FIELD OF THE INVENTION
[001] The present invention discloses a process to enhance the wear life of exhaust fan blades using high wear resistant Co-WC tiles which can flex and twist at high speed rotation of fan blades without any damage.
BACKGROUND OF THE INVENTION
[002] Wear of the components is one of the most commonly encountered industrial problems in coal fired power stations. In power plants, coal is ground to powder in impact mills by grinding rolls and then feed to different chambers for further processing. The coal has higher ash and Silica (Quartz) content present in it, which has the Vickers hardness of > 1100 Hv. Due to the harsh abrasive action of the component with coal, surface of component gets worn out faster and it reaches the condition where it needs to be replaced. This involves the plant shutdown and ultimately affects the plant efficiency.
[003] In the thermal power plants, the exhaust fan (also called blower) which is used to conveying the pulverized coal to the boiler, is one of the severely affected component. The pulverized coal strikes the fan blades (which rotates at the
speed of > 2500 rpm) and cause wear and tear over the fan blade surface. The degree of wear depends on the content of Silica present into the coal. Once the surface worn-out, the resulting geometrical alteration leads to dramatic efficiency drops. Furthermore, balance problems appear, affecting the service life of the component.
[004] The life of exhaust fan can be enhanced by protecting the fan blade surface by hard material. Once the hardness of fan blade surface is higher than the quartz, then it will not be adversely affected by the quartz content of the ash.

The most widely used methods to protect the fan blades surface are coatings with hard chromium, weld overlays, and thermal spray coatings etc.
[005] Document (US. Pat. No. 3,743,556) discloses a method to enhance the wear resistant of the component. It is based upon the infiltration of braze into a layer of tungsten carbide particles, which has very high hardness. In that process, coatings are made by first applying cloth containing particles of tungsten carbide to a surface needing protection against wear. Another piece of cloth containing particles of braze alloy is placed over the layer of carbide particles. The substrate with the two layers of cloth is placed in an inert-atmosphere furnace and heated to the brazing temperature of the braze alloy. Braze infiltrates down into the carbide particles and brazes them to each other and to the substrate, enhancing the life of component.
[006] Document (US. Pat. No 6,649,682 Bl) also describe the almost similar process for wear protection. The method takes advantage of the thermal spraying and cloth processes by using a process based upon paint technology. First, hard facing particlesfi.e. tungsten carbide) and braze-alloy particles are made into separate paints. The hard facing particle layer is first "painted' over the area of metal needing protection, over that a layer of braze is painted. The coated component is then heated in a furnace to a temperature that is above the melting temperature of the braze alloy. The braze alloy then infiltrates down into the layer of hard facing particles and brazes them into a composite of hard particles in a matrix of braze alloy onto the Substrate metal. It poses resistance to abrasion and erosion far above that of steels, hard chromium, and most thermally sprayed carbide coatings.
[007] US. Pat. No 6,649,682 also discloses another approach in which, a layer of adhesive is applied to the metal substrate, and hard facing particles are applied to that adhesive layer. After drying, another layer of adhesive is applied over the adhered hard particles. Braze powder is then applied to the layer of wet

adhesive thus forming a layer of braze particles over the layer of hard particles. Heating in an inert atmosphere then causes metallurgical fusion, which produces a composite of hard particles in a matrix of braze, metallurgically bonded to the substrate metal.
[008] In all above cases, hard facing particles meets three criteria: first, the hard facing particles have a Vickers hardness of above 1100, second, they are wettable by the selected braze alloy to achieve infiltration, and third, hard facing particles do not dissolve by the molten braze alloy.
[009] Kermetico, USA also developed the abrasion resistance coating using wolfram (tungsten) carbide with hardness of up to 2300HV. A Wolfram carbide hard metal coating is applied over the substrate using high-velocity oxygen fuel (HVOF) and found that the abrasion resistance of the coating was hundred times better than that of regular steel, and many times better than weld overlays.
[010] Document (CN105967660A) also discloses a coating material for prolonging life of exhaust air blades. The coating material is prepared from the glass glaze, alumina ceramic powder, nickel-chromium alloy, epoxy resin, tantalum carbide, and bamboo charcoal powder, with a proper amount of anhydrous ethanol. After preparing the material, plasma spraying method is used to coat over the substrate to enhance the life of exhaust air blades.
[011] However, in all above methods coating thickness is not more than 0.5 mm, which enhance the life of blades only by 3-4 months. Further, life of blades can be enhanced by fixing the ceramic tiles over the blades, HMA Australia also developed a method, where ceramic sheet (preferably of Alumina) having a thickness of 4.5 ~ 6.5mm fixed over the wear surface with the help of an adhesive.

[012] Alumina based ceramics haver hardness over 1700 HV, which enhanced the wear rate of component dramatically and life up to more than a year. Also, unlike most hard-facings, ceramics have even wear rate over the surface, as there are no hard or soft spots that wear at uneven rates, which avoids the rebalancing of the fan between overhauls. In addition to this, ceramic has only 40% the density of steel. A 1/8" overlay of ceramic on a 5/ 16" steel plate weighs 55% as much as a 3/8" deck plate blade, it means ceramic tiles will not create any undue stress on the spider.
[013] However, in coal based power plants the exhaust fans rotate at > 2500 rpm. Such high-speed dynamic of fan blades can damage the tiles, as ceramics are not flexible. The damaged tiles will fall off from the blade surface, which result in unscheduled downtime. The fixing of damage tiles over the surface may lead to uneven surface.
[014] Generally, shortcomings of the prior art arise from (1) coating thickness over the blade surface cannot be more than 0.5 mm, which limits the life of blades up to 3-4 months and (2) high-speed dynamic of fan blades damage the ceramic tiles, which limits the life of fan blades. Thus, there is a need for an improved method for enhancing the exhaust fan blades life which can avoid the coating over the surface and also can avoid the damage of tiles at rotations > 2500 rpm.
OBJECTS OF THE INVENTION
[015] The main object of the present invention is to provide a process to enhance the wear life exhaust fan blades.
[016] Another object of the invention is to provide a process to enhance the wear life of exhaust fan blades, which can avoid the coating over the fan blades surface to enhance its wear resistance.

[017] Yet another object of the invention is to provide a process to enhance the wear life of exhaust fan blades, which can replace the ceramic tiles with a material which can withstand against high speed rotation.
[018] A further object of the invention is to provide a process to enhance the
wear life of exhaust fan blades, which can avoid damage of the tiles over the
surface.
[019] These and other objects and advantages of the invention will be apparent
from the ensuing description when read in conjunction with the accompanying
drawings.
SUMMARY OF THE INVENTION
[020] The present invention discloses a process to enhance the wear life of exhaust fan blades. In the present invention, ceramic tiles are replaced with high wear resistant flexible Co-WC tiles, which can also withstand high speed rotation (i.e. >2500 rpm) without any damage over the surface. The present invention uses small tiles rather than a continuous coating, having an advantage that the blade can flex and twist with no damage to the individual tiles.
BRIEF DESCRIPTION OF THE DRAWINGS
[020] Figure. 1 Shows the flow chart to fabricate the exhaust fan blades with enhanced life.
[021] Figure. 2 Shows the profile and dimensions of single exhaust fan blades.
[022] Figure. 3 Shows the schematic of exhaust fan blades having high wear resistant tiles over the surface and in Fig. 3(b) cross section of exhaust fan blade is shown.

[023] Figure 4. Optical micrograph of Vickers indentation on the surface of high
wear resistant Co-WC tiles. The length of the diagonals was used to calculate the
hardness of tiles.
[024] Figure. 5 Shows the schematic of honeycomb shape tiles along with dimensions.
BRIEF DESCRIPTION OF THE INVENTION
Thus according to this invention is provided a process if enhance the wear life of exhaust fan blades.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Thus according to the invention is provided a process to enhance the wear life of exhaust fan blades, comprising the steps of cutting a MS plate as per blade profile, cleaning the surface and applying a coating of bonding agent over the surface, followed by fixing flexible tiles over the surface and subjecting the same to heat treatment for better adhesion of tiles over the fan blade surface. [025] As discussed above, the exhaust fan is used to conveying the pulverized coal into the boiler. The pulverized coal strikes the fan blades and cause wear and tear over the fan blade surface. Once the surface worn-out, the geometrical alteration of surface leads to dramatic efficiency drop as well as balancing problem of fan blades, which result in unscheduled downtime. Embodiments of the present invention discloses a method which enhance the exhaust fan blades life by fixing the high wear resistant flexible tiles over the surface.
[026] As discussed above there are many routes to enhance the wear resistant of fan blade surface, including coatings with hard chromium, coatings with hard carbide particles, weld overlays and fixing of ceramic tiles over the surface. Out of these methods, fixing of ceramic tiles over the surface is most efficient way to enhance the wear resistant of blades. However, high-speed dynamic of fan blades damages the tiles, which fall off from the blade surface and result in unscheduled downtime.

[027] Embodiments of the present invention disclose a process to enhance the fan blade life. In the present invention, ceramic tiles are replaced with flexible tiles, which can withstand against high speed rotation (i.e. >2500 rpm) without any damage over the surface. The flexible tiles enhance the wear life of fan blades.
[028] Figure. 1 shows the flow chart to fabricate the exhaust fan blades with enhanced life. Referring to Fig. 1, the flow chart starts from step100. At step 100 MS plate is cut as per blade profile. After that surface is cleaned with 120/1200 grit size emery paper, step 101. After that coating of bonding agent applied over the surface, step 102. In step 103, flexible tiles are fixed over the surface. After that heat treatment is given to the component for better adhesion of tiles over the fan blade surface, step 104.
[029] In step 100, MS plate is cut as per blade profile. The Fig. 2 shows the profile and dimensions of exhaust fan blades. The fan blades have almost rectangular shape with dimensions around 520 mm width and 700 mm length. For better adhesion of bonding agent, the blade surface is cleaned with different grit size papers. After that, adhesive[step 102] is applied over the surface. In the present invention, bonding agent can withstand up to 180°C, since pulverized coal /flue gas which strikes the fan blades have temperature of around 120-140°C. The bond strength of adhesive agents varies between 3500-4500 psi. The adhesive agent is selected from commercially available adhesives.
[030] In step 103, the tiles are fixed over the exhaust fan blade surface. The schematic of exhaust fan blades showing tiles over the surface is shown in Fig. 3(a). Figure 3 (b) shows the cross section of exhaust fan blade. The tiles over the surface have honeycomb shape, which helps in interlocking of tiles with each other and improve the bonding with base substrate. The tiles used to protect the surface against the wear, have fully-dense structure.

[031] In accordance with an embodiment, the tiles used, are made of cobalted-tungsten carbide (Co-WC). The WC is selected because of their extremely high hardness (> 2000 Hv), which protect the blade surface from wear. Use of Co inside WC matrix improve the ductility (or flexibility) of tiles, which avoids the damage of tiles against the high speed rotation (i.e. >2500 rpm). However, ceramic tiles made of Alumina do not withstand due to poor ductility. The amount of cobalt in WC matrixis preferably between about 5 to about 20 % of the total weight, more preferably between about 10 to about 12% in weight. It is also within the scope of this invention that, other metals may be used such as, for example, copper, nickel, alloys of any of these.
[032] The Vickers hardness (Hv) of the Co-WC tiles was measured by Vickers hardness tester under the force of 30 kg, using the equation Hv = 1.854 F/d2where F is the indentation load and d is the arithmetic mean of two diagonals(d1 and d2).The length of the diagonals was calculated using the impressions shown in Fig. 4. The average hardness of tiles ranges from 1350-1650 Hv, which is much higher than the hardness of Silica (main cause of wear) present in the coal. The high hardness of tiles will improve the wear resistant of exhaust fan blades.
[033] In the present invention, Co-WC tiles have thickness of around 4.5-5 mm over the fan blade surface. The side length of honeycomb shape tiles ranges from 20-40 mm, preferably 18-20 mm. The schematic of honeycomb shape tiles is shown in Fig.5. Present invention, use small tiles rather than a continuous coating, having an advantage that the blade can flex and twist with no damage to the individual tiles.
[034] Although the present invention has been described with reference to the preferred embodiments, thereof, it is intended that the specification and examples be considered as exemplary only, the true scope and spirit of the invention being indicated by the following claims:

We Claim:
1. A process to enhance the wear life of exhaust fan blades, comprising the steps of cutting a MS plate as per blade profile (step 100), cleaning the surface (step 101) and applying a coating of bonding agent over the surface (step 102), followed by fixing flexible tiles over the surface (step 103) and subjecting the same to heat treatment for better adhesion of tiles over the fan blade surface (step 104).
2. The process as claimed in claim 1, wherein said tiles have honeycomb shape with, to improve the interlocking with neighbouring tiles .
3. The process as claimed in claim 1, wherein the side length of the honeycomb tiles ranges from 20 - 40 mm, preferably from 18-20 mm.
4. The process as claimed in claim 1, wherein said tiles have a thickness of around 4-5 mm and hardness > 1350 Hv.
5. The process as claimed in claim 1, wherein said tiles are made of Cobalt (Co) and Tungsten carbide (WC), the WC content in the tiles is between 80-90 wt % of the total weight and the Co content in the tiles is between 5-20 wt % of the total weight.
Dated this 30* day of January 2019