FORM – 2
THE PATENTS ACT, 1970
(39 OF 1970)
AND
THE PATENT RULE, 2003
COMPLETE SPECIFICATION
(SEE SECTION 10 AND RULE 13)

“A WASTE HEAT RECOVERY SYSTEM FOR DUST LADEN FLUE GASES
USING REPOSE FLOW DESIGN”

TRANSPARENT ENERGY SYSTEMS PRIVATE LIMITED
AN INDIAN COMPANY,
“PUSHPA HEIGHTS”, 1ST FLOOR,
BIBWEWADI CORNER,
PUNE SATARA ROAD,
PUNE- 411 037, 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 for Dust Laden Flue Gases” using 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 tubes inclined at an angle more than angle of repose of the dust particles in flue gases.

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.

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.

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 repose flow design, 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 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. 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.

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 “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. The said tubes are inclined at an angle more than the angle of repose of the dust particles in the flue gases. The dust laden hot flue gases flows over the said inclined tubes and heat from these flue gases is recovered by the water flowing inside these tubes.
The dust laden hot flue gases enter the waste heat recovery system from top and move downwards over the inclined tube cassettes. 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.
The dust particles gets 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. The dust collected at lower end is dislodged by providing pulse jet air, which intermittently blows in dust in downward direction towards dust collection points by gravity. Thus automatic cleaning of tubes surfaces is achieved with 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. As the gas flows over inclined tubes, better overall heat transfer coefficient can be achieved than in case of co-flow design.

STATEMENT OF INVENTION:-
A waste heat recovery system for dust laden flue gases comprises a heat recovery heat exchanger of vertical body with top and bottom casing ends; in the top portion of the said the heat exchanger a evaporator is formed with multiple numbers of inclined tubes arranged in parallel in one row , the inclined tubes are connected to headers on both ends, which forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; in the bottom portion an economiser formed with multiple numbers of inclined tubes arranged in parallel in one row , the inclined tubes are connected to headers on both ends, which forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; the inclination of said tubes greater than the repose angle of the flue dust particle; a steam drum having its down comer connected to the bottom header of the evaporator and risers from evaporator are connected to steam drum; the inlet header of economiser section connected to water supply source; the outlet of economizer is connected to the steam drum; 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 Pulse jet air system provided at lower end of the said inclined tubes.

A waste heat recovery system for dust laden flue gases comprises a heat recovery heat exchanger of horizontal cylindrical body with both ends having conical shape; on side of the said the heat exchanger a boiler is formed with multiple numbers of inclined tubes arranged in parallel in one row , connected to out let header at upper end and inlet header at lower end, that forms a cassette of the tubes (3) and number of such cassettes arranged over one another to form a heat recovery section; at other side an economizer (2) formed with multiple numbers of inclined tubes (3) arranged in parallel in one row , connected to out let header at upper end and inlet header at lower end, that forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; the inclination of said tubes greater than the repose angle of the dust particle of the flue gases; a steam drum having its down comer connected to the bottom header of the evaporator and risers from evaporator are connected to steam drum ; the inlet header of economiser section connected to water supply source; ; the outlet of economizer is connected to the steam drum flue gas inlet provided one end of the said heat exchanger from evaporator side and flue gas outlet provided at the other end of the said heat exchanger; and a Pulse jet air system provided at lower end of the said inclined tubes.

According to another embodiment the invention provides a waste heat recovery system for dust laden flue gases comprises a heat recovery heat exchanger of horizontal cylindrical body with both ends having conical shape; on side of the said the heat exchanger a boiler is formed with multiple numbers of inclined tubes arranged in parallel in one row , connected to out let header at upper end and inlet header at lower end, that forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; at other side an economizer formed with multiple numbers of inclined tubes arranged in parallel in one row , connected to out let header at upper end and inlet header at lower end, that forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; the inclination of said tubes greater than the repose angle of the dust particle of the flue gases; 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 inlet connected to the inlet header of boiler section; the inlet header of economiser section connected to water supply source; flue gas inlet provided one end of the said heat exchanger from boiler side and flue gas outlet provided at the other end of the said heat exchanger; and a Pulse jet air 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,
Figure1 shows simplified flow diagram of the Waste Heat Recovery System for dust laden flue gases using repose flow design according 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 repose flow design, according to the present invention.
1. Boiler (also called as Evaporator)
2. Economizer
3. 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 repose flow design” comprises Evaporator (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. Number of such cassettes arranged 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.
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 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.

ADVANTAGES OF PRESENT INVENTION:-
1. In repose flow design, the tubes surface is maintained clean without or with very low intensity and low frequency of external forces / system.
2. There is no excessive dependency on operation / maintenance of external dust removal system; hence availability of waste heat recovery boiler is improved.
3. In 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.
4. 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.
5. Automatic cleaning of tubes surface area is achieved by repose flow design.
6. The sagging of tubes is lower as compared to horizontal cross flow construction.
7. 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 section of a waste heat recovery boiler according to behavior of dust particles in different sections.

3. 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.

4. 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

5. 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.

6. Different types of pulse jet air system can be used according to suitability.

Features of the invention

1. A waste heat recovery system for dust laden flue gases using repose flow design comprises tubes which are inclined at an angle with respect to horizontal.

2. A waste heat recovery system for dust laden flue gases using 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 repose flow design wherein the 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 repose flow design wherein 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 repose flow design wherein the dust particles are collected at lower end of inclined tubes.

6. A waste heat recovery system for dust laden flue gases using repose flow design 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 repose flow design wherein 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 repose flow design wherein no intensive external forces and resultant stresses are involved, which improves the overall life of tube weld joints.
 
WE CLAIM:-

1. A waste heat recovery system for dust laden flue gases comprises a heat recovery heat exchanger of vertical body with top and bottom casing ends; in the top portion of the said the heat exchanger a evaporator is formed with multiple numbers of inclined tubes arranged in parallel in one row , the inclined tubes are connected to headers on both ends, which forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; in the bottom portion an economiser formed with multiple numbers of inclined tubes arranged in parallel in one row , the inclined tubes are connected to headers on both ends, which forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; the inclination of said tubes greater than the repose angle of the flue dust particle; a steam drum having its down comer connected to the bottom header of the evaporator and risers from evaporator are connected to steam drum; the inlet header of economiser section connected to water supply source; the outlet of economizer is connected to the steam drum; 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 Pulse jet air system provided at lower end of the said inclined tubes.

2. A waste heat recovery system for dust laden flue gases comprises a heat recovery heat exchanger of horizontal cylindrical body with both ends having conical shape; on side of the said the heat exchanger a boiler is formed with multiple numbers of inclined tubes arranged in parallel in one row , connected to out let header at upper end and inlet header at lower end, that forms a cassette of the tubes (3) and number of such cassettes arranged over one another to form a heat recovery section; at other side an economizer (2) formed with multiple numbers of inclined tubes (3) arranged in parallel in one row , connected to out let header at upper end and inlet header at lower end, that forms a cassette of the tubes and number of such cassettes arranged over one another to form a heat recovery section; the inclination of said tubes greater than the repose angle of the dust particle of the flue gases; a steam drum having its down comer connected to the bottom header of the evaporator and risers from evaporator are connected to steam drum ; the inlet header of economiser section connected to water supply source; ; the outlet of economizer is connected to the steam drum flue gas inlet provided one end of the said heat exchanger from evaporator side and flue gas outlet provided at the other end of the said heat exchanger; and a Pulse jet air system provided at lower end of the said inclined tubes.

3. A waste heat recovery system as claimed in claim 1 and 2 wherein a steam super heater formed by the inclined tube cassette provided in the said heat exchanger between flue gas inlet and the said boiler section.

4. A waste heat recovery system a waste heat recovery system as claimed in claims 1 to 3 wherein water pre-heater provided after economizer to recover the remaining heat for pre-heating of water.

5. A waste heat recovery system as claimed in claims 1 to 4 wherein the inclination of said cassette tubes can be adjusted depending on the type of dust of flue gases.

6. A waste heat recovery system as claimed in claim 5 wherein for the cement clinker dust the angle of repose about 20 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 20 deg with horizontal.

7. A waste heat recovery system as claimed in claims 1 to 6 wherein the said pulse jet air cleaning system provided with control system for operating intermittently.

8. A waste heat recovery system for dust laden flue gases using repose flow design as claimed in claim no. 1 to 7, 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.

9. A waste heat recovery system for dust laden flue gases using repose flow design as claimed in claim no. 1 to 8, wherein 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.

10. A waste heat recovery system for dust laden flue gases using repose flow design as claimed in claim no. 1 to 9, wherein no intensive external forces and resultant stresses are involved, which improves the overall life of tube weld joints.