FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)
A FLUE GAS HEAT RECOVERY SYSTEM USING A CONDENSING
HEAT EXCHANGER
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 heat recovery systems. Particularly, the present invention relates to the field of flue gas heat recovery systems.
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 large power plants and the process furnaces in large refineries, petrochemical and chemical industries and incinerators burn extensive quantities of fossil fuels such as natural gas, fuel oil, coal and waste materials and therefore emit large amounts of high temperature flue gases. The limiting reserves of the fossil fuel and the growing concerns about energy conservation have forced industries to recover the heat released through the flue gases and utilize it for preheating combustion air or feed water; to be used in the boiler, furnace or steam generator.
In a conventional flue gas heat recovery system the heat from the flue gases is recovered by passing the hot flue gases through heat exchangers. 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. Recently, a condensing heat exchanger is being adapted to extract the latent heat and the remaining sensible heat from the flue gases. In a condensing heat exchanger, the flue gases are

condensed and for the condensation reaction 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. Most of the modern flue gas heat recovery systems consist of a condensing heat recovery system wherein the feed water temperature is considerably high. This imposes a limitation over the applications using condensing heat exchanger.
Some of the flue gas heat recovery systems use two heat exchangers. Hot flue gas first enters a sensible heat exchanger, where a significant portion of flue gas sensible heat is recovered by boiler feed water. The condensing heat exchanger is used for heating the make-up water by recovering the remaining sensible heat from the flue gas & 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 and the use of a condensing heat exchanger is not feasible.
Several attempts have been made to design flue gas heat recovery systems using a condensing heat exchanger. These systems are either used for feed water heating or combustion air preheating. Some of the related systems are discussed in the following section dealing with the prior art.
US patent application 4548262 presents a scheme of condensing heat exchanger for transferring heat between gaseous fluid having condensable gases. This scheme can be used for combustion air preheating. But the basic

limitation of this scheme lies in the fact that the combustion air does not provide the sufficient heat sink for the flue gas condensation and only small portion of the flue gas heat can be recovered by using this scheme.
US Patent No. 5368096 discloses a scrubber and a heat recovery system for treating flue gases, said system comprising of a flue gas 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. The heat recovery performance of this system is the function of make up water availability. In most of the application, sufficient make up water is not available due to condensate recycling and imposes limitation on heat recovery.
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.

US 4548262 discloses a combustion air preheating scheme using a flue gas condensing heat exchanger but the combustion air does not provide the sufficient heat sink to maximize heat recovery. US 5368096 & US 5510087 disclose a make-up water heating system using a flue gas condensing heat exchanger. As most of the modern boiler systems are based on condensate recycling and have limited amount of make-up water quantity, a condensing heat exchanger based on make-up water heating systems as discussed in the US 5368096 & US 5510087 have several limitations and demonstrate poor performance.
US Patent No. 5607011 presents a closed heat exchanging circuit to cool and condense water vapor contained in the flue gas to recover heat energy to preheat the boiler combustion air streams and cold feed water. The system is quite complex and requires multiple close heat exchanging circuit for the combustion air preheating. Each closed circuit requires circulation pump & evaporator & condenser coil. Heat is received from the flue gas at the evaporator coil and rejected to the combustion air at the condenser coil. This also has feed water heating cycle place in the lowest temperature zone of the condensing heat exchanger, which requires heat pump for the recovery of low temperature heat. The major limitation of this scheme is it complexity as it requires multiple heat exchanging circuit, circulation pumps & heat pump.
Therefore, there is felt a need for a flue gas heat recovery system, which will overcome the drawbacks of the existing systems discussed in the prior art.

OBJECTS OF THE INVENTION
Art object of the present invention is to provide a flue gas heat recovery system using a condensing heat exchanger.
Another object of the present invention is to provide a flue gas heat recovery system which effectively and efficiently extracts the optimum amount of heat from the flue gases received therein.
Still another object of the present invention is to provide optimum heat recovery also with limited make-up water availability and to provide a sufficient heat sink for heat recovery to maximize flue gas heat recovery.
Yet another object of the present invention is to provide a flue gas heat recovery system which can be used for heating a plurality of fluids.
One more object of the present invention is to provide a control loop to distribute the heat recovery among the various heat sinks to maximize heat recovery in variable operating condition.
SUMMARY OF THE INVENTION
In accordance with the present invention, is provided a flue gas heat recovery system using a condensing heat exchanger for simultaneously heating water and air, said system comprising :
• a sensible heat exchanger has a first inlet for hot flue gases, a second inlet for pre-heated feed water, means to extract heat from the hot flue gases to the pre-heated feed water, a first

outlet for partly cooled flue gases, and a second outlet for heated feed water;
• a condensing heat exchanger having a first inlet for at least a portion of the partly cooled flue gases, a second inlet for cold water to be used in said system as circulating water and make-up water, means to extract heat from the partly cooled flue gases to the cold water to generate heated water to be used in said system as heated circulating water and heated make-up water, a first outlet for cooled flue gases, and a second outlet for the heated water;
• an air preheater having a first inlet to receive the heated circulating water, a second inlet for cold air, means to pre-heat the cold air by extracting heat from the heated circulating water, a first outlet for the circulating water to be fed to the condensing heat exchanger, and a second outlet means for pre-heated air;
• a first storage tank operatively connected to the sensible heat exchanger and adapted to store the pre-heated feed water;
• a second storage tank operatively connected to the condensing heat exchanger and adapted to store the cold water;
• a first conduit means to connect the first outlet of the sensible heat exchanger to the first inlet of the condensing heat exchanger;
• a second conduit means to connect the second outlet of the condensing heat exchanger to an inlet of the first storage tank to provide the heated make-up water as the pre-heated feed water;
• a third conduit means to connect an outlet of the first storage tank to the second inlet of the sensible heat exchanger to

provide the pre-heated feed water to the sensible heat exchanger;
• a bypass means provided along the second conduit means to bypass a portion of the circulating heated make-up water to the first inlet of the air pre-heater;
• a fourth conduit means to connect the first outlet of the air-preheater to the second inlet of the condensing heat exchanger;
• a fifth conduit means to connect the first outlet of the sensible heat exchanger to a first vent; and
• a sixth conduit means to connect the first outlet of the condensing heat exchanger to a second vent.
Typically, in accordance with the present invention, an induced draft fan is provided along the first conduit means.
Preferably, in accordance with the present invention, a damper is provided along the first conduit means.
Typically, in accordance with the present invention, a feed pump is provided along the third conduit means to pump the pre-heated feed water from the first storage tank to the second inlet of the sensible heat exchanger.
Additionally, in accordance with the present invention, a circulating pump is provided to pump the cold water comprising the make-up water from the second storage tank and the circulating water from the air preheater to the second inlet of the condensing heat exchanger.

Typically, in accordance with the present invention, a first control valve is provided along the third conduit means to control the flow rate of the pre-heated feed water to the second inlet of the sensible heat exchanger.
Preferably, in accordance with the present invention, a second control valve is provided along the second conduit means to control the flow rate of the pre-heated feed water to the first storage tank on the basis of the water level in the first storage tank.
Additionally, in accordance with the present invention, a third control valve is provided along the bypass means to control the flow rate of the heated circulating water to the first inlet of the air preheater.
In accordance with the present invention, is provided a method for recovering heat from flue gases in a flue gas heat recovery system, said method comprising the following steps:
i. extracting heat from hot flue gases in a sensible heat exchanger for heating pre-heated feed water, to generate partly cooled flue gases and heated feed water; ii. regulating flow rate of at least a portion of the partly cooled flue gases carried through a first conduit means from the sensible heat exchanger to a condensing heat exchanger by using a damper and an induced draft fan provided along the first conduit means; iii. extracting heat from the partly cooled flue gases, in cold water to be used in said system as circulating water and make-up water, in the condensing heat exchanger, to provide heated water to be used in

said system as heated circulating water and heated make-up water,
and discharge cooled flue gases; iv. regulating the quantity of the heated make-up water carried through
a second conduit means to a first storage tank as the pre-heated feed
water and to an air preheater as the heated circulating water through
a bypass means; v. extracting heat from the heated circulating water in the air heater for
pre-heating cold air received therein, to provide pre-heated air and
the circulating water; vi. recycling the circulating water from the air preheater to the
condensing heat exchanger through a fourth conduit means; vii. discharging a portion of the partly cooled flue gases from the
sensible heat exchanger through a fifth conduit means, if the
available make-up water and cold air does not provide sufficient
heat sink for the heat recovery; and viii. discharging the cooled flue gases from the condensing heat
exchanger through a sixth conduit means.
Typically, in accordance with the present invention, the method for recovering heat from flue gases in a flue gas heat recovery system includes the step of controlling the flow rate of the heated circulating water to the air preheater depending on the circulating water temperature at the outlet of air preheater.
Additionally, in accordance with the present invention, the method for recovering heat from flue gases in a flue gas heat recovery system includes the step of controllably feeding the pre-heated feed water to the sensible heat

exchanger from the first storage tank depending on the water level in a boiler.
Preferably, in accordance with the present invention, the method for recovering heat from flue gases in a flue gas heat recovery system includes the step of controlling the flow rate of the pre-heated feed water to the first storage tank depending on the water level in the first storage tank.
Preferably, in accordance with the present invention, the method for recovering heat from flue gases in a flue gas heat recovery system includes the step of controlling the flow rate of the partly cooled flue gases to the condensing heat exchanger depending upon the heated circulating water and the heated make-up water temperature.
Additionally, in accordance with the present invention, the method for recovering heat from flue gases in a flue gas heat recovery system includes the step of providing two heat recovery loops: a first heat recovery loop comprising the first storage tank, a feed pump, the sensible heat exchanger, and the boiler; and a second heat recovery loop comprising the second storage tank, a circulating pump, the condensing heat exchanger, the air preheater, and the first storage tank.

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 another embodiment of the flue gas heat recovery system for heating air;
FIGURE 3 illustrates yet another embodiment of the flue gas heat recovery system for simultaneously heating water and air, in accordance with the present invention;
FIGURE 4 illustrates the performance of the flue gas heat recovery system in accordance with the present invention (shown in FIGURE 3) in comparison with the performance of the flue gas heating system for heating water (shown in FIGURE 1);
FIGURE 5 illustrates the temperature characteristics of water and air heated using the flue gas heat recovery system, in accordance with the present invention;
FIGURE 6 illustrates the performance characteristics of a recirculation loop
for the flue gas heat recovery 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 flue gas heat recovery system for simultaneously heating water and air. Hot flue gases, typically from a boiler, are received in the heat recovery system where the flue gases are cooled and heat therein is utilized to obtain heated water and air. The heated water is supplied to the boiler for vaporizing and the air is fed to the boiler as combustion air. Thus, the energy or fuel consumption by the boiler is substantially reduced. Further, the system of the present invention aims at providing sufficient heat sink for maximizing the amount of heat recovered from the flue gases received therein. Also, the system of the present invention is designed to work efficiently under low make-up water quantity availability.
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 100. A first heat recovery loop consists of a feed water tank 124, a feed water pump 128, a control valve 130, and a sensible heat exchanger 102. The feed water pump 128 receives water from the feed water tank 124 and pumps the feed water to the sensible heat exchanger 102. The feed water receives sensible heat of the hot flue gas and is fed to the boiler 104. The

control valve 130 is used to regulate the feed water quantity by sensing the boiler 104 water level.
The conventional heat recovery system 100 further comprises a second heat recovery loop consisting of a make-up water tank 118, a make-up water pump 116, a flow control valve 120, and a condensing heat exchanger 108. Make-up water passing through the condensing heat exchanger 108 receives latent and sensible heat from the flue gas received therein from the sensible heat exchanger 102 to provide hot make-up water which is collected in the feed water tank 124. The make-up water quantity is regulated by sensing water level in the feed water tank 124. The hot flue gas exits the boiler 104 and first enters the sensible heat exchanger 102, 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 102 is passed through the condensing heat exchanger 108 using an induced draft fan 112. The flue gas enters the condensing heat exchanger 108 through an inlet 114 and exits from an outlet 122 rejecting the remaining sensible heat and latent heat to the cold make-up water received therein.
The flue gas exiting at the outlet 122 is discharged through a vent 106b. The vent 106b is generally lined with corrosion-resistant material to protect the vent 106b 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 108 needs to be regulated. The flue gas quantity is regulated with the help of a damper 110 by sensing the make-up water outlet temperature. The efficiency of the conventional heat recovery system 100 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 108. A significant quantity of the flue gas is bypassed and directly sent to a vent 106a from the sensible heat exchanger 102, 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. The effect of % make-up water on the boiler 104 on the efficiency gain has been plotted in FIGURE 4. It can be seen that the efficiency gain of the condensing heat exchanger 108 decreases from 10.25 % to 4 % with decrease in make-up water % from 100 % to 40 %. 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.
FIGURE 2 illustrates another embodiment of the flue gas heat recovery system for heating air, represented in FIGURE 2 by numeral 200. In the heat recovery system 200 combustion air is pre-heated instead of make-up water. The heat recovery system 200 comprises a circulation pump 210, a condensing heat exchanger 202, and an air preheater 208. The circulation pump 210 is used to circulate water between the condensing heat exchanger 202 and the air preheater 208. The circulating water receives sensible and latent heat from the flue gas in the condensing heat exchanger 202 and rejects the heat to the combustion air in the air preheater 208. Flue gas enters the condensing heat exchanger 202 at a flue gas inlet 204 and exits through a flue gas outlet 206 after rejecting the heat to the circulating water. The combustion air enters the air preheater 208 at an air inlet 212 and exits at an air outlet 214. In the heat recovery system 200 the combustion air does not provide sufficient sink to receive all the sensible heat and latent heat

available in the flue gas. Thus, the system 200 only provides a heat recovery opportunity in the range of 3 - 4 % efficiency gain.
Referring to FIGURE 3, therein is disclosed a further embodiment of the flue gas heat recovery system in accordance with the present invention, generally represented in FIGURE 3 by numeral 300. The flue gas heat recovery system 300 is designed to provide a heat sink to effectively extract the sensible and the latent heat from the flue gases to simultaneously heat water and air.
The flue gas heat recovery system 300 is designed to overcome the limitations of the conventional heat recovery systems illustrated in FIGURE 1 and FIGURE 2. The flue gas heat recovery system 300 combines a make-up water heating system and a combustion air pre-heating system, to provide sufficient heat sink and maximize the flue gas heat recovery. The flue gas heat recovery system 300 comprises a sensible heat exchanger 302, a condensing heat exchanger 308, an air preheater 332, and a plurality of conduit means to connect the aforementioned components.
The flue gas heat recovery system 300 is provided with two heat recovery loops: a first heat recovery loop comprising a first storage tank 324, a feed pump 328, the sensible heat exchanger 302, and a boiler 304; and a second heat recovery loop comprising the second storage tank 318, a circulating pump 316, the condensing heat exchanger 308, and the air preheater 332. The sensible heat exchanger 302 is provided with a first inlet to receive hot flue gases from the boiler 304, a second inlet to receive pre-heated feed water, means to extract heat from the hot flue gases to the pre-heated feed

water, a first outlet for partly cooled flue gases, and a second outlet for heated feed water which is typically fed to the boiler 304 to reduce energy-consumption by the boiler 304. The condensing heat exchanger 308 is adapted to receive the partly cooled flue gases from the sensible heat exchanger 302. The condensing heat exchanger 308 receives the partly cooled flue gases at a first inlet 314 through a first conduit means 350 provided to connect the first outlet of the sensible heat exchanger 302 to the first inlet 314 of the condensing heat exchanger 308 and cold water to be used in the flue gas heat recovery system 300 as circulating water and make-up water enters at a second inlet. The first conduit means 350 is provided with an induced draft fan 312 and a damper 310 to regulate the flow rate of the partly cooled flue gases carried there through. The circulating pump 316 is provided to pump the cold water comprising the circulating water from the air preheater 332 and the make-up water from the second storage tank 318 to the second inlet of the condensing heat exchanger 308. The condensing heat exchanger 308 further comprises means to extract heat from the partly cooled flue gases to the cold water to generate heated water to be used as heated circulating water and heated make-up water in the flue gas heat recovery system 300, a first outlet 322 for discharging the cooled flue gases, and a second outlet for discharging the heated circulating water and the heated make-up water. As the total cold water quantity (circulating water and make-up water) passing through the condensing heat exchanger 308 is the function of the make-up water quantity, the quantity of the partly cooled flue gases in the condensing heat exchanger 308 needs to be regulated. Depending upon the heated water (heated circulating water and heated make-up water) outlet temperature, the flow rate of the partly cooled flue gases to the condensing heat exchanger 308 is regulated using the damper

310. The heated make-up water is carried by a second conduit means 352, wherein the second conduit means 352 connects the second outlet of the condensing heat exchanger 308 to an inlet 356 of the first storage tank 324, to subsequently provide the heated make-up water as pre-heated feed water to the sensible heat exchanger 302. The first storage tank 324 is used to store the pre-heated feed water and condensate and serves as a feed water tank for the boiler 304. A third conduit means 360 is used to connect an outlet 358 of the first storage tank 324 to the second inlet of the sensible heat exchanger 302 to provide the pre-heated feed water to the sensible heat exchanger 302. At least a portion of the total heated water quantity from the condensing heat exchanger 308 is circulated through the air preheater 332 as the heated circulating water. This is achieved by providing a bypass means 354 along the second conduit means 352, wherein the bypass means 354 are adapted to bypass the heated circulating water to the air preheater 332.
The air preheater 332 is provided with a first inlet to receive the heated circulating water from the bypass means 354 and a second inlet 334 for receiving cold air. The air preheater 332 further comprises means to pre-heat the cold air by extracting heat from the heated circulating water, a first outlet for cold circulating water, and a second outlet for pre-heated air 336. A third control valve 340 is provided along the bypass means 354 for controlling the flow rate of the heated circulating water to the air preheater 332 depending on the heated circulating water temperature at the first outlet of the air preheater 332. If the cold circulating water temperature at the first outlet of the air preheater 332 is high, it will increase temperature at the inlet of condensing heat exchanger 308 and reduce the rate of condensation. The cold circulating water is received in the condensing heat exchanger 308

through a fourth conduit means 362 connecting the first outlet of the air preheater 332 to the second inlet of the condensing heat exchanger 308, wherein the circulating water is mixed with the make-up water at the inlet of the circulating pump 316. The circulating pump 316 is adapted to pump the cold water comprising the circulating water received from air preheater 332 and the make-up water from the second storage tank 318 to the second inlet of the condensing heat exchanger 308, wherein the second storage tank is provided for storing the make-up water. The temperature of the circulating water at the inlet of the circulating pump 316 should be sufficiently below the flue gas dew point temperature for the desired flue gas condensation.
The flow rate of the pre-heated feed water received in the first storage tank 324 through the second conduit means 352 is regulated by using a second control valve 342 which is provided along the second conduit means 352, based on the water level in the first storage tank 324. Further, a first control valve 330 is provided along the third conduit means 360 to control the flow rate of the pre-heated feed water from the first storage tank 324 to the second inlet of the sensible heat exchanger 302 depending upon the water level in the boiler 304. Still further, the feed pump 328 is provided to pump the preheated feed water from the first storage tank 324 to the second inlet of the sensible heat exchanger 302. A fifth conduit means 364 is provided for connecting the sensible heat exchanger 302 to a first vent 306a, wherein a portion of the partly cooled flue gases are discharged from the sensible heat exchanger 302 through the fifth conduit means 364, if the available make-up water and cold air does not provide sufficient heat sink for heat recovery. The cooled flue gases from the condensing heat exchanger 308 are carried

by a sixth conduit means 366 which connects the first outlet 322 of the condensing heat exchange to a second vent 306b.
The advantage of the flue gas heat recovery system 300 over the conventional heat recovery system 100 is illustrated in FIGURE 4, the graph is represented in FIGURE 4 by numeral 400. It is seen that with 100 % make-up water quantity, the flue gas heat recovery system 300 gives only a limited benefit. In this condition, efficiency gain of the flue gas heat recovery system 300 over the conventional system 100 is low (in the range of 0.5 - 1 %). However, the efficiency difference of the system 300 increases with the decrease in % make-up water quantity. At 40 % make-up water quantity, the difference in efficiency is as high as 4 % with a net efficiency gain of 8%. This primarily happens as the total flow through the condensing heat exchanger 308 is constant and only bypass quantity is increased with lower requirement of the make-up water quantity. The increase in % bypass flow with decrease in % of make-up water quantity is illustrated in FIGURE 6, the graph represented by numeral 600. As the bypass flow is low at high % make-up water, it has less potential for combustion air preheating resulting in lower combustion air and make-up water temperature. This effect is illustrated in FIGURE 5, graph represented by numeral 500. It can be seen in FIGURE 5, that the combustion air and make-up water temperature increases with decrease in make-up water quantity. The make-up water temperature with 100 % make-up water quantity is 80 °C, while at 40 % make-up water quantity the make-up water temperature achieved 95 °C.

TECHNICAL ADVANCEMENTS
A flue gas heat recovery system for simultaneously heating water and air, in accordance with the present invention has several technical advantages including but not limited to the realization of:
• the flue gas heat recovery system provides a sufficient heat sink and is adapted to extract both sensible and latent heat from the hot flue gases, thus maximizing the amount of energy recovered;
• an integrated circulating make-up water, feed water & combustion air heating system to provide maximum possible heat sink for the flue gas condensation;
• an integrated condensing heat exchanger for circulating make-up water and combustion air preheating;
• the flue gas heat recovery system provides optimum heat recovery also with limited make-up water quantity availability; and
• the flue gas heat recovery system comprises automation means for easy control of the flow rates of the plurality of fluids through the heat recovery system.
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 of the wide 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 flue gas heat recovery system using a condensing heat exchanger for simultaneously heating water and air, said system comprising:
• a sensible heat exchanger has a first inlet for hot flue gases, a second inlet for pre-heated feed water, means to extract heat from the hot flue gases to the pre-heated feed water, a first outlet for partly cooled flue gases, and a second outlet for heated feed water;
• a condensing heat exchanger having a first inlet for at least a portion of the partly cooled flue gases, a second inlet for cold water to be used in said system as circulating water and make-up water, means to extract heat from the partly cooled flue gases to the cold water to generate heated water to be used in said system as heated circulating water and heated make-up water, a first outlet for cooled flue gases, and a second outlet for the heated water;
• an air preheater having a first inlet to receive the heated circulating water, a second inlet for cold air, means to pre-heat the cold air by extracting heat from the heated circulating water, a first outlet for the circulating water to be fed to the condensing heat exchanger, and a second outlet means for pre-heated air;
● a first storage tank operatively connected to the sensible heat exchanger and adapted to store the pre-heated feed water;
● a second storage tank operatively connected to the condensing heat exchanger and adapted to store the cold water;

• a first conduit means to connect the first outlet of the sensible heat exchanger to the first inlet of the condensing heat exchanger;
• a second conduit means to connect the second outlet of the condensing heat exchanger to an inlet of the first storage tank to provide the heated make-up water as the pre-heated feed water;
• a third conduit means to connect an outlet of the first storage tank to the second inlet of the sensible heat exchanger to provide the pre-heated feed water to the sensible heat exchanger;
● a bypass means provided along the second conduit means to bypass a portion of the circulating heated make-up water to the first inlet of the air pre-heater;
• a fourth conduit means to connect the first outlet of the air-preheater to the second inlet of the condensing heat exchanger;
• a fifth conduit means to connect the first outlet of the sensible heat exchanger to a first vent; and
• a sixth conduit means to connect the first outlet of the condensing heat exchanger to a second vent.

2. The flue gas heat recovery system as claimed in claim 1, wherein an induced draft fan is provided along the first conduit means.
3. The flue gas heat recovery system as claimed in claim 1, wherein a damper is provided along the first conduit means.

4. The flue gas heat recovery system as claimed in claim 1, wherein a feed pump is provided along the third conduit means to pump the pre-heated feed water from the first storage tank to the second inlet of the sensible heat exchanger.
5. The flue gas heat recovery system as claimed in claim 1, wherein a circulating pump is provided to pump the cold water comprising the make-up water from the second storage tank and the circulating water from the air preheater to the second inlet of the condensing heat exchanger.
6. The flue gas heat recovery system as claimed in claim 1, wherein a first control valve is provided along the third conduit means to control the flow rate of the pre-heated feed water to the second inlet of the sensible heat exchanger.
7. The flue gas heat recovery system as claimed in claim 1, wherein a second control valve is provided along the second conduit means to control the flow rate of the pre-heated feed water to the first storage tank on the basis of the water level in the first storage tank.
8. The flue gas heat recovery system as claimed in claim 1, wherein a third control valve is provided along the bypass means to control the flow rate of the heated circulating water to the first inlet of the air preheater.

9. A method for recovering heat from flue gases in a flue gas heat recovery system, said method comprising the following steps:
i. extracting heat from hot flue gases in a sensible heat exchanger for heating pre-heated feed water, to generate partly cooled flue gases and heated feed water;
ii. regulating flow rate of at least a portion of the partly cooled flue gases carried through a first conduit means from the sensible heat exchanger to a condensing heat exchanger by using a damper and an induced draft fan provided along the first conduit means;
iii. extracting heat from the partly cooled flue gases, in cold water to be used in said system as circulating water and make-up water, in the condensing heat exchanger, to provide heated water to be used in said system as heated circulating water and heated make-up water, and discharge cooled flue gases;
iv. regulating the quantity of the heated make-up water carried through a second conduit means to a first storage tank as the pre-heated feed water and to an air preheater as the heated circulating water through a bypass means;
v. extracting heat from the heated circulating water in the air heater for pre-heating cold air received therein, to provide preheated air and the circulating water;
vi. recycling the circulating water from the air preheater to the
condensing heat exchanger through a fourth conduit means; vii. discharging a portion of the partly cooled flue gases from the sensible heat exchanger through a fifth conduit means, if the

available make-up water and cold air does not provide sufficient heat sink for the heat recovery; and viii. discharging the cooled flue gases from the condensing heat exchanger through a sixth conduit means.
10. The method as claimed in claim 9, which includes the step of controlling the flow rate of the heated circulating water to the air preheater depending on the circulating water temperature at the outlet of air preheater.
11. The method as claimed in claim 9, which includes the step of controllably feeding the pre-heated feed water to the sensible heat exchanger from the first storage tank depending on the water level in a boiler.
12. The method as claimed in claim 9, which includes the step of controlling the flow rate of the pre-heated feed water to the first storage tank depending on the water level in the first storage tank.
13. The method as claimed in claim 9, which includes the step of controlling the flow rate of the partly cooled flue gases to the condensing heat exchanger depending upon the circulating heated make-up water temperature.

14. The method as claimed in claim 9, which includes the step of providing two heat recovery loops: a first heat recovery loop comprising the first storage tank, a feed pump, the sensible heat exchanger, and the boiler; and a second heat recovery loop comprising the second storage tank, a circulating pump, the condensing heat exchanger, the air preheater and the first storage tank.