FIELD OF THE INVENTION:-
The present invention relates to a heat recovery heat exchanger system for dust laden flue gases using corrugated tubes and system thereof. More particularly, the present invention relates to a heat recovery system (comprising Boiler, Economizer and other accessories) for dust laden flue gases using vertically arranged corrugated tubes, wherein flue gas flow is parallel to the axis of the tube. It is also an embodiment of the present invention that the said corrugated tubes are vertically or horizontally arranged such that flue gas flow is perpendicular to the axis of the tube.
While doing heat recovery in various plants like Cement Plant, Steel Plant, Foundries, etc, the flue gases come out of these plants are invariably heavily dust laden. This dust removal as well as prevention of settling of the dust on the heat transfer surface becomes an important aspect in designing these 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 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.
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. Vertical Co-flow Boiler with Longitudinal Finned Tubes as of our patent application
no 2191/MUM/2008. 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 longitudinally finned 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 AND NEED OF THE PRESENT INVENTION:-
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) As bare tubes are used, the heating area per unit length of tubes is lower. Hence longer tubes are required increasing overall size of the boiler. This also result in increased structure for boiler, boiler casing and in result increased overall project cost and erection time period.
ii) The tube configuration obstruct the flow of gases hence even self-cleaning velocity of flue gases cannot avoid dust accumulation. Hence knocker

system should be available continuously to avoid performance deviation due
to fouling of heating surfaces. 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) As bare tubes are used, the heating area per unit length of tubes is lower.
Hence longer tubes are required increasing overall size of the boiler. This
also result in increased structure for boiler, boiler casing and in result
increased overall project cost and erection time period. ii) Tubes are placed horizontally with axis perpendicular to gas flow. This
obstructs the dusty gas flow significantly, and Dust Accumulation tendency
increases. iii) The dust accumulation on top of each tube (shadow of above tube) has to
be dislodged by external means. iv) In Cross Flow Design, the boiler tubes can 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) As bare tubes are used, the heating area per unit length of tubes is lower.
Hence longer tubes are required increasing overall height of the boiler. This
also result in increased structure for boiler, boiler casing and in result
increased overall project cost and erection time period. ii) Longer tubes results in higher overall size and weight of equipment for
erection. iii) The overall cost of system is higher due to higher overall size.

4. Vertical Co-flow heat exchanger with Longitudinal Finned Tubes as of our patent
application no 2191/MUM/2008:
i) As longitudinally finned tubes are used the heat transfer area is lesser as compared to the bare tubes however, the cost of manufacturing longitudinal tubes outweighs the benefits achieved by reduction in heat transfer area.
ii) The overall cost of system is high.
OBJECT OF THE INVENTION:-
Object of present invention aims at developing a heat recovery heat exchanger for dust laden flue gases using vertically arranged corrugated tubes, wherein flue gas flow is parallel to the axis of the tube. It is also an embodiment of the present invention that the said corrugated tubes are vertically or horizontally arranged such that flue gas flow is perpendicular to the axis of the tube.
It is also object of present invention to eliminate limitations or drawbacks of the prior art. STATEMENT OF THE INVENTION:
According to this invention therefore a heat recovery heat exchanger system for dust laden flue gases dust laden flue gases using corrugated tubes comprises a evaporator (1) having matrix of cassettes of corrugated tubes (3) formed by number of corrugated water tubes (3) fixed between upper header (4) and bottom header (7), placed side by side; an economizer (2) having matrix of cassettes of corrugated tubes (3) formed by number corrugated water tube (3) fixed between upper header (4) and bottom header (7) and placed side by side, located below the said evaporator (1) in the flue gas path flowing in downward direction parallel to the axis of said corrugated tubes; the matrix of tubes of the said evaporator (1) and economizer (2) supported on the said top headers (4) to hang freely under self weight in the fuel gas path; a feed water inlet provided to the said bottom header (7) of economizer (2); an out let of the said top header(4) of the economizer (2) connected to an inlet provided at upper side of a steam drum(5); a hot feed water outlet provided to the said drum (5) at lower side connected to the inlet of the bottom header (7)

of the said evaporator (1); an outlet for saturated water at the said top header (4) connected to the inlet provided on the upper side of the said steam drum (5); mechanical hammers / vibrators (6) provided to the said tube cassets of the boiler and economizer to dislodge the dust.
According to a another embodiment of the present invention a heat recovery heat exchanger system for dust laden flue gases using corrugated tubes comprises a evaporator (1) having cassettes of corrugated tubes (3) formed by number of corrugated tubes arranged horizontally one above other corrugated water tubes (3) connected at both ends with 180 degree bends wherein the said cassettes placed side by side are fixed between upper header (4) and bottom header (7) respectively to forma matrix ; an economizer (2) having cassettes of corrugated tubes (3) formed by number of corrugated tubes arranged horizontally one above other corrugated water tubes (3) connected at both ends with 180 degree bends wherein the said cassettes placed side by side are fixed between upper header (4) and bottom header (7) respectively to form a matrix, located below the said evaporator in the flue gas path flowing in down ward direction perpendicular to the axis of said corrugated tubes; the matrix of tubes of the said evaporator (1) and economizer (2) supported on the said top header (4) to hang freely under self weight in the fuel path; a feed water inlet provided to the said bottom header (7) of economizer (2); an out let of the said top header (4) of the economizer (2) connected to an inlet provided at upper side of a steam drum (5); a hot feed water outlet provided to the said steam drum (5) at lower side connected to the inlet of the bottom header (7) of the said evaporator (1); an outlet for saturated water at the said top header (4) connected to the inlet provided on the upper side of the said steam drum (5); mechanical hammers / vibrators (6) provided to the said tube matrix of the boiler and economizer to dislodge the dust.
DESCRIPTION OF FIGURES:-
Figure 1: Shown embodiment 1 of the present invention in which the heat recovery heat exchanger and system thereof comprises of corrugated tubes arranged vertical and with their axis parallel to the direction of gas flow.

Figure 2: Shown embodiment 2 of the present invention in which the heat recovery heat exchanger and system thereof comprises of corrugated tubes arranged horizontal and with their axis perpendicular to the direction of gas flow. Figure 3: Lateral section drawing of a typical corrugated tube.
BRIEF DESCRIPTION OF THE PRESENT INVENTION:-
Referring to the accompanying figure 1 and 2 following components can be identified in
the waste heat recovery system for dust laden flue gases using corrugated tubes,
according to the present invention.
Evaporator (1)
Economizer (2)
Corrugated tubes (3)
Top Headers (4)
Bottom Headers (7)
Steam Drum (5)
Vibrators / Hammers (6)
Referring embodiment 1 of the present invention as shown in accompanying Figure 1, the
present invention a heat recovery heat exchanger for dust laden flue gases using
corrugated tubes and system thereof comprises of evaporator (1) and economizer (2)
arranged vertically one above the other.
The evaporator and economizer comprise vertically arranged corrugated tubes (3). The said corrugated tubes (3) of evaporator and economizer are arranged such that tube axis along its length is vertical. These vertically arranged tubes (3) are corrugated.
The typical construction of corrugated tubes is as shown in Figure 3. In Figure 3, X indicates vertically arranged tube and water flows inside this tube. Y indicates corrugations on tube outer surface along its length. Z indicates the distance between adjacent corrugations also called as pitch of corrugations. Flue gas flows outside the tube (X) over corrugations (Y).
These corrugations on tubes along its length provide higher heat transfer area per unit length of the tube as compared to the bare tube type tubes. This is a water tube type

construction, wherein water flows inside the tubes (3) and dust laden hot flue gases flow outside the tubes (3).
Numbers of such corrugated tubes are arranged in one row with a common header (4) at top and a common header (4) at bottom form one cassette of the tubes. Plurality of such said cassettes are placed adjacent to each other forming a matrix of tubes. All such tube cassettes are supported on headers (4) at the top to hang freely under self-weight. The water enters the economizer (2) from bottom and exits from top of economizer from where it enters the steam drum (5). The water in the steam drum is circulated through evaporator (1) by natural circulation.
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 dust laden hot flue gases flows vertically downwards parallel to axis of the said corrugated tubes (3), along its length and heat from these flue gases is recovered by the water flowing inside the tubes in each section of heat recovery system.
Velocity of gases in the waste heat recovery section is kept lower, so that the dust from flue gases is separated by gravity. This reduces deposition of dust particles on tube surfaces.
Since tube axis is vertical and parallel to the flue gas flow, the arrangement can be referred as "Co-Flow" or "Co-Current" arrangement for the gases. The "Co-current down flow arrangement" for air and dust greatly enhances the movement of separated dust to the lower portion without accumulation or deposition in system, minimizing / eliminating fouling or chocking. The gravitational flow of the dust also eliminates additional power requirement for its conveying. The free flow of dust under gravity force eliminates possibility of re-deposition of dust on tube surface.
Mechanical hammers / vibrators (6) are provided in the waste heat recovery system. These hammers / vibrators can be operated cyclically or intermittently to dislodge the dust deposited on the tube surface, if any. Dust dislodged from the said tube (3) surfaces due to hammering / vibration fall freely downwards without obstruction through uniform flue gas passage.

The embodiment 2 of the present invention as shown in figure 2, shows evaporator and economizer arranged one over another such that their tubes are horizontal with their axis perpendicular to the flue gas flow. Since the axis is horizontal and perpendicular to the flue gas flow, the arrangement can be referred as "Cross-Flow" arrangement for the gases. The Cross flow arrangement provides much improved heat transfer coefficient as compared to "Co-flow" arrangement. Additionally, the "cross flow" arrangement using corrugated tubes has higher heat transfer coefficient as compared to "cross flow" arrangement using bare tubes.
ADVANTAGES AND FEATURES OF THE PRESENT INVENTION:-
1. The corrugated tubes have higher heating area per unit length of tube as compared to bare tube. This reduces overall length of tubes and in result overall size of heat recovery heat exchanger.
2. The reduced length of tube and overall size of heat exchanger reduces cost of heat recovery system supporting structure and casing.
3. The reduced overall size of heat exchanger, result in lower erection time and in turn it reduces the project period.
4. The reduced overall size, structure, casing and erection time lowers the overall project cost and improves project feasibility.
5. Pressure drop on the flue gas side is lower as compared to cross flow construction, conserving fan power.
6. Flue gas and dislodged dust both move in the downward direction resulting in the self-cleaning process.
7. Automatic vibrators / mechanical hammers dislodge the settled powder easily.
8. Free hanging tube design under self-weight prevents bending / buckling of the tubes.
9. Vertical arrangement of corrugated tubes allows flue gas flow with free flowing gravity force driven dust cake eliminates any possibility of re-deposition of dust on the tubes. Dust deposition and fouling being avoided, heat transfer effectiveness is ensured.
10. The cross flow arrangement of corrugated tubes has improved heat transfer coefficient as compared to co-flow arrangement. The choice of cross flow and co-

flow arrangement can be made depending on the properties of dust in flue gas and dust percentage.
VARIATIONS POSSIBLE TO BE COVERED AND PROVIDED WITHIN THE PRESENT INVENTION:-
1. Variable pitch of corrugations
The pitch of corrugations on tube surface can be varied according to the level of dust loading / concentration in flue gas and nature / composition of dust.
2. Variable height of corrugation depth
The height of corrugation depth on the tube can be varied according to dust loading/ concentration and nature / composition of dust.
3. Variable corrugation thickness
The thickness of corrugation can be varied according to nature / composition of dust and temperature of flue gases.
4. Variation in corrugation geometry
The corrugation can have different type of geometry according to nature / composition of flue gases.
5. A steam super heater can be provided before evaporator to superheat the steam from steam drum, depending on further application of steam. The superheater may have same type of construction as that of evaporator or economizer wherein axis of tube is parallel to the flue gas flow as per embodiment 1 or perpendicular to the flue gas flow as per embodiment 2.
6. Water pre-heater 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 evaporator or economizer wherein axis of tube is parallel to the flue gas flow as per embodiment 1 or perpendicular to the flue gas flow as per embodiment 2.
TYPICAL APPLICATION AREAS:-
1. Heat recovery heat exchangers for dust laden exhaust gases such as those coming from cement plants (preheater tower and clinker cooler), glass melting furnaces, steel plants, and incinerator exhausts etc.

2. The said heat recovery heat exchangers can be used for driving steam rankine cycle based power generation plants and organic rankine cycle based power generation plants.