Description:FIELD OF THE INVENTION:
The present invention relates to the field of industrial thermal systems and heat generation equipment, and more particularly to indirect hot air generators employed for supplying heated process air in industrial applications such as drying, heating, and process conditioning.
More specifically, the invention pertains to a solid fuel fired hot air generator incorporating an integrated heat recovery system, comprising heat absorbing surfaces disposed within a furnace and a closed-loop intermediate fluid circuit configured to extract heat directly from the combustion zone and transfer the extracted heat to process air through a radiator or primary air heater.
The invention further relates to systems and methods for controlling furnace temperature by heat extraction using a circulating intermediate fluid, operating either under natural circulation (thermosiphon) or forced circulation, thereby substantially reducing or eliminating the requirement of dilution air, lowering excess air levels, reducing flue gas volume, and improving overall thermal efficiency.
Additionally, the invention encompasses specific structural configurations, material selections, and operating parameter ranges, including defined temperature and pressure conditions of the intermediate fluid, construction of heat absorbing surfaces within the furnace, enabling optimized performance, reduced equipment sizing, and lower energy consumption in hot air generation systems.

BACKGROUND OF THE INVENTION:
Indirect hot air generators are widely employed in industrial applications for supplying heated air for drying, curing, and process heating operations. In conventional systems, a solid fuel fired furnace generates high-temperature flue gases, which transfer heat to process air through a metallic heat exchanger, typically fabricated from stainless steel to withstand corrosion and elevated temperatures.
A fundamental limitation in such conventional systems lies in the method adopted for furnace temperature control. In order to maintain the required acceptable temperature in the combustor, it is standard practice to introduce substantial quantities of dilution air, in addition to the excess air required for complete combustion. Typically, the total air supplied is in the range of 150% to 170% above stoichiometric air requirements in the case of fluidised bed combustion hot air generators.
While effective for temperature moderation, this approach introduces several technical and economic disadvantages:
• The addition of dilution air significantly increases the mass flow rate of flue gases, resulting in higher sensible heat losses through the exhaust.
• Elevated flue gas volumes necessitate larger heat exchangers, typically stainless steel units, thereby increasing capital expenditure.
• Increased gas flow requires higher capacity induced draft fans and combustion air fans, leading to greater capital cost and electrical power consumption.
• Larger ducting systems are required to handle the increased flow, resulting in higher fabrication, insulation, and heat loss costs.
• The overall thermal efficiency of the system is reduced, as a considerable portion of generated heat is carried away in the exhaust gases.
Conventional attempts to improve system efficiency have primarily focused on optimizing heat exchanger configurations or enhancing combustion efficiency. However, such approaches do not address the core inefficiency arising from dependence on dilution air for furnace temperature regulation.
In certain known systems, heat recovery arrangements are employed downstream of the furnace, such as air preheaters or economizers, to recover residual heat from flue gases. However, these systems operate after combustion and do not directly influence the temperature conditions within the furnace. Consequently, the requirement for dilution air remains substantially unchanged.
Further, prior art systems do not provide a controlled mechanism for extracting heat directly from the furnace combustion zone using an intermediate heat transfer medium operating within defined pressure and temperature ranges. There is also no teaching or suggestion of a closed-loop fluid circuit integrated with heat absorbing surfaces within the furnace, configured to regulate furnace temperature while simultaneously transferring heat to process air via a dedicated radiator or primary air heater.
Additionally, conventional designs do not disclose arrangements capable of operating under natural circulation (thermosiphon principles) or selectively under forced circulation, nor do they provide guidance on selection of intermediate fluids, structural configuration of heat absorbing surfaces, or operating parameters such as fluid inventory, temperature gradients, and pressure ranges required to achieve stable and efficient heat extraction.
Accordingly, there exists a need for an improved hot air generator that:
• Eliminates or substantially reduces reliance on dilution air for furnace temperature control;
• Provides direct and controlled extraction of heat from the furnace;
• Operates within defined and safe temperature and pressure regimes;
• Reduces flue gas volume and associated losses;
• Enables downsizing of heat exchangers, fans, and ducting; and
• Improves overall thermal efficiency while reducing capital and operational costs.
The present invention addresses the aforementioned deficiencies by providing a hot air generator incorporating a closed-loop heat-absorbing fluid circuit integrated with the furnace, enabling efficient way of temperature regulation and enhanced energy utilization.

PRIOR ART:
Indirect hot air generators and furnace-based heating systems are well established in the art, with numerous disclosures directed toward heat recovery, indirect heat exchange, and combustion efficiency improvement. However, such prior art systems primarily rely on post-combustion heat recovery mechanisms and do not address the fundamental inefficiencies arising from furnace temperature control using dilution air.
US20090298002A1 discloses an indirect heat exchanger wherein hot flue gases transfer heat to combustion gases through an intermediate medium such as an inert gas. The system improves heat exchange stability and reduces thermal fluctuations. However, the heat transfer occurs downstream of the combustion zone, and there is no teaching of extracting heat directly from the furnace using a closed-loop circulating fluid for temperature regulation.
CN103776160A describes an indirect heat exchange hot air furnace integrating a radiation section with a heat exchanger to improve heat transfer intensity and reduce equipment size. While the furnace body itself participates in heat transfer, the system still relies on conventional combustion control strategies and does not disclose a dedicated intermediate fluid circuit for controlled heat extraction within the furnace.
CA2096372A1 discloses a furnace incorporating a heat exchanger for transferring heat from combustion gases to another fluid stream. Such systems are primarily concerned with efficient downstream heat exchange and flow management, without addressing real-time thermal regulation of the furnace through internal heat absorption mechanisms.
US4776391A describes a heat exchanger system for transferring heat from flue gases to water or another fluid medium. Although heat recovery is achieved, the arrangement again operates external to the combustion zone, and does not provide a mechanism for modulating furnace temperature through controlled heat withdrawal at the source.
US11612019B2, disclose indirect air heating arrangements incorporating heat generating portions and air heating sections. While such systems improve air heating efficiency, they remain dependent on conventional excess air supply strategies and do not eliminate the need for dilution air within the furnace.
US4275705, introduces secondary and tertiary heat exchange stages to recover additional heat from combustion gases. These systems may include thermosiphon elements; however, they are configured as auxiliary heat recovery units, and not as integral furnace-embedded heat absorbing circuits designed to regulate combustion temperature and reduce excess air requirements.
US4154055, heat is extracted from exhaust gases and transferred to incoming air streams. These systems operate on complex cycles and remain dependent on cooling exhaust gases prior to heat exchange, rather than addressing direct heat extraction from the combustion zone itself.

DISTINCTION OVER PRIOR ART:
The present invention is structurally and functionally distinct from conventional hot air generators, furnace heat recovery systems, economizers, thermic fluid heaters, and indirect heat exchange arrangements disclosed in the prior art.
Conventional hot air generators primarily regulate furnace temperature by introducing substantial quantities of dilution air in addition to excess combustion air. Such systems generally operate with excess air levels in the range of 150–170% above stoichiometric requirements, resulting in increased flue gas volume, higher thermal losses, larger heat exchangers, increased fan capacity requirements, and elevated power consumption. In contrast, the present invention regulates furnace temperature by means of controlled heat extraction through heat absorbing surfaces (20) disposed directly within the furnace (10), thereby substantially reducing or eliminating the requirement for dilution air.
Unlike conventional economizers and downstream heat recovery systems, the present invention extracts heat directly from the combustion zone through a closed-loop intermediate fluid circuit (30). Prior art systems typically recover heat only after combustion by utilizing flue gas heat in secondary exchangers or air preheaters, and therefore do not influence furnace thermal conditions or reduce the air requirements.
Further, prior art water-cooled furnace walls and boiler-type arrangements are primarily intended for structural cooling or steam generation and are not configured for controlled transfer of extracted heat to process air through a dedicated radiator or primary air heater (40). Such systems also generally operate under high-pressure boiler conditions and do not disclose the use of a selectively operable natural circulation or forced circulation intermediate fluid circuit integrated with a hot air generator.
The present invention further differs from known systems in that the rate of heat extraction through the intermediate fluid circuit (30) is regulated based on furnace operating conditions so as to maintain the furnace within a predetermined safe operating range. Furnace temperature is continuously monitored using temperature sensors (36) The prior art neither teaches nor suggests regulation of furnace temperature through controlled internal heat extraction. .
Additionally, the present invention establishes a direct functional relationship between furnace heat extraction and reduction in excess air requirement. By continuously removing heat from the furnace through the intermediate fluid circuit (30), stable combustion conditions are maintained without the need for dilution air, thereby reducing excess air levels to approximately 40–60% above stoichiometric requirements. This results in substantial reduction in flue gas volume, enabling reduction in the size of the primary flue gas heat exchanger (50), induced draft fans (70), combustion air fans (71), and associated ducting (80). The prior art fails to disclose or suggest such integrated thermal and flow optimization.
The present invention also differs in material and thermal configuration. The heat absorbing surfaces (20) and radiator (40) are configured to operate within controlled temperature ranges permitting the use of carbon steel, while the main flue gas heat exchanger (50) remains fabricated from stainless steel due to exposure to hotter and more corrosive flue gases. Such staged and functionally differentiated heat extraction architecture is absent in conventional systems.
Accordingly, the present invention provides a technically distinct and non-obvious solution involving furnace-integrated heat extraction, regulated thermal control, reduced excess air operation, and integrated process air heating through a closed-loop intermediate fluid circuit, thereby achieving improved thermal efficiency, reduced fuel consumption, and reduced auxiliary power requirements not attainable through the teachings of the prior art.

DEFINITIONS:
For the purposes of the present specification and claims, the following terms shall have the meanings assigned hereinbelow, unless the context otherwise requires:
The term “hot air generator” refers to an indirect heating system configured to generate heated process air by transferring thermal energy from combustion products to air through one or more heat exchange arrangements without direct mixing of combustion gases with the process air.
The term “furnace” or “combustor” refers to a combustion chamber configured for burning fuel and generating thermal energy in the form of high-temperature combustion gases and radiant heat.
The term “heat absorbing surfaces” refers to one or more thermally conductive members, including tubes, coils, panels, or channels, disposed within or in thermal communication with the furnace and configured to absorb heat from the combustion zone for transfer to an intermediate fluid.
The term “intermediate fluid” refers to a heat transfer medium circulating within a closed-loop circuit for transferring heat from the furnace to process air, and includes water, pressurized water-steam mixtures, thermic oil, or equivalent heat transfer fluids.
The term “closed-loop intermediate fluid circuit” refers to a substantially sealed fluid circulation system configured to circulate the intermediate fluid between the heat absorbing surfaces and a radiator or primary air heater, with or without phase change of the fluid.
The term “radiator” or “primary air heater” refers to a heat exchange assembly configured to transfer heat from the intermediate fluid to process air.
The term “thermosiphon” refers to a natural circulation mechanism wherein fluid circulation occurs due to density differences generated by temperature gradients between heated and cooled portions of the fluid circuit.
The term “excess air” refers to air supplied to the combustion process in excess of the theoretical stoichiometric air required for complete combustion.
The term “dilution air” refers to additional air supplied to a furnace primarily for reducing furnace temperature and not directly required for the combustion of fuel.
The term “safe operating temperature” refers to a temperature range within which furnace components and heat absorbing surfaces operate without material degradation, thermal instability, or unsafe combustion conditions.
The term “process air” refers to air intended for industrial use after heating by the hot air generator.
The term “stoichiometric air” refers to the theoretical quantity of air required for complete combustion of a given quantity of fuel under ideal conditions.

OBJECTS OF THE INVENTION:
The principal object of the present invention is to provide a hot air generator incorporating a closed-loop heat-absorbing fluid circuit configured to extract heat directly from a furnace, thereby regulating furnace temperature without reliance on dilution air.
Another object of the present invention is to reduce excess air requirements from conventional levels of about 150–170% above stoichiometric air to approximately 40–60%, thereby lowering flue gas volume and associated thermal losses resulting in improved thermal efficiency and reduce fuel consumption.
Another object of the present invention is to reduce the size and capacity of downstream equipment, including heat exchangers, fans, and ducting, thereby lowering capital cost and power consumption.
A further object of the present invention is to provide a system operable under natural or forced circulation of an intermediate fluid, within defined temperature and pressure ranges, ensuring efficient and reliable heat transfer.

SUMMARY OF THE INVENTION:
The present invention relates to a hot air generator configured to improve thermal efficiency and reduce excess air requirements by incorporating a closed-loop heat-absorbing fluid circuit in thermal communication with a furnace.
In accordance with the invention, the hot air generator comprises a combustor (10) for burning fuel and generating high-temperature flue gases, and a main flue gas heat exchanger (50) for transferring heat from the flue gases to process air. The system is characterized by the provision of heat absorbing surfaces (20) disposed within the combustor (10), through which an intermediate heat transfer fluid is circulated via a closed-loop circuit (30).
The intermediate fluid absorbs heat directly from the combustion zone and is conveyed to a radiator or primary air heater (40), wherein the absorbed heat is transferred to incoming process air. The fluid is thereafter recirculated to the combustor (10), thereby establishing a continuous heat extraction cycle. The circulation of the intermediate fluid is affected either by natural circulation based on thermosiphon principles or by means of an optional mechanical pump (31).
By virtue of continuous heat extraction from the furnace, the temperature within the combustor (10) is maintained within safe operating limits without the need for dilution air, thereby substantially reducing the air requirements from conventional levels of about 150–170% above stoichiometric air to approximately 40–60%. This reduction in air leads to a corresponding decrease in flue gas volume, thereby minimizing heat losses and improving overall thermal efficiency.
In a preferred embodiment, a controlled portion of heat, typically in the range of 15–45% of the total thermal output, is extracted through the intermediate fluid circuit, thereby reducing the thermal load on the main flue gas heat exchanger (50) and enabling reduction in its size. The remaining heat is recovered through the main flue gas heat exchanger (50). Residual recoverable heat present in the flue gases is transferred in the combustion air preheater (60) for preheating combustion air.
The invention further enables reduction in the size and capacity of induced draft fans (70), combustion air fans (71), and ducting (80) due to reduced gas flow rates, resulting in lower capital cost and reduced power consumption. The heat absorbing surfaces (20) and radiator or primary air heater (40) may be constructed from carbon steel, while the main flue gas heat exchanger (50) is preferably fabricated from stainless steel to withstand higher flue gas temperatures and corrosive conditions.
The invention thus provides a structurally simple and energy-efficient hot air generator, capable of operating within defined temperature and pressure ranges of the intermediate fluid, and adaptable for various industrial applications requiring controlled hot air supply.

BRIEF DESCRIPTION OF THE DRAWINGS:
A comprehensive understanding of the present invention may be achieved by referring to the following detailed description, which should be read in conjunction with the accompanying drawing. The drawing, which forms an integral part of this specification, provides a visual representation of the invention and, together with the description, elucidates its construction, operation, and key functional aspects.
FIG. 1(a) illustrates a schematic representation of a hot air generator system in accordance with the present invention, showing a combustor, the main flue gas heat exchanger, a combustion air preheater, and an integrated closed-loop intermediate fluid circuit. FIG. 1 (b) shows the location of the heat absorbing surface at a different location based on the type of combustion technology used. Typically, in the case of non-fluidised bed hot air generators.
FIG. 2 illustrates a sectional view of the combustor depicting the arrangement of heat absorbing surfaces disposed within the furnace and exposed to the combustion zone for direct heat extraction.
FIG. 3 illustrates a schematic of the closed-loop intermediate fluid circuit, including flow paths, elevation differences enabling thermosiphon operation, and an optional mechanical circulation pump.
FIG. 4 illustrates a radiator or primary air heater configured to transfer heat from the intermediate fluid to incoming process air, including tube-fin arrangements and airflow paths.
FIG. 5 illustrates a combustion air preheater utilizing residual heat from flue gases to preheat incoming combustion air prior to entry into the combustor.
FIG. 6 illustrates an alternative embodiment of the system configured for natural circulation, wherein the radiator is positioned at a higher elevation relative to the furnace to facilitate thermosiphon-driven fluid flow.
FIG. 7 illustrates a comparative schematic showing reduction in heat exchanger size, duct sizing, and fan capacity relative to conventional hot air generator systems.
The following reference numerals are used in the accompanying drawings to denote corresponding parts of the invention:
• 10 – Combustor / Furnace
• 11 – Fuel feeding arrangement
• 12 – Combustion bed
• 13 – Furnace refractory lining
• 20 – Heat absorbing tubes
• 30 – Closed-loop intermediate fluid circuit
• 31 – Circulation pump (optional)
• 32 – Supply line (hot fluid to radiator)
• 33 – Return line (cooled fluid to furnace)
• 34 – Expansion tank / header
• 36 – Temperature sensor (furnace side)
• 40 – Radiator / primary air heater
• 41 – Radiator tube bundle
• 42 – Fins / extended surfaces
• 43 – Process air inlet
• 44 – Process air outlet
• 50 – Main flue gas heat exchanger (stainless steel)
• 51 – Flue gas inlet
• 52 – Flue gas outlet
• 53 – Process air passage
• 60 – Combustion air preheater
• 61 – Combustion air inlet
• 62 – Preheated combustion air outlet
• 70 – Induced draft fan
• 71 – Combustion air fan
• 80 – Flue gas ducting
• 81 – Stack / chimney

DETAILED DESCRIPTION OF THE INVENTION
The following detailed description sets forth exemplary embodiments of the present invention with reference to the accompanying drawings and reference numerals. The description is intended to enable a person skilled in the art to make and use the invention and is not intended to limit the scope of the invention as defined by the appended claims. Various modifications, substitutions, and equivalent arrangements apparent to persons skilled in the art shall be considered within the scope of the present invention.
Referring to FIG. 1, the hot air generator comprises a combustor or furnace (10) configured for combustion of solid fuel supplied through a fuel feeding arrangement (11) onto a combustion bed (12) disposed within a refractory-lined combustion chamber. The furnace (10) operates at elevated temperatures, typically within a range of about 700°C to 1000°C in the flame zone. The furnace is fluidly connected to a main flue gas heat exchanger (50) configured to transfer heat from flue gases to process air. Flue gases exiting the main flue gas heat exchanger (50) may further pass through a combustion air preheater (60) before being discharged through ducting (80) and a stack (81) with the assistance of an induced draft fan (70). Combustion air is supplied to the furnace by a combustion air fan (71), wherein the system is configured to operate at substantially reduced excess air levels relative to conventional hot air generators.
Referring to FIG. 2, the furnace (10) is provided with heat absorbing surfaces (20) disposed within the combustion zone and positioned to receive radiant and convective heat directly from the combustion process. The heat absorbing surfaces (20) may comprise tubes, coils, or equivalent heat exchange members arranged in serpentine, helical, panel, or other suitable configurations to maximize heat transfer area and thermal exposure. During operation, the surface temperature of the heat absorbing surfaces is maintained within allowable limits of the selected material so as to preserve structural integrity and operational reliability. The placement, geometry, and effective surface area of the heat absorbing surfaces are selected such that approximately 15% to 45% of the total thermal output of the furnace is extracted through the intermediate fluid circuit. The heat absorbing surfaces (20) are positioned within the combustion zone such that extraction of heat through the intermediate fluid circuit (30) maintains furnace temperature within a predetermined operating range without introduction of dilution air.
Referring to FIG. 3, the heat absorbing surfaces (20) are connected to a closed-loop intermediate fluid circuit (30). The circuit comprises a supply line (32) for conveying heated intermediate fluid from the furnace, a return line (33) for returning cooled fluid to the furnace, and an expansion vessel or header (34) configured to accommodate thermal expansion and maintain stable circulation conditions. The intermediate fluid may comprise water, pressurized water, water-steam mixture, thermic oil, or equivalent heat transfer media suitable for the operating temperature range.
In one embodiment, the system operates under natural circulation without the use of a mechanical pump, as illustrated in FIG. 6. In such configuration, the radiator or primary air heater (40) is positioned at a higher elevation relative to the furnace (10), thereby enabling thermosiphon-driven circulation. Heated fluid within the heat absorbing surfaces becomes less dense and rises through the supply line (32), while cooled fluid from the radiator or primary air heater (40) becomes denser and returns through the return line (33). The piping arrangement is configured to minimize hydraulic resistance through appropriate pipe sizing, smooth bends, and optimized flow paths so as to maintain effective natural circulation.
Referring to FIG. 4, the heated intermediate fluid transfers thermal energy to process air within the radiator or primary air heater (40). The radiator or primary air heater (40) comprises a tube bundle (41) provided with fins or extended heat transfer surfaces (42) to enhance heat transfer efficiency. Process air enters through an inlet (43), passes over the heated surfaces and exits through an outlet (44) at an elevated temperature which is further heated to the required temperature in the main flue gas heat exchanger (50) suitable for industrial process applications.
Referring again to FIG. 1, flue gases exiting the furnace pass through the main flue gas heat exchanger (50), which is preferably fabricated from stainless steel due to exposure to elevated flue gas temperatures and corrosive combustion constituents. The flue gas inlet (51) and flue gas outlet (52) define the flue gas flow path through the heat exchanger, while process air passes through a separate passage (53) for indirect heat exchange. Since a portion of heat is extracted directly from the furnace through the intermediate fluid circuit (30), the thermal load on the main flue gas heat exchanger (50) is reduced, thereby enabling reduction in its overall size relative to conventional systems.
The flue gases may further pass through a combustion air preheater (60), wherein incoming combustion air supplied through inlet (61) is preheated prior to entering the furnace through outlet (62). Recovery of residual heat from the flue gases further improves the overall thermal efficiency of the system.
During operation, the furnace temperature is maintained within a predetermined safe operating range, typically between about 700°C and 1000°C in the bulk combustion zone, by continuous extraction of heat through the heat absorbing surfaces (20). Temperature sensors (36) positioned within or adjacent to the furnace provide real-time monitoring of furnace temperature conditions.
The controlled extraction of heat from the furnace through the intermediate fluid circuit directly enables reduction in the air requirements by maintaining stable furnace temperature without the need for dilution air. Consequently, the air requirements are reduced to approximately 40% to 60% above stoichiometric air requirements, as compared to approximately 150% to 170% in conventional systems. In representative operating conditions, total air supply may be reduced from approximately 13.9–14.5 kg of air per kg of fuel to approximately 7.5–8.6 kg of air per kg of fuel, thereby proportionally reducing flue gas volume. The above quantification is for a fluidised bed combustion hot air generators.
Reduction in flue gas volume correspondingly reduces the capacity requirements of the induced draft fan (70), combustion air fan (71), ducting (80), and associated insulation systems. In typical implementations, auxiliary fan power consumption may be reduced by up to approximately 40% due to lower volumetric gas flow rates. Reduced duct sizes additionally lower insulation requirements and minimize surface heat losses.
The extraction of approximately 15% to 45% of the thermal energy output through the intermediate fluid circuit is achieved through appropriate selection of heat absorbing surface area, intermediate fluid flow rate, and temperature differential across the circuit.
The heat absorbing surfaces (20) and radiator or primary air heater (40) may be fabricated from carbon steel or other suitable materials capable of operating within the intended temperature range, typically below about 450°C, thereby reducing manufacturing cost. In contrast, the main flue gas heat exchanger (50) is preferably fabricated from stainless steel to withstand prolonged exposure to higher temperature flue gases and corrosive combustion products.
The present invention therefore provides a hot air generator having improved thermal efficiency, reduced excess air requirement, reduced fuel consumption, and optimized equipment sizing through a structurally integrated and thermally regulated heat extraction arrangement.
, Claims:We claim,
1. A hot air generator with a heat-absorbing fluid circuit for improved thermal efficiency characterised in that
the generator consists of:
- a combustor or furnace (10) configured to burn fuel and generate high-temperature flue gases;
- a main flue gas heat exchanger (50) configured to transfer heat from the flue gases to process air;
- heat absorbing surfaces (20) disposed within the furnace (10) in thermal communication with a combustion zone;
- a closed-loop intermediate fluid circuit (30) operatively connected to the heat absorbing surfaces (20) and to a radiator or primary air heater (40),
wherein an intermediate fluid circulating through the closed-loop intermediate fluid circuit (30) extracts heat directly from the combustion zone through the heat absorbing surfaces (20) and transfers the extracted heat to process air through the radiator or primary air heater (40),
wherein continuous extraction of heat from the furnace maintains furnace temperature within a predetermined operating range without introduction of dilution air into the furnace, thereby reducing excess air requirement and flue gas volume.
2. The hot air generator as claimed in claim 1, wherein the intermediate fluid circuit (30) operates under natural circulation based on thermosiphon effect.

3. The hot air generator as claimed in claim 1, wherein the intermediate fluid circuit (30) operates using a mechanical pump (31) for forced circulation.

4. The hot air generator as claimed in claim 1, wherein the intermediate fluid comprises water, pressurized water, water-steam mixture, thermic oil, or combinations thereof.

5. The hot air generator as claimed in claim 1, wherein the intermediate fluid operates within a temperature range of 120°C to 380°C depending upon fluid type and operating configuration.
6. The hot air generator as claimed in claim 1, wherein the furnace (10) operates with excess air in the range of 40% to 60% above stoichiometric air requirement.

7. The hot air generator as claimed in claim 1, wherein the heat absorbing surfaces (20) are configured to extract approximately 15% to 45% of total thermal energy output of the hot air generator.

8. The hot air generator as claimed in claim 1, wherein the heat absorbing surfaces (20) comprise carbon steel tubes arranged in straight, serpentine, helical, or panel configurations within the furnace (10).

9. The hot air generator as claimed in claim 1, wherein the radiator or primary air heater (40) comprises a tube bundle (41) provided with or without the extended heat transfer surfaces (42) configured to transfer heat from the intermediate fluid to process air.

10. The hot air generator as claimed in claim 1, wherein the furnace is configured to operate at reduced flue gas flow rates resulting from reduction in excess air supplied to the furnace.

11. The hot air generator as claimed in claim 1, wherein the main flue gas heat exchanger (50) is configured to receive reduced thermal load due to partial heat extraction through the intermediate fluid circuit (30) and the primary air heater.

12. The hot air generator as claimed in claim 1, wherein furnace temperature is maintained within a predetermined operating range by continuous heat extraction through the intermediate fluid circuit (30) without introducing dilution air into the furnace (10).

13. The hot air generator as claimed in claim 1, wherein the heat absorbing surfaces (20) and the radiator or primary air heater (40) are fabricated from carbon steel, and the main flue gas heat exchanger (50) is fabricated from stainless steel.

14. The hot air generator as claimed in claim 1, wherein reduction in excess air and flue gas volume reduces heat loss through the flue gases and auxiliary power consumption associated with gas handling fans.
15. The hot air generator as claimed in claim 1, wherein the Primary air heater (40) supplies pre-heated hot air to the main flue gas heat exchanger (50) thus elevates the metal temperature at the air inlet zone of the main flue gas heat exchanger (50), thereby minimizing cold-end corrosion of the main flue gas heat exchanger (50).

16. The hot air generator as claimed in claim 1, wherein the heat absorbing surfaces (20) are positioned within the combustion zone such that extraction of heat through the intermediate fluid circuit (30) maintains furnace temperature within a predetermined operating range without introduction of dilution air.

17. A method for operating a hot air generator,
characterised in that
the method consists the steps of:
- burning fuel in a furnace (10) to generate flue gases;
- extracting heat directly from a combustion zone through heat absorbing surfaces (20) connected to a closed-loop intermediate fluid circuit (30);
- circulating an intermediate fluid through the intermediate fluid circuit (30);
- transferring heat from the intermediate fluid to process air through a radiator or primary air heater (40); and
- maintaining furnace temperature within a predetermined operating range without introducing dilution air into the furnace (10).
18. The hot air generator as claimed in claim 1, wherein, in a non-fluidized bed combustion hot air generator system, continuous extraction of heat through the closed-loop intermediate fluid circuit (30) reduces excess air requirement from approximately 90–120% above stoichiometric air requirement to approximately 40–60% above stoichiometric air requirement.

19. The hot air generator as claimed in claim 1, wherein the closed-loop intermediate fluid circuit (30) is configured to reduce flue gas temperature entering the main flue gas heat exchanger (50), thereby reducing thermal stress and enhancing operational life of heat exchanger tubes.