FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)
A CONDENSING HEAT EXCHANGER SYSTEM HAVING A MULTI-PASS ARRANGEMENT
THERMAX LIMITED
an Indian Company
of D-13, MIDC Industrial Area, R.D. Aga Road,
Chinchwad, Pune - 19,
Maharashtra, India
The following specification particularly describes the invention and the manner in which
it is to be performed.

FIELD OF THE INVENTION
The present invention relates to the field of condensing heat exchangers.
BACKGROUND OF THE INVENTION & PRIOR ART
Flue gases are gases that exit to the atmosphere via a flue which is a pipe or a channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. These flue gases are generally a result of combustion/burning of fossil fuels, coal or waste materials and typically have a high temperature. The steam generators in power plants and process plant burn extensive quantities of fossil fuels such as natural gas, fuel oil, coal, and waste materials; therefore emit large amounts of high temperature flue gases. The growing concerns about the global warming and limited reserves have forced industries to maximize flue gas heat recovery by heating feed water and preheating combustion air.
In a conventional flue gas heat recovery system the heat from the flue gases is recovered by passing the hot flue gases through a series of heat exchangers. Primarily, these heat exchangers generally only target the sensible heat available in the flue gases, thus, a significant portion of the latent heat of water vapor from the flue gases is lost to the atmosphere. In order to make the heat recovery systems more efficient a condensing heat exchanger is presently being provided which extracts the latent heat and the remaining sensible heat from the flue gases received therein. In a condensing heat exchanger, the flue gases are condensed and for the condensation to initiate, the temperature of water used for exchanging the heat should be below the dew point temperature which is less than 60 °C in the condensing heat exchanger. The trend of using condensing heat exchanger has increased

in the case of natural gas fired boiler and heater due to lower possibility of corrosion.
If there is no condensate recovery and sufficient make-up water at lower temperature is available, the complete heat recovery is done by using condensing heat exchanger. If the low temperature make-up water quantity is not sufficient due to process condensate recovery, heat exchanger is generally split in two sections; sensible heat exchanger to be used for heating high temperature feed water and condensing heat exchanger for low temperature make-up water. In such cases, the condensing heat exchangers are typically used for heating make-up water by recovering the left-over sensible heat from the flue gases & the latent heat of water available in the flue gas. Condensing heat exchangers for such applications are selected on the basis of make-up water requirement. The total heat recovery is the function of the requirement of make-up water. In some cases the make-up water requirement can be quite low or the temperature of make-up water is high, then the use of a condensing heat exchanger is not feasible. Since, when water quantity available is less, water temperature is high and flue temperature is low, the LMTD is less in the condensing heat exchanger and a large heat transfer area is required. To accommodate a large heat transfer area, typically, a two-pass configuration is provided with both flue gas entry & exit duct at the operative top of the heat exchanger. Cold water enters near the flue gas exit and navigates in a counter-flow direction to maximize heat recovery. A major drawback of the aforementioned arrangement is that maximum condensation takes place in the top tube bank, which causes the condensate to fall on the bottom tubes and form a liquid film over the heat transfer tube that reduces the heat transfer efficiency of the bottom tubes

significantly affecting the overall performance of the condensing heat exchanger.
A conventional waste heat recovery system using a the condensing heat exchanger typically consists of two heat recovery loops as seen in FIGURE 1, the conventional heat recovery system represented in FIGURE 1 by numeral 10. A first heat recovery loop consists of a feed water tank 34, a feed water pump 36, a control valve 40, and a sensible heat exchanger 12. The feed water pump 36 receives water from the feed water tank 34 and pumps the feed water to the sensible heat exchanger 12. The feed water receives sensible heat of the hot flue gas and is fed to the boiler 14. The control valve 40 is used to regulate the feed water quantity by sensing the boiler 14 water level.
The conventional heat recovery system 10 further comprises a second heat recovery loop consisting of a make-up water tank 28, a make-up water pump 26, a flow control valve 30, and a condensing heat exchanger 18 and feed water tank 34. Make-up water passing through the condensing heat exchanger 18 receives latent and sensible heat from the flue gas received therein from the sensible heat exchanger 12 to provide hot make-up water which is collected in the feed water tank 34. The make-up water quantity is regulated by sensing water level in the feed water tank 34. The hot flue gas exits the boiler 14 and first enters the sensible heat exchanger 12, where a significant portion of sensible heat of the flue gas is rejected to the feed water. Further, the partly cooled flue gas from the sensible heat exchanger 12 is passed through the condensing heat exchanger 18 using an induced draft fan 22. The flue gas enters the condensing heat exchanger 18 through

an inlet 24 and exits from an outlet 32 rejecting the remaining sensible heat and latent heat to the cold make-up water received therein.
The flue gas exiting at the outlet 32 is discharged through a vent 16b. The vent 16b is generally lined with corrosion-resistant material to protect the vent 16b from corrosion. As the make-up water quantity is not equal to the feed water quantity, the flue gas flow through the condensing heat exchanger 18 needs to be regulated. The flue gas quantity is regulated with the help of a damper 20 by sensing the make-up water outlet temperature. The efficiency of the conventional heat recovery system 10 is function of the make-up water quantity. If the make-up water quantity is low, only a small quantity of the flue gas is passed through the condensing heat exchanger 18. A significant quantity of the flue gas is bypassed and directly sent to a vent 16a from the sensible heat exchanger 12, as the available make-up water does not provide sufficient heat sink for the flue gas heat recovery. A significant opportunity of heat recovery is lost due to unavailability of the required heat sink. In most of the process industries a significant portion of condensate is recovered with reduction in make-up water quantity. This imposes a significant limitation on the application of condensing heat exchanger. Other major limitation of the aforementioned system is the handling of the condensed flue gas which contains water droplets and subsequently the corrosion problem associated with it. Due to the corrosion problem, the system requires two flue gas vents. Special flue gas vents lined with corrosion resistant material are required to handle the condensed flue gas. Flue gas ducts joining condensing heat exchanger and flue gas vents are also required to be lined with corrosion resistant material. This makes the overall system very costly and infeasible.

Several attempts have been made to overcome the aforementioned drawbacks of condensing heat exchanger systems; some of the related systems are discussed in the following section dealing with the prior art.
US Patent No. 4548262 discloses a condensing gas-to-gas heat exchanger apparatus disposed in a flue gas discharge conduit of a coal-burning hand fired firebox-type steam heating boiler. The heat exchanger disclosed in US 4548262 comprises of a shell and tube type heat exchanger which acts as an air preheater; hot flue gases move outwards towards the discharge ducts while cool air flow downwards, thus exchanging heat between the hot flue gases and the cool air. The system as disclosed in US 4548262 can only be used for combustion air preheating. The major drawback of the system as disclosed in US 4548262 is that the heat sink provided is insufficient to extract all the heat available in the hot flue gases, thus making the system inefficient.
US Patent No. 5368096 discloses a scrubber and a heat recovery system for treating flue gases, said system comprising of a cooling heat exchanger and a condensing heat exchanger. In the system as disclosed in US 5368096, the flue gases enter the cooling heat exchanger and travel downwards towards the condensing heat exchanger which is located below the cooling heat exchanger and is provided with a perforated tray to obtain uniform gas distribution and increased heat transfer area. Cooling water enters the system at the condensing heat exchanger and moves upwards, while gaining heat from the flue gases and exits the system at the cooling heat exchanger as hot water. The system as disclosed in US 5368096 is used only to heat water.

Also, since the heat sink available is less, a substantial amount of heat is lost to the atmosphere.
US Patent No. 5510087 discloses a two-stage downflow flue gas treatment condensing heat exchanger system for removal of contaminants and recovery of heat from flue gases formed during the combustion of waste materials, coal and other fossil fuels burned by electric power generating plants. The system as disclosed in US 5510087 comprises of two sections: a first condensing heat exchanger and a second condensing heat exchanger; both arranged vertically, one above the other. Flue gases travel downwards through the system while heating a liquid and forming a condensate. The condensate is passed through a mist eliminator to remove the moisture before exit of the flue gases. The system as disclosed in US 5510087 is not adapted to recover the latent heat from the flue gases, thus, sufficient heat sink is not provided. Also, the mist eliminator might not be able to completely eliminate the moisture from the flue gases; this can cause corrosion problems due to condensation in the vent.
US Patent No. 5607011 discloses a condensing heat exchanger for removing acidic vapors and recovering waste heat energy from flue gases released from a fossil fuel fired boiler. The heat is recovered from the flue gases in the apparatus as disclosed in US 5607011 by providing multiple evaporative and condensing heat exchangers to cool and condense the water vapor contained in the flue gases and subsequently recovers the heat energy which is used to preheat the combustion air and the feed water fed to the boiler. The apparatus as disclosed in US 5607011 is adapted to recover the sensible and the latent heat from the flue gases, however, the apparatus is complex

and requires multiple heat exchanging units for the purpose of preheating air and water. Each heat exchanging unit requires a separate circulation pump and evaporating and condensing heat exchange coil. Also, since water is heated in a low temperature zone of the condensing heat exchangers, a heat pump is to be provided for the recovery of low temperature heat. The drawback of the apparatus as disclosed in US 5607011 is the complexity and high capital and operating costs.
The condensing heat exchanger systems as discussed in the prior art provide insufficient heat recovery, have a low efficiency and high wastage of energy through disposal of hot flue gases, cause corrosion problems in the carry lines, are complex and involve high capital and operating costs. Therefore, there is felt a need for a condensing heat exchanger system, which will overcome the drawbacks of the existing systems discussed in the prior art.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a condensing heat exchanger system.
Another object of the present invention is to provide a condensing heat exchanger system for extracting sensible and latent heat from flue gases.
Still another object of the present invention is to provide a condensing heat exchanger system that is effective and highly efficient in extracting optimum amount of heat from the flue gas received therein.

Yet another object of the present invention is to provide a heated fluid to be used in a boiler to reduce the energy consumption by the boiler.
One more object of the present invention is to provide a condensing heat exchanger system which is corrosion-resistant.
Yet one more object of the present invention is to provide a condensing heat exchanger system with automation means for easy and efficient operation.
Still one more object of the present invention is to maximize the heat recovery from flue gases by providing a sufficient heat sink.
SUMMARY OF THE INVENTION
In accordance with the present invention, is provided a condensing heat exchanger system for extracting sensible and latent heat from flue gases, said condensing heat exchanger system comprising:
a) a condensing heat exchanger having a multi-pass arrangement, the condensing heat exchanger comprising: • an insulated casing having a plurality of chambers, at least one of the plurality of chambers having a plurality of heat exchanger coils adapted to receive a fluid, and a plurality of vertical baffles adapted to partition the plurality of chambers, the insulated casing comprising:
■ a flue gas inlet adapted to receive hot flue gases;
■ a first chamber adapted to accommodate a first heat exchanger coil adapted to extract and provide sensible heat from the hot flue gases to a heated fluid passed

through the first heat exchanger coil, to provide a hot fluid for use in process cycle;
■ a second chamber adapted to accommodate a second heat exchanger coil adapted to extract and provide sensible and latent heat from the heat-extracted non-condensed flue gases from the first chamber to a warm fluid passed through the second heat exchanger coil, to provide the heated fluid which is received in the first heat exchange coil;
■ a third chamber adapted to accommodate a third heat exchanger coil adapted to extract and provide the latent heat from the heat-extracted partially condensed flue gases from the second chamber to a fluid passed through the third heat exchanger coil, to provide the warm fluid passed which is received in the second heat exchange coil; and
■ a flue gas outlet adapted to release cooled flue gases from the third chamber;
a plurality of interconnecting headers adapted to connect the heat exchanger coils of one of the plurality of chambers to the heat exchanger coils of another of the plurality of chambers;
an inlet header provided with a fluid inlet and adapted to transfer the fluid in the third heat exchanger coil, wherein the heat exchanger coils of the third chamber is connected to the heat exchanger coils of the second

chamber by means of one of the interconnecting header; and
an outlet header provided with a fluid outlet and adapted to transfer the fluid from the heat exchanger coils to the fluid outlet, wherein the heat exchanger coils of the first chamber is connected to the heat exchanger coils of the second chamber by means of one of the interconnecting header;
b) a conduit means adapted to cany the cooled flue gases discharged through the flue gas outlet; and
c) a reheater provided along the conduit means to receive the cooled flue gases and provide dry heated flue gases.
Typically, in accordance with the present invention, a mist eliminator is provided along the conduit means to receive the cooled flue gases and provide moisture-free cooled flue gases.
Preferably, in accordance with the present invention, the reheater is provided downstream of the mist eliminator along the conduit means.
Typically, in accordance with the present invention, a drain is provided at the operative bottom of the condensing heat exchanger to collect and discharge the condensate.
Preferably, in accordance with the present invention, hot flue gases from a boiler are received through an induced draft fan in the flue gas inlet of the condensing heat exchanger.

Additionally, in accordance with the present invention, the plurality of vertical baffles are metallic having a flow passage for the flue gases.
Typically, in accordance with the present invention, the hot fluid from the condensing heat exchanger is fed to the boiler.
In accordance with the present invention, the dry heated flue gases from the reheater are discharged through a common vent with remaining flue gases.
Typically, in accordance with the present invention, hot flue gases are received in the reheater before entering the condensing heat exchanger.
In accordance with the present invention, is provided a method of recovering heat from flue gases for the purpose of heating a fluid using a condensing heat exchanger system comprising a condensing heat exchanger having a multi-pass arrangement, the method comprising the following steps:
(i) receiving hot flue gases in a first chamber of the condensing
heat exchanger; (ii) extracting and providing sensible heat from the hot flue gases to a heated fluid in a first heat exchanger coil in the first chamber, providing a hot fluid for use in process cycle and heat-extracted non-condensed flue gases; (iii) receiving the heat-extracted non-condensed flue gases from the
first chamber in a second chamber; (iv) extracting and providing sensible and latent heat from the heat-extracted non-condensed flue gases to a warm fluid in a second heat exchanger coil in the second chamber, providing a heated

fluid which is sent to the first heat exchanger coil and heat-extracted partially condensed flue gases;
(v) receiving the heat-extracted partially condensed flue gases from the second chamber in a third chamber;
(vi) extracting and providing latent heat from the heat-extracted partially condensed flue gases to a fluid in a third heat exchanger coil in the third chamber, providing a warm fluid which is sent to the second heat exchanger coil and cooled flue gases;
(vii) removing moisture from the cooled flue gases leaving the third chamber in a mist eliminator for providing moisture-free cooled flue gases; and
(viii) heating the moisture-free cooled flue gases in a reheater for providing dry heated flue gases which are discharged through a vent.
Typically, in accordance with the present invention, the method of recovering heat from flue gases for the purpose of heating a fluid includes the step of heating the moisture-free cooled flue gases in the reheater by using heat from the hot flue gases which are passed through the reheater before entering the condensing heat exchanger.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with reference to the accompanying drawings, in which;

FIGURE 1 illustrates an embodiment of the conventional flue gas heat recovery system for heating water;
FIGURE 2 illustrates a schematic of the condensing heat exchanger showing the heat exchanger coils and the flue gas path, in accordance with the present invention;
FIGURE 3 illustrates a schematic of the condensing heat exchanger showing the plurality of vertical baffles, in accordance with the present invention;
FIGURE 4 illustrates a schematic of the condensing heat exchanger system, in accordance with the present invention; and
FIGURE 5 illustrates a schematic of a flue gas heat recovery system using the condensing heat exchanger system in accordance with the present invention.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with reference to the accompanying drawings which do not limit the scope and ambit of the invention. The description provided is purely by way of example and illustration.
The present invention envisages a condensing heat exchanger system for extracting sensible and latent heat from hot flue gases. The condensing heat exchanger system comprises a condensing heat exchanger having a multi-

pass arrangement for heating a fluid, typically, boiler make-up water or feed water by extracting heat from the flue gases received therein, a conduit means for carrying the cooled flue gases leaving the condensing heat exchanger, and a reheater for drying and heating the cooled flue gases before disposal through a vent. Hot flue gases, typically flue gases released from a boiler or sensible heat exchanger, are received in the condensing heat exchanger, where the flue gases are cooled and the heat therein is utilized to heat a fluid, typically, boiler make-up water or feed water. The heated water is supplied to the boiler for vaporizing. Thus, the energy or fuel consumption by the boiler is substantially reduced. Further, the condensing heat exchanger of the present invention aims at providing a heat exchanger for maximizing the amount of heat recovered from the flue gases received therein. Also, the condensing heat exchanger of the present invention is designed to work efficiently when the fluid quantity available is low or the fluid temperature is high. The present invention provides a large heat transfer area by providing a three-pass arrangement in the condensing heat exchanger; a large heat transfer area is required if the fluid quantity available is less or fluid temperature is high because the LMTD in such cases is less. The condensing heat exchanger incorporates serpentine tubes for carrying the fluid which reduce the size of the condensing heat exchanger and make it compact while allowing a large heat transfer area.
In accordance with a preferred embodiment of the present invention is provided a condensing heat exchanger for heating a fluid, typically, boiler make-up water or feed water. The condensing heat exchanger comprises a multi-pass arrangement which is adapted to prevent the flue gas water vapor condensate from falling on heat exchanger tubes, thus minimizing formation

of a liquid film on the heat exchanger tubes, providing a high efficiency heat recovery system at all times. The liquid film over the heat transfer tubes reduces thermal performance of the condensing heat exchanger, as the heat transfer coefficient for film condensation is less than the drop wise condensation. Referring to FIGURE 2 & FIGURE 3, therein is disclosed a condensing heat exchanger showing the heat exchanger coils and the flue gas path, generally represented in FIGURE 2 & 3 by numeral 100. Hot flue gases containing about 15 - 20 % moisture and 10 — 20 ppm solid particles and having a temperature in the range of 120 - 260 °C enter the condensing heat exchanger 100 through an induced draft fan (shown in FIGURE 4 as numeral 150) at the flue gas inlet 102. The condensing heat exchanger is encompassed in an insulated casing 104 to prevent any heat loss to the surrounding. The insulated casing 104 comprises a plurality of chambers, at least one of the plurality chambers having a plurality of heat exchanger coils adapted to receive the fluid, and a plurality of vertical baffles adapted to partition the plurality of chambers. The hot flue gases enter the insulated casing 104 at the flue gas inlet 102 and are directed towards a first chamber 106 through a duct 108. The first chamber 106 is adapted to accommodate a first heat exchanger coil 113 and further adapted to extract and provide the sensible heat from the hot flue gases to the fluid to provide a hot fluid to be fed to the boiler (represented in FIGURE 5 by numeral 504).
Cold fluid having temperature in the range of 20 - 40 °C enters the heat exchanger coils of the condensing heat exchanger 100 at a fluid inlet 109 of a bottom interconnecting header 110 provided at the operative bottom of the condensing heat exchanger 100. The bottom interconnecting header 110 is adapted to transfer the fluid through the heat exchanger coils, wherein the

heat exchanger coils of the plurality of chambers are interconnected by means of the bottom interconnecting header 110. The heat exchanger coils of the condensing heat exchanger 100 are serpentine in order to incorporate a large heat transfer area. The cold fluid traverses from a third chamber 132 to the first chamber 106, while the flue gases traverse from the first chamber 106 and subsequently proceed to the third chamber 132 via a second chamber 118.
Hot flue gases entering the first chamber 106 through the duct 108 traverse downwards, shown by a direction arrow 112, exchanging heat with heated fluid having temperature in the range 55 - 70 °C which flows in the first serpentine heat exchanger coil 113 in the counter-wise direction, of the first chamber 106 of the condensing heat exchanger 100. The hot flue gases traverse towards the operative bottom of the first chamber 106 after exchanging heat with the heated fluid received therein. The hot flue gases do not condense in the first chamber 106 and only sensible heat is extracted from the flue gases in the first chamber 106 as the heated fluid temperature in the first chamber is higher than water vapor dew point temperature of the flue gas. Thus, heat extracted non-condensed flue gases and hot fluid is obtained after passing the flue gases through the first chamber 106. The plurality of vertical metallic baffles provided partition the insulated casing 104 into the various chambers. A first vertical metallic baffle 114 is provided to separate the first chamber 106 from the second chamber 118. Hot fluid having temperature in the range of 70 - 95 °C is obtained from a fluid outlet 120 of an outlet header 116 located at the operative top for discharging the hot fluid from the first chamber 106 of the condensing heat exchanger 100. The outlet header 116 is adapted to discharge the hot fluid from the

condensing heat exchanger coils, wherein the heat exchanger coils of the plurality of chambers are interconnected by means of a top interconnecting header 156 and the bottom interconnecting header 110. The hot fluid obtained from the outlet header 116 is fed to the boiler. This provides substantial fuel and energy savings in the boiler. Since, no condensation happens in the first chamber 106; a lower-grade material can be used for the serpentine coils 113, thus reducing the cost of the condensing heat exchanger 100.
The heat-extracted non-condensed flue gases having temperature in the range of 70-100 °C exit the first chamber 106 near the operative bottom of the condensing heat exchanger 100 through an opening 122 provided towards the bottom of the vertical baffle 114 and are received in the second chamber 118. The plurality of vertical baffles are provided with a flow passage for the flue gases. The heat-extracted non-condensed flue gases after losing sensible heat in first serpentine heat exchanger coil 113 enter the second chamber 118 from the operative bottom and traverse upwards, shown by a direction arrow 124, through a second serpentine heat exchanger coil 126 provided in the second chamber 118. In the second chamber 118, sensible and latent heat from the heat-extracted non condensed flue gases is extracted to a warm fluid. The warm fluid having temperature in the range of 45 - 55 °C is passed through the serpentine coil 126 in the counter-wise direction. The heat-extracted non condensed flue gases further lose heat to the warm fluid flowing through the serpentine coils 126, producing heated fluid having temperature in the range of 55 - 70 °C which is received in the first heat exchanger coil 113 of the first chamber 106 and heat-extracted partially condensed flue gases at relatively lower temperature is received in

the third chamber 132. A drain 128 is provided at the operative bottom of the condensing heat exchanger 100 to collect and discharge the condensate produced during the process.
A second vertical metallic baffle 130 is provided to separate the second chamber 118 from a third chamber 132. The heat-extracted partially condensed flue gases from the second chamber 118 enter the third chamber 132 through an opening 134 provided at the operative top of the vertical baffle 130. In the third chamber 132, a third serpentine heat exchanger coil 136 are provided to extract the remaining latent heat from the heat-extracted partially condensed flue gases received therein. The cold fluid having temperature in the range of 20 - 40 °C is received through the serpentine coils 136 of the third chamber 132 of the condensing heat exchanger 100 through the fluid inlet 109 of the inlet header 154, The heat-extracted partially condensed flue gases enter the third chamber 132 at the opening 134 and traverse downwards, shown by a direction arrow 138, while the cold fluid received in the serpentine coils 136 travels in a counter-wise direction. The heat-extracted partially condensed flue gases lose the latent heat to the cold fluid flowing through the serpentine coils 136, to provide the warm fluid which is received in the second heat exchanger coil 126 of the second chamber 118. Maximum condensation of the flue gases happens towards the lower section of the third chamber 132. The condensate is received in the drain 128 of the condensing heat exchanger 100, which is discharged from the drain 128 and subsequently disposed or reused.
Due to condensation in the third chamber 132, a thin moisture-film is formed on the serpentine coils 136, which might act as an insulator affecting the heat

transfer from the flue gases in the water. However, the moisture-film formed in the condensing heat exchanger 100 of the present invention is minimal and mostly present in the lower section of the third chamber 132, which has a minimal effect on the efficiency of the condensing heat exchanger 100. Also, since minimum condensation occurs in the first chamber 106 and the second chamber 118, no condensate falls or moisture-film forms on the serpentine coils 113 & 126, respectively. Thus, the condensing heat exchanger 100 of the present invention overcomes one of the major drawbacks of the existing condensing heat exchangers.
In an alternative embodiment of a condensing heat exchanger with two-pass and top entry of the flue gas, maximum condensation occurs at the top-section of the condensing heat exchanger and the condensate thus formed falls at the bottom section of heat exchanger coils creating a liquid film over the heat transfer surface and adversely affecting the heat transfer performance of the system. In a single-pass condensing heat exchanger with top entry of the flue gas, maximum condensation takes place at the bottom most section of the heat exchanger, however, the condensation starts at the intermediate section of heat exchanger and adversely affects the heat transfer performance of the bottom most section of the heat exchanger coil.
The heat-extracted partially condensed flue gases exchange heat with fluid flowing through the serpentine coils 136 and become cooled flue gases having temperature in the range of 45 - 65 °C while generating the warm fluid having temperature in the range of 45 - 55 °C. The warm fluid from the serpentine coils 136 enters the serpentine coils 126 through the interconnecting top header 156 where it becomes heated to a temperature

between 55 - 70 °C by extracting heat from the flue gases and enters the serpentine coils 113 through the interconnecting bottom header 110 where it is heated to a temperature between 65 - 95 °C and exits from the outlet header 116 through the fluid outlet 120. The hot fluid exiting through the fluid outlet 120 is used in the boiler to reduce fuel/energy consumption by the boiler.
The hot flue gases having temperature between 120 - 200 °C enter the first chamber 106 of the condensing heat exchanger 100 through the duct 108 and lose heat during the passage through the second chamber 118 and the third chamber 132. After passing through the third chamber 132, the cooled flue gases having temperature between 45 - 65 °C exit the condensing heat exchanger 100 at a flue gas outlet 142. FIGURE 4 shows a schematic of the condensing heat exchanger system, in accordance with the present invention. The condensing heat exchanger system is generally referred in FIGURE 4 by numeral 300.
The cooled flue gases leaving through the flue gas outlet 142 contain substantial quantity of moisture and cannot be directly sent to a stack or vent, since the moisture will cause corrosion problems in the stack or vent. The cooled flue gases discharged through the flue gas outlet 142 are carried through a conduit means 152. The reheater 148 is provided along the conduit means 152 to provide dry flue gases which can be conveniently disposed through a stack (represented in FIGURE 5 by numeral 502). A mist eliminator 144 is also provided along the conduit means 152 for removing moisture from the cooled flue gases to provide moisture-free cooled flue gases. The reheater 148 is provided downstream to the mist eliminator 144

along the conduit means 152 and is adapted to receive the moisture-free cooled flue gases from the mist eliminator 144 and provide the dry cooled flue gases. The mist eliminator 144 comprises of a mesh, typically wire mesh, which separates the water droplets or moisture in the cooled flue gases received therein from the flue gas outlet 142 to provide moisture-free cooled flue gases. A drain 146 is provided at the operative bottom of the mist eliminator 144 to remove the moisture collected therein from the mist eliminator 144. Even after passing through the mist eliminator 144, the moisture-free cooled flue gases can carry miniscule amount of moisture. The reheater 148 reheats the moisture-free cooled flue gases from the mist eliminator 144 to provide the dry cooled flue gases which can be conveniently discharged through the stack. The reheater 148 utilizes heat from the hot flue gases entering the condensing heat exchanger 100 at the flue gas inlet 102. As shown in FIGURE 4, hot flue gases are received through an induced draft fan 150 in the reheater 148, where the heat from the flue gases is used to heat and dry the moisture-free cooled flue gases from the mist eliminator 144 to provide the dry cooled flue gases which are discharged through the vent (represented in FIGURE 5 by numeral 502) and the hot flue gases are fed to the condensing heat exchanger 100 through the flue gas inlet 102. The present system 300 can be conveniently used for clean flue gases with lower level of sulfur contents primarily produced from the combustion of clean gaseous fuels like natural gas and liquefied petroleum gas.
The condensing heat exchanger system 300 of the present invention provides a suitable design of condensing heat exchanger for maximizing sensible and latent heat recovery from the hot flue gases, improves the performance of

condensing heat exchanger by reducing the thickness of a liquid film on the heat exchanger coils, minimizes corrosion problems in the heat exchanger by using appropriate material for the different heat exchanger coils depending on their corrosion expectation and the vent by removing moisture from the cooled flue gases before disposal by using mist eliminator and reheater.
FIGURE 5 discloses a flue gas heat recovery system using the condensing heat exchanger system 300 of the present invention in addition to a sensible heat exchanger. The flue gas heat recovery system referred by numeral 500 in FIGURE 5 comprising the condensing heat exchanger 100 and a sensible heat exchanger 506 is used for heating water and is designed to maximize heat recovery. The condensing heat recovery loop consists of a make-up water pump 516, a control valve 518, the condensing heat exchanger 100, a make-up water tank 514 and feed water tank 508. The circulating pump 516 circulates water through the control valve 518 to the condensing heat exchanger 100, where the make-up water receives heat from the flue gases. The circulating pump 516 receives the make-up water from the make-up water tank 514.
Heated water from the condensing heat exchanger 100 is received in a feed water tank 508 and is regulated using a flow control valve 518 by sensing water level in feed water tank 508. The heated water is pumped to the sensible heat exchanger 506 from the feed water tank 508 by a feed pump 512. The feed water flow through sensible heat exchanger 506 is regulated by using flow control valve 510. Flue gases from the boiler 504 first enter the sensible heat exchanger 506 and reject a significant portion of sensible heat in the sensible heat exchanger 506 to further heat the heated water

which is subsequently used as boiler feed water in the boiler 504. The partly cooled flue gas leaving the sensible heat exchanger 506 are received in the condensing heat exchanger 100 through the induced draft fan 150 and a damper 520. Also, the reheater 148 is located in-line with the induced draft fan 150 to receive the flue gases for heating and drying the cooled flue gases received therein from the condensing heat exchanger 100 through the conduit means 152. In the condensing heat exchanger 100, the latent heat and the sensible heat from the flue gases is extracted to heat the make-up water received therein. If the make-up water quantity does not provide sufficient heat sink for the cooling and condensation of complete flue gas, flue gas quantity in the condensing heat exchanger 100 is regulated. This is done by using the damper 520. Flue gases are fed to the condensing heat exchanger 100 using the induced draft fan 150. Flue gases after losing heat in the condensing heat exchanger 100 are carried by the conduit means 152 and discharged through the vent 502. The cooled flue gases leaving the condensing heat exchanger 100 are passed through the mist eliminator 146 and the reheater 148 to completely remove any moisture therein to prevent corrosion in the vent 502. In contrary to the conventional system 10 (as discussed in the prior art), the present system does not require two independent vents for discharging the usual flue gases and the flue gases exiting the condensed heat exchanger 100.

TRAIL RESULTS
Hot flue gases leaving a six-ton boiler at 140 °C are received in the condensing heat exchanger 100 of the present invention. The inlet and outlet water and flue-gas temperature through the various passes in provided in the following TABLE 1:

Parameters First Pass Second Pass Third Pass Units
Flue gas inlet temperature 140 79.5 63.2 °C
Flue gas outlet temperature 79.5 63.2 63.2 °C
Estimated heat recovery (Sensible) 122985.2 32518.5 0.0 Kcal/Hr
Estimated heat recovery (Latent) 0.0 19793.1 148429.6 Kcal/Hr
Estimated heat recovery (Total) 122985.2 52311.6 148429.6 Kcal/Hr
Water inlet temp. 63.5 54.9 30.0 °C
Water outlet temp. 83.6 63.5 54.9 °C
From TABLE 1 it is observed that 38 % of heat of the hot flue gases, which is only sensible heat, is recovered in the first chamber, while, 16 % of heat (sensible and latent heat both) from the hot flue gases is recovered in the second chamber. However, most of the heat transfer occurs in the third chamber, where 46 % of latent heat from the flue gases is extracted. The flue gases exited the condensing heat exchanger 100 at 63.2 °C and hot water at 83.6 °C was obtained at the outlet.

TECHNICAL ADVANCEMENTS
A condensing heat exchanger system for extracting sensible and latent heat from hot flue gases, in accordance with the present invention has several technical advantages including but not limited to the realization of:
• the condensing heat exchanger comprises a multi-pass arrangement for maximizing heat recovery;
• the condensing heat exchanger system is effective and highly efficient in extracting optimum amount of heat from the flue gas received therein;
• the hot fluid mainly make-up water and feed water exiting the condensing heat exchanger can be used in a boiler to obtain substantial fuel savings in the boiler;
• no condensation occurs in the first chamber of the condensing heat exchanger, which eliminates possibility of corrosion due to water vapor condensation and provides flexibility in material selection;
• the condensing heat exchanger is made from corrosion-resistant material and further the condensing heat exchanger system is provided with a mist eliminator & a reheater to minimize any possibility of corrosion in flue gas vent; and
• the condensing heat exchanger system is provided with automation means for easy and efficient operation.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.

in view 01 the ware variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We Claim:
1. A condensing heat exchanger system for extracting sensible and latent heat from flue gases, said condensing heat exchanger system comprising:
a) a condensing heat exchanger having a multi-pass arrangement, the condensing heat exchanger comprising: • an insulated casing having a plurality of chambers, at least one of the plurality of chambers having a plurality of heat exchanger coils adapted to receive a fluid, and a plurality of vertical baffles adapted to partition the plurality of chambers, the insulated casing comprising:
■ a flue gas inlet adapted to receive hot flue gases;
■ a first chamber adapted to accommodate a first heat exchanger coil adapted to extract and provide sensible heat from the hot flue gases to a heated fluid passed through the first heat exchanger coil, to provide a hot fluid for use in process cycle;
* a second chamber adapted to accommodate a second heat exchanger coil adapted to extract and provide sensible and latent heat from the heat-extracted non-condensed flue gases from the first chamber to a warm fluid passed through the second heat exchanger coil, to provide the heated fluid which is received in the first heat exchange coil;
■ a third chamber adapted to accommodate a third heat
exchanger coil adapted to extract and provide the
latent heat from the heat-extracted partially condensed

flue gases from the second chamber to a fluid passed through the third heat exchanger coil, to provide the warm fluid passed which is received in the second heat exchange coil; and ■ a flue gas outlet adapted to release cooled flue gases
from the third chamber; a plurality of interconnecting headers adapted to connect the heat exchanger coils of one of the plurality of chambers to the heat exchanger coils of another of the plurality of chambers;
an inlet header provided with a fluid inlet and adapted to transfer the fluid in the third heat exchanger coil, wherein the heat exchanger coils of the third chamber is connected to the heat exchanger coils of the second chamber by means of one of the interconnecting header; and
an outlet header provided with a fluid outlet and adapted to transfer the fluid from the heat exchanger coils to the fluid outlet, wherein the heat exchanger coils of the first chamber is connected to the heat exchanger coils of the second chamber by means of one of the interconnecting header; a conduit means adapted to carry the cooled flue gases discharged through the flue gas outlet; and a reheater provided along the conduit means to receive the cooled flue gases and provide dry heated flue gases.

2. The condensing heat exchanger system as claimed in claim 1, wherein a mist eliminator is provided along the conduit means to receive the cooled flue gases and provide moisture-free cooled flue gases.
3. The condensing heat exchanger system as claimed in claim 1 & 2, wherein the reheater is provided downstream of the mist eliminator along the conduit means.
4. The condensing heat exchanger system as claimed in claim 1, wherein a drain is provided at the operative bottom of the condensing heat exchanger to collect and discharge the condensate.
5. The condensing heat exchanger system as claimed in claim 1, wherein hot flue gases from a boiler are received through an induced draft fan in the flue gas inlet of the condensing heat exchanger.
6. The condensing heat exchanger system as claimed in claim 1, wherein the plurality of vertical baffles are metallic having a flow passage for the flue gases.
7. The condensing heat exchanger system as claimed in claim 1, wherein the hot fluid from the condensing heat exchanger is fed to the boiler.
8. The condensing heat exchanger system as claimed in claim 1, wherein the dry heated flue gases from the reheater are discharged through a common vent with remaining flue gas.

9. The condensing heat exchanger system as claimed in claim 1, wherein hot flue gases are received in the reheater before entering the condensing heat exchanger.
10. A method of recovering heat from flue gases for the purpose of heating a fluid using a condensing heat exchanger system comprising a condensing heat exchanger having a multi-pass arrangement, the method comprising the following steps:
(i) receiving hot flue gases in a first chamber of the
condensing heat exchanger; (ii) extracting and providing sensible heat from the hot flue gases to a heated fluid in a first heat exchanger coil in the first chamber, providing a hot fluid for use in process cycle and heat-extracted non-condensed flue gases; (iii) receiving the heat-extracted non-condensed flue gases
from the first chamber in a second chamber; (iv) extracting and providing sensible and latent heat from the heat-extracted non-condensed flue gases to a warm fluid in a second heat exchanger coil in the second chamber, providing a heated fluid which is sent to the first heat exchanger coil and heat-extracted partially condensed flue gases; (v) receiving the heat-extracted partially condensed flue gases
from the second chamber in a third chamber; (vi) extracting and providing latent heat from the heat-extracted partially condensed flue gases to a fluid in a third heat exchanger coil in the third chamber, providing a

warm fluid which is sent to the second heat exchanger coil and cooled flue gases;
(vii) removing moisture from the cooled flue gases leaving the third chamber in a mist eliminator for providing moisture-free cooled flue gases; and
(viii) heating the moisture-free cooled flue gases in a reheater for providing dry heated flue gases which are discharged through a vent.
11. The method of recovering heat from flue gases as claimed in claim 10, which includes the step of heating the moisture-free cooled flue gases in the reheater by using heat from hot flue gases which are passed through the reheater before entering the condensing heat exchanger.