FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)
AN AIR HEATER SYSTEM
THERMAX LIMITED
an Indian Company
of D-13, MIDC Industrial Area, R.D. Aga Road,
Chinchwad, Pune - 19,
Maharashtra, India
Inventors: a) Jha R.S., and b) Kharat Rahul
The following specification particularly describes the invention and the manner in which
it is to be performed.

FIELD OF INVENTION
The present invention relates to the field of air or gas heaters.
DEFINITIONS OF TERMS USED IN THE SPECIFICATION
The term "creep failure" used in the specification means the slow deformation of the material and successive failure under the continuous exposure of higher temperature and higher stress.
The term "convective tubes" used in the specification mean the tubes conveying heat transfer media there through and facilitating heat transfer.
The term "natural circulation" used in the specification means the circulation caused by density gradient in conjunction with gravitational field, which causes a less dense fluid (hot fluid) to rise above a more dense fluid (cold fluid).
BACKGROUND
Industries, such as, packaging, automotive, plastics, rubber, textile, and electronics, require hot dry air having temperature in the range of 120 °C to 140 °C, for curing, preheating, drying, soldering, etc. An air heater is provided for these purposes, typically, in which, cold air is heated by flue gases generated from the combustion of fuels such as biomass, fossils or waste. In this type of air heater system, cold air is passed over the furnace and convective tubes to directly receive heat from the hot combustion gases. In such air heaters, there is a higher possibility of the convective tubes getting overheated, leading to creep failure. This problem can be solved by

using a water heating system, where, primarily, the hot combustion gases transfer heat to cold water, to provide hot water, which is then used to indirectly heat air by passing through a heat exchanger/radiator. However, such water heating systems are complex and require a circulation pump, designed for pumping water at high temperatures, to navigate the hot water to the heat exchanger/radiator. Therefore, a substantial amount of power is consumed in pumping the water. Alternatively, a steam-based system can be used for air heating, which does not require any circulation pump. However, such system additionally requires a steam generator and a radiator. In the steam-based system, steam is generated in the steam generator and then fed to steam radiator, where the steam gets condensed by transferring heat to the cold air. The condensed water is discharged via a steam trap, where a significant quantity of steam flashes and heat is lost to the atmosphere. Due to the flash steam loss, such a system is less efficient compared to a water-based system.
Several attempts have been made to provide air/gas heater systems which overcome one or more of the drawbacks listed above. Some of the disclosures are listed in the prior art below:
GB Patent No. 728399 discloses an air or gas heater wherein the unit for heating air or gas comprises a boiler having a header tank which is fitted with a plurality of vertically extending tubular water or steam circulators, each of which comprise an outer and an inner tube, said circulators providing a battery of tubes serving as a heat exchanger for warming/heating a current of gas or gaseous mixture passed between said

outer tubes. The upper ends of the outer tubes are closed and are connected together and to an open pipe by tubes, forming air vents. The inner tubes have holes near their upper ends. The tubes have gills. Further, the flue tubes contain rotatable cleaning worms which also act as baffles.
CN Patent No. 2453321 discloses a full automatic and electric heating type miniature air-heater for small-area heating. An automatic control and electric heating device, composed of an electricity leakage protector, a thermostat and an electric heater, and a heater chamber with small capacity, are arranged in an outer shell of a heater body, wherein an upper flange dish on the upper end of the heater cavity is connected tightly and fixed with a cover board provided with the electric heater, and the lower end is sealed by a lower flange dish. The upper part and the lower part of the heater chamber are respectively provided with a water outlet pipe opening and a water return pipe opening which are communicated with an outer pipeline. The air-heater provides a small volume of heated air, at a low cost, is fully automatic in operation, and allows automatic temperature setting and convenient installation and use. Further, the heater causes water to circulate naturally without the need for a circulation water pump, when used for smaller heating applications, and provides an ideal circulating heating effect.
CN Patent No. 2715038 discloses a quick-heating electric heater comprising a casing body and an electric heating device, wherein the heater is characterized in that the inner surrounded by the casing body is provided with a circulating water chamber and a circulating pipe communicating with

the circulating water chamber. The inner of the circulating water chamber is provided with the electric heating device, and more than one heat sinking fin is sheathed with the circulating pipe. A heat sinking cavity is formed between the adjacent heat sinking fins, and the upper part of the casing body is provided with an air circulation hole. The heater utilizes natural circulation principle, that the water in the circulating chamber can circulate from bottom to top after heat exposure, which makes the water in the circulating pipe continuously flow, and then the water can continuously make heat change with the air; the heat sinking fins are sheathed with the circulating pipe, which achieves the action of increasing the area of the water heating, and thus the heater has favorable heat sinking effect, and quick heat exposure of indoor air.
GB Patent No. 606975 discloses a tubulous, natural circulation steam boiler comprising vaporizing tubes arranged to discharge into a steam and water drum and to be fed from the drum through downcomer connections, forming an air heater disposed in the path of combustion air flowing to the boiler furnace, wherein the air heater comprises a multiplicity of small diameter downcomer tubes arranged in a bank. The steam boiler is adapted to operate as part of a mobile power plant.
The air/gas heaters disclosed in the GB Patent No. 728399 and the GB patent No. 606975 discuss the application of natural circulation for air heating. However, the heater disclosed in the GB patent No. 606975 is placed outside the shell and a portion of heat from the fluid passing through the downcomer is used to preheat combustion air. The system disclosed in

GB606975 is primarily a steam generator, where natural circulation is used for combustion air preheating. Further, the heater disclosed in GB728399 is used for process heating applications and comprises an external air heater with complex arrangement for riser and downcomer circuit. Other applications are mainly focused on miniature air heaters using electrical air heating. These heaters require electric power and have higher operational cost. The present invention aims at providing an air heater system which will substantially reduce the power consumption by eliminating the need to pump water to the heat exchanger/radiator, for heating air for large, medium or small scale heating applications. Also, the present invention aims at providing an air heater system with simple and reliable construction having a higher working life.
OBJECTS OF THE INVENTION
An object of the present invention is to provide an air heater system for large or small scale industrial air/gas heating applications.
Another object of the present invention is to provide an air heater system which conserves power in comparison with conventional water-based heating systems, thereby reducing the operating costs.
Still another object of the present invention is to provide an air heater system which has a longer life and reliability in comparison with direct-fired air heater systems.

Yet another object of the present invention is to provide an air heater system which has a higher efficiency and zero flash steam loss in comparison with steam-based air heater system.
One more object of the present invention is to provide an air heater system in which the metal temperature is a function of the water temperature and not the gas temperature; thus, the metal does not get overheated, preventing creep failure.
Still one more object of the present invention is to provide an air heater system that eliminates the need for pumping means to circulate water.
Yet one more object of the present invention is to provide an air heater system that is efficient and can be alternatively used as a steam generator.
SUMMARY OF THE INVENTION
In accordance with the present invention, is provided an air heater system comprising:
■ a condensing section including a set of air heating tubes for receiving air to be heated;
■ a first shell adapted to receive hot fluid for heating, by natural circulation, the air passing through said set of air heating tubes;
■ a boiling section including a combustor, a furnace, and at least one set of convective tubes, arranged in a predetermined manner for driving the natural circulation; and

■ wherein, said condensing section and a portion of said boiling section are adapted to be placed in a housing selected from in said first shell and in separate shells.
Typically, in accordance with the present invention, said first shell comprises a portion of said boiling section.
Alternatively, in accordance with the present invention, a separate second shell is provided for enclosing a portion of said boiling section.
Typically, in accordance with the present invention, said condensing section and a portion of said boiling section are enclosed in said first shell.
Alternatively, in accordance with the present invention, said condensing section is enclosed in said first shell and a portion of said boiling section is enclosed in said second shell.
Preferably, in accordance with the present invention, said set of air heating tubes is connected to an air inlet at one operative end for receiving the air to be heated and an air outlet at other operative end for discharging heated air.
Typically, in accordance with the present invention, said set of air heating tubes is located operatively above said boiler section in said first shell for driving the natural circulation.

Alternatively, in accordance with the present invention, said first shell is located operatively above said second shell for driving the natural circulation.
Alternatively, in accordance with the present invention, at least one riser pipe and at least one downcomer pipe is provided between said first shell and said second shell, for circulation of fluid between said first shell and said second shell.
Preferably, in accordance with the present invention, said set of convective tubes comprises a first subset of convective tubes and a second subset of convective tubes.
Typically, in accordance with the present invention, an internal reversal chamber is provided between said furnace and said set of convective tubes.
Preferably, in accordance with the present inversion, an external reversal chamber is provided between said first subset of convective tubes and said second subset of convective tubes.
Typically, in accordance with the present invention, said second subset of convective tubes is provided in communication with a flue gas outlet.
Preferably, in accordance with the present inversion, the cross-section of said first shell is selected from the group consisting of circular, ovular, elliptical, rectangular, and polygonal.

Typically, in accordance with the present invention, the cross-section of said second shell is selected from the group consisting of circular, ovular, elliptical, rectangular, and polygonal.
In accordance with the present invention, is disclosed a method for providing heated air using an air heater system, said method comprising the following steps:
■ circulating cold air via an air inlet through a set of air heating tubes of a condensing section enclosed in a first shell;
■ combusting a fuel in a combustor included in a boiler section, to generate hot flue gases;
■ traversing the hot flue gases through a furnace and a set of convective tubes, to effect transfer of heat from the hot flue gases to the fluid to be heated, to provide hot fluid and release cooled flue gases;
■ conveying the hot fluid to said condensing section, by natural circulation, for heating the cold air passing through said set of air heating tubes, to provide heated air and cooled fluid;
■ discharging the heated air from said set of air heating tubes via an air outlet; and
■ returning the cooled fluid to said boiling section by natural circulation.
Typically, in accordance with the present invention, the method for providing heated air includes the step of effecting natural circulation by locating said condensing section operatively above said boiling section.

Preferably, in accordance with the present invention, the method for providing heated air includes the step of providing a portion of said boiling section, including said set of convective tubes and said furnace, in said first shell.
Alternatively, in accordance with the present invention, the method for providing heated air includes the step of providing a portion of said boiling section, including said set of convective tubes and said furnace, in a second shell.
Alternatively, in accordance with the present invention, the method for providing heated air includes the step of conveying the hot fluid from said second shell to said first shell by at least one riser pipe.
Alternatively, in accordance with the present invention, the method for providing heated air includes the step of returning the cooled fluid from said first shell to said second shell by at least one downcomer pipe.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with the help of the accompanying drawings, in which,
Figure 1 illustrates the cross-section of an embodiment of the air heater system in accordance with the present invention;

Figure 2 illustrates a schematic of the embodiment of the air heater system in accordance with the present invention, showing the internal reversal chamber, the external reversal chamber, and the air inlet and air outlet; and
Figure 3 illustrates the cross section of an alternative embodiment of the air heater 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 modified layout of an air heater system, which reduces the power consumption in comparison with the conventional circulating water-based air heater systems. The air heater system of the present invention eliminates the need for pumping means to circulate water, thereby, eliminating the pumping costs, and giving substantial energy savings. Further, in the air heater system of the present invention, the metal temperature is a function of the water temperature and not the gas temperature; thus, the metal does not get overheated, and is not subject to creep failure, giving the air heater system longevity. These and other benefits of the air heater system of the present invention will be apparent from the description provided here below.

The air heater system of the present invention primarily comprises: a condensing section including a set of air heating tubes for receiving air to be heated; a first shell, wherein, the first shell is adapted to receive hot fluid for heating, by natural circulation, the air passing through the set of air heating tubes; and a boiling section including a combustor, a furnace, and at least one set of convective tubes, arranged in a predetermined manner for driving the natural circulation; wherein, the condensing section and a portion of the boiling section is placed in a housing selected from in the first shell or in separate shells. Typically, for driving the natural circulation, the condensing section and a portion of the boiling section are enclosed in the first shell, such that, the condensing section is located opefatively above the boiling section. Alternatively, the condensing section is enclosed in the first shell and a portion of the boiling section is enclosed in a second shell, wherein the first shell is located operatively above the second shell to drive the natural circulation. The preferred embodiments of the air heater system of the present invention are discussed below, where, in a first embodiment, illustrated in Figure 1 & 2, a portion of the heating means is located within the first shell, and in a second embodiment, illustrated in Figure 3, a portion of the heating means is enclosed in a second shell.
Figure 1 and Figure 2 illustrate a first preferred embodiment of the air heater system of the present invention, generally represented by numeral 100, in which, Figure 1 illustrates a cross-sectional view and Figure 2 illustrates the overall layout of the air heater system. Referring to Figure 1 & 2, cold air to be heated, having temperature between 10-40 °C, is received through the set of air heating tubes, represented by numeral 106,

wherein, the set of air heating tubes 106 are enclosed in the first shell, represented by numeral 102 in the embodiment 100 illustrated in Figure 1 & 2, so as to be arranged proximal to the operative roof of the first shell 102 parallel to the operative longitudinal horizontal axis. The first shell 102 has a closed tubular structure with circular, ovular, elliptical, rectangular, or polygonal cross-section. A first operative end of the set of air heating tubes 106 is connected to an air inlet, represented by numeral 112 in Figure 2, for receiving the air to be heated there from. The air heater 100 is mainly divided in two sections, viz., a bottom section 104 of air heater 100 acts like a boiler primarily consisting of a combustor (not shown in figure), a furnace 108 and a set of convective tube 110; and a top section of air heater 100 acts like a condenser, consisting a set of air heating tubes 106. Fuel is burnt in the furnace 108 with the help of combustor and the hot flue gas is passed through the set of convective tube 110 transferring heat there from to the surrounding fluid to be heated, typically water. The water gets vaporized and mixture of water and vapor, so obtained, traverses upwards due to buoyancy effect and enters to the condenser section of the air heater 100, where it condenses by transferring heat to the cold air. The condensed water returns back to the boiler section of the air heater 100 causing a continuous flow of water and referred as natural circulation.
The first shell 102 is located around the combustor 108, wherein, the hot flue gases communicated to the first shell 102 transfer heat to the fluid to be heated, typically water, converting the fluid to the steam bubble, water and steam bubble move upwards to induce circulation without the circulation pumps. An internal reversal chamber is provided, within the first shell 102,

to communicate the hot flue gases to the set of corrective tubes 110. The set of convective tubes 110, as illustrated in the embodiment 100, comprises a first subset of convective tubes and a second subset of convective tubes, parallel to the operative longitudinal horizontal axis of the first shell 102. The first subset of convective tubes, represented by numeral 124, is connected to the furnace 108 via the internal reversal chamber, represented by numeral 118 in Figure 2, for receiving the hot flue gases there from. In the first subset of convective tubes 124, the hot flue gases transfer heat to the fluid to be heated surrounding the first subset of tubes 124, causing the heated fluid, typically water and steam mixture, to rise due to natural circulation or buoyancy effect. The first subset of convective tubes 124 is connected to the second subset of convective tubes represented by numeral 126, by means of an external reversal chamber, represented by numeral 120 in Figure 2, located along the operative side wall of the first shell 102, to provide the partly cooled flue gases therein. in the second subset of convective tubes 126, the flue gases get cooled by transferring the remaining heat to the heated fluid surrounding the second subset of convective tubes 126, causing the heated fluid to rise further towards the top section of the first shell 102. The set of convective tubes 110 can comprise more than two subsets of convective tubes, which are to be, positioned at a level above the furnace 108. In the embodiment 100, cooled flue gases are discharged after passing through the second subset of convective tubes 126, wherein the second subset of convective tubes 126 is connected to a flue gas outlet, represented by numeral 116, for discharging the cooled flue gases.

The hot fluid rises towards the top section of the first shell 102, by natural circulation, to be in communication with the set of air heating tubes 106. The set of air heating tubes 106 is essentially to be positioned operatively above the set of convective tubes 110 in the first shell 102, to cause the natural circulation. The cold air extracts heat from the hot fluid, to become heated air having temperature in the range of 100 - 160 °C. The heated air is discharged from the set of air heating tubes 106 via an air outlet, represented by numeral 114 in Figure 2, connected to a second operative end of the set of air heating tubes 106. The fluid, typically water and steam mixture, after losing heat condenses and falls down and re-enters the boiling section 104 due to natural circulation, thereby, completing the heating cycle. If water level falls down below a specified level, make up water can be pumped in air heater to regain the water level.
Referring to Figure 3, is illustrated a cross-sectional view of a second preferred embodiment of the air heater system of the present invention, represented by numeral 200 in Figure 3, wherein, the boiling section 104 is enclosed in a second shell, represented in Figure 3 by numeral 128, and the second shell is positioned operatively below the first shell 102, to facilitate the natural circulation of the hot fluid from the second shell 128 to the first shell 102. The cold air to be heated, having temperature between 10 - 40°C, is received through the set of air heating tubes, represented by numeral 106, wherein, the set of air heating tubes 106 are enclosed in the first shell, represented by numeral 102 in the embodiment 200 illustrated in Figure 3. The first shell 102 has a closed tubular structure with circular, ovular, elliptical, rectangular, or polygonal cross-section. A first operative end of

the set of convective tubes 106 is connected to an air inlet (not shown in Figure 3), for receiving the air to be heated there from. The boiling section 104 is enclosed in the second shell 128, as shown in the embodiment 200, wherein, the second shell 128 has a closed tubular structure with circular, ovular, elliptical, rectangular, or polygonal cross-section. The boiling section 104 primarily comprises the combustor, the furnace, the at least one second set of tubes and the inlet means. The furnace, represented by numeral 108, is placed inside the second shell 104, at the bottom section of shell, for combusting a fuel therein. The combustion of fuel by the combustor in the furnace 108 generates hot flue gases, which are carried from the furnace 108 to the set of convective tube 110 through internal reversal chamber 118. The second shell 128, in the embodiment 200, encases the set of convective tubes, represented by numeral 110 and the inlet means (not shown in Figure). A fluid to be heated is supplied to the boiler section 104 placed in the second shell 128 via the inlet means, thereby, filling the second shell 128 with the fluid to be heated, such that, the set of convective tubes 110 are completely immersed in the fluid to be heated.
The second shell 128 is located operatively around the furnace 108, such that, the set of convective tubes 110 remain at a level operatively above the furnace 108, wherein, the hot flue gases communicated to the second shell 128 transfer heat to the fluid to be heated, causing the fluid to move upward due to buoyancy effect to enter the first shell 102. In the first shell 102, the hot fluid is cooled by transferring heat to the air through air heating tube to move downward to re-enter the second shell 128. This effect causes the

natural circulation between top and bottom shell and does not require any circulation pump to facilitate flow between the shells. An internal reversal chamber 118, is located inside the second shell 128 and allows communication of the hot flue gases from the furnace 108 to the set of convective tubes 110. The set of convective tubes 110, as illustrated in the embodiment 200, comprises a first subset of convective tubes and a second subset of convective tubes, stacked and positioned around the furnace 108 and near the top of the second shell 128 parallel to the operative longitudinal horizontal axis. The first subset of convective tubes, represented by numeral 124, is connected to the furnace 108 via the internal reversal chamber 118, for receiving the hot flue gases there from. In the first subset of convective tubes 124, the hot flue gases transfer heat to the fluid to be heated surrounding the first subset of convective tubes 124, causing the heated fluid to rise due to buoyancy effect. The first subset of convective tubes 124 is connected to the second subset of convective tubes, represented by numeral 126, by means of an external reversal chamber (not shown in Figure 3), located along the operative side wall of the second shell 128, to provide the partly cooled flue gases therein. In the second subset of convective tubes 126, the flue gases get cooled by transferring the remaining heat to the heated fluid surrounding the second subset of convective tubes 126, causing the heated fluid to rise further towards the top section of the first shell 102. The set of convective tubes 110 can comprise inore than two subsets of convective tubes, which are to be, positioned at a level above the furnace 108. In the embodiment 200, cooled flue gases are discharged after passing through the second subset of convective tubes 126, wherein the second subset of convective tubes 126 is connected to a flue gas outlet (not shown

in the Figure 3), provided on the boiling section 104 in the second shell 128, for discharging the cooled flue gases.
The hot fluid rises towards the first shell 102, which is located operatively above the second shell 128, by natural circulation, to be in communication with the set of air heating tubes 106. The second shell 128 is connected to the first shell 102 by means of at least one riser pipe, represented by numeral 130 in Figure 3, connecting the second shell 128 to the first shell 102, for circulating the hot fluid from the second shell 128 to the first shell 102. The first shell 102 is essentially positioned operatively above the second shell 128, to cause the natural circulation. The cold air extracts heat from the hot fluid, to become heated air having temperature in the range of 100 - 160 °C. The heated air is discharged from the set of air heating tubes 106 via an air outlet (not shown in Figure 3), connected to a second operative end of the set of air heating tubes 106. The fluid, typically water and steam mixture, after losing heat is condensed and conveyed to the second shell 128 by natural circulation, wherein, the first shell 102 is connected to the second shell 128 by at least one downcomer pipe, represented in Figure 3 by numeral 132, to circulate the cooled and condensed fluid. The downcomer pipe 132 leads from the operative side of the first shell 102 to the operative side of the second shell 128, to communicate the cooled fluid to the second shell 128, by natural circulation, thereby, completing a first heating cycle.

TECHNICAL ADVANTAGES
An air heater system for air/gas heating applications, comprising: a boiling section with a combustor, furnace, reversal chamber and set of convective tubes, a condensing section placed at the top of boiling section consisting of a set of air heating tubes to receive cold air and produce hot air, a hot fluid being circulated between boiling section and condensing section by natural circulation means, wherein condensing and boiling section can be placed in a common shell or a separate shell; as described in the present invention has several technical advantages including but not limited to the realization of:
• an air heater system which conserves required power for circulation, in comparison with conventional air heating systems based on circulating water, by eliminating the need for pumping means to circulate water, thereby reducing the operating costs;
• an air heater system in which the metal temperature is a function of the water temperature and not the gas temperature; thus, the metal does not get overheated, preventing creep failure, thus, giving the system a longevity in comparison with conventional direct air heating system; and
% an air heater system that is efficient and can be alternatively used as a steam generator.
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. Wherever a range of values is specified, a value up to 10% below and above the lowest and highest numerical value respectively, of the specified range, is included in the scope of the invention.
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 ■ An air heater system comprising:
■ a condensing section including a set of air heating tubes for receiving air to be heated;
■ a first shell adapted to receive hot fluid for heating, by natural circulation, the air passing through said set of air heating tubes;
■ a boiling section including a combustor, a furnace, and at least one set of convective tubes, arranged in a predetermined manner for driving the natural circulation; and
■ wherein, said condensing section and a portion of said boiling section are adapted to be placed in a housing selected from in said first shell and in separate shells.

2. The air heater system as claimed in claim 1, wherein said first shell Comprises a portion of said boiling section.
3. The air heater system as claimed in claim 1, wherein a separate second shell is provided for enclosing a portion of said boiling Section.
4. The air heater system as claimed in claim 1, wherein said condensing section and a portion of said boiling section are enclosed in said first shell.

5. The air heater system as claimed in anyone of the preceding claims, wherein said condensing section is enclosed in said first shell and a portion of said boiling section is enclosed in said second shell.
6. The air heater system as claimed in claim 1, wherein said set of air heating tubes is connected to an air inlet at one operative end for receiving the air to be heated and an air outlet at other operative end for discharging heated air.
7. The air heater system as claimed in claim 2, wherein said set of air heating tubes is located operatively above said boiler section in said first shell for driving the natural circulation.
8. The air heater system as claimed in anyone of the preceding claims, wherein said first shell is located operatively above said second shell for driving the natural circulation.
9. The air heater system as claimed in anyone of the preceding claims, wherein at least one riser pipe and at least one downcomer pipe is provided between said first shell and said second shell, for circulation of fluid between said first shell and said second shell.
10.The air heater system as claimed in anyone of the preceding claims, wherein said set of convective tubes comprises a first subset of convective tubes and a second subset of convective tubes.

11 .The air heater system as claimed in anyone of the preceding claims, wherein an internal reversal chamber is provided between said furnace and said set of convective tubes.
12.The air heater system as claimed in claims 10, wherein an external reversal chamber is provided between said first subset of convective tubes and said second subset of convective tubes.
13.The air heater system as claimed in claims 10 & 12, wherein said second subset of convective tubes is provided in communication with a flue gas outlet.
14.The air heater system as claimed in any one of the preceding claims, wherein the cross-section of said first shell is selected from the group consisting of circular, ovular, elliptical, rectangular, and polygonal.
15.The air heater system as claimed in claim 3, wherein the cross-section of said second shell is selected from the group consisting of circular, ovular, elliptical, rectangular, and polygonal.
16.A method for providing heated air using an air heater system, said method comprising the following steps:
■ circulating cold air via an air inlet through a set of air heating tubes of a condensing section enclosed in a first shell;
■ combusting a fuel in a combustor included in a boiler section, to generate hot flue gases;

■ traversing the hot flue gases through a furnace and a set of convective tubes, to effect transfer of heat from the hot flue gases to the fluid to be heated, to provide hot fluid and release cooled flue gases;
■ conveying the hot fluid to said condensing section, by natural circulation, for heating the cold air passing through said set of air heating tubes, to provide heated air and cooled fluid;
■ discharging the heated air from said set of air heating tubes via an air outlet; and
■ returning the cooled fluid to said boiling section by natural circulation.
17.The method as claimed in claim 16, which includes the step of effecting natural circulation by locating said condensing section operatively above said boiling section.
18.The method as claimed in claim 16, which includes the step of providing a portion of said boiling section, including said set of convective tubes and said furnace, in said first shell.
19.The method as claimed in claim 16, which includes the step of providing a portion of said boiling section, including said set of convective tubes and said furnace, in a second shell.

20.The method as claimed in claim 19, which includes the step of conveying the hot fluid from said second shell to said first shell by at least one riser pipe.
21.The method as claimed in claim 19, which includes the step of returning the cooled fluid from said first shell to said second shell by at least one downcomer pipe.