FIELD OF INVENTION
The invention is generally related to the field of transformer oil cooling and
particularly to the field where oil and water are used as two media of heat
exchange.
BACKGROUND OF THE INVENTION
In the area of transformer oil cooling where oil and water are used as media of
heat exchange it is absolutely necessary to ensure that cooling fluid, which is
water should not mix with oil in case of leakage.
This is achieved by a concentric double tube cooler of prior art. The double tube
heat exchanger are used when it is absolutely unacceptable that both media in
the heat exchange mix in case of leakages (for instance transformer oil) or when
leakage to the environment should be prevented at all costs.
Compared to single tube transformer oil coolers the tube bundle of the double
tube construction has two tube plates at each end. In the case of an occurred
damage of the tube e.g. due to corrosion, the water will not contaminate the

environment or oil, but will keep in the leakage clearance and will cause an alarm
signal. Cooler can be still operated till the next shut down without expensive
standstill. This safety technology enables a higher water operating pressure than
the pressure at the oil side thus eliminating requirement of head break tank,
differential Pr. Gauges and other monitoring devices.
In the present day double tube type cooler there are two tubes having same
centre in one pair. Cooling water flows through the inside tube which is plain
90/10 Cu-Ni tube. The outer tube has special features such as spiral inner flutes
cut longitudinally on its inner diameter to provide path for leakage water in case
of inner tube carrying cooling water gets punctured.
Since to attain maximum metal to metal contact, to not to sacrifice heat transfer
between fluids, the inner tube is expanded in the outer tube in its full length.
Thus the path for the leakage water is through the flutes cut spirally inside the
outer tube to between the spaces of two tube sheets at fixed end side.

It is vitally important that this leaking water is relayed to alarm system tank,
mounted below the tube sheet level to convey leakage water which in present
case would first fill the space between the tube sheet and then would go to fixed
end side through different tube pairs and would take time. Air locking is also
possible due to very small space between inner tube outside diameter and flute
depth on the inside of outer tube.
For this purpose and to overcome the problem of prior art dummy tube (4) is
provided at the lower-most portion of tube layout which immediately starts
conveying the water to fixed end side of tube sheet.
Thermal designing is done carefully keeping extra margin to make one tube
space dummy and during layout of tube on tube plate it is so done that a tube
comes at the lower most portion, which is used for the purpose of relaying
leaking water to space between tube sheets and then to connected alarm
system. This tube does not contribute in heat transfer but serves a very
important purpose of conveying water to the alarm quickly and without any
impediment.

SUMMARY OF THE INVENTION
The main object of the invention is prompt disposal of accumulated leaked water
to the alarm system by offsetting the problem of time lag between the leakage
and cooler sounding system.
In the double tube water cooler of the prior art, in normal course the leaking
water shall accumulate between the two tube sheet and as it rises in this space it
will have to travel to the alarm side of tube sheet (fixed tube sheets) in case the
leakage is near floating end side of the tube sheets.
This results in time lag between the leakage and cooler sounding the alarm. Also
water has to travel through space between two tubes to reach fixed end side and
shall remain entrapped between two tubes.
By providing a special dummy tube at the bottom most portion of tube nest
which runs between inner tube sheet to inner tube sheet, any water leaked and
accumulated between floating side inner and outer tube sheet quickly go to the
fixed end side of space between inner tube sheet and outer tube sheet, where
the alarm is connected. Thus offsetting the disadvantages mentioned above.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the figures of the
accompanying drawings in which
Figure 1 shows a general layout of the transformer
cooling system involving oil and water.
Figure 2 shows schematic diagram showing the tube
nest and dummy tube of the present invention.
Figure 3 shows the schematic of the water inlet, outlet arrangement,
oil inlet, outlet arrangement, alarm system, tube layout etc.
Figure 4 shows the details of the tube fixing.
Figure 5 shows the plan view of the tube layout.

DETAILED DESCRIPTION
Figure 1 shows the general layout diagram of the cooling system where oil and
water are used as two media of heat exchange.
Hot oil enters through the inlet pipe of shell and is guided over the tube nest of
cooling water (3) and gets out through outlet oil pipe provided on shell after
getting cooled by water. Cooling water enters through inlet pipe of inlet / outlet
water box and goes to the inner tubes (1) and returns through reverse end
water box back to inner tubes to outlet branch of inlet / outlet box. Double tube
water coolers are employed to safe guard the transformer. The winding of which
is cooled by oil, which in turn is cooled by water.
Concentric double tube water coolers of special construction are used. In such
double tube construction there are two tubes having same centre forming one
pair. Cooling water flows through the inner tube (1) which is plain 90 / 10 Cu-Ni
tube. The outer tube (2) has special features such as spiral inner flutes cut
longitudinally on its inner diameter to provide path for leaking water in case
inner tube carrying cooling water gets punctured.

In case of leakage in any of the inner tubes the leaking water will not mix with
oil but shall go to the space between two tube sheets on either side (4, 5) and
(6, 7) through the annular space between inner and outer tube in the form of
grooves cut on the inside diameter of the outer tube. Inner tubes are expanded
in the tube holes of outer tube sheets and outer tubes are expanded in tube
holes of inner tube sheets. Provision of dummy tube (9) is made at the bottom
most portion of the tube sheets at both ends so that water accumulating
between two tube sheets on the fixed end or floating end are communicated to
alarm system (8) through the clear path of this tube very quickly. The alarm is
connected at fixed end and the function of dummy tube (9) of the present
invention is to promptly carry water accumulated between tube sheets in the
floating ends (6, 7) to tube sheets at fixed end from where it will be led to the
alarm tank (8). Since the tubes do not take part in heat transfer they are called
dummy tubes. The inner tubes (1) are expanded in the tube holes of outer
sheets (4, 7) and outer tubes are expanded in the tube holes of inner tube
sheets (5, 6).

Compared to single tube transformer oil coolers the tube bundle of the double
tube construction has two tube plates at each end. In the case of an occurred
damage of the tube e.g. due to corrosion, the water will not contaminate the
environment or oil, but will keep in the leakage clearance and will cause an alarm
signal. Cooler can be still operated till the next shut down without expensive
standstill. This safety technology enables a higher water operating pressure than
the pressure at the oil side thus eliminating requirement of head break tank,
differential Pr. Gauges and other monitoring devices.

WE CLAIM
1. A dummy tube (9) adaptable to double-tube water cooled transformers
employing concentric double tube in which water flows through the inner
tubes (1) having concentric outer tubes (2) where, in case of leakage in
any of the inner tubes (1) the leaking water goes to the space between
inner and outer tubes (2) and guided through grooves cut on the inside
diameter of the outer tube, accumulate in the space between inner and
outer tube sheets at both ends, characterized in that the said dummy tube
provided at the bottom most portion of tube nest (3) runs between inner
tube sheet at floating end (6) to inner tube sheet at fixed end (5) and
conveys any leaked water accumulated between floating inner tube sheet
(6) and outer tube sheet (7) promptly to the fixed end side of space
between inner tube sheet (5) and outer tube sheet (4) and then to alarm
system (8) connected at the fixed end side.
2. The dummy tube (9) as claimed in claim 1 does not take part in heat
transfer.

3. The inner tube (1) as claimed in claim 1 are expanded in the tube holes of
outer tube sheets (4, 7) and outer tubes (2) are expanded in the tube
holes of inner tube sheets (5, 6).
4. A dummy tube (9) adaptable to double tube water cooled transformers to
facilitate quick passage of leaked water to the alarm system, substantially
as herein described and illustrated in the figures of the accompanying
drawings.

The invention relates to dummy tubes provided in the double tube water coolers
for cooling transformers. In the double tube water cooler of the prior art, in
normal course the leaking water shall accumulate between the two tube sheet
and as it rises in this space it will have to travel to the alarm side of tube sheet
(fixed tube sheets) in case the leakage is near floating end side of the tube
sheets. This results in time lag between the leakage and cooler sounding the
alarm. Also water has to travel through space between two tubes to reach fixed
end side and shall remain entrapped between two tubes. By providing a special
dummy tube at the bottom most portion of tube nest which runs between inner
tube sheet to inner tube sheet, any water leaked from inner tube (1) carrying
water and accumulated between floating side inner and outer tube sheet (6, 7)
quickly go to the fixed end side of space between inner tube sheet and outer
tube sheet (4, 5), where the alarm is connected.