FIELD OF THE INVENTION
Present invention relates to a magnetic coupling method for uniform application of
hydrophobic coating inside a relatively long (10-40 mm) with a small diameter (15 mm)
tube without comprising the heat transfer performance of the tube.
BACKGROUND OF THE INVENTION
In thermal power plants, condensers are traditionally used for cooling the exhaust gas
(steam) of a steam-turbine to condense the steam it and recycle the condensed water.
The condenserconsists of thousands of tubes, which can be made of Copper-Nickel
(95/5, 90/10 & 70/30), Stainless steel (304, 304 L, 316, 316 L), Titanium, Super Ferritic
Stainless Steel, etc.depending on plant & cooling water requirements. These tubes
provide a barrier between the cooling media (in the form of water, most often) and the
heated fluid (exhaust steam) to facilitate heat transfer. The length of the condenser
tubes ranges from 5-40 m in length, having a small inner diameter of the order of 10-40
mm.
The condenser tubes, which constantly pass the cooling liquid such as sea water
containing many contaminants at a relatively high flow speed, for example1-2.5 m/sec.,
are susceptible to various types of corrosion, erosion, biofouling, and bio-corrosion.The
contaminants may be natural sediment (i.e., river silt), bio-growth (i.e. algae, fungi, and
micro-organisms), coal dust from plant operations transported to source water, or
crystalline solids precipitated as particulates in a cooling tower basin (i.e., calcium
carbonate, silicates, phosphates, sulphates, etc.).
Growth of biological organisms such as algae, fungi, bacteria and other micro-
organismsalong the inner surface of the tube leads to the formation of biofilms. Biofilms
contain a considerable amount of entrapped water, which is an insulator of heat, so
even thin biofilms can have a significant impact on heat transfer. In many cases,

biofilms will capture fine particulates, which may further reduce heat transfer and hence
power plant efficiency.
In addition to this, the excretions from the biofilm organisms can accelerate the
corrosion of the substrate, a process known as ‘bio-corrosion. The form of corrosion
can be caused either directly or indirectly. When the attack is direct, the deposit itself
contains corrosive substances which, when concentrated at a localized site, can cause
loss of tube material. A typical example of this type of corrosion would be chloride
pitting. Indirect attacks can be caused by several factors, including accumulation of
deposits that form a barrier between the cooling water and tube material, allowing a
corrosion cell to form underneath, or by microbiologically influenced corrosion (MIC).
Types of bacteria causing MIC includes, sulfate reducing bacteria (SRB), metal-
reducing bacteria (MRB), metal depositing bacteria (MDB), slime-producing bacteria
(SPB), acid producing bacteria (APB) and fungi. In most of the cases, pitting is often a
result, which can cause tube failure well before the material’s life expectancy.
With years of use of contaminated sea water in condenser, biofilms, rust, scales and
other contaminants are generated and stick to the inner peripheral surface of pipes.
Two major problems result from substances that adhere to interior tube surfaces; one
is aloss of heat transfer (reduced thermal conductivity), and other is a reduction of the
diameter of the pipe, which results in a reduction of the flow rate, thereby deteriorating
the heat exchange efficiency. When this heat transfer efficiency becomes low, power
plant efficiency comes down drastically, and hence tubes have to be cleaned
mechanically/or chemically to restore the original effectiveness.
Several chemicals, often in combination, are used to control condenser tube fouling (or
deposits). Chemicals are primarily utilized with recirculating cooling towers,
conventional chemical treatment methods used today include adding scale inhibitors,
dispersants, biocides, corrosion inhibitors, and chemicals for pH control. However,
potential risks to base metal must be carefully considered when using such harsh
chemicals. In the case of online chemical cleaning, one major disadvantage is that you

can’t be sure of the composition of the fouls and other deposits. The process of
cleaning is also very tedious and leads to the shutdown of the power plant. In addition
to this, increased use of biocides and cleaning agents pollute the waste water.
For these reasons, there is an increasing demand for the development of a method
which can prevent the recurrence of rusting, scaling, corrosion, and contamination of
the inner surface of the tubes for many years, once the tubes have been treated before
put touse. In past few decades, condenser tube coatings (inner surface) by polymers
have been used to mitigate the problem of, rusting, scaling, corrosion, and fouling,
etc.Thefunction of a coating is to act as a barrier that prevents either chemical
compounds or corrosion current from contacting the tube substrate. The coating’s
effectiveness of fulfilling these functions depends on its degree of integrity (being an
entirely continuous film or freedom from imperfection or defects), its ability to bond to
the pipe substrate, and its ability to insulate against the passage of corrosion current
(dielectric strength) or chemical ions. However, uniform coating to the inner surface of
a long tube (10-40 m) of smalldiameter (< 40 mm) is a very tedious job.
Condenser tube coating began in Europe in the mid-twentieth century, using a “flood
and drain” method to coat the inner surface of tubes with phenolic materials. However,
there are two major disadvantages of flood and drain method; one is the wastage of
the material, and another is non-uniform and thick coating. In addition to this, the
bottom of condenser tube has much thicker layer compared to the top surface due to
gravitation settling of polymer used for coating.
However, the coatingmust avoid degradation of the heat transfer, which is the essential
function of the condenser tubes. If thecoating is too thick, heat transfer will be restricted,
affecting pressure drop. Coatings organic pigments may also inhibit heat transfer.It is
reportedthat the “protective layer” of the condenser (heat exchanger) tubesmust have a
thickness of the coated film to‘the order of 40-75□m, which does not significantly inhibit
heat transfer[1].In addition to this, the coating should be uniform, varying the thickness
of the coated film is likely to cause variation of the heat conducting or transferring

capability and hence plant efficiency. Thus, strict control of the coating thickness and
uniformity is of great importance to avoid degradation of the heat transfer.
U.S. Pat.No2,880,109 (1959) discloses a method of coating the inside surface of
cylindrical tubes, where coating material having alower melting point than the metal of
the cylindrical tubes is kept inside. Then heating the cylinder until the coating material
melts and becomes completely liquefied, and after that spinning the cylinder on its axis
for the deposition of coating material on the inner surface centrifugally. However, in this
method controlling the uniformity is again a challenge.
U.S. Pat.No4,335,677(1982) describes a spray coating tool used for coating the inner
surface of a tube. The tool sprays a desired paint through a nozzle while the tool is
being shifted inside the tube to be coated along the axis of the same tube. The nozzle
hasa prism portion on the external surface of which at least one spiral groove is
formed.The spiralgroove function to impart atomizing gas in a straightand a spirally
going force, so as to spirally spray the paint on the inner surface of the tube.
U.S. Pat.No4,421,790(1983)teaches a spray coating method for coating the inner
surface of long length and smaller diameter tubes. During the spray coating a long
flexible supply hose, longer than the tube to be coated, is reciprocated in the long tube
with a spray nozzle attached to the tip for spraying the paint by the action of
compressed air. The paint and the air are respectively delivered from a paint reservoir
and a compressed air tank located outside the tube. For the better adhesion, paint
(coating material), and air is respectively heated to a predetermined temperature
before spraying action. However, getting the uniform thickness ofin spray coating is
again a challenge.
Instead of using a spray head to disperse the coating throughout the tube,employing a
pigging technology disclosed in U.S. Pat.No4,425,385(1984) is employed. In this
method, a “slug” of the coating is introduced into one end of the tube, and a specially
designed and sized coating pig is propelled down its length with compressed air or
drawn down the tube with a rod. The pig pushes the “slug” of coating along the tube,

and tube interior, in general,is left with a very thin film of coating. The pig is designed to
remove, or squeegee off, as much coating as possible as it travels through the tube. In
this method coating thickness on the tube wall is commonly less than one mil.
However, in pigging and spraying method use of atomizing gas or compressed air
leads to uneven distribution of coating on the walls in the form of rings of high and low
coverage. Varying the thickness of the coated film is likely to cause variation of the
heat conducting (or transferring) capability and hence plant efficiency.
Even though aforesaid methods are available for coating the inner surface of
condenser tubes, still there is a need for coating method, which can control the coating
thickness, wastage of material and can apply the uniform layer without using atomizing
gas (or compressed air) which leads to uneven distribution of coating on the walls in
the form of rings of high and low coverage. In addition to this, applying a coating
(hydrophobic) which can repel the water to delay further the problem of, rusting,
scaling, corrosion, and fouling, etc.
OBJECTS OF THE INVENTION
It is therefore an object of this invention to propose a magnetic coupling method for
uniform application of hydrophobic coating inside a relatively long (10-40 mm) with a
small diameter (15 mm) tube without comprising the heat transfer performance of the
tube.
Another object of the invention to propose a magnetic coupling method for uniform
application of hydrophobic coating inside a relatively long (10-40 mm) with a small
diameter (15 mm) tube without comprising the heat transfer performance of the tube,
which maintains an uniform coating thickness in the range 10-75 µm, without affecting
the heat transfer properties of the tube.
A still another object of the invention is to propose a magnetic coupling method for
uniform application of hydrophobic coating inside a relatively long (10-40 mm) with a

small diameter (15 mm) tube without comprising the heat transfer performance of the
tube, which eliminates atomizing gas (or compressed air) to coat the inner surface, to
avoid uneven distribution of coating on the walls of the tube in the form of rings of high
and low coverage.
A further object of the invention is to propose a magnetic coupling method for uniform
application of hydrophobic coating inside a relatively long (10-40 mm) with a small
diameter (15 mm) tube without comprising the heat transfer performance of the tube,
which presents the sea water to develop rusting, scaling, corrosion of the tube
including fouling at the time of coating within the small diameter.
A still further object of the invention is to propose a magnetic coupling method for
uniform application of hydrophobic coating inside a relatively long (10-40 mm) with a
small diameter (15 mm) tube without comprising the heat transfer performance of the
tube, which reduces the consumption of coating material.
SUMMARY OF THE INVENTION
Present invention, discloses a method for applying the hydrophobic coating
uniformly inside the inner surface of a long tube of small diameter (15 mm) to
mitigate the problem of, rusting, scaling, corrosion, and fouling, etc., without
affecting the heat transfer performance. In the present invention, a magnetic
coupling method is used, which avoids the use of atomizing gas (or compressed
air) to coat the inner surface. The magnetic coupling method is capable of
applying the uniform coating inside the tube having a thickness anywhere in the
range of 10-75 um.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1. (a) Showing the schematic for coating material 3, and coating applicator 2,
inside the tube 1, (b) is the cross-section of tube to be coated along with the magnetic

coil 4, used to move the coating applicator inside the pipe and, (c) represents the
dome-shaped coating applicator having ferromagnetic or paramagneticmaterials 7 over
the surface.
Figure 2. Showing the uniform coating inside the tube, inspected by fiberscope.
Figure 3. Showing the water contact angle of around 110°-120° over the PDMS coated
304 steel.
Figure 4. The potentiodynamicelectrochemical curve of PDMS coated 304 steel
substrate in sea water solution.
DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS
As noted above,condenser tubes are susceptible to various types of corrosion, erosion,
and biofouling (or bio-corrosion) due to contaminated seawaterand need to be replaced
periodically. Embodiments of the present inventiondisclose a method for applying the
hydrophobic coating uniformly inside the inner surface of a long tube of small diameter
to mitigate the problem of, rusting, scaling, corrosion, and fouling, etc., without affecting
the heat transfer performance.Below embodiment describes the process adopted for
applying the hydrophobic coating uniformly inside the inner surface of condenser
tubes.
The magnetic coupling method is used to coat the inner surface of condenser tube. In
this method, a light weight bullet 2 (called coating applicator) is inserted into the tube 1
of diameter 15 mm (length 1 m), with a selected quantity of coating material 3, as
shown in Fig 1 (a). The coating applicator 2 is pushed through the tube in the direction
of arrow X using a circular magnetic coil 4, around the tube 1, as shown in Fig 1 (b).
While the coating applicator 2 travels along the inner surface of the tube, the applicator
transports the coating material leaving behind a thin layer 5 of the coating material 3.
In the present invention, the coating applicator 2has adome shape. The advantage of
dome shape is, as the applicator 2 moves through the tube 1, the coating material 3

collects around the tip 6 of the dome shape applicator and space between applicator 2
and tube1 is coated with a thin film 5 of coating material 3.The coating applicator 2 can
be made of some differentmaterials, including but not limited to plastics, composites,
polymers, etc., where thesurface of applicator 2 is coated with ferromagnetic or
paramagneticmaterials7, Fig 1 (c).
In the present invention the ferromagnetic or paramagneticmaterials used to coat the
applicator 2, could be magnesium, molybdenum, lithium, and tantalum
(paramagneticmaterials) or Iron, nickel, and cobalt (ferromagnetic materials).
In the present invention, the coating thickness is limited to 75 µm to avoid the effect of
heat transfer. To limit the coating thickness, the diameter of the coating applicator 2
measures approximately 75 µm less than an inner diameter of the tube 1. The coating
thickness can be anywhere 10-75 µm, depending on the diameter of applicator 2.
The present invention avoids the use of atomizing gas (or compressed air) to coat the
inner surface, which leads to uneven distribution of coating on the walls in the form of
rings of high and low coverage. In the present method, the coating applicator is pushed
through the tube by the attraction force between the applicator 2, and circular magnetic
coil 4 [Fig 1 (b)], around the tube 1. Magnetic coil4 outside the tube1 do not interfere
with the coating material 3 and hence uniform layer thin film 5, is achieved throughout
the inner surface of tubes.
Theapplied coating may be cured in the ambient air or, to accelerate the cure time,
warm air may be blown down through the tube. At the same time, infrared heat, such
as a heat lamp or glow bar, may be applied to the outside, or the pipe may be
preheated.
The uniformity of coating is analyzed by fiberscopes. Small diameter flexible probe is
insertedinto the tube, which is connected to a photographic equipment to allow the
images to be permanently recorded. Fig 2 indicates that the coating inside the tube is

uniform, which will avoid the variation in heat transfer across the length of the tube.
The present method is not limited to coat the tubes of diameter 15 mm and length of 1
m. Tubes of diameter less than 15 mm of the desired length can be coatedsimply by
changing the diameter of coating applicator according to the inner diameter of the tube.
According to literature [2], suitable modification of the surfaces which can repel the
presence of aqueous conditions (moisture, contamination, dissolved oxygen), then
corrosion initiation and bio-film formation at the surfaces can be avoided or delayed as
the modified surfaces do not provide conditions conducive to initiate corrosion attack
and biofilm formation. Thus generating a ‘superhydrophobic /hydrophobic’ surface with
water repellency is an attractive option to prevent and delay.
According to the definition, a surface is superhydrophobic if the contact angle of water
is larger than 150° and water droplets readily slide off the surface if the surface is tilted
slightly. For water, the substrate surface is usually considered to be hydrophobic if the
contact angle is greater than 90°. If liquid spreads completely across the surface
(0°contact angle)and forms a film, then the surface is called hydrophilic.
In the present invention, to enhance the water repellency of inner surface, hydrophobic
polydimethylsiloxane (PDMS) i.e. Powersil-567 of WackerSiliconesis used for coating.
The cure mechanism of PDMS involved afunctional oxime cross-linker (methyl- O, O′,
O-butan-2-onetrioximosilane for Powersil-567) in the presence of acatalyst (tin) and
humidity.The hydrophobic nature of the coating according tothe invention prevents the
formation of water film. Thus the attachment of corrosive material and biofilm
formationcan be lessened or avoided.
It iswithinthe scopeofthisinventionthat hydrophobic material could be, Stearic Acid,
Polyvinylchloride (PVC), Polytetrafluoroethylene (PTFE),
Octadecyl trichlorosilane (ODTS), Dodecyltrichlorosilane (DTS),
Octadecyl trichlorosilane (ODTS) and much more.

To determine the nature of thecoating, thecontact angle of water is measured over the
PDMS coated 304 steel. Figure 3shows the water droplet on PDMS coated 304 steel
substrate, captured by CCD camera. The coated substrate exhibits the contact angle in
the range of 110°-120°, which is hydrophobic in nature. The hydrophobic nature of
coating will delay corrosion initiation and bio-film formation at the inner surface of
tubes.
To evaluate the effectiveness of PDMS coating over 304 steel against the corrosion,
the corrosion test is carried out in seawater, which is one of the most corroded and
most abundant naturally occurring electrolytes. The sea water is prepared according to
ASTM standard as given in Table 1.
For corrosion study,Potentiodynamicelectrochemical studies were conducted in the
potential range of -0.9 V to 0.2V. Polarization curve for PDMS coated 304 steel is
shown in Fig 4. For PDMS coated 304 steel, corrosion current decreased from 3 x 10-
13A (uncoated substrate) to 6.17 x 10-22 A. This indicates that a protective film is
formed on the metal surface, which can minimize the corrosion and biofouling.
Although the present invention has been described concerning the preferred
embodiments, thereof, it is intended that the specification and examples be
consideredas exemplary only, the true scope and spirit of the invention being indicated
by the following claims:

Proceeding of international conference on heat exchanger fouling and cleaning
V111-2009, Editors: H. Muller-Steinhagen, M.R. Malayeri and A.P. Watkinson.
P. V. Mahalakshmi, S. C. Vanithakumari, Judy Gopal, U. KamachiMudali and
Baldev Raj “Enhancing corrosion and biofouling resistance through
superhydrophobic surface modification” Current science, Vol. 101, (2011) 1328.

WE CLAIM
1. A magnetic coupling method for uniform application of hydrophobic coating
inside a relatively long (10-40 mm) with a small diameter (15 mm) tube
without comprising the heat transfer performance of the tube, the method
comprising:-
(i) - providing a coating applicator, which carries the coating material
and pushed through the tube by an attraction force between the
applicator and a circular magnetic coil disposed around the tube;
(ii) - limiting the coating thickness inside the tube to 75 µm to avoid the
effect on heat transfer, and
(iii) - selecting the coating thickness anywhere between 10-75 µm, depending on the diameter of the coating applicator.
2. The method as claimed in claim 1, wherein the use of atomizing gas (or
compressed air) is avoided to coat the inner surface, which leads to uneven
distribution of coating on the walls in the form of rings of high and low
coverage.
3. The method as claimed in claim 1, wherein the magnetic coil outside the
tube is disposed to avoid interference with the coating material and hence
uniformly coated thin film is achieved throughout the inner surface of tubes.
4. The method as claimed in claim 1 ,whereinthe applied coating may be cured
in the ambient air or, to accelerate the cure time, hot air may be blown down
through the tube, with simultaneously application of infrared heat, such as a
heat lamp or glow bar, to theoutside, or the pipe may be preheated.
5. The method as claimed in claim 1, wherein the method is applicable for
coating the tubes of diameter 15 mm and length of 1 m tubes of diameter

less than 15 mm with desired length by changing the diameter of coating
applicator according to the inner diameter of the tube.
6. The method as claimed in claim 1, wherein to enhance the water repellency
of inner surface of the tube, hydrophobic polydimethylsiloxane (PDMS) is
used for coating.
7. The method as claimed in any of the preceding claims, wherein the coating
material prevents the formation of water film on the inner surface, including
attachment of corrosive material and biofilm formation.
8. The method as claimed in claim 1, wherein the hydrophobic material is
selected from a group consisting of Stearic Acid, Polyvinylchloride(PVC),
Polytetrafluoroethylene (PTFE), Octadecyl trichlorosilane (ODTS),
Dodecyltrichlorosilane (DTS), and Octadecyl trichlorosilane (ODTS).
9. The method as claimed in claim 1, wherein the coating material exhibits an
contact angle with the tube in the range of 110º - 120°, which are
hydrophobic in nature.
10. The method as claimed in any of the preceding claims wherein the coating
material shows a decrease in corrosion current compared to the uncoated
surface, which interalia establishes that a protective film is formed on the
metal surface, which can minimize the corrosion and biofouling.