FORM-2
THE PATENTS ACT. 1970 (39 of 1970)
COMPLETE SPECIFICATION (Section 10, rule 13)
"Non corrosive Air Pre-heater using intermediate fluid 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 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 primary and secondary heat exchanger.
Even more particularly, it relates to bypassing the secondary fluids in forced circulation circuit to maintain metal temperature in off design condition.
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
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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 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
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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 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
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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 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.
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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.
OBJECTS OF THE INVENTION
The object of this invention is to develop a non corrosive Air Pre-heater using intermediate fluid 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 primary and secondary heat exchanger.
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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 system comprises of two heat exchangers: (i). Flue Gas Heat Exchanger and (ii). Air Heat Exchanger and a secondary heat transfer medium fluid. The Flue-Gas Heat Exchanger is a liquid gas heat exchanger where a secondary fluid flows inside the Coil and gets heated by the flue gas flowing over the coils. This heated secondary fluid then flows through the Coils in the Air Heat Exchanger, heating the air flowing over these coils. The flow of secondary fluid takes place due to forced circulation using external pump. The selection of secondary fluid is very critical as the degradation of fluid should not take place at the maximum possible temperature of the fluid.
In the proposed system, the bypass arrangement for the secondary fluids has been proposed to eliminate corrosion at off-design conditions. 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
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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.
Numeral 1: Control Valve.
Numeral 2: Pump for forced circulation. Numeral 3: Flue gas heat exchanger. Numeral 4: Air Heat exchanger.
Numeral 5: Coils.
FIG. 3: The effect of flue gas inlet temperature on required bypass % to maintain metal temperature.
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 6: Closed Loop Heat Exchanger
Numeral 7: Plain Cross Counter Heat Exchanger
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DETAILED DESCRIPTION OF THE INVENTION
The system comprises of two heat exchangers: (i). Flue Gas Heat Exchanger and (ii). Air Heat Exchanger and a secondary heat transfer medium fluid.
The Flue Gas heat exchanger (Numeral 3) is of Cross flow configuration with secondary fluids inside the Coils (Numeral 5) and flue gas over the Coils. In this Heat Exchanger, the secondary medium is heated by taking heat from the flue gas. The Air Heat Exchanger (Numeral 4) is of Cross flow configuration with secondary fluids inside the Coils (Numeral 5) and air over the Coils. In this Heat Exchanger, the secondary medium is cooled by giving off heat to the air.
This system is based on the forced circulation and requires external Pump (Numeral 2) for the circulation of secondary medium. This system also has a Control Valve (Numeral 1) to bypass secondary fluid to control the minimum metal temperature at off-design condition.
The minimum metal temperature is the function of secondary fluid temperature. The secondary fluid temperature is primarily function of 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
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boiler load as the flue gas quantity and temperature inlet to the Air Pre-heater decreases with decrease in load. When the secondary fluid quantity is bypassed heat transfer in air heat exchanger reduces. Due to reduction in heat transfer, secondary fluid outlet temperature shifts to the higher temperature. Similarly, decrease in air inlet temperature reduces the heat transfer thereby reducing the circulating water temperature. The optimum bypass of secondary fluid quantity will assure the required temperature of secondary fluid as shown in Fig 3 & Fig. 4.
However, the above system has a limitation to keep the flue gas temperature higher than the secondary fluid 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 Closed Loop heat exchanger with simple tubular heat exchanger as shown in Fig. 5. In this type of system, Closed Loop Heat Exchanger (Numeral 6) 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 Plain Cross Heat Exchanger (Numeral 7), 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 is placed to reduce corrosion. At the higher temperature, the normal Plain Cross Heat Exchanger is used as it does not have corrosion problem at the suggested temperature profile. This arrangement is to maximise heat 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 closed loop heat exchanger using intermediate fluid and comprising of a Control Valve, a Pump for forced circulation, a Flue Gas Heat Exchanger, an Air Heat Exchanger.
2. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said The Flue Gas Heat Exchanger is of Cross flow configuration with secondary fluids inside the Coils and flue gas over the Coils.
3. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the secondary medium is heated by taking heat from the flue gas in the said Flue Gas Heat Exchanger.
4. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said Air Heat Exchanger is of Cross flow configuration with secondary fluids inside the Coils and air over the Coils.
5. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the secondary medium is cooled in the Air Heat Exchanger by giving off heat to the air.
6. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said Pump causes forced circulation of secondary medium.
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7. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said Control Valve is provided to bypass secondary fluid to control the minimum metal temperature at off-design condition.
8. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the said Closed Loop Heat Exchanger is placed at the low flue gas temperature zone, where atmospheric air is heated to considerable high temperature to eliminate corrosion in plain cross counter heat exchanger.
9. The Non Corrosive Air Pre-heater as claimed in claim 1, wherein the air coming out of the Closed Loop Heat Exchanger is passed through a Plain Cross Heat Exchanger where flue gas is at higher temperature and due to higher air and flue gas temperature, corrosion does not take place in this heat exchanger.
Dated this 19th August, 2007

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ABSTRACT
A Non Corrosive Air Pre-heater with a closed loop heat exchanger (Numeral 6) to heat atmospheric air to sufficiently high temperature wherein an intermediate fluid is used to transfer heat from flue gas to the air, comprising a Control Valve (Numeral 1) to bypass the intermediate fluid to control the minimum metal temperature at off design condition, a Pump (Numeral 2) for forced circulation of intermediate fluid, a Flue Gas Heat Exchanger (Numeral 3) to heat intermediate fluid by taking heat from flue gas , an Air Heat Exchanger (Numeral 4) to heat air by taking heat from the intermediate fluid, and then the heated air is passed through a Plain Cross Counter Heat Exchanger (Numeral 7), where higher air and flue gas temperature, avoids corrosion in this heat exchanger.