CLIAMS:1. A hybrid air heater (100) including a furnace (104) for combusting a fuel to produce hot flue gases, a reversal chamber (105) for conveying said hot flue gases, a shell (102) containing at least one heat exchanger (110) for heating process air by means of heated water and said hot flue gases;
wherein, said hybrid air heater (100) comprises:
a first water-wall (106) including a first set of water tubes positioned surrounding said furnace (104) for heating water by said hot flue gases, said shell (102) being positioned at a location selected from the operative top of said first water-wall (106) and the operative front of said first water wall (106); and
a second water-wall (108) including a second set of water tubes defining said reversal chamber (105) for heating water by said hot flue gases being conveyed therefrom, said second water-wall (108) being operatively connected to said first water-wall (106); and
said shell (102) being adapted to receive said heated water from said first water-wall (106) and said second water-wall (108), in which said at least one heat exchanger (110) containing a set of fire tubes (113) for conveying said hot flue gases and a set of air tubes (114) for conveying process air to be heated provides heated process air by extracting heat from said heated water and said hot flue gases.

2. The hybrid air heater as claimed in claim 1, wherein said furnace (104) comprises a combustor selected from stationary grate, bubbling bed, chain grate, moving grate, reciprocating grate, underfeed stoker and fluidized bed.

3. The hybrid air heater as claimed in claim 1, wherein said air heater (100) comprises a secondary heat exchanger (112) for heating said process air by means of said heated water and said hot flue gases.

4. The hybrid air heater as claimed in claim 3, wherein set of fire tubes (113) of said heat exchanger (110) is in fluid communication with set of fire tubes (124) of said secondary heat exchanger (112) for conveying said hot flue gases.

5. The hybrid air heater as claimed in claim 1, wherein said at least one heat exchanger (110) comprises a smoke chamber (120).

6. The hybrid air heater as claimed in claim 1, wherein said at least one heat exchanger (110) comprises a process air inlet chamber (116) and a heated process air outlet chamber (118).

7. The hybrid air heater as claimed in anyone of the preceding claims, wherein heated process air outlet chamber (130) of said secondary heat exchanger (112) is in fluid communication with heated process air outlet chamber (118) of said heat exchanger (110) for controlling process air temperature by means of controlled air distribution.

8. The hybrid air heater as claimed in claim 7, wherein an external air damper is provided for controlling the air distribution.

9. The hybrid air heater as claimed in claim 1, wherein metal sheet covering panels (134) are provided for packaging said hybrid air heater (100).

10. The hybrid air heater as claimed in claim 1, wherein one or more means selected from refractory walls and baffle panels are provided with said first water-wall (106) and said second water-wall (108) for allowing multiple passes for said hot flue gases.

11. A method for heating process air in a hybrid air heater by combusting a fuel in a furnace to produce hot flue gases, and conveying said hot flue gases via a reversal chamber to a shell containing at least one heat exchanger for heating process air by means of heated water and said hot flue gases,
wherein, said method comprises the following steps:
extracting heat from said hot flue gases in water by providing a first water-wall including a first set of water tubes positioned surrounding said furnace, said shell being positioned above or in front of said first water-wall for compacting said air heater;
extracting heat from said hot flue gases in water by providing a second water-wall including a second set of water tubes which define said reversal chamber;
receiving said heated water from said first water-wall and said second water-wall in said shell;
conveying said hot flue gases in a set of fire tubes of said at least one heat exchanger to heat process air being conveyed through a set of air tubes of said at least one heat exchanger, thereby heating said process air by said heated water and said hot flue gases. ,TagSPECI:FIELD OF THE DISCLOSURE
The present disclosure relates to an air heater.
More particularly, the present disclosure relates to a packaged hybrid air heater with flexibility for the type of fuel, multiple combustors and air temperature control.

BACKGROUND
Industries require hot dry air having temperature between 120 ºC to 140 ºC for curing, preheating, drying, soldering, etc. An air heater is provided for these purposes in which cold air is heated by flue gases generated from the combustion of fuels such as biomass, fossils or waste. In a conventional air heater system, cold air is passed over the furnace in convective tubes to directly receive heat from the hot combustion gases. In such air heaters, there is a high possibility of the convective tubes getting overheated, leading to creep failure. This problem is solved by using steam based system, where steam is generated in a boiler and passed through external radiator to produce hot air. In this type of system, contact between flue gas and air is avoided by using steam as a secondary medium. This system has an inherent drawback like flash steam loss and poor system efficiency. 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. A drawback of this system is the higher power consumption due to use of external circulation pump.

This problem has been solved by the introduction of integral furnace air heater, where the furnace is placed inside the shell. The air heater works on indirect air heating concept where air tubes are placed inside the shell. This design offers inherent limitations to furnace dimensions. The air heater works primarily on fuels like coal, wood logs and biomass briquettes. The flue gases generated inside the furnace are taken to a set of fire tubes through an internal reversal chamber. These flue gases heat the water present inside the shell, and this water then heats the air flowing in the air tubes. As the furnace dimensions cannot be increased beyond certain limits, the combustion volume is low. The smaller combustion volume leads to poor volatile combustion and poor residence time of solid / biomass fuels. Ultimately, this air heater works with poor efficiency and emissions. It can accommodate combustors like stationary grate, bubbling bed and chain grate. The other combustors like reciprocating grate, moving grate and fluidized bed cannot be placed inside the furnace. This results in lack of fuel flexibility.

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.

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 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. It is therefore desirable to provide a packaged hybrid air heater that ameliorates the above-noted drawbacks of the conventional air heaters.
OBJECTS
Some of the objects of the hybrid air heater of the present disclosure, which at least one embodiment herein satisfies, are as follows:
It is an object of the present disclosure to provide a hybrid air heater which gives flexibility in the type of fuel, multiple combustors and air temperature control.

It is another object of the present disclosure to provide a hybrid air heater which has a longer life, reliable operation, low operating costs, and high efficiency.

It is yet another object of the present disclosure to provide a hybrid air heater which has a compact and simple construction.

Other objects and advantages of the present disclosure will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present disclosure.

SUMMARY

In accordance with the present disclosure, there is provided a hybrid air heater including a furnace for combusting a fuel to produce hot flue gases, a reversal chamber for conveying said hot flue gases, a shell containing at least one heat exchanger for heating process air by means of heated water and said hot flue gases;
wherein, said hybrid air heater comprises:
a first water-wall including a first set of water tubes positioned surrounding said furnace for heating water by said hot flue gases, said shell being positioned at a location selected from the operative top of said first water-wall and the operative front of said first water wall; and
a second water-wall including a second set of water tubes defining said reversal chamber for heating water by said hot flue gases being conveyed therefrom, said second water-wall being operatively connected to said first water-wall; and
said shell being adapted to receive said heated water from said first water-wall and said second water-wall, in which said at least one heat exchanger containing a set of fire tubes for conveying said hot flue gases and a set of air tubes for conveying process air to be heated provides heated process air by extracting heat from said heated water and said hot flue gases.

The said furnace comprises a combustor selected from stationary grate, bubbling bed, chain grate, moving grate, reciprocating grate, underfeed stoker and fluidized bed.

Preferably, said air heater comprises a secondary heat exchanger for heating said process air by means of said heated water and said hot flue gases. More preferably, set of fire tubes of said at least one heat exchanger are in fluid communication with set of fire tubes of said secondary heat exchanger for conveying said hot flue gases.

Additionally, said at least one heat exchanger comprises a smoke chamber. Furthermore, said at least one heat exchanger comprises a process air inlet chamber and a heated process air outlet chamber.

Advantageously, heated process air outlet chamber of said secondary heat exchanger is in fluid communication with heated process air outlet chamber of said at least one heat exchanger for controlling process air temperature by means of controlled air distribution. Furthermore, an external air damper may be provided for controlling the air distribution.

In accordance with the present disclosure, metal sheet covering panels are provided for packaging said hybrid air heater.

Additionally, one or more means selected from refractory walls and baffle panels are provided with said first water-wall and said second water-wall for allowing multiple passes for said hot flue gases.
In accordance with the present disclosure, there is provided a method for heating process air in a hybrid air heater by combusting a fuel in a furnace to produce hot flue gases, and conveying said hot flue gases via a reversal chamber to a shell containing at least one heat exchanger for heating process air by means of heated water and said hot flue gases,
wherein, said method comprises the following steps:
extracting heat from said hot flue gases in water by providing a first water-wall including a first set of water tubes positioned surrounding said furnace, said shell being positioned above or in front of said first water-wall for compacting said air heater;
extracting heat from said hot flue gases in water by providing a second water-wall including a second set of water tubes which define said reversal chamber;
receiving said heated water from said first water-wall and said second water-wall in said shell;
conveying said hot flue gases in a set of fire tubes of said at least one heat exchanger to heat process air being conveyed through a set of air tubes of said at least one heat exchanger, thereby heating said process air by said heated water and said hot flue gases.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The hybrid air heater of the present disclosure will now be described with the help of the accompanying drawings, in which:

FIGURE 1 illustrates a front view of a preferred embodiment of the hybrid air heater;

FIGURE 2 illustrates an isometric view of the preferred embodiment of the hybrid air heater of FIG.1;

FIGURE 3 illustrates a side-view of the preferred embodiment of the hybrid air heater of FIG.1;
FIGURE 4 illustrates a sectional-view of the preferred embodiment of the hybrid air heater of FIG. 1 with a secondary air heater 112; and

FIGURE 5 illustrates the preferred embodiment of the hybrid air heater of FIG. 1 with metal sheet covering panels.

DETAILED DESCRIPTION
A system and a method of the present disclosure will now be described with reference to the embodiments which do not limit the scope and ambit of the disclosure.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

In the integral furnace air heaters, a furnace is placed inside a shell. The air is heated indirectly through an intermediate means such as water which is filled in the shell and air passed through air tubes placed inside the shell. Hot flue gases are generated in the furnace by combustion of a fuel which are passed via an internal reversal chamber through fire tubes placed inside the shell to heat the water and thereby the air. The furnace dimensions cannot be increased beyond a certain limit, thus the combustion volume is low. The other disadvantages of smaller furnace are poor volatile combustion and poor residence time of solid/biomass fuels. Ultimately, such air heater works with poor efficiency and emissions. It can accommodate combustors like stationary grate, chain grate for solid/biomass fuels and also liquid and gaseous fuel combustors. The other combustors like reciprocating grate, moving grate and fluidized bed cannot be placed inside the furnace. This results in lack of fuel flexibility.

The hybrid air heater of the present disclosure overcomes the above-noted drawbacks of the conventional hybrid air heaters. In the hybrid air heater of the present disclosure, the furnace is placed outside the shell. The furnace is surrounded by a water-wall. The shell is placed above the water-wall to make the air heater compact and reduce its footprint. The water-wall includes a set of closely-packed water tubes which circulate water therein. The water extracts heat from the hot flue gases produced in the furnace during combustion becoming heated water. This heated water or mixture of hot water & steam is circulated by means of natural circulation to the shell to transfer heat to process air being circulated through the same shell. The shell contains a set of fire tubes which receive the hot flue gases. The shell contains a set of air tubes which circulate the process air to be heated. As the furnace is placed outside the shell, there is flexibility in the type of combustor and fuel. The temperature of the process air can also be controlled.

Figures 1, 2 & 3 illustrate the hybrid air heater of the present disclosure, showing the front view, the isometric view and the side-view, respectively; the air heater being generally referenced by the numeral 100. The air heater 100 comprises a furnace positioned at location 104. The furnace 104 comprises a combustor which is selected from a stationary grate, a bubbling bed, a chain grate, a moving grate, a reciprocating grate, a underfeed stoker and a fluidized bed. The furnace 104 is surrounded by a first water-wall 106. The first water-wall 106 includes a first set of closely-packed water tubes for circulating water to be heated. A shell 102 is positioned at the operative top of the first water-wall 106. Alternatively, the shell 102 can be also placed in front of first water wall 106. A fuel such as coal, wood, biomass, etc., is combusted in the furnace 104 to produce hot flue gases. A portion of heat from the hot flue gases is extracted by the water circulated in the first set of water tubes of the first water-wall 106. A second water-wall 108 is operatively connected to the first water-wall 106. The second water-wall 108 includes a second set of closely-packed water tubes for circulating water to be heated. The second water-wall 108 is taller than the first water-wall 106, being adapted to define an internal reversal chamber 105 (shown in Fig. 4). The hot flue gases navigate through the internal reversal chamber 105 to the shell 102. During the process, a portion of heat from the hot flue gases is extracted by the water circulated in the second set of water tubes of the second water-wall 108. A refractory wall and/or baffle panels are provided with the first water-wall 106 and the second water-wall 108 for allowing multiple passes of the hot flue gases. This offers higher residence time for combustion of solid/biomass fuels. The turning of flue gases inside the second water-wall 108 because of baffles, results into better emission efficiency.

The shell 102 comprises at least one heat exchanger 110 (shown in Fig.4). The heat exchanger 110 comprises a set of fire tubes 113 for conveying the hot flue gases from the internal reversal chamber 105 and a set of air tubes 114 for conveying the process air to be heated. The shell 102 is filled with water, preferably the heated water from the first water-wall 106 and the second water-wall 108. The process air is heated by extracting heat from the hot flue gases and the heated water.

With this type of arrangement, the air heater 100 can be packaged inside the manufacturer’s factory and there is no need to assemble different components at customer’s site. As the furnace 104 is placed outside the shell 102, all type of combustors can be used without need to alter the design to accommodate different combustors. The external furnace also gives higher combustion volume.

Figure 4 of the accompanying drawings illustrates a sectional view of the preferred embodiment of the present disclosure with a secondary air heater 112. The hot flue gases are produced in the furnace 104. The first water-wall 106 is positioned surrounding the furnace 104. The hot flue gases are conveyed through the internal reversal chamber 105 defined by the second water-wall 108. A portion of heat from the hot flue gases is extracted by the water circulated through the water tubes of the first water-wall 106 and the second water-wall 108. The heated water is received in the shell 102. The hot flue gases are received in the shell 102 at the set of fire tubes 113 of the heat exchanger 110 from the internal reversal chamber 105. A stream of process air to be heated is received in the shell 102 at a process air inlet chamber 116. The process air flows through the set of air tubes 114 from the process air inlet chamber 116. The hot flue gases lose heat to the water in the shell 102 and thereby heat the process air passed through the air tubes 114. The partially heated flue gases after losing heat to water in the shell 102 exit the set of fire tubes 113 into a smoke chamber 120. The heated process air exits at heated process air outlet chamber 118.

The smoke chamber 120 of the heat exchanger 110 is provided in fluid communication with a smoke inlet chamber 128 of the secondary heat exchanger 112 to provide the partially cooled flue gases to the secondary air heater 112. A stream of process air to be heated is received in the secondary heat exchanger process air inlet chamber 126. The process air to be heated is carried through the set of air tubes 122. The partially cooled flue gases flow through the set of fire tubes 124 from the smoke inlet chamber 128. The secondary heat exchanger 112 heats the process air via an intermediate medium such as water. Due to lower flue gas temperature in the secondary heat exchanger 112, it operates at lower temperature in comparison with the main heat exchanger 110. This helps to achieve higher heat transfer and efficiency of the air heating system. The heated process air exits the secondary heat exchanger 112 through the secondary heat exchanger heated process air outlet chamber 130. The cooled flue gases exit the secondary heat exchanger 112 through the smoke outlet chamber 132. The secondary heat exchanger heated process air outlet chamber 130 may be provided in fluid communication with the heated process air outlet chamber 118 for directing flow of the heated process air.

The secondary heat exchanger 112 thus acts as a heat recovery unit. The system is designed in such a way that a first stream of the process air to be heated passes through the shell 102 and a second stream of the process air passes through the secondary heat exchanger 112. Both the heated process air streams may be mixed to achieve the required process air temperature. The air tubes 114 inside the main heat exchanger 110 and the air tubes 122 of the secondary heat exchanger 112 are designed so as to achieve the required heat recovery and at the same time required process air temperature for the given flow. The air distribution in both these streams is governed by the designed air tube configuration in the main heat exchanger 110 and the secondary heat exchanger 112. An additional damper is used for optimal air distribution to achieve desired process air temperature. If the process requires very low air temperature and high volume of air, it is achieved by introducing a suction damper at the outlet of air heater.

Metal sheet covering panels 134 may be provided for packaging the hybrid air heater 100. The packaged air heater is illustrated in the Figure 5 of the accompanying drawings.

TECHNICAL ADVANCEMENT
The hybrid air heater, as described in the present disclosure, has several technical advantages including, but not limited to, the realization of: the hybrid air heater gives flexibility in the type of fuel, multiple combustors and air temperature control; and has a longer life, reliable operation, low operating costs, high efficiency, and a compact and simple construction.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the invention as it existed anywhere before the priority date of this application.

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.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.