FORM-2
THE PATENTS ACT. 1970 (39 of 1970)
COMPLETE SPECIFICATION (Section 10, rule 13)
Non Corrosive Air Pre-heater using water as Secondary Fluid with Natural Circulation System for the Flue Gas Containing Sulfur Pollutant"
Thermax Limited
with Corporate office at Thermax House, 4 Pune-Mumbai Road, Shivajinagar, Pune 411005,
Maharashtra, India.
an Indian Company registered under the provisions of the Companies Act, 1956,
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE NATURE OF THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED: -
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INTRODUCTION
This invention related to air preheater.
More particularly it is related to non corrosive air preheaters.
Still more particularly, it relates to non corrosive Air Pre-heater using intermediate fluid for the flue gas containing sulfur pollutant.
Even more particularly, it relates to air preheating system with water with natural circulation as secondary medium to maintain metal temperature higher than the dew point temperature.
Even more particularly, it relates to optimizing metal temperature by distribution of heat transfer area of air heater and flue gas cooler.
Even more particularly, it relates to bypassing the air from air heater to maintain metal temperature in off design condition.
Even more particularly, it relates to a non corrosive Air Pre-heater using two heat exchangers in the form of Thermosyphon heat exchanger and normal cross counter heat exchanger and wherein the Thermosyphon Heat Exchanger is placed at the low flue gas temperature zone and normal cross counter heat exchanger at the higher flue gas temperature zone.
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BACKGROUND OF THE INVENTION
The boilers and heaters are primarily used in process heating applications. One of the major concerns in these applications is the increasing fuel cost (and in general the cost of energy). Due to substantial increase in the fuel price and subsequent increase in the operating cost, process heating application demands the development of boiler and heater with higher efficiency. As the boiler efficiency is primarily a function of flue gas temperature, lots of efforts have gone into recovery of the heat from such flue gas. One of such application is air-preheater.
Air Pre-heater is used to increase boiler efficiency by recovery of flue gas heat for preheating combustion air. Simple direct tubular heat exchanger can be used for this purpose for the clean fuel containing no sulfur. The major limitation of the tubular heat exchanger is the cold end corrosion for the fuel containing sulfur.
In the process of combustion, sulfur of the fuel gets converted into sulfur dioxide. Approximately, 1% to 5 % of Sulfur Dioxide gets converted into sulfur trioxide. This Sulfur trioxide combines with water vapour and generates sulfuric acid. During the flue gas heat recovery this sulfuric acid can condense over the heat transfer surface. The Sulfuric acid condensation takes place if the metal temperature goes below sulfuric acid dew point temperature. The dew point temperature of sulfuric acid is the function of partial pressure of water vapour and sulfur trioxide. Fig. 1 explains the effect of the sulfur contents on sulfuric
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acid dew point temperature. This also explains the required minimum metal temperature to avoid sulfuric acid condensation. In air preheater, as air enters at ambient condition (20 to 30 deg. eel.) metal temperature can be less than the recommended metal temperature and Air Pre-heater will be prone to corrosion.
PRIOR ART
An U.S. patent application No. 4034803 claiming a corrosion resistant tubular Air Pre-heater in which combustion air is heated by heat transfer from flue gases from a furnace. The heat exchanger is of a longitudinal type having a central enclosure of rectangular cross-section in which a plurality of tubular heat transfer elements are mounted. Surrounding the central enclosure is an outer plenum. Cold combustion air enters at the bottom of the outer plenum and circulates around and up the plenum to a series of openings on top of the heat transfer elements. The partially heated combustion air then passes downwardly through metal tubes in the heat transfer elements and then passes outwardly to the furnace. The minimum temperature in metal structure enclosing the combustion gases is controlled by two means. One is the use of insulation in selected areas to limit heat transfer rate to cold air impinging upon metal surfaces. The second method is by control of cold air flow incidence on or along heat transfer surfaces so as to minimize rapid heat transfer from any metal surface, which might chill the surface below the dew point.
An U.S. patent application No. 3,623,549 claiming a gas to liquid to gas heat exchange apparatus comprising; a first flow means for a heated gas; a second flow means for a cooler gas, said second flow means being independent of and separated from the first flow means; means for transferring heat from a gas in the first flow means to a gas in the second
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flow means which includes at least two flow circuits for fluid heat exchange media, each such circuit comprising a heat exchanger in the first flow means, a heat exchanger in the second flow means, conduit means extending between the first and second flow means and connecting the two heat exchangers into a closed circulation system, and pump means for circulating a heat exchange medium through the closed circulation system; a first heat exchange liquid in a first of said flow circuits; a second heat exchange liquid having a boiling or degradation point above that of the first heat exchange liquid in a second of said circuits; means for circulating the heated gas first to that heat exchanger of the second flow circuit in the first flow means and then to that heat exchanger of the first flow circuit in said first flow means; means for circulating the cooler gas first to that heat exchanger of the first flow circuit in said second flow means and then to that heat exchanger of the second flow circuit in said second flow means; means for independently controlling the rate of flow of the heat exchange liquid through each of the flow circuits; and means for independently controlling the flow of the gas through first and second flow means
An U.S. patent application No. 4044820 claiming a method for cooling a hot gas close to its dew point without causing condensation comprising
flowing air through a first chamber while flowing the hot gas to be cooled through a second chamber which is separated from the first chamber by a common wall, and
continuously flowing a heat transfer fluid having a high heat capacity as compared to said hot gas through the inlet end of heat transfer tubes which extend back and forth between the two chambers through said common wall in a serpentine path between the two chambers
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which extends in one direction and out the outlet end of the tubes at the opposite end of said serpentine path, whereby the heat which is absorbed by the heat transfer fluid in each section of the tubes in the hot gas chamber is directly transferred to the air as the heat transfer fluid advances through the tubes in the first air chamber.
However, the Patent No. 4034803 is limited to the system claimed above suggests controlling of the metal temperature by insulation and minimising rapid heat transfer. Moreover, the above system is complex and inefficient way of non corrosive air preheaters thereby not satisfying the scientific endeavors of making available a system which is efficient and less complex.
Also, Patent No. 3623549 is limited to the use of two different heat exchanger surfaces for hot and cold gases and use of intermediate fluid for the heat transfer.
And also, Patent No. 4044820 is limited to the use of intermediate fluid and multiple circuits to reduce sulfuric acid condensation for air preheating application. This circuit is based on forced circulation and requires pumping power. This also suggests regulating air quantity to reduce fear of corrosion. It requires multiple pump to maximise recovery and minimise fear of corrosion.
Moreover, the above system is complex and inefficient way of non corrosive air preheaters thereby not satisfying the scientific endeavors of making available a system which is efficient and less complex.
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OBJECTS OF THE INVENTION
The object of this invention is to develop a non corrosive Air Pre-heater using intermediate fluid mainly water with natural circulation for the flue gas containing sulfur pollutant.
Another object of this invention is to develop a non corrosive Air Pre-heater with secondary medium to maintain metal temperature higher than the dew point temperature.
Another object of this invention is to develop a non corrosive Air Pre-heater which optimises metal temperature by distribution of heat transfer area of flue gas cooler and air heater.
Another object of this invention is to develop a non corrosive Air Pre-heater which bypasses the secondary fluids in forced circulation circuit to maintain metal temperature in off design condition.
Another object of this invention is to develop a non corrosive Air Pre-heater for better and higher heat recovery.
SUMMARY OF THE INVENTION
The proposed system comprises of two heat exchangers, (i). flue gas cooler and (ii). Air heater. In this system, water is used as intermediate heat transfer medium. The flue gas
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cooler is a liquid gas heat exchanger where the flue gas is cooled by transferring heat to the water. This heated water vaporises and the water and steam mixture flows through the Riser in the air heater, where air is heated by taking heat of water and steam mixture. The flow of water and steam mixture takes place due to natural circulation (circulation due to density difference in Riser and Down Comer). After the cooling and condensation of water and steam mixture, it flows back to the flue gas cooler through Down Comer. This circuit can be used, when Air Inlet and Flue Gas Outlet is not very far from each other. To eliminate corrosion at off-design condition air quantity can be bypassed using air Bypass Damper.
The proposed Air Pre-heater can be combined with a conventional Air Pre-heater to maximize heat recovery with no fear of corrosion.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of this invention and a better understanding of the principles and details of the invention will be evident from the following description taken in conjunction with the appended drawings in which:
FIG. 1: The effect of sulfur on flue gas dew point and minimum metal temperature to avoid corrosion.
FIG. 2: Schematic diagram of air preheating scheme and control system.
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FIG. 3: The effect of flue gas inlet temperature on required bypass % to maintain metal temperature.
Numeral 1: Air Heater
Numeral 2- Downcomer Numeral 3- Flue Gas Cooler Numeral 4- Riser
FIG. 4: The effect of air inlet temperature on the require % bypass to maintain metal temperature.
FIG. 5: Hybrid air preheating scheme with indirect air preheating scheme at low temperature
zone.
Numeral 5: Thermosyphon Heat exchanger with water as a secondary medium
Numeral 6: Plain Cross Counter Heat Exchanger
Numeral 7: Bypass Damper
DETAILED DESCRIPTION OF THE INVENTION
The proposed Air Pre-heater consists of two heat exchangers: Flue Gas Cooler (Numeral 3) and Air Heater (Numeral 1). These two heat exchangers are connected by a Riser (Numeral 4) and Down Comer (Numeral 2). The Air Heater is of shell and tube design with water as a heat transfer medium. The Flue Gas Cooler can be of shell and tube design or cross flow design. The water gets heated and vapourised in Flue Gas Cooler by taking heat from the flue gas and then water and the steam mixture flows to the Air Heater through the
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Riser. In the Air Heater, the steam and water mixture gets condensed by transferring heat to air and the condensed water returns back to Flue Gas Cooler through Down Comer.
As explained, the minimum metal temperature is primarily responsible for the corrosion and plays a vital role in the design of heat recovery systems. The minimum metal temperature will depend on the temperature of the circulating water. The water temperature inter alia depends on the flue gas inlet temperature, air inlet temperature, flue gas and air quantity and distribution of heat exchanger area. The minimum metal temperature increases with increase in flue gas heat exchanger area and decreases with increase in air heat exchanger area. The optimal selection of heat transfer area of air and flue gas heat exchanger eliminates corrosion on design condition. Still it is not fail safe in off design condition. The circulating secondary fluid temperature decreases with decrease in boiler load as the flue gas quantity and temperature inlet to the Air Pre-heater decreases with decrease in load. When the air quantity is bypassed using a Bypass Damper (Numeral 7) shown in Fig. 2, the heat transfer in secondary heat exchanger reduces. Due to reduction in heat transfer, pressure shifts to the higher pressure which in turns increases circulating water temperature. Similarly, decrease in air inlet temperature reduces the heat transfer thereby reducing the circulating water temperature. The optimum bypass of air quantity assures the required temperature of water as shown in Fig. 4 and Fig. 5.
Fig. 6 illustrates the effect of inlet flue gas temperature change on circulating water temperature. The circulating water temperature decreases with decrease in inlet temperature. When the optimum air quantity is bypassed as shown in Fig. 6, heat transfer in secondary heat exchanger reduces. Due to reduction in heat transfer, pressure shifts to the higher pressure which in turns increases circulating water temperature. This can be seen in Fig. 7,
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where circulating water temperature increases with increase in bypass water quantity for the given values of flue gas inlet temperatures.
However, the above system has a limitation to keep the flue gas temperature higher than the water temperature and air temperature less than the water temperature. Due to this limitation air outlet temperature can not exceed flue gas outlet temperature, which is generally possible with counter cross heat exchanger. This put a limitation on heat recovery. Due to this reason this system is not suitable for high heat recovery.
This problem can be solved by combining the proposed thermosyphon heat exchanger with simple tubular heat exchanger as shown in Fig. 8. In this type of system, Thermosyphon Heat Exchanger (Numeral 5) is placed at the low flue gas temperature zone, where atmospheric air is heated to considerable high temperature. This air is then passed in a counter cross tubular heat exchanger (Numeral 6), where flue gas is at higher temperature. Due to higher air and flue gas temperature, corrosion does not take place in this heat exchanger.
At lower end heat exchanger with secondary fluid has been introduced to reduce corrosion. At higher temperature, the normal Counter Cross heat exchanger is used as it does not have corrosion problem at suggested temperature profile. This arrangement is to maximise recovery and minimise corrosion.
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We Claim,
1. A Non Corrosive Air Pre-heater for flue gas containing sulfur pollutant with a Plain Cross Counter Heat Exchanger and a thermosyphon heat exchanger using water as intermediate fluid and comprising of a bypass damper, a riser, a downcomer, a Flue Gas cooler, an Air heater.
2. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said heat exchangers are Flue Gas Cooler and Air Heater which are connected by the said Riser and said Downcomer for the natural circulation of water.
3. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein water gets heated and vapourised in the said Flue Gas Cooler by taking heat from the flue gas and then water and the steam mixture flows to the said Air Heater through the said Riser.
4. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the steam and water mixture gets condensed in the said Air Heater by transferring heat to air and the condensed water returns back to said Flue Gas Cooler through the said Downcomer.
5. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said Flue Gas Cooler and the said Air Heater are placed at height difference to induce natural circulation

6. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein water is used as intermediate heat transfer medium.
7. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein air is bypassed through the said Air Bypass Damper to produce optimum bypass of air quantity to assure the required temperature of water.
10. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said Thermosyphon Heat Exchanger is placed at the low flue gas temperature zone, where atmospheric air is heated to considerable high temperature and then the air is then passed in a said Counter Cross Heat Exchanger where flue gas is at higher temperature.

ABSTRACT
A Non Corrosive Air Pre-heater combines a plain counter cross heat exchanger (Numeral 6) and a thermosyphon heat exchanger (Numeral 5) with a closed loop consisting of two heat exchangers in the form of Flue Gas Cooler (Numeral 3) and Air Heater (Numeral 1) ,a Riser (Numeral 4), a Downcomer (Numeral 2), an Air Bypass Damper (Numeral 7) to bypass optimum air quantity to assure required circulating water temperature, wherein Flue Gas Cooler and Air Heater are connected by the Riser and Downcomer and wherein water gets heated and vaporized in the said Flue Gas Cooler by taking heat from the flue gas and then water and the steam mixture flows to the Air Heater through the Riser and wherein the steam and water mixture gets condensed in the Air Heater by transferring heat to air and the condensed water returns back to Flue Gas Cooler through the Downcomer and wherein the Flue Gas Cooler and the Air Heater are placed at height difference to induce natural circulation and wherein water is used as an intermediate heat transfer medium and wherein the Thermosyphon Heat Exchanger (Numeral 5) is placed at the low flue gas temperature zone, where atmospheric air is heated to considerable high temperature and then the air is passed in a said Counter Cross Heat Exchanger (Numeral 6) eliminating fear of corrosion due to higher air and flue gas temperature resulting in high metal temperature.