FORM-2
THE PATENTS ACT. 1970 (39 of 1970)
COMPLETE SPECIFICATION (Section 10, rule 13)
"A Non Corrosive Air Pre-heater using Different Arrangement of Air and Flue Gas Passage 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 Pre-heater
More particularly it is related to non corrosive Air Pre-heater
Still more particularly, it relates to non corrosive Air Pre-heater using different arrangement of air and flue gas passage for the flue gas containing sulfur pollutant
Even more particularly, it relates to Air Pre-heater configuration with Cross configuration and mixed counter parallel flow Air Pre-heater to maximise heat recovery without fear of dew point corrosion.
Even more particularly a corrosion resistant Air Pre-heater with Cross flow with counter parallel flow Air Pre-heater to maximise heat recovery without fear of dew point corrosion by using a unique air bypass control system to maintain metal temperature at 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 efficiency. As the boiler efficiency is primarily a function of flue gas temperature, lots of
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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 Air Pre-heater can be used for this purpose for the clean fuel containing no sulfur. The major limitation of the tubular Air Pre-heater 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 Pre-heater, 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 Air Pre-heater 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.
However, 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 to eliminate corrosion in air Pre-heaters 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 configuration with Cross configuration and mixed counter parallel flow Air Pre-heater to maximize heat recovery without fear of dew point corrosion.
Another object of this invention is to develop a non corrosive air Pre-heater with optimal metal temperature to eliminate corrosion
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Another object of this invention is to develop a non corrosive Air Pre-heater for better and higher heat recovery.
Another object of this invention is to develop a non corrosive Air Pre-heater with unique Air Bypass Damper to maintain metal temperature at off design condition to eliminate corrosion.
SUMMARY OF THE INVENTION
The most popular arrangement for an Air Pre-heater is multipass tubular cross flow Air Pre-heater, where air enters in the coldest zone of the flue gas path and moves towards the hotter zone of the flue gas, which can be called as a counter cross configuration. This type of design has excellent heat transfer performance but it has certain limitation for sulfur containing flue gas. In this type of Air Pre-heater coldest flue gas and air meet together causing very low metal temperature. Due to this effect, this design is very prone to dew point corrosion. The alternative could be parallel cross arrangement, where coldest air enters in the hottest zone of the flue gas. This has an excellent metal temperature profile as the coldest flue gas meet with hottest air and hottest flue gas meet with coldest air. However, this has very poor LMTD (log mean temperature difference) and heat transfer performance. This also put constraints on the heat recovery.
The proposed configuration combines the benefit of the above two configurations. In the proposed configuration, cold air is introduced in intermediate flue gas zone and then it moves towards the colder flue gas zone like parallel cross configuration. The hot air is
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introduced in the hottest flue gas zone. This maximises metal temp with higher heat
recovery.

introduced in the hottest flue gas zone. This maximises metal temp with higher heat recovery.
The proposed configuration gives very good metal temperature profile with a high heat recovery. With the proposed configuration, the metal temperature can be maintained at design condition but in off-design conditions, metal temperature can fall below the recommended metal temperature. The problem of metal temperature falling below the recommended temperature in off-design conditions could be over come by using air bypass control system.
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 represents the effect of sulfur on flue gas dew point and minimum metal temperature to avoid corrosion.
FIG. 2 represents a heat transfer pass arrangement for multiple pass Air Pre-heater with counter cross configuration.
FIG. 3 illustrates heat transfer pass arrangement for multiple pass Air Pre-heater with parallel cross configuration.
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FIG. 4 illustrates effect of heat transfer area on lowest metal temperature and flue gas exit temperature for parallel cross & counter cross configuration of Air Pre-heater.
FIG. 5 illustrates heat transfer pass arrangement for multiple pass Air Pre-heater with proposed configuration.
FIG. 6 illustrates internal of the proposed Air Pre-heater.
FIG. 7 illustrates comparison of flue gas exit temperature and minimum metal temperature of proposed configuration with conventional configurations.
FIG. 8 illustrates the effect of load on minimum metal temperature.
FIG. 9 illustrates bypass control system.
FIG. 10 illustrates the effect of flue gas inlet temperature on minimum metal temperature.
FIG. 11 illustrates percentage of bypass air as a function of load %.
FIG. 12 illustrates percentage of bypass air as a function of Flue gas entry temperature
Numeral 1: Hottest flue gas zone
Numeral 2: where air is introduced in zone Numeral 3: 3rd zone
Numeral 4: coldest zone
Numeral 5- Air outlet duct
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Numeral 6- Tube arrangement in the Air Pre-heater
Numeral 7- Air inlet duct
Numeral 8-Flue gas duct for turning
Numeral 9-Plate to divide air passage
Numeral 10-Air duct for durning
Numeral 11- Flue gas outlet duct
Numeral 12- Flue gas inlet duct
Numeral 13- Main Air Pre-heater
Numeral 14- Air Bypass Damper
DETAILED DESCRIPTION OF THE INVENTION
Fig. 2 represents the Counter cross configuration arrangement of Air Pre-heater where air enters in the coldest zone (Numeral 4) of the flue gas and moves towards the hotter zone of the flue gas and finally comes out from the hottest zone (Numeral 1) of the flue gas.
Numerals 1, 2, 3 & 4 indicate the Air passage in descending order of flue gas temperature.
Fig. 3 represents the Parallel cross configuration arrangement of Air Pre-heater where air enters in the hottest zone (Numeral 1) of the flue gas and moves towards the coldest zone of the flue gas and finally comes out from the coldest zone (Numeral 4) of the flue gas.
Fig. 4 represents a graph comparing the performance of Counter Cross configuration and Parallel Cross configuration. The Parallel Cross configuration has poor recovery but
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excellent metal temperature profile. The Counter Cross has better heat recovery but very poor metal temperature profile. The Parallel Cross configuration can be used for low flue gas temperature application which has low heat recovery potential. This also indicates the need of different configuration to optimise metal temperature with higher heat recovery
The proposed configuration combines the benefit of these two configurations. In the proposed configuration, cold air is introduced in intermediate flue gas zone (Numeral 2) and then it moves towards the colder flue gas zone like parallel cross configuration. Finally the hot air is introduced back to the hottest flue gas zone (Numeral 1). This maximises metal temp with higher heat recovery. Fig. 5 is the schematic diagram of the proposed configuration showing a four pass Air Pre-heater, where air is introduced in zone 2 (Numeral 2), starting from flue gas entry. This hot air passes through zone 3 (Numeral 3) and zone 4 (Numeral 4) and finally enters in the zone 1, (Numeral 1) which is the hottest flue gas zone.
FIG. 6 illustrates the internals of the proposed Air Pre-heater configuration which combines the benefit of Parallel and Cross configurations. The proposed Air Pre-heater configuration is a four pass Air Pre-heater, where air is introduced in the Air Pre-heater via the Air Inlet Duct (Numeral 7). This hot air flows over the Tube Arrangements (Numeral 6) and turns back from the Air Duct (Numeral 10) to re-enter the Tube Arrangements and it finally comes out of the system via Air Outlet Duct (Numeral 5). The Tubes Arrangements are divided into four passes by the Plate (Numeral 9). Meanwhile, the flue gas enters in the Tube Arrangement via the Flue Gas Inlet (Numeral 12). It passes through the Tube Arrangement and turns back in the Flue Gas Duct (Numeral 8) and re-enters the Tube Arrangement and finally come out from the Flue Gas Outlet (Numeral 11).
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Fig. 7 explains how the proposed configuration has good metal temperature profile in combination with higher heat recovery. The proposed configuration has better metal temperature profile but less heat recovery in comparison with Counter cross configuration. Also, the proposed configuration has better heat recovery but poor metal temperature profiles (but sufficient to avoid corrosion) in comparison with Parallel cross configuration.
Therefore, the proposed configuration has optimal benefits in heat recovery and metal temperature profile.
The proposed configuration eliminates the possibility of corrosion at design condition but it does not eliminate the possibility of corrosion in off design condition like lower load or lower flue gas entry temperature. This can be seen from Fig. 8 & Fig. 10, which illustrates the effect of percentage load and flue gas entry temperature on the minimum metal temperature. This shows that the possibility of corrosion at lower load or lower flue gas entry temperature. This problem can be solved by bypassing air quantity in reduced load or reduced flue gas entry temperature. By bypassing air, air temperature increases and air side heat transfer coefficient decreases causing increase in metal temperature. Therefore, the proposed configuration in combination with proposed Air Bypass Damper (Fig. 9) eliminates the possibility of corrosion.
Fig. 9 shows the schematic of the Air Bypass Control System wherein the air bypass duct with controllable Damper (numeral 14) is connected in parallel with Main Air Pre-heater (Numeral 13). The Damper is provided to control bypass air quantity as a function of metal temperature. Fig 11 & Fig. 12 show the effect of the percentage load and flue gas entry temperature on the bypass air quantity.
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We Claim,
1. A Non Corrosive Air Pre-heater using mixed counter and Parallel cross configuration wherein air is introduced in intermediate flue gas zone and then is moved towards the colder flue gas zone and finally the resultant hot air is introduced back to the hottest flue gas zone to maximize the metal temperature with higher heat recovery.
2. The Non Corrosive Air Pre-heater configuration as claimed in claim 1, wherein the Air Pre-heater is a four pass pre-heater comprising of Tube Arrangement, Air Inlet Duct, Flue
, gas Duct, Plate to divide air passage, Air Duct, Flue gas outlet duct, Flue gas inlet duct, Main Air pre-heater and Air Bypass Control System.
3. The Non Corrosive Air Pre-heater as claimed in claim 1 & 2, wherein air is introduced in the said Air Pre-heater via the said Air Inlet Duct.
4. The Non Corrosive Air Pre-heater as claimed in claim 1 & 2, wherein the said Tube Arrangment is made up of plurality of tubes arranged in the said Air Pre-heater.
5. The Non Corrosive Air Pre-heater as claimed in claim 1 & 2, wherein the air coming from the said Air Inlet Duct flows over the said Tube Arrangement and turns back from the said Air Duct to re-enter the said Tube Arrangements and it finally comes out of the system via the said Air Outlet Duct.

6. The Non Corrosive Air Pre-heater as claimed in claim 1 & 2, wherein the said Plate divides the said Tubes Arrangements into four passes.
7. The Non Corrosive Air Pre-heater as claimed in claim 1 & 2, wherein flue gas enters the Tube Arrangement via the said Flue Gas Inlet and passes through the said Tube Arrangement and turns back in the said Flue Gas Duct and re-enters the said Tube Arrangement and finally come out from the said Flue Gas Outlet.
8. The Non Corrosive Air Pre-heater as claimed in claim 1 & 2, wherein the said Air Bypass Control System with controllable Air Bypass Damper is connected in parallel to the said Main Air Pre-heater.
9. The Non Corrosive Air Pre-heater as claimed in claim 1, 2 & 7, wherein air is bypassed through the said Air Bypass Damper in reduced load or reduced flue gas entry temperature whereby the air temperature increases and air side heat transfer coefficient decreases causing increase in metal temperature and eliminating the possibility of corrosion in off design conditions.





ABSTRACT
A Non Corrosive Air Preheater using a mixed Counter and Parallel cross configuration (Fig-5) wherein cold air is introduced in intermediate flue gas zone (Numeral 2) and then it moves towards the colder flue gas zone (Fig. 2) like Parallel cross configuration and the hot air is introduced in the hottest flue gas zone (Numeral 1) which maximises metal temp with higher heat recovery thereby giving very good metal temperature profile with a high heat recovery and wherein metal temperature falling below the recommended temperature in off-design conditions could be over come by using Air Bypass Damper (Fig. 9).