The present invention relates to a root soldered wrapped-on high fin tube for
enhanced heat transfer area in gas cooled turbo generator coolers.
BACKGROUND OF THE INVENTION
Generator Gas Coolers are plug-in type surface heat exchanger. These are plugged in
the cooler housing of the Turbo-generator. The cooler dissipate the heat from Generator
windings to increase the efficiency of generator and protect the insulation from
heat/burning damage. Heat is dissipated through the cold cooling water (Auxiliary Cooling
Water). Water flows inside the tubes while hydrogen gas flows over the tubes through the
fins.
In higher rating Turbo-generator, the heat extracted is very high as compared to that
of low rating generator whereas the duct size available for Generator gas cooler is very
limited. Therefore, either overall heat transfer coefficient or heat transfer area is to be
increased to dissipate the higher heat generated in Turbo-generator keeping the pressure
drop within allowable limits. The cooler is to be accommodated in the duct size available in
the generator. By increasing the more number of tubes using existing tube technology i.e.
wire wound finned tube, gas side pressure drop gets increased and exceed the allowable
pressure drop limit of gas flow in Turbo-generator. It is therefore required to design

Generator Gas Cooler tube bundle which can provide either high heat transfer coefficient or
high heat transfer area with less (allowable) gas side pressure drop.
Existing fin tubes available for Generator Gas cooler are:
1. Wire Wound Finned Tube: In wire wound finned tube, wire loop is spirally
wounded over the base tube. It has high pressure drop across gas flow side due to its
geometry. Root of the fin is soldered with base tube. (Refer Fig. 1).
2. Crimped Fin Tube: In crimped fin tube, thin strip is wounded over the base tube.
Fins are crimped (waviness in the fin at root) and the thickness of the fin is same at the top
and root. Root of the fin is soldered with base tube. It has high pressure drop across gas
side. Due to crimping of the fins, more number of the fins along the length cannot be
provided. (Refer Fig. 2).
3. L-Fin Tube: In L-fin tube, L- shaped thin strip is wounded over the base tube. Root
of the fin is not soldered with the base tube. Due to which, there are chances of air gap
between fin and tube over the period of the time and heat transfer coefficient gets
effected.(Refer Fig. 3).

4. G-Fin Tube: In G-fin tube, a helical groove is ploughed into the outside of the base-
tube wall such that metal is only displaced, not removed. The fin is wound mechanically into
the groove under tension, after which the displace metal is rolled back on each side of the
fin to hold it in place. That is why this type is also called embedded finned tube. The
affective thickness of the base-tube wall is the thickness beneath the groove. This type
provides excellent contact, both thermal and mechanical, between fin and groove. Rollers
are used to press groove edge of the base tube for fin to tube bonding. Therefore, gap
between fins is required and more number of fins cannot be accommodated along the
length. Hence surface area is limited. Further, due to the groove, thicker base tube is
required. (Refer Fig. 4).
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a root soldered wrapped-on
high fin tube for enhanced heat transfer area in gas cooled turbo generator coolers which is
capable of providing either high heat transfer co-efficient or high heat transfer area with
less allowable gas side pressure drop.
Another object of the invention is to propose a root soldered wrapped-on high fin
tube for enhanced heat transfer area in gas cooled turbo generator coolers which is capable

of providing a strong bonding between fins and base tubes, when the solder material melts
and joins the fins to base tube.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: Shows Wire Wound Finned Tube according to prior art.
Fig. 2 : Shows Crimped Fin Tube according to prior art.
Fig.3 : Shows L-Fin Tube according to prior art.
Fig.4 : Shows G-Fin Tube according to prior art.
Fig.5 : Shows Root Soldered Wrapped-on High Fin Tube according to invention.
Fig.6 : Shows Model Gas Cooler according to invention.
Fig.7 : Shows Experimental Test Set-up according to invention.
Fig.8 : Shows Plate Fin Tube Bundle according to invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
The root soldered wrapped-on high fin tube is developed in which thin sheet (Item-3,
Fig. 5) of copper is wrapped over the base tube (Item-2, Fig. 5) and tip of the fin (Item-1,
Fig. 5) is stretched due to which thickness at the top of the fin less than that of the root.
Root of the fin is soldered with base tube so that no air gap is developed in between the fin
and base tube.

It is required to develop fin tube which has high heat transfer coefficient with low
pressure drop on fin side (to provide more heat transfer area).
Tube: Copper or copper alloy circular thin tube. Thickness considers as 0.9 mm or 1 mm.
Fin: Continuous thin strip of 99.9% pure copper of thickness 0.25 mm to 0.4 mm. Width of
strip may be considered 3 mm to 6 mm.
Solder: Lead tin solder wire may be considered of dia 0.3 mm to 0.9 mm for
soldering/bonding of copper fin with 90:10 Cu Ni base tube.
Manufacturing Process: Continuous thin strip (Item-1, Fig. 5) of copper to be wrapped on
thin circular base tube (low thickness) (Item-2, Fig. 5) such that pitch of fins may be
maintained and tip of the fin is stretched to the extent that it may not crack. The reduction
in the thickness is proportional to the diameter of root of the tip. Due to the stretching of
the fin, the thickness of the fin at top becomes lesser than the bottom which results high
heat transfer coefficient and there is no waviness at the bottom of the fin which results less
shell side pressure drop. Width of the strip shall maintain the height of the fin (Refer Fig. 5).
Solder wire is also wrapped over the tube, in such away that it is properly placed in
between the two subsequent fins.

The tube with wrapped-on fin and solder wire material to be passed through heating
element of temperature range 200°C to 250°C for very short duration. The solder material
will be melted and join the fins to base tube. The air gap, if any, will also be removed
automatically. This also provides mechanical strength to fin and tube joint.
These types of permanent joint of fins to tube sheet are possible on thin tube sheets
also. By using thin tubes, the overall heat transfer coefficient will be higher.
The finned tubes so developed is named as "Wrapped on Root Soldered High Fin
Tubes".
The advantages of the root soldered wrapped-on high fin tube are:
1. High Heat transfer coefficient as compared to the L-fin tubes.
2. Can easily wrapped-on over the tubes.
3. Since there is no leg like in L fin tubes, more number of fins can be accommodated
by reduced pitch resulting increased number of pitches which results more surface area.
4. Bonding between fins and base tubes is strong.
5. very low pressure drops of gas across fins.
6. Unlike G fin tubes, there are wrapped on lower thickness of tubes which results high
heat transfer coefficient.
7. As there is less pressure drop across gas side, more heat transfer area can be
provided to dissipate the higher heat load.
8. Fins can be wrapped on thin tubes also like 0.9 mm, 1mm.

Various steps involved for the invention of root soldered wrapped-on high fin tube are
as below:
1. Finalizing of input data: Inputs are based on previous experience and
knowledge evolved at workplace.
2. Thermal Design of prototype cooler using root soldered wrapped-on
high fin tube: Based on the above inputs, thermal design of the prototype cooler has
been done using the thermal software HTRI customizing fin details. Cooling water side,
four pass cooler is considered. However, hot air is passed through shell side.
3. Cross validation of thermal design of cooler: Thermal calculation of the
cooler is cross validated.
4. Design Documentation of high fin tube and prototype cooler: Based on
the thermal calculation, design documentation of prototype cooler is prepared for the
manufacturing. Drawings and Combined Bill of Material are developed using AutoCAD
drawing software.
5. Development of manufacturing process of root soldered wrapped-on
high fin tube: As these types of tubes are to be manufactured for the first time,
manufacturing process for root soldered wrapped-on high fin tubes is developed.

6. Manufacturing of prototype cooler using high fin tube in BHEL: Based
on the design documentation, all the assemblies and high fin tubes are developed and
manufactured. Cooler sub-assemblies and tubes are assembled and testing is carried
out as per the requirement. Cooler is dispatched for prototype performance test.
7. Performance testing of prototype cooler: Experimental setup was prepared
at Laboratory. Various parameters are measured across the cooler at different
conditions.
8. Evaluation of Results of the performance of the cooler: Based on the
data collected, heat transfer of the cooler is evaluated and compared with the results
of HTRI calculation performed earlier.
EXPERIMENTAL SET-UP AND PROCEDURE FOLLOWED
Procedure
The experimental setup and procedure followed for air cooled heat exchanger is as
follows:
A radial fan is employed to supply air into the duct system. A damper is used to
control the quantity of air passing through the fan. In the duct system electric heaters are
used to heat up the air. Desired temperature of air is maintained, by switching heaters on

or off. Thermocouples are placed in the path of air flow before the Heat Exchanger to
measure inlet air temperature. These thermocouples are connected to data logging system
which is used to record the temperature. In similar fashion, thermocouples are also put in
place to measure air outlet temperature, water inlet and water outlet temperature as well.
The thermocouples are evenly distributed at a cross-section so as to take values at different
points. Later on these values are used to get average value at a particular cross-section. Air
enters into Heat exchanger and comes in contact with high fins and tubes. Water which is
at ambient temperature flows inside the tubes. Because of temperature difference heat
transfer takes place from air side to water side. Some pressure drop also takes place across
the heat exchanger. This pressure drop is measured by a micro-manometer.
Air velocity is measured at two sections by using Hot wire Anemometer, manually.
First section is just after the Heat exchanger and other one is at the end of the duct from
where air finally exits to atmosphere. Velocity measurement at the end of the duct is to
ensure that there is no turbulence in air velocity and is uniform throughout the cross
section.
Model cooler is a four-pass cross flow heat exchanger. Cold water is taken from a
sump and is passed through the tubes of the Heat Exchanger. Flow rate of water is
measured by rotameter and adjusted by opening or closing a valve. Temperature and
differential pressure readings between inlet and outlet of water are taken. During heat
exchange process water gets heated up and finally exits at water outlet section, and from

there it is sent to cooling tower, where it rejects heat to environment by convection and
evaporation. Finally water goes into sump and gets mixed with the water already present in
the sump. This water is again re-circulated into the heat exchanger and cycle goes on.
Necessary modifications were carried out, to suit the hot air heat exchanger test
facility existing in the laboratory, for the testing of model gas cooler. Model Cooler test rig
with instrumentation is shown in Figure-6 & 7. Figure-6 shows Model gas cooler Cooled
Heat Exchanger which is of four pass construction. Here cold water enters cold air side and
hot water leaves from the hot air side. Figure-7 shows the test rig with all the
instrumentation and data logging system.
Conclusion
1. Comparison with the HTRI output has indicated that the cooler is performing better
than the design prediction.
2. Overall heat transfer coefficient determined experimentally at design condition is
found to be about 82.5 W/m2K as against the design estimated value of 74W/m2K which is
approximately 11% higher.
3. Air side pressure drop was measured 120 Pa as against predicted value of 131 Pa
which is approximately 8% lower.

BENEFITS ACCRUED
Financial Benefits:
Root Soldered Wrapped-on High Fin tubes are used in Generator Gas Cooler for
660MW, 700MW and 800MW Supercritical TG sets. Savings in these sets are as below:
1. Savings in 660 MW & 700 MW Generator Gas Cooler:
(Rs. Means Indian Rupee, INR)
Earlier, tube technology for higher rating was not available in BHEL and the coolers
for supercritical sets were procured. These procured coolers are manufactured using plate
fin tube bundle with the tabulators and this technology is proprietary of the vendor. (Refer
Figure-8).
Earlier
Cost of procuring the cooler = Euro 53250.00
As there are two coolers per set. Therefore, cost of the coolers per set
2X 53250
Euro 106500.00
Rs. 8223930.00
(1 Euro = Rs. 77.22 on 2 Dec, 2014)

Now
Cost of root soldered high fin tubes = Rs. 3233.75
There are 564 no. of root soldered high fin tubes per cooler. Therefore, cost of high fin
tubes per cooler = 564x3233.75
= Rs. 1823835.00
Total Weight of cooler =3345 Kg
Total Weight of fin tubes per cooler = 1859 Kg
Rest Weight of cooler (mainly carbon steel IS 2062) = 1486 Kg
Cost of carbon steel per kg = Rs.405.00
(includes fabrication, machining, assembly etc)
Cost of the cooler (approximately) = 405X1486+1823835
= Rs.2425665.00(approx.).
As there are two coolers per set. Therefore, cost of the coolers per set
Rs. 2425665.00 X 2
Rs. 4851330.00
Savings in 660/700 MW TG set = 8223930-4851330
= Rs. 3372600/-(approx.)
2. Savings in 800 MW Generator Gas Cooler: It was planned to procure Generator
Gas Cooler for 800 MW TG sets as the design and tube technology was not available with
BHEL. Plate fin tube Bundle was envisaged.

Earlier
Size and weight of the cooler for 800 MW TG sets is approximately 30% higher than
that of the cooler for 660 MW TG sets.
Therefore, cost of procuring the cooler =1.3 X 53250 Euro
=69225.00 Euro
As there are two coolers per set. Therefore, cost of the coolers per set
=2 X 69225 Euro
=138450.OOEuro
=Rs 10691109.00
(1 Euro = Rs 77.22 on 2 Dec, 2014)
Now
Cost of root soldered high fin tubes = Rs. 3550.00
There are 684 no. of root soldered high fin tubes per cooler. Therefore, cost of high fin
tubes per cooler =684 X 3550.00
=Rs.2428200.0
Total Weight of cooler
4155Kg
Total weight of fin tubes per cooler = 2494 Kg
Rest weight of cooler (mainly carbon steel IS 2062) = 1661 Kg
Cost of carbon steel per Kg = Rs.405.00
(includes fabrication, machining, assembly etc.)
Cost of the cooler (approximately) = 405X1661+2428200
=Rs. 3100905.00 (approx.)

As there are two coolers per set. Therefore, cost of the coolers per set
Rs. 3100905.00 X 2
Rs. 6201810.00
Savings in 800 MW TG set = 10691109-6201810
= Rs. 4489299/- (approx.)
Therefore, total savings in supercritical 660/700 MW & 800 MW TG sets is Rs. 7861899/-.

WE CLAIM
1. A root soldered wrapped-on high fin tube for enhanced heat transfer area in gas
cooled turbo generator coolers comprising;
a thin tube of predetermined thickness;
continuous thin strip of fin;
lead tin solder wire;
characterized in that,
continuous thin strip of fin wrapped on thin circular base tube of low thickness
maintaining pitch of fins, when tip of the fin is stretched to the extent that it may not crack,
wherein the tube with wrapped-on fin and solder wire material wrapped over the tube is
passed through heating element for the solder material to melt and joint the fins to base
tube resulting a permanent joint of wrapped on root soldered high Fin tubes.
2. A root soldered wrapped-on high fin tube as claimed in claim 1, wherein the fin is a
continuous thin strip of 99.9% pure copper of thickness 0.25 mm to 0.4 mm when the width
of the strip is between 3 mm - 6 mm.
3. A root soldered wrapped-on high fin tube as claimed in claim 1, wherein the circular
thin tube is made of copper or copper alloy having thickness of 0.9 mm or 1 mm.

4. A root soldered wrapped-on high fin tube as claimed in claim 1, wherein the solder
wire wrapped over the tube is placed in between the two subsequent fins.
5. A root soldered wrapped-on high fin tube as claimed in claim 1, wherein the
temperature range of heating element is 200°C to 250°C.