FORM 2
THE PATENT ACT, 1970,
(39 OF 1970)
&
THE PATENTS RULE, 2003
COMPLETE SPECIFICATION
(SEE SECTION 10; RULE 13)
"WASTE HEAT RECOVERY SYSTEM FOR DUST LADEN FLUE GASES USING CRISSCROSS REPOSE FLOW DESIGN"
TRANSPARENT ENERGY SYSTEMS PRIVATE LIMITED
AN INDIAN COMPANY,
"PUSHPA HEIGHTS", 1ST FLOOR,
BIBWEWADI CORNER, PUNE-SATARA ROAD,
PUNE-411037,
MAHARASHTRA, INDIA.
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE NATURE OF THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

FIELD OF INVENTION:-
The present invention relates to "A Waste Heat Recovery System (WHRS) for Dust Laden Flue Gases using Crisscross Repose Flow Design". More particularly, present invention relates to a Waste Heat Recovery System (comprising Boiler, Economizer and other accessories) for dust laden flue gases using crisscross arrangement of tubes inclined at an angle more than angle of repose of the dust particles in flue gases such as to create dynamic repose situation. This invention improvement over our application no. 396/MUM/2012 filed on February 13, 2012 (13.02.2012) titled a waste heat recovery system for dust laden flue gases, a simple repose flow design and the co-flow design.
While recovery of waste heat in various plants like Cement Plant, Steel Plant, Foundries, etc, the flue gases come out of these plants are invariably heavily dust laden. These dust particles accumulate on heat recovery surfaces of the tubes and the extent of this dust accumulation is always unpredictable. This dust removal as well as prevention of settling of the dust on the heat transfer surface becomes an important aspect in efficient design of such waste heat recovery systems.
PRIOR ART:-
Conventionally various types of Waste heat recovery systems are used for recovery of heat from dust laden flue gases. In these waste heat recovery systems, the water flows inside the tubes and flue gas flow is outside the bare tubes. The following types of heat recovery systems are classified according to configuration of heating surfaces (Boiler / Economizer tubes) with respect to flue gas flow.
1 Horizontal Cross Flow Boiler with bare tubes.
In this type of configuration, heating surfaces (Boiler / Economizer tubes) are arranged such that flue gases flow horizontally in the heat recovery equipment on outside of the bare tubes. The water tubes are placed vertically in the flow of gas such that there is cross flow heat exchange between flue gas and water flowing inside the tubes. In this type of configuration, axis of tubes is

perpendicular to flue gas flow direction. A small inclination may be given for circulation of steam bubbles in the tubes.
2 Vertical Cross Flow Boiler with bare tubes.
In this-type of configuration, heating surfaces (Boiler / Economizer tubes) are arranged such that flue gases flow vertically in the heat recovery equipment on outside of the bare tubes. The water tubes are placed horizontally in the flow of gases such that there is cross flow heat exchange between flue gas and water flowing inside the tubes. In this type of configuration, the axis of tubes is perpendicular to the flue gas flow direction.
3 Vertical Co-Flow Boiler with bare tubes.
In this type of configuration, heating surfaces (Boiler / Economizer tubes) are arranged such that flue gases flow vertically in the heat recovery equipment on outside of the bare tubes. The water tubes are placed vertically in the flow of gases such that there is co-flow heat exchange between flue gas and water flowing inside the tubes. In this type of configuration, the axis of tubes is parallel to the flue gas flow direction.
4 Repose Flow Boiler with bare tubes
In this type of arrangement, the heating surfaces (Boiler/Economizer tubes) are arranged such that flue gases flow vertically in the heat recovery equipments on outside of the bare tubes inclined at an angle more than angle of repose of dust particles arranged in a manner that central axis of tubes in any particular row of tubes in the panels adjacent to each other are in one plane. In this type of configuration, this plane is inclined to the flue gas flow direction at an angle more than the angle of repose of the dust in the gas.
LIMITATIONS OF PRIOR ART:-
Some of limitations of aforesaid types of waste heat recovery systems for dust laden flue gases are:-

1. Horizontal Cross Flow Boiler with bare tubes.
i) The tube configuration obstruct the flow of gases hence even self
cleaning velocity of flue gases cannot avoid dust accumulation.
Hence knocker / hammering system should be available
continuously to avoid performance deviation due to fouling of
heating surfaces, ii) The continuous knocking / hammering with higher frequency and
higher intensity on the tubes may induce additional stresses in the
tube weld, resulting in lower life of tube welds, iii) As tubes significantly obstruct the gas flow, there is heavy erosion
of boiler tubes. iv) The dust accumulated in shadow region of previous tube in flue gas
flow cannot be easily removed, hence heat transfer is affected, v) The horizontal arrangement of heat recovery system sections,
require more floor space as compared to vertical co-flow boiler.
Hence cannot be installed in compact space, vi) Multi Point Dust collection is necessary as floor space of waste heat
recovery system is more, increasing operation cost.
2. Vertical Cross Flow Boilers with bare tubes.
i) Tubes are placed horizontally with axis perpendicular to gas flow.
This obstructs the dusty gas flow significantly, and Dust
Accumulation tendency increases, ii) The dust accumulation on top of each tube (shadow of above tube)
has to be dislodged by external means such as knocking /
hammering system, iii) The continuous knocking / hammering with higher frequency and
higher intensity on tubes may induce additional stresses in the tube
weld, resulting in lower life of tube welds, iv) In Cross Flow Design, the horizontal boiler tubes may sag between
supports due to self weight, weight of circulating water inside tubes
and weight of accumulations inside tube.

3. Vertical Co-Flow Boilers with bare tubes.
i) Longer tubes results in higher overall size and weight of equipment
for erection, ii) The overall cost of system is higher due to higher overall size, iii) Dust accumulation on tube surfaces is lower in vertical co-flow
boilers, but still knocking / hammering system is required to clean
the dust particles deposited on tube surfaces, iv) The continuous knocking / hammering with higher frequency and
higher intensity on tubes may induce additional stresses in the tube
weld, resulting in lower life of tube welds.
4. Repose Flow Boiler
i) Dust flowing over the inclined tubes display bridging tendency forming obstacle to gas passage. This increases the pressure drop across WHRS.
ii) Therefore, even though repose flow design has advantage of higher overall heat transfer coefficient than above mentioned designs, it requires intermittent knocking / hammering with higher frequency and higher intensity on tubes. This may induce additional stresses in the tube weld, resulting in lower life of tube welds
PRESENT SCENARIO AND NEED OF PRESENT INVENTION:-
The conventional waste heat recovery systems used for dust laden flue gases have several limitations and drawbacks as described in above section.
Present invention aims at developing a waste heat recovery system for dust laden flue gases using crisscross repose flow design creating dynamic repose situation, which will overcome the limitations and drawbacks of conventional waste heat recovery systems. Following issues also need to be addressed while designing waste heat recovery system for dust laden flue gases:

1 Methods of avoiding Deposition of dust from flue gases on the Heat Transfer Surface which otherwise results in fouling and lesser heat recovery.
2 Methods of easy dislodging of the deposited dust and keep the heat transfer surface free of dust.
3 Keep flue gas pressure drop in boiler low.
4 The overall size of heat recovery system should be lower to reduce erection time and cost.
OBJECT OF THE PRESENT INVENTION:-
Object of present invention aims at developing a waste heat recovery system for dust laden flue gases using Crisscross Repose Flow Design, wherein the boiler tubes are arranged inclined at an angle more than "angle of repose of the dust particles" in the flue gases in crisscross manner meaning; in a bundle of panels having tubes inclined at an angle more than angle of repose, the central axis of tubes in any particular row of alternate panels are in one plane. The flow of flue gases and gravitational force on dust particles due to inclination more than angle of repose reduces the tendency of dust particles to settle on the tube surface. Additionally, the crisscross arrangement reduces bridging of dust in the gas flow passage.
It is also object of present invention to eliminate limitations or drawbacks of the prior art.
SUMMARY OF PRESENT INVENTION:-
According to the present invention, a waste heat recovery system for dust laden flue gases using vertically arranged "Crisscross Repose Flow Design", wherein flue gases are flowing over the bare tubes which are arranged inclined to the angle more than angle of repose of the dust particles in flue gases with respect to horizontal. In

this type of heat recovery system, the flue gas flows on outside of the tube and water flows inside the tube.
The said tubes of boiler are arranged such that tube axis along its length is inclined with respect to horizontal. If this angle of inclination is 'x' degree, then the angle of inclination of tubes in adjacent panel is '180-x'degree. This means, in a bundle of panels arranged such as to have tubes in alternate panels inclined at an angle 'x' and '180-*' respectively, the central axis of tubes in any particular row of alternate panels are in one plane. The angle lx' is more than the angle of repose of the dust particles in the flue gases. The dust laden hot flue gases flow over the said inclined tubes and heat from these flue gases is recovered by the water flowing inside these tubes.
For example the table below gives typical values of angle of repose for few dusts

Sr. No. Material Angle of Repose in degrees
1 Cement 30o to 44o
2 Crushed Slag 25o
3 Ash 45°
4 Coal 38o
5 Lime (ground) 43o
The dust laden hot flue gases enter the waste heat recovery system from top and move downwards over the inclined tube cassettes arranged as described above. The dust particles tends to settle on the tubes surface, however as the angle of inclination of tubes is more than the "angle of repose of the dust particles", the dust particles starts moving over the tubes by gravity at lower end of the inclined tube. The conveying flow of flue gases in downward direction and gravity forces due to tube inclination minimize the deposition of dust particles on tubes. Hence, deposition of dust particles on tube surface is minimized and clean surface is made available for recovery of heat.

points by gravity. The dust particles get collected on lower end of the inclined tubes and the overall length of tubes is maintained as deposition free cleaned surface for effective heat transfer and efficient heat recovery from flue gases. Thus automatic cleaning of tubes surfaces is achieved with crisscross repose flow design and controlled operation of pulse jet air system intermittently removes the collected dust particles. This reduces the frequency and intensity of external vibrations required for removal dust deposited on tube surface. As there is no intensive mechanical hammering involved on tubes or headers, the additional stresses are reduced improving the life of tube weld joints.
The flow of dust laden hot gases over the crisscross arrangement of tube panels is highly turbulent. Therefore, the heat transfer coefficients for the crisscross arrangement are superior to the heat transfer coefficients for simple repose flow design and the co-flow design.
STATEMENT OF INVENTION:-
Accordingly invention provides a waste heat recovery system for dust laden flue gases comprises a heat recovery heat exchanger of vertical cylindrical body with top and bottom ends having conical shape; in the top portion of the said the heat exchanger a boiler is formed with multiple numbers of inclined tubes arranged in parallel in one plane connected to out let header at upper end and inlet header at lower end, that forms one cassette of the tubes, such panels of inclined tubes are arranged to form a crisscross arrangement; similarly in the bottom portion, an economiser formed with multiple numbers of inclined tubes arranged in parallel in one plane connected to out let header at upper end and inlet header at lower end, that forms one cassette of the tubes, such panels of inclined tubes are arranged to form a crisscross arrangement ; the inclination of said tubes is greater than the angle of repose of the dust particles in the flue gas; a steam drum having first inlet connected outlet of the outlet header economiser section, second inlet connected to outlet header of boiler section and an outlet connected to the inlet header of boiler section; the inlet header of economiser section connected to water supply source; flue gas inlet provided on top end of the said heat exchanger and flue gas

outlet provided at the bottom end of the said heat exchanger; and a dust dislodgement system provided at lower end of the said inclined tubes.
BRIEF DESCRIPTION OF THE SCHEMATIC DRAWING:-
The invention is described with reference to accompanying drawings wherein,
Figurel shows simplified flow diagram of the Waste Heat Recovery System for dust laden flue gases using crisscross repose flow design according to this invention.
Figure 2 shows simplified flow diagram of the Waste Heat Recovery System for dust laden flue gases using group staggered crisscross repose flow design as a possible variation to the design related to this invention
Referring to the accompanying figure 1, following components can be identified in the waste heat recovery system for dust laden flue gases using crisscross repose flow design, according to the present invention.
1. Boiler
2. Economizer
3. Crisscross arrangement of Inclined Tubes
4. Inlet/Outlet Headers
5. Steam Drum
6. Riser / Downcomer Headers
7. Pulse Jet Air system
Referring to accompanying Figure 1, the present invention "A Waste Heat Recovery System for Dust Laden Flue Gases using crisscross repose flow design" comprises Boiler 1 and Economizer 2 arranged vertically one above the other. The boiler and economizer comprise tubes 3 inclined at certain angle with respect to horizontal. Each inclined tube is connected to inlet and outlet header 4 on either end of the tube. Multiple numbers of such tubes are arranged in one row that forms a cassette of the tubes. The alternate cassette or panel of the tubes are arranged to form a crisscross arrangement. Number of such cassettes arranged in crisscross manner over one another form a heat recovery section of the boiler. These heat recovery sections are

connected to steam drum 5 through risers and down comers 6 for circulation of water in the inclined tubes. Depending of type of dust in the flue gas the angle of inclination of the said tubes is selected such that it is more than "angle of repose of the dust particles". E.g. for cement clinker dust, the angle of repose is about 30 Deg. Hence in the waste heat recovery boiler designed for recovery of heat from flue gases containing cement clinker dust, the inclination of tubes will be kept more than 30 Deg. Pulse jet air system 7 is provided at lower end of the said inclined tubes. This pulse jet air cleaning system can be intermittently operated with the help of control system.
Referring to accompanying Figure 2, the present invention "A Waste Heat Recovery System for Dust Laden Flue Gases using crisscross repose flow design" comprises Boiler (marked as item no. 1) and Economizer (marked as item no. 2) arranged vertically one above the other. The boiler and economizer comprise of a bundle of paired tubes (marked as item no. 3) inclined at certain angle with respect to horizontal. Each inclined pair of tubes is connected to inlet and outlet header (marked as item no. 4) on either end of the pair. Multiple numbers of such paired tubes are arranged in one row that forms a cassette of the paired tubes. The alternate cassette or panel of the paired tubes are arranged to form a crisscross arrangement. Number of such cassettes arranged in crisscross manner over one another form a heat recovery section of the boiler. These heat recovery sections are connected to steam drum (marked as item no. 5) through risers and down comers (marked as item no. 6) for circulation of water in the inclined tubes. Depending on type of dust in the flue gas the angle of inclination of the said paired tubes is selected such that it is more than "angle of repose of the dust particles". E.g. for cement clinker dust, the angle of repose is about 30 Deg. Hence in the waste heat recovery boiler designed for recovery of heat from flue gases containing cement clinker dust, the inclination of paired tubes will be kept more than 30 Deg. Pulse jet air system (marked as item no. 7) is provided at lower end of the said inclined tubes. This pulse jet air cleaning system can be intermittently operated with the help of control system.

In figure 2 the crisscross tubes arrangement is such that the grouped crisscross tubes on the inlet and out let headers joined at a common point. All other features same as of the figure 1.
As shown in figure 1, dust laden flue gases enter the waste heat recovery system from top and move downwards through tube cassettes and different sections explained above. The hot flue gases are passed over these inclined tubes for recovery of the heat from flue gases. The water circulating inside the inclined tubes recovers the heat from flue gases and temperature of water is increased. This water circulates through the steam drum and steam is released in the steam drum. As the velocity of gases is maintained lower to keep lower pressure drop on flue gas side, the dust particles tends to settle on inclined tubes of the boiler.
As the angle of tube inclination is more than "angle of repose of dust particles", the dust particles move downwards along tubes length by gravity. The conveying flow of the flue gases and gravitational forces involved due to inclination angle do not allow the dust particles to settle on heating surface of the tubes. This downward movement reduces tendency of dust particles to settle on the tubes surface, keeping clean surface for efficient heat transfer. The dust particles moving downwards get collected at the lower end of inclined tubes. The pulse jet air system is intermittently operated to dislodge these dust particles, which are collected at lower end of inclined tubes. After dislodging these dust particles move downwards by gravity. The pulse jet air system can be activated intermittently by timer control or it can be operated with respect to set level of pressure drop across each section of the boiler through a control system.
The crisscross repose flow design reduces tendency of the dust particles to settle on the tube surface. Hence, the frequency and intensity of external forces required for dust dislodging through mechanical hammers / knockers, is reduced. This reduces the requirement external forces and resultant stresses in tube weld joints are reduced. As there are reduced external forces acting on weld joints, possibility of tube joint failure is drastically reduced.
Steam superheater and Water pre-heater may be provided in the flue gas flow within the waste heat recovery equipment as per intended application of steam and extent

of waste heat recovery expected. Steam superheater and water pre-heater may have similar type of construction.
In case of group/paired tube staggered arrangement, the values of heat transfer coefficient of a pair of tubes are almost twice as high as the corresponding values in similar staggered bundle with single tube. Also, the compactness of a paired tube staggered arrangement is almost twice as high as that of single tube staggered arrangement.
ADVANTAGES OF PRESENT INVENTION:-
1. In crisscross repose flow design, the bridging of dust between the adjacent tubes or tube panels is drastically reduced. Therefore, the tubes surface is maintained clean without or with very low intensity and low frequency of external forces / system.
2. The crisscross repose flow design will have very low fouling.
3. The crisscross design offers higher turbulence in the gas passage which keeps the dust in suspended condition.
4. The inline-arrangement of tubes in crisscross design offers low pressure drop on the gas side.
5. Water circulation through tubes in crisscross design will be better due to tube inclination and large number of sub-headers.
6. There is no excessive dependency on operation / maintenance of external dust removal system; hence availability of waste heat recovery boiler is improved.
7. In crisscross repose flow designs very less intensity and frequency of external forces and resultant stresses (using systems such as hammering system, knockers or vibrators) are applied on the tube joints. This improves the life of tube joints.

8. As external systems such as hammering system are not continuously required, the utility power consumption is saved. The pulse jet air system in operated intermittently.
9. Automatic cleaning of tubes surface area is achieved by crisscross repose flow design.
10. The sagging of tubes is lower as compared to horizontal cross flow construction.
11.As tubes are inclined in the gas flow, heat transfer coefficient is better than vertical co-flow construction. This reduces the overall size of boiler and associated parameters such as cost, weight, erection period, etc.

VARIATIONS POSSIBLE TO BE COVERED AND PROVIDED WITHIN THE INVENTION OF WASTE HEAT RECOVERY SYSTEM FOR DUST LADEN FLUE GASES USING REPOSE FLOW DESIGN:-
1. The angle of inclination of heat recovery tubes can be varied according to type or size of dust particles in the flue gases.
2. The angle of tube inclination can be changed in different sections of a waste heat recovery boiler according to behavior of dust particles in different sections.
3. The tube diameter can be changed in different sections of a waste heat recovery boiler and also in one boiler to another
4. The tube pitch can be changed in different sections of a waste heat recovery boiler and also in one boiler to another
5. The tube length can be changed in different sections of a waste heat recovery boiler and also in one boiler to another
6. The tubes of elliptical cross section can be used in different sections of a waste heat recovery boiler and also in one boiler to another
7. The group staggered arrangement as shown in the fig 2 can also be used in different sections of a waste heat recovery boiler and also in one boiier to another
8. The cross sectional area of the gas passage can be variable in different section of a waste heat recovery boiler and also in one boiler to another
9. On water side, the system could be designed for forced circulation or natural circulation of water as appropriate
10. The system could be designed for applications such as generation of steam, hot water or hot thermic fluid.
11.The dust dislodging arrangement could be any of mechanical hammering system, vibrators or knockers depending on the nature of dust.
12. The flue gas flow can be in vertically upward or downward direction or in horizontal direction or in combination on the inclined tubes, depending on space availability or location of inlet and outlet connections of the flue gas.

13.A steam superheater can be provided before boiler to superheat the steam from steam drum, depending on further application of steam. The superheater may have same type of construction as that of boiler
14. Water preheater can be provided after economizer to recover the remaining heat for pre-heating of water. The water preheater may have same type of construction as that of boiler.
15. Different types of pulse jet air system can be used according to suitability.
MAIN FEATURES OF INVENTION:-
1. A waste heat recovery system for dust laden flue gases using crisscross repose flow design wherein the boiler tubes are arranged inclined at an angle more than "angle of repose of the dust particles" in the flue gases in crisscross manner meaning; in a bundle of panels having tubes inclined at an angle more than angle of repose, the central axis of tubes in any particular row of alternate panels are in one plane as explained in the accompanying detailed description and figures.
2. A waste heat recovery system for dust laden flue gases using crisscross repose flow design wherein the angle of inclination of tubes with horizontal is more than the "angle of repose of dust particles" in the flue gases.
3. A waste heat recovery system for dust laden flue gases using crisscross repose flow design flue gases containing dust particles are flowing over inclined tubes in vertical or horizontal direction.
4. A waste heat recovery system for dust laden flue gases using crisscross repose flow design the dust particle slide on the inclined tubes by gravity towards lower end of inclined tubes as the inclination is more than the "angle of repose of dust particles".

5. A waste heat recovery system for dust laden flue gases using crisscross repose flow design as claimed in claim no. 1, 2, 3 and 4, wherein the dust particles are collected at lower end of inclined tubes.
6. A waste heat recovery system for dust laden flue gases using crisscross repose flow design as claimed in claim no. 1, 2, 3, 4 and 5, wherein the dust particles collected at lower end of inclined tubes are dislodged by intermittently operating pulse jet air system and dust particles fall downwards by gravity.
7. A waste heat recovery system for dust laden flue gases using crisscross repose flow design the dust particles do not deposit on tubes surfaces providing clean heat transfer area for efficient heat recovery, with less frequency and intensity of external forces or dust removing systems like hammering system or vibrators or knockers.
8. "A waste heat recovery system for dust laden flue gases using crisscross repose flow design no intensive external forces and resultant stresses are involved, which improves the overall life of tube weld joints.
9. A waste heat recovery system for dust laden flue gases using crisscross repose flow design the crisscross design creates dynamic repose flow situations which also create turbulence in the gas passage thereby increasing the overall heat transfer coefficient.

WE CLA1M:-
1. Accordingly invention provides a waste heat recovery system for dust laden flue gases * comprises a heat recovery heat exchanger of vertical cylindrical body with top and bottom ends having conical shape; in the top portion of the said the heat exchanger a boiler is formed with multiple numbers of inclined tubes arranged in parallel in one plane connected to out let header at upper end and inlet header at lower end, that forms one cassette of the tubes, such panels of inclined tubes are arranged to form a crisscross arrangement ; similarly in the bottom portion, an economiser formed with multiple numbers of inclined tubes arranged in parallel in one plane connected to out let header at upper end and inlet header at lower end, that forms one cassette of the tubes, such panels of inclined tubes are arranged to form a crisscross arrangement ; the inclination of said tubes is greater than the angle of repose of the dust particles in the flue gas; a steam drum having first inlet connected outlet of the outlet header economiser section, second inlet connected to outlet header of boiler section and an outlet connected to the inlet header of boiler section; the inlet header of economiser section connected to water supply source; flue gas inlet provided on top end of the said heat exchanger and flue gas outlet provided at the bottom end of the said heat exchanger; and a dust dislodgement system provided at lower end of the said inclined tubes.
2. A waste heat recovery system as claimed in claims 1, wherein the joining of the said crisscross inclined tubes of boiler and economizer on headers made separately.
3. A waste heat recovery system as claimed in claims 1 & 2, wherein the joining of the said crisscross inclined tubes of boiler and economizer on headers made at common with point of group staggered crisscross.
4. A waste heat recovery system as claimed in claims 1 to 3, wherein a steam super heater formed by the inclined tube cassette/ crisscross inclined tubes provided in the said heat exchanger between flue gas inlet and the said boiler section.

5. A waste heat recovery system a waste heat recovery system as claimed in claims 1 to 4, wherein water pre-heater provided after economizer to recover the remaining heat for pre-heating of water.
6. A waste heat recovery system as claimed in claims 1 to 5, wherein the inclination of said cassette tubes is adjustable according depending on the type of dust of flue gases.
7. A waste heat recovery system as claimed in claim 6, wherein for the cement clinker dust the angle of repose about 30 Deg. the waste heat recovery boiler designed for recovery of heat from flue gases containing cement clinker dust, the inclination of tubes will be kept more than 30 Deg.
8. A waste heat recovery system as claimed in claims 1 to 7, wherein the said pulse jet air cleaning system provided with control system for operating intermittently.