nakada, I. and Kobayashi, T., 2003, "Composite Roll for Manufacturing
Heat Transfer Tubes", US Patent Number 6565498 B2.
References
6. Wadekar, V.V., 1998, "Improving Industiral Heat Transfer-Compact and
Not-Compact Heat Exchangers", Journal of Enhanced Heat Tranfer, 5(1),
pp. 53-69.
7. Meng, J.-A., Chen, Z.-J., Li, Z.-X. and Guo, Z.-Y., 2005, "Field-
Coordination Analysis and Numerical Study on Turbulent Convective Heat
Transfer Enhancement", Journal of Enhanced Heat Transfer, 12(1), pp.
73-83.
8. Collier, J.G. and Thome, J.R., Convective Boiling and Condensation, 3rd
Ed., Clarendon Press, Oxford, pg. 288.
9. Cheng, L. and Chen, T., 2007, "Study of Vapour Liquid Two-Phase
Frictional Pressure Drop in a Vertical Heated Spirally Internally Ribbed
Tube", Chemical Engineering Science, Vol. 62, pp. 783-792.

10. Bennett, A.W., Hewitt, G.F., Kearsey, H.A. and Keeys, R.K.F., 1967, "Heat
Transfer to Steam Water Mixture in Uniformly Heated Tubes in which the
CHF has been exceeded, UK Rep. AERE-R-5373, Harwell, England.
11. Era, A., Gaspari, G.P., Hassid, A., Milani, A. and Lavattavelli, R., 1966,
Heat Transfer Data in the Liquid Deficient Region for Steam-Water
Mixtures at 70 kg/cm2 Flowing in Tubular and Annular Conduits, Rep.
CISE-R-184, Milan, Italy.
12.Swenson, H.S., Carver, J.R., and Szoeke, G., 1961, "The Effects of
Nucleate Boiling versus Film Boiling on Heat Transfer in Power Boiler
Tubes", Paper 61-W-201 presented at ASME Winter Annual Meeting, New
York, 26 November - 1 December.
13. Cheng, L. and Xia, G., 2002, "Experimental Study of CHF in a Vertical
Spirally Internally Ribbed Tube under the condition of High Pressures",
Int. J. Therm. Sci., 41, pp. 396-400.
H.Schmidt, K.R., 1959, Warmtechnische Untersuchungen an hoch belasteten
Kesselheizflachen, Mitteilungen der Vereinigung der Grosskessel-bezitzer,
December, 391-401.
15. Foster Wheeler Power Group, 2006,
http://www.fwc.com/publications/pdf/BrochureArchSCRG031606.pdf.
16.Cumo, M., Farello, G.E. and Ferrari, G., 1972, "The Influence Curvature in
Post Dry-Out Heat Transfer", Int. J. Heat Mass Trans., 15, pp. 2045-2062.
17.Stephon, K., 1992, Heat Transfer in Condensation and Boiling, Springer-
Verlag, Berlin, Section 13.9.
18.Tong, L.S. and Tang, Y.S., 1997, Boiling Heat Transfer and Two-Phase
Flow, 2nd Ed., Taylor & Francis, Washington D.C., pg. 318.

19.Brognaux, L.J., Webb, R.L, Chamra, L.M., and Chung, B.Y., 1997, "Single-
Phase Heat Transfer in Micro-Fin Tubes", Int. J. Heat Mass Transfer, Vol.
40, No. 18, pp. 4345-4357.
20.Rabas, TJ. and Mitchell, H., 2000, "Internally Enhanced Carbon Stell
Tubes for Ammonia Chillers", Heat Transfer Engg., Vol. 21, pp. 3-16.

We Claim
1. A welded, hollow enhanced heat transfer tube or enhanced surface heat
transfer tube for tubeside (tube inside surface) enhancement, which
provides longitudinally or lengthwise varying heat transfer enhancement
along its length and having at least one embossed enhanced surface
pattern for enhanced heat transfer; characterized by comprising a a
plurality of ribs for the post-CHF or post dryout heat transfer portion or
segment for once-through type applications in boilers, evaporators and
reboilers, among others, where the exit quality is one or unity or in other
words, the vapour exiting this enhanced heat transfer tube is dry and
saturated.
2. The enhanced heat transfer tube as claimed in 1, wherein the ribbed
surface is configured designed using the correlation: (hrib I hsmooth) = 2.5
±10% for Xcr > 0.56, where hribf, hsmooth and xσ are the ribbed tube heat
transfer coefficient, smooth tube heat transfer coefficient and smooth
tube critical quality, respectively.
3. The enchanced heat transfer tube as claimed in claims 1 or 2, wherein for
a given value of total mass velocity or mass flux the relation: Lrib = 0.4
[Lsmooth (drib I dsmooth)] ±10% for xσ > 0.56, gives the length of the ribbed
portion of the enhanced heat transfer tube.

4. The enchanced heat transfer tube as claimed in any of the preceding
claim.
5. The enhanced heat transfer tube as claimed in 1, wherein the enhanced
surface comprising patterned ends and intermediate smooth segment or
portions.
6. The enhanced heat transfer tube as claimed in 1, wherein the enchanced
surface comprises smooth ends and an intermediate patterned segment.
7. The enhanced heat transfer tube as claimed in 1, comprising at least two
enhancement patterns or enhanced surface patterns along its length such
that the second enhanced surface pattern provides drag or pressure drop
either equal to or greater than the first enhanced surface pattern and
having overall efficiency index value greater than one.
8. The enhanced heat transfer tube as claimed in 7, wherein the first
enhanced surface pattern I having e/d = 0.024, p/e = 1.71, α = 17.5°,
and ratio of fin width at its top to base = 0.3 in the upstream portion,
wherein the second enhanced surface pattern II having e/d = 0.024, p/e
= 1.71, α = 20°, and ratio of fin width at its top to base= 0.3 in the
downstream portion, and wherein the enchanced heat transfer patterns
both having trapezoidal fin cross-section.

9. The enhanced heat transfer tube as claimed in 7, wherein the embossed
enhanced surface pattern or enhancement pattern having higher heat
transfer coefficient located towards its downstream portion or end, which
has reduced driving potential for heat transfer in a smooth surface or non-
enhanced heat transfer tube.
10. An enhanced heat transfer tube comprising compound enhancement on
tubeside (tube inside surface) or enhanced heat transfer outer surface
providing doubly augmented enhanced heat transfer tube using
galvannealed strip (or plate), which have porous surface characteristics
for enhanced heat transfer applications and are processed to manufacture
these enhanced heat transfer tubes.
11. A process of manufacturing of enhanced heat transfer tube as claimed in
claim 1 to 10, comprising the steps of:
passing a metal strip through an embossing section in a
manufacturing plant for ERW tubes;
forming an inside surface of the emhanced heat transfer tube by
getting one side of the metal strip embossed; and
feeding the embossed metal strip to a tube forming roll for
forming and welding.

The present invention meets the above-described need by providing enhanced heat transfer tubes with varying heat transfer enhancement along their lengths. The enhanced heat transfer tube of the present invention provides better
alternative to a ribbed or rifled tube, which has ribs on all of its inside surface for
the reasons of lesser pressure drop observed with the smooth portion of the
enhanced heat transfer tube of present invention. Presently, ribbed tubes are
employed in utility boilers and other boiler/evaporator applications, where the
use of ribs can be justified on the background of additional pressure drop penalty
only for pushing the critical quality towards higher values, as extremely high heat
transfer rates during boiling till CHF in principle do not provide any significant
benefits in the heat transfer enhancements with ribbed tubes over smooth surface tubes. In addition to this, the manufacturing of cold drawn ribbed or rifled tubes is costlier because of the economic aspects of the ERW over cold
drawn process. Furthermore, as discussed above, reduction in tube length of around 60% can be achieved with the heat transfer tube of present invention, which results into material cost reduction, in addition to the compact and energy
efficient designs. The present invention provides enhanced heat transfer tubes having on their inside surface patterned ends and intermediate smooth segment, which gives
lengthwise varying heat transfer enhancement and has applicability for example in boilers where water/steam undergoes three different heat transfer modes, such as water preheating, water boiling, and steam superheating.
The present invention also provides enhanced heat transfer tubes having on their
inside surface smooth ends and an intermediate patterned segment, which gives
lengthwise varying heat transfer enhancement and has applicability for example
in boilers where water/steam undergoes three different modes of heat transfer,
such as pre-CHF boiling, post-CHF boiling and steam superheating.
In the present invention the disadvantages with respect to cost, tube material,
tube manufacturing process, flow regime operations and energy efficiency or
performance of the enhanced heat transfer tubes for enhancement of water flow
inside tubes under turbulent flow conditions for the applications in shell-and-tube
condensers, which are most common type of water-cooled condensers, have
been largely eliminated by having two enhanced heat transfer patterns or
enhanced surface patterns varying in type along the lengthwise or along the
longitudinal direction of the enhanced heat transfer tube of the present invention
in such a way that the second enhanced surface pattern provides drag or
pressure drop either equal to or greater than the first enhanced surface pattern,
however yielding overall efficiency index having a value greater than one.
In the present invention for the applications such as water-cooled condensers,
the embossed heat exchange enhancement pattern having higher heat transfer
coefficient is located towards the downstream portion of the enhanced heat
transfer tubes because of the reduced driving potential for heat transfer at these
ends of the heat transfer tubes of the present invention, which have two
enhanced heat transfer patterns along their length.