Title of Invention	A THERMOSYPHON ECONOMISER FOR THE FLUE GAS CONTAINING SULFUR DIOXIDE AND SULFUR TRIOXIDE
Abstract	A boiler economizer wherein feed water is not directly heated by flue gas but circulating water as a secondary fluid is used to heat the feed water and which comprises of Risers (Numeral 3), Downcomers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5), Secondary heat exchanger (Numeral 2) and Drum (Numeral 1) forming a closed loop for natural circulation of water and wherein the said Primary Heat Exchanger Coil is used to transfer the heat from flue gas to secondary fluid and wherein the boiler feed water is passed through the said Drum where it is heated in Secondary heat exchanger coil (Numeral 2) and the hot feed water is sent to the boiler and such economizer provided by a Control Valve (Numeral 14) for controlling the water flow thereby controlling the metal temperature.

Title of Invention

A THERMOSYPHON ECONOMISER FOR THE FLUE GAS CONTAINING SULFUR DIOXIDE AND SULFUR TRIOXIDE

Abstract

A boiler economizer wherein feed water is not directly heated by flue gas but circulating water as a secondary fluid is used to heat the feed water and which comprises of Risers (Numeral 3), Downcomers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5), Secondary heat exchanger (Numeral 2) and Drum (Numeral 1) forming a closed loop for natural circulation of water and wherein the said Primary Heat Exchanger Coil is used to transfer the heat from flue gas to secondary fluid and wherein the boiler feed water is passed through the said Drum where it is heated in Secondary heat exchanger coil (Numeral 2) and the hot feed water is sent to the boiler and such economizer provided by a Control Valve (Numeral 14) for controlling the water flow thereby controlling the metal temperature.

Full Text	FORM-2 THE PATENTS ACT. 1970 (39 of 1970) COMPLETE SPECIFICATION (Section 10, rule 13) "Thermosyphon Economiser for the flue gas containing sulfur dioxide and sulfur trioxide" 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: - 1 FIELD OF THE INVENTION This invention relates to an economiser using secondary medium to maintain metal temperature higher than the dew point temperature Particularly, this invention relates to an economizer using water as a secondary medium working in thermosyphon loop. More particularly, this invention relates to an economizer with low temperature flue gas heat recovery system. Even more particularly, this invention relates to an economizer with bypass corrosion management system to control corrosion at off design condition primarily arising from lower feed water temperature and less load. 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 economiser. 2 The economiser is used to recover the boiler flue gas heat by feed water heating. This is used to increase the thermal efficiency of the system. For the clean fuel without sulfur contamination, it is very easy to recover the heat as flue gas does not have any condensation or subsequent corrosion problem. But the fuel containing sulfur has major limitation as it is very prone to dew point corrosion. In the process of combustion, sulfur of the fuel gets converted into sulfur dioxide. Approximately, 1% - 5 % of sulfur Dioxide gets converted into sulfur tri oxide. 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. However, the problem of cold end corrosion has been persistent in most of the economizers as most of such economizers have the problem of not having the water temperature at the recommended level to avoid sulfuric acid condensation. The Feed water temperature can vary between 30 Deg-C to 90 Deg-C depending on condensate recovery, which is not sufficient to avoid dew point corrosion. 3 PRIOR ART An U.S. patent application No. 4,318,366 claiming a boiler having a combustion chamber and a flue comprising of, a first heat exchanger disposed in said flue for exchanging heat between boiler feed liquid and gaseous combustion products in said flue, flow dividing means having first and second outlets and an inlet connected to receive boiler feed liquid and having adjustable proportioning means for providing a first portion of said liquid to said first outlet and a second portion to said second outlet, a second heat exchanger disposed exteriorly of said boiler and said flue and having a first flow path and a second flow path for exchanging heat with liquid flowing in said first flow path, said first flow path being connected in series between said first outlet and said first heat exchanger and said boiler, said second outlet being connected to said first heat exchanger in a parallel relation to said first flow path, temperature responsive means disposed in said flue for measuring the temperature of gaseous combustion products in said flue after passage thereof over said first heat exchanger and for positioning said proportioning means to maintain the proportion of feed liquid flowing at each outlet so as to maintain the temperature of said combustion products at a preselected level solely with heat extracted from said flue gases. 4 An U.S. patent application No. 4,470,449 claims, an apparatus for removing heat from a gas stream to heat a liquid stream comprising: an economizer having a shell and tube construction having first, second and third sections, said second section being intermediate said first and third sections, said gas stream flowing sequentially through said first, second and third sections and said liquid stream flowing sequentially through said second, third and first sections, said gas stream and said liquid stream being in concurrent flow in said second section and in countercurrent flow in said first and third sections. An U.S. patent application No. 4,173,949, claims, In a boiler system which includes a boiler for boiling feedwater therein to produce steam, an exhaust stack connected to said boiler for removing flue gas therefrom, and a fuel economizer mounted in said stack for preheating feedwater in said economizer prior to introduction of said feedwater into said boiler, and wherein said fuel economizer preheats the feedwater therein by passing the feedwater in heat exchange with flue gas from the boiler; an improved method of preheating said feedwater prior to its introduction into said economizer which comprises passing said feedwater through a preheater heat exchanger prior to its introduction into said economizer, conducting a first portion of steam from said boiler through a first conduit and into said preheater heat exchanger to heat said feedwater therein, conducting a second portion of steam from said boiler through a second conduit and into said preheater heat exchanger to heat said feedwater therein, controlling the flow of steam through said first conduit in response to the temperature of feedwater entering said economizer whereby the temperature of the feedwater entering said economizer is above the dew point of the corrosive acids in the flue gas passing 5 through said economizer, and controlling the flow of steam through said second conduit in response to the temperature of the exit gas from said exhaust stack to maintain the temperature of the flue gas above a predetermined temperature which will minimize corrosion within the exhaust stack due to the condensation of corrosive acids therein. The 4,318366 application recommends using two heat exchangers wherein the first heat exchanger is used to recover heat from the flue gas and second is used to heat feed water before entering to the first heat exchanger. Hot feed water from first heat exchanger is passed through second heat exchanger for heating cold feed water. Cold feed water has a bypass line with control valve, which operates depending on flue gas exit and feed water temperature. The 4470449 application suggests dividing the complete heat exchanger in three part and cold water should be entered in intermediate section. The hot feed water then enters in last section and finally enters in first section. This is done to maximise recovery and metal temperature. This also suggests using flue gas bypass line for the first heat exchanger to increase higher flue gas temperature. But this has a drawback as the metal temperature is primarily governed by water temperature and the flue gas temperature plays very insignificant role in maintaining metal temperature. Lastly, the 4173949 application suggests the preheating of feed water before entering to economiser. The preheating is done by live steam and amount of steam is controlled by sensing the feed water temperature. 6 These systems are complex and are lacking in providing a integrated and high quality self-contained thermosyphon economiser for the flue gas containing sulfur dioxide and sulfur trioxide. Thus, the problem of cold end corrosion still persists as the water temp is less than the recommended temperature to avoid sulfuric acid condensation ( Feed water temp, can vary between 30 Deg-C to 90 Deg-C depending on condensate recovery). Also, the commonly used solution for such cold end corrosion by preheating water to minimum metal temp to avoid corrosion is not economical and thereby cost prohibitive in the energy conscious world. OBJECTS OF THE INVENTION The objective of this invention is to provide an improved economizer for boilers. Another objective of this invention is to provide a boiler economizer which is efficient and which is not subject to cold end corrosion. Another object of the invention is to provide a boiler economizer which recovers maximum heat from flue gases without corrosion at all firing conditions. It is the objective of this invention to provide a system that could be used to eliminate corrosion for low temperature flue gas heat recovery system. Another objective of this invention is to provide a system which has bypass corrosion management system to control corrosion at off design condition primarily arising from lower feed water temperature and less load. 7 These and other objects and advantages of the present invention will become more apparent from the detailed description thereof taken with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1: The effect of sulfur on flue gas dew point and minimum metal temperature to avoid corrosion. Figure 2: Schematic diagram of economiser and control loop. Figure 3: Internal arrangement of economiser showing primary and secondary heat exchanger arrangement Numeral 1- Drum Numeral 2- Secondary heat exchanger coil Numeral 3- Riser Numeral 4- Riser header Numeral 5- Primary heat exchanger coil Numeral 6- Primary heat exchanger casing Numeral 7- Flue gas inlet duct Numeral 8- Leg support Numeral 9- Down comer header Numeral 10- Cleaning door Numeral 11- Flue gas duct outlet Numeral 12- Down comer Numeral 13- Nozzle for attachments like air vent, pressure gauge etc. 8 Numeral 14- Control Valve for bypass line. Figure 4: The effect of secondary heat exchanger area on flue gas exit temperature and circulating water temperature. Figure 5: Effect of load and % bypass on circulating water temperature. Figure 6: Percentage of the bypass requirement at different load condition to maintain circulating water temperature. Figure 7: Percentage of the bypass requirement at different load condition to maintain circulating water temperature. Figure 8: Percentage of the bypass to maintain circulating water temperature as a function of water inlet temperature. BRIEF SUMMARY OF THE INVENTION The proposed economizer uses water as a secondary fluid. In this system, the feed water is not directly heated by flue gas. The secondary medium (Fluid) is used to heat the feed water which receives the heat from the flue gas. The proposed economizer has a Primary circuit comprising of Risers (Numeral 3), Downcomers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5) and Drum (Numeral 1) forming a closed loop for natural circulation of water. The Primary Heat Exchanger Coil is used to transfer the heat from flue 9 gas to secondary fluid. The boiler feed water is passed through the Drum, where it is heated in secondary heat exchanger Coil (Numeral 2) and the hot feed water is sent to the boiler. The Secondary circuit comprises primarily of Control Valve for bypass line. The Control Valve (Numeral 14) shown in the Fig.2 is for controlling the water flow thereby controlling the metal temperature. DETAILED DESCRIPTION OF THE INVENTION The schematic for the boiler economizer is illustrated in Fig. 2. wherein, the system is divided into two circuits as explained hereinabove. The internal arrangement of economiser showing primary and secondary heat exchanger arrangement is shown in Fig. 3. The primary circuit consists of Risers (Numeral 3), Down Comers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5) and Drum (Numeral 1). In the Primary Heat Exchanger Coil, water (secondary fluid) is heated using flue gas, the hot water gets vaporized in the Coil. The hot water and steam mixture enters the Riser through the Riser Header (Numeral 4) and finally reaches the Drum where the water and the steam mixture is condensed by transferring heat to the feed water via Secondary Heat Exchanger Coil (Numeral 2). The Condensed water flows back in the Primary Heat Exchanger Coil via the Down Comer. The Down Comer Header (Numeral 9) is used to connect the Down Comer and the Primary Heat Exchanger Coil. 10 The flue gas enters in the Primary Heat Exchanger from the Flue Gas Inlet Duct (Numeral 7) and comes out of Flue Gas outlet Duct (Numeral 11) after transferring heat to the secondary fluid in the Primary Heat Exchanger. The Primary Heat Exchanger Casing (Numeral 6) provides the enclosure to the Primary Heat Exchanger. A provision for cleaning the economizer is provided in the form of a Cleaning Door (Numeral 10). Also, Nozzles (Numeral 13) are provided for various attachments like air vent, pressure gauge, etc. The entire economizer is supported by plurality of Leg Support (Numeral 8). As the flue gas is not in direct contact with secondary circuit, the metal temp is the function of pressure and corresponding saturation temperature of thermosyphon circuit. The pressure and temperature of circulation circuit is the function of heat transfer area of primary heat exchanger and heat transfer area of secondary heat exchanger. Fig. 4 illustrates the circulating water temperature and flue gas outlet temperature as a function of these two areas. Both circulating water temperature and flue gas outlet temperature decreases with increase in secondary heat exchanger area. The circulating water temperature increases and flue gas exit temperature decreases with increase in primary heat exchanger area. Depending on the requirement of minimum metal temperature to avoid corrosion and flue gas exit temperature requirement, these heat exchangers areas are optimized. Optimal selection of heat transfer area of Primary and Secondary Heat Exchanger eliminates corrosion on-design condition. Still it is not fail safe in off design condition. Fig. 5 & Fig. 6 illustrates the effect of load change on circulating water temperature. 11 The circulating water temperature decreases with decrease in boiler load as the flue gas quantity and temperature inlet to the economizer decreases with decrease in load. Fig. 2 shows a Control Valve for bypassing water quantity. When the water quantity is bypassed heat transfer in secondary heat exchanger reduces. Due to reduction in heat transfer, drum pressure shifts to the higher pressure which in turns increases circulating water temperature. This can be seen in Fig-5, where circulating water temperature increases with increase in bypass water quantity. Fig. 6 show the optimum bypass quantity to eliminate corrosion as a function of load. Similar phenomenon will be experienced with decrease in feed water temperature. Fig. 7 illustrates the effect of inlet water temperature change on circulating water temperature. The circulating water temperature decreases with decrease in inlet water temperature as the rate of heat transfer in drum increases with decrease in inlet water temperature. Due to increase in heat transfer in drum, drum pressure shifts to the lower pressure which in turns reduces circulating water temperature. This can be corrected by bypassing some quantity of water, which will reduce heat transfer in drum and brings back drum to the desired pressure and corresponding circulating water temperature. This can be seen in Fig. 7, where circulating water temperature increases with increase in bypass water quantity. Fig. 8 shows the optimum bypass quantity to eliminate corrosion as a function of load. 12 We Claim, 1. A Thermosyphon economizer for recovering heat from low temperature flue gas comprising: a Drum, a Secondary Heat Exchanger Coil, a Riser, a Riser Header, a Primary Heat Exchanger Coil, a Primary Heat Exchanger Casing, a Flue gas Inlet Duct, a Leg Support, a Downcomer Header, Cleaning Door, a Flue gas Duct Outlet, a Downcomer, a Nozzle means for attachments like air vent and a Control Valve. 2. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the said Economizer uses water as a secondary fluid and wherein the feed water is not directly heated by flue gas but the secondary fluid is used to heat the feed water. 3. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the said Risers, the said Downcomers, the said Primary Heat Exchanger Coil and the said Drum form a closed loop for natural circulation of water. 4. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the said Primary Heat Exchanger Coil uses flue gas to heat the secondary fluid. 5. The Economizer for recovering heat from low temperature flue gas claimed in claim 1 & 4, wherein the hot secondary fluid and steam mixture enters the said Riser through the said Riser Header to finally reach the said Drum where the secondary fluid and the steam mixture is condensed by transferring heat to the feed water via the said Secondary Heat Exchanger Coil. 6. The Economizer for recovering heat from low temperature flue gas claimed in claim 1 & 5, wherein the said condensed mixture of secondary fluid and steam flows back in the said Primary Heat Exchanger Coil via the said Down Comer and downcomer header. 7. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the flue gas enters in the said Primary Heat Exchanger from the said Flue Gas Inlet Duct and comes out of the said Flue Gas outlet Duct after transferring heat to the secondary fluid in the said Primary Heat Exchanger. 8. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the Economizer is supported by plurality of Leg Support and wherein the said Primary Heat Exchanger Casing provides the enclosure to the said Primary Heat Exchanger and the said Nozzles are provision for various attachments like air vent, pressure gauge. 10. The economizer for recovering heat from flue gases claimed in claim 1, wherein the said Control Valve is connected to the Primary Heat Exchanger and wherein feed water quantity is bypassed through a Control Valve to reduce heat transfer in said Secondary Heat Exchanger which produces higher Drum pressure which in turns increases circulating water temperature. Dated this 19th August, 2007 ABSTRACT A boiler economizer wherein feed water is not directly heated by flue gas but circulating water as a secondary fluid is used to heat the feed water and which comprises of Risers (Numeral 3), Downcomers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5), Secondary heat exchanger (Numeral 2) and Drum (Numeral 1) forming a closed loop for natural circulation of water and wherein the said Primary Heat Exchanger Coil is used to transfer the heat from flue gas to secondary fluid and wherein the boiler feed water is passed through the said Drum where it is heated in Secondary heat exchanger coil (Numeral 2) and the hot feed water is sent to the boiler and such economizer provided by a Control Valve (Numeral 14) for controlling the water flow thereby controlling the metal temperature.

Full Text

FORM-2
THE PATENTS ACT. 1970 (39 of 1970)
COMPLETE SPECIFICATION (Section 10, rule 13)
"Thermosyphon Economiser for the flue gas containing sulfur dioxide and sulfur trioxide"
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: -
1

FIELD OF THE INVENTION
This invention relates to an economiser using secondary medium to maintain metal temperature higher than the dew point temperature
Particularly, this invention relates to an economizer using water as a secondary medium working in thermosyphon loop.
More particularly, this invention relates to an economizer with low temperature flue gas heat recovery system.
Even more particularly, this invention relates to an economizer with bypass corrosion management system to control corrosion at off design condition primarily arising from lower feed water temperature and less load.
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 economiser.
2

The economiser is used to recover the boiler flue gas heat by feed water heating.
This is used to increase the thermal efficiency of the system. For the clean fuel without sulfur
contamination, it is very easy to recover the heat as flue gas does not have any condensation
or subsequent corrosion problem. But the fuel containing sulfur has major limitation as it is
very prone to dew point corrosion.
In the process of combustion, sulfur of the fuel gets converted into sulfur dioxide. Approximately, 1% - 5 % of sulfur Dioxide gets converted into sulfur tri oxide. 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.
However, the problem of cold end corrosion has been persistent in most of the economizers as most of such economizers have the problem of not having the water temperature at the recommended level to avoid sulfuric acid condensation. The Feed water temperature can vary between 30 Deg-C to 90 Deg-C depending on condensate recovery, which is not sufficient to avoid dew point corrosion.
3

PRIOR ART
An U.S. patent application No. 4,318,366 claiming a boiler having a combustion chamber and a flue comprising of,
a first heat exchanger disposed in said flue for exchanging heat between boiler feed liquid and gaseous combustion products in said flue,
flow dividing means having first and second outlets and an inlet connected to receive boiler feed liquid and having adjustable proportioning means for providing a first portion of said liquid to said first outlet and a second portion to said second outlet,
a second heat exchanger disposed exteriorly of said boiler and said flue and having a first flow path and a second flow path for exchanging heat with liquid flowing in said first flow path, said first flow path being connected in series between said first outlet and said first heat exchanger and said boiler,
said second outlet being connected to said first heat exchanger in a parallel relation to said first flow path,
temperature responsive means disposed in said flue for measuring the temperature of gaseous combustion products in said flue after passage thereof over said first heat exchanger and for positioning said proportioning means to maintain the proportion of feed liquid flowing at each outlet so as to maintain the temperature of said combustion products at a preselected level solely with heat extracted from said flue gases.
4

An U.S. patent application No. 4,470,449 claims, an apparatus for removing heat from a gas stream to heat a liquid stream comprising:
an economizer having a shell and tube construction having first, second and third sections, said second section being intermediate said first and third sections, said gas stream flowing sequentially through said first, second and third sections and said liquid stream flowing sequentially through said second, third and first sections, said gas stream and said liquid stream being in concurrent flow in said second section and in countercurrent flow in said first and third sections.
An U.S. patent application No. 4,173,949, claims, In a boiler system which includes a boiler for boiling feedwater therein to produce steam, an exhaust stack connected to said boiler for removing flue gas therefrom, and a fuel economizer mounted in said stack for preheating feedwater in said economizer prior to introduction of said feedwater into said boiler, and wherein said fuel economizer preheats the feedwater therein by passing the feedwater in heat exchange with flue gas from the boiler; an improved method of preheating said feedwater prior to its introduction into said economizer which comprises passing said feedwater through a preheater heat exchanger prior to its introduction into said economizer, conducting a first portion of steam from said boiler through a first conduit and into said preheater heat exchanger to heat said feedwater therein, conducting a second portion of steam from said boiler through a second conduit and into said preheater heat exchanger to heat said feedwater therein, controlling the flow of steam through said first conduit in response to the temperature of feedwater entering said economizer whereby the temperature of the feedwater entering said economizer is above the dew point of the corrosive acids in the flue gas passing
5

through said economizer, and controlling the flow of steam through said second conduit in response to the temperature of the exit gas from said exhaust stack to maintain the temperature of the flue gas above a predetermined temperature which will minimize corrosion within the exhaust stack due to the condensation of corrosive acids therein.
The 4,318366 application recommends using two heat exchangers wherein the first heat exchanger is used to recover heat from the flue gas and second is used to heat feed water before entering to the first heat exchanger. Hot feed water from first heat exchanger is passed through second heat exchanger for heating cold feed water. Cold feed water has a bypass line with control valve, which operates depending on flue gas exit and feed water temperature.
The 4470449 application suggests dividing the complete heat exchanger in three part and cold water should be entered in intermediate section. The hot feed water then enters in last section and finally enters in first section. This is done to maximise recovery and metal temperature. This also suggests using flue gas bypass line for the first heat exchanger to increase higher flue gas temperature. But this has a drawback as the metal temperature is primarily governed by water temperature and the flue gas temperature plays very insignificant role in maintaining metal temperature.
Lastly, the 4173949 application suggests the preheating of feed water before entering to economiser. The preheating is done by live steam and amount of steam is controlled by sensing the feed water temperature.
6

These systems are complex and are lacking in providing a integrated and high quality self-contained thermosyphon economiser for the flue gas containing sulfur dioxide and sulfur trioxide. Thus, the problem of cold end corrosion still persists as the water temp is less than the recommended temperature to avoid sulfuric acid condensation ( Feed water temp, can vary between 30 Deg-C to 90 Deg-C depending on condensate recovery). Also, the commonly used solution for such cold end corrosion by preheating water to minimum metal temp to avoid corrosion is not economical and thereby cost prohibitive in the energy conscious world.
OBJECTS OF THE INVENTION
The objective of this invention is to provide an improved economizer for boilers.
Another objective of this invention is to provide a boiler economizer which is efficient and which is not subject to cold end corrosion.
Another object of the invention is to provide a boiler economizer which recovers maximum heat from flue gases without corrosion at all firing conditions.
It is the objective of this invention to provide a system that could be used to eliminate corrosion for low temperature flue gas heat recovery system.
Another objective of this invention is to provide a system which has bypass corrosion management system to control corrosion at off design condition primarily arising from lower feed water temperature and less load.
7

These and other objects and advantages of the present invention will become more apparent from the detailed description thereof taken with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1: The effect of sulfur on flue gas dew point and minimum metal temperature to avoid corrosion.
Figure 2: Schematic diagram of economiser and control loop.
Figure 3: Internal arrangement of economiser showing primary and secondary heat exchanger arrangement
Numeral 1- Drum
Numeral 2- Secondary heat exchanger coil
Numeral 3- Riser
Numeral 4- Riser header
Numeral 5- Primary heat exchanger coil
Numeral 6- Primary heat exchanger casing
Numeral 7- Flue gas inlet duct
Numeral 8- Leg support
Numeral 9- Down comer header
Numeral 10- Cleaning door
Numeral 11- Flue gas duct outlet
Numeral 12- Down comer
Numeral 13- Nozzle for attachments like air vent, pressure gauge etc.
8

Numeral 14- Control Valve for bypass line.
Figure 4: The effect of secondary heat exchanger area on flue gas exit temperature and circulating water temperature.
Figure 5: Effect of load and % bypass on circulating water temperature.
Figure 6: Percentage of the bypass requirement at different load condition to maintain circulating water temperature.
Figure 7: Percentage of the bypass requirement at different load condition to maintain circulating water temperature.
Figure 8: Percentage of the bypass to maintain circulating water temperature as a function of water inlet temperature.
BRIEF SUMMARY OF THE INVENTION
The proposed economizer uses water as a secondary fluid. In this system, the feed water is not directly heated by flue gas. The secondary medium (Fluid) is used to heat the feed water which receives the heat from the flue gas. The proposed economizer has a Primary circuit comprising of Risers (Numeral 3), Downcomers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5) and Drum (Numeral 1) forming a closed loop for natural circulation of water. The Primary Heat Exchanger Coil is used to transfer the heat from flue
9

gas to secondary fluid. The boiler feed water is passed through the Drum, where it is heated in secondary heat exchanger Coil (Numeral 2) and the hot feed water is sent to the boiler. The Secondary circuit comprises primarily of Control Valve for bypass line. The Control Valve (Numeral 14) shown in the Fig.2 is for controlling the water flow thereby controlling the metal temperature.
DETAILED DESCRIPTION OF THE INVENTION
The schematic for the boiler economizer is illustrated in Fig. 2. wherein, the system is divided into two circuits as explained hereinabove.
The internal arrangement of economiser showing primary and secondary heat exchanger arrangement is shown in Fig. 3.
The primary circuit consists of Risers (Numeral 3), Down Comers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5) and Drum (Numeral 1). In the Primary Heat Exchanger Coil, water (secondary fluid) is heated using flue gas, the hot water gets vaporized in the Coil. The hot water and steam mixture enters the Riser through the Riser Header (Numeral 4) and finally reaches the Drum where the water and the steam mixture is condensed by transferring heat to the feed water via Secondary Heat Exchanger Coil (Numeral 2). The Condensed water flows back in the Primary Heat Exchanger Coil via the Down Comer. The Down Comer Header (Numeral 9) is used to connect the Down Comer and the Primary Heat Exchanger Coil.
10

The flue gas enters in the Primary Heat Exchanger from the Flue Gas Inlet Duct (Numeral 7) and comes out of Flue Gas outlet Duct (Numeral 11) after transferring heat to the secondary fluid in the Primary Heat Exchanger.
The Primary Heat Exchanger Casing (Numeral 6) provides the enclosure to the Primary Heat Exchanger. A provision for cleaning the economizer is provided in the form of a Cleaning Door (Numeral 10). Also, Nozzles (Numeral 13) are provided for various attachments like air vent, pressure gauge, etc. The entire economizer is supported by plurality of Leg Support (Numeral 8).
As the flue gas is not in direct contact with secondary circuit, the metal temp is the function of pressure and corresponding saturation temperature of thermosyphon circuit. The pressure and temperature of circulation circuit is the function of heat transfer area of primary heat exchanger and heat transfer area of secondary heat exchanger. Fig. 4 illustrates the circulating water temperature and flue gas outlet temperature as a function of these two areas. Both circulating water temperature and flue gas outlet temperature decreases with increase in secondary heat exchanger area. The circulating water temperature increases and flue gas exit temperature decreases with increase in primary heat exchanger area.
Depending on the requirement of minimum metal temperature to avoid corrosion and flue gas exit temperature requirement, these heat exchangers areas are optimized.
Optimal selection of heat transfer area of Primary and Secondary Heat Exchanger eliminates corrosion on-design condition. Still it is not fail safe in off design condition. Fig. 5 & Fig. 6 illustrates the effect of load change on circulating water temperature.
11

The circulating water temperature decreases with decrease in boiler load as the flue gas quantity and temperature inlet to the economizer decreases with decrease in load. Fig. 2 shows a Control Valve for bypassing water quantity. When the water quantity is bypassed heat transfer in secondary heat exchanger reduces. Due to reduction in heat transfer, drum pressure shifts to the higher pressure which in turns increases circulating water temperature. This can be seen in Fig-5, where circulating water temperature increases with increase in bypass water quantity.
Fig. 6 show the optimum bypass quantity to eliminate corrosion as a function of load. Similar phenomenon will be experienced with decrease in feed water temperature. Fig. 7 illustrates the effect of inlet water temperature change on circulating water temperature. The circulating water temperature decreases with decrease in inlet water temperature as the rate of heat transfer in drum increases with decrease in inlet water temperature. Due to increase in heat transfer in drum, drum pressure shifts to the lower pressure which in turns reduces circulating water temperature. This can be corrected by bypassing some quantity of water, which will reduce heat transfer in drum and brings back drum to the desired pressure and corresponding circulating water temperature. This can be seen in Fig. 7, where circulating water temperature increases with increase in bypass water quantity. Fig. 8 shows the optimum bypass quantity to eliminate corrosion as a function of load.
12

We Claim,
1. A Thermosyphon economizer for recovering heat from low temperature flue gas
comprising:
a Drum, a Secondary Heat Exchanger Coil, a Riser, a Riser Header, a Primary Heat Exchanger Coil, a Primary Heat Exchanger Casing, a Flue gas Inlet Duct, a Leg Support, a Downcomer Header, Cleaning Door, a Flue gas Duct Outlet, a Downcomer, a Nozzle means for attachments like air vent and a Control Valve.
2. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the said Economizer uses water as a secondary fluid and wherein the feed water is not directly heated by flue gas but the secondary fluid is used to heat the feed water.
3. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the said Risers, the said Downcomers, the said Primary Heat Exchanger Coil and the said Drum form a closed loop for natural circulation of water.
4. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the said Primary Heat Exchanger Coil uses flue gas to heat the secondary fluid.
5. The Economizer for recovering heat from low temperature flue gas claimed in claim 1 & 4, wherein the hot secondary fluid and steam mixture enters the said Riser through the

said Riser Header to finally reach the said Drum where the secondary fluid and the steam mixture is condensed by transferring heat to the feed water via the said Secondary Heat Exchanger Coil.
6. The Economizer for recovering heat from low temperature flue gas claimed in claim 1 & 5, wherein the said condensed mixture of secondary fluid and steam flows back in the said Primary Heat Exchanger Coil via the said Down Comer and downcomer header.
7. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the flue gas enters in the said Primary Heat Exchanger from the said Flue Gas Inlet Duct and comes out of the said Flue Gas outlet Duct after transferring heat to the secondary fluid in the said Primary Heat Exchanger.
8. The Economizer for recovering heat from low temperature flue gas claimed in claim 1, wherein the Economizer is supported by plurality of Leg Support and wherein the said Primary Heat Exchanger Casing provides the enclosure to the said Primary Heat Exchanger and the said Nozzles are provision for various attachments like air vent, pressure gauge.
10. The economizer for recovering heat from flue gases claimed in claim 1, wherein the said Control Valve is connected to the Primary Heat Exchanger and wherein feed water quantity is bypassed through a Control Valve to reduce heat transfer in said Secondary

Heat Exchanger which produces higher Drum pressure which in turns increases circulating water temperature.
Dated this 19th August, 2007

ABSTRACT
A boiler economizer wherein feed water is not directly heated by flue gas but circulating water as a secondary fluid is used to heat the feed water and which comprises of Risers (Numeral 3), Downcomers (Numeral 12), Primary Heat Exchanger Coil (Numeral 5), Secondary heat exchanger (Numeral 2) and Drum (Numeral 1) forming a closed loop for natural circulation of water and wherein the said Primary Heat Exchanger Coil is used to transfer the heat from flue gas to secondary fluid and wherein the boiler feed water is passed through the said Drum where it is heated in Secondary heat exchanger coil (Numeral 2) and the hot feed water is sent to the boiler and such economizer provided by a Control Valve (Numeral 14) for controlling the water flow thereby controlling the metal temperature.

Documents:

1762-MUM-2007-ABSTRACT(7-5-2012).pdf

1762-mum-2007-abstract.doc

1762-mum-2007-abstract.pdf

1762-MUM-2007-CLAIMS(AMENDED)-(10-11-2010).pdf

1762-MUM-2007-CLAIMS(AMENDED)-(6-3-2012).pdf

1762-MUM-2007-CLAIMS(AMENDED)-(7-5-2012).pdf

1762-mum-2007-claims.doc

1762-mum-2007-claims.pdf

1762-MUM-2007-CORRESPONDENCE 20-6-2008.pdf

1762-MUM-2007-CORRESPONDENCE(20-4-2012).pdf

1762-MUM-2007-CORRESPONDENCE(23-7-2010).pdf

1762-mum-2007-correspondence(26-3-2009).pdf

1762-MUM-2007-CORRESPONDENCE(6-3-2012).pdf

1762-mum-2007-correspondence(ipo)-(6-3-2009).pdf

1762-mum-2007-correspondence-received.pdf

1762-mum-2007-description (complete).pdf

1762-MUM-2007-DRAWING(10-11-2010).pdf

1762-MUM-2007-DRAWING(6-3-2012).pdf

1762-MUM-2007-DRAWING(7-5-2012).pdf

1762-mum-2007-drawings.pdf

1762-mum-2007-form 18(14-9-2007).pdf

1762-MUM-2007-FORM 2(TITLE PAGE)-(10-11-2010).pdf

1762-MUM-2007-FORM 26(23-7-2010).pdf

1762-mum-2007-form 26(26-3-2009).pdf

1762-mum-2007-form 9(14-9-2007).pdf

1762-mum-2007-form-1.pdf

1762-mum-2007-form-2.doc

1762-mum-2007-form-2.pdf

1762-mum-2007-form-26.pdf

1762-mum-2007-form-3.pdf

1762-MUM-2007-POWER OF ATTORNEY 20-6-2008.pdf

1762-MUM-2007-POWER OF ATTORNEY(6-3-2012).pdf

1762-MUM-2007-POWER OF ATTORNEY(7-5-2012).pdf

1762-MUM-2007-REPLY TO EXAMINATION REPORT(10-11-2010).pdf

1762-MUM-2007-REPLY TO HEARING(7-5-2012).pdf

1762-MUM-2007-SPECIFICATION(AMENDED)-(10-11-2010).pdf

1762-MUM-2007-SPECIFICATION(AMENDED)-(7-5-2012).pdf

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Patent Number

252946

Indian Patent Application Number

1762/MUM/2007

PG Journal Number

24/2012

Publication Date

15-Jun-2012

Grant Date

11-Jun-2012

Date of Filing

14-Sep-2007

Name of Patentee

THERMAX LIMITED

Applicant Address

THERMAX HOUSE, 4 PUNE-MUMBAI ROAD, SHIVAJINAGAR,PUNE

Inventors:

#	Inventor's Name	Inventor's Address
1	C.P.PRAJEENDRAN	THERMAX HOUSE, 4 PUNE-MUMBAI ROAD, SHIVAJINAGAR, PUNE-411005
2	R.B.KHARAT	THERMAX HOUSE, 4 PUNE-MUMBAI ROAD, SHIVAJINAGAR, PUNE-411005
3	A. KRISHNAKUMAR	THERMAX HOUSE, 4 PUNE-MUMBAI ROAD, SHIVAJINAGAR, PUNE-411005
4	R.S.JHA	THERMAX HOUSE, 4 PUNE-MUMBAI ROAD, SHIVAJINAGAR, PUNE-411005

PCT International Classification Number

F27B1/00,F27B1/24

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA