US20180340686A1

US20180340686A1 - System for increasing flue gas side draft of heater assemblies using a draft booster impeller assembly

Info

Publication number: US20180340686A1
Application number: US15/981,338
Authority: US
Inventors: Hyungsik Lee; Kelly T. Murphy; Ada M. Cardenas; Todd A. Grubb
Original assignee: ExxonMobil Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 2017-05-26
Filing date: 2018-05-16
Publication date: 2018-11-29

Abstract

A system for increasing flue gas side draft in a heater or boiler assembly is disclosed utilizing an impeller assembly to enable an increase in firing rates beyond the ability of the exhaust stack and to reduce the size of the low flue gas side pressure drop heat pipe air preheaters.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application relates and claims priority to U.S. Provisional Patent Application No. 62/511,501, filed on May 26, 2017, entitled “System For Increasing Flue Gas Side Draft Of Heater Assemblies Using A Draft Booster Impeller Assembly”, which is incorporated herein specifically herein by reference in its entirety.

BACKGROUND

Field

The presently disclosed subject matter relates to a system for increasing flue gas side draft in a heater assembly. In particular, the presently disclosed subject matter relates to a system for increasing flue gas side draft in a heater assembly utilizing a draft booster impeller assembly positioned within flue gas ducting.

Description of Related Art

There is a desire to increase flue gas side draft in fired heaters and boilers, which may result in increased firing rates within the fired heaters and boilers. This has been accomplished using various techniques.
Centrifugal induced draft fans have been applied in creating a high level of the flue gas side draft gain in fired heaters and boilers. One such application is illustrated in FIG. 1. A forced draft fan 1 is used to supply ambient air to a heater 2 for combustion. A portion of the ambient air is fed through an air preheater assembly 3 and a portion may be fed directly to the heater 3. The air and fuel are combusted within the heater. Flue gas is created which exits the heater from an exhaust stack 4. The exhaust stack 4 includes a bypass damper 5 whereby the flue gas is directed via ducting 6 to the preheater assembly 3 whereby the warm flue gas heats the incoming ambient air. A centrifugal draft fan 7 is utilized in connection with an air preheater 2 to pull flue gas through the preheater before returning the same to the exhaust stack. The pressure gain from the centrifugal induced draft fan 7 is typically large by overcoming over 4 inch of water column of pressure drop in the air preheater and associated ducting. Substantial design modifications are required to use the conventional centrifugal induced draft fan for generating low level of pressure gain. These fans are often located at ground level when used in conjunction with an air preheater. Therefore additional ducting is required from the exhaust stack to the ground where the induced draft fan is located. As such, the installation of centrifugal induced draft fans is costly due to addition of the fan and flue gas ducting.
Others have utilized steam eductors to boost a small level of the draft within the furnace. The use of a steam eductor 11 is illustrated in FIG. 2. In FIG. 2, a steam eductor 11 is located within the exhaust stack 12. The kinetic energy of the steam in the eductor facilitates the movement of flue gas, which creates the desired draft improvement. Steam eductors are noisy and inefficient, which often require the use of a silencer to reduce noise. Eductors are also costly because steam or other medium is necessary to operate the eductor.
Axial fans have also been used to generate a low level of draft improvement. Typical axial fans connect the fan to the motor with belts, and often a separate cooling fan is used to cool the bearings, both which are unreliable in high temperature applications.
There is a need for a low cost solution to generate a low level of draft improvement within a fired heater or boiler assembly that overcomes the deficiencies in the prior art.

SUMMARY

In accordance with one aspect of the present invention, a system for increasing flue gas side draft in a heater assembly is disclosed. The assembly includes a heater, an inlet supply of combustion air, an exhaust duct and an exhaust stack. Flue gas is produced in the heater during the operation and exits the exhaust duct and stack. It is contemplated that the heater can be one of a fired heater, a boiler or other suitable assembly for generating heat for one or more process streams in a refinery or petrochemical processing facility. The heater assembly may further include a low flue gas side pressure drop air preheater and associated ducting for preheating at least a portion of the combustion air. As illustrated in FIG. 1 of the prior art, at least portions of the flue gas and air may be directed to the combustion air preheater to preheat the combustion air. Alternatively, the air preheater can be installed in the connecting duct between the fired heater and a ground mounted stack with the bypass ducts around the air preheater. The warmed combustion air is then fed to the heater. The system in accordance with the presently disclosed subject matter includes an impeller assembly coupled to the exhaust stack. The operation of the impeller increases flue gas draft in the heater assembly to enable an increase in firing rates beyond the ability of the exhaust stack. In addition, it can be used to overcome the flow resistance in the low flue gas side pressure drop air preheaters. The impeller assembly preferably includes a motor and one of an axial impeller having a plurality of vanes and a centrifugal impeller having a plurality of rotating impellers.
In accordance with the presently disclosed subject matter, it contemplated that at least a portion of the impeller assembly is located within one of the flue gas duct or the exhaust stack. Each location is effective in increasing the flue side draft.
In accordance with one aspect of the presently disclosed subject matter, the impeller assembly includes an axial impeller. It is contemplated that both the axial impeller and the motor can be located within the flue gas duct or the exhaust stack. Alternatively, it is contemplated that only the axial impeller is located within the flue gas duct or the exhaust stack. With such an arrangement, the motor is located externally of the flue gas duct or exhaust stack. A seal is provided, located within either the duct wall or stack wall. A shaft connects the motor to the axial impeller. The shaft passes through the seal to prevent the escape of flue gas.
In accordance with one another aspect of the presently disclosed subject matter, the impeller assembly includes a centrifugal impeller. It is contemplated that only the centrifugal impeller is located within the flue gas duct or the exhaust stack. With such an arrangement, the motor is located externally of the flue gas duct or exhaust stack. A seal is provided located within either the duct wall or stack wall. A shaft connects the motor to the centrifugal impeller. The shaft passes through the seal to prevent the escape of flue gas. A self-lubrication system is employed with a gear box to control the speed of the impeller
The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the disclosed subject matter. Together with the description, the drawings serve to explain the principles of the disclosed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a fired heater having an air preheater using an induced draft fan in accordance with the prior art;

FIG. 2 is a schematic view of a fired heater having steam eductor in accordance with the prior art;

FIG. 3 is a schematic view of a system for increasing flue gas side draft in a heater assembly using an axial impeller in accordance with one aspect of the present invention;

FIG. 4 is a schematic view of a system for increasing flue gas side draft in a heater assembly using an axial impeller in accordance with another aspect of the present invention;

FIG. 5 is a schematic view of a system for increasing flue gas side draft in a heater assembly using a centrifugal impeller in accordance with yet another aspect of the present invention; and

FIG. 6 is a schematic view of a system for increasing flue gas side draft in a heater assembly using a centrifugal impeller in accordance with yet another aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the various exemplary embodiments of the disclosed subject matter, exemplary embodiments of which are illustrated in the accompanying drawings. The structure and corresponding method of installation of the disclosed subject matter will be described in conjunction with the detailed description of the system.
Various aspects of the system for increasing flue gas side draft in a heater assembly below 1.5 inch of water column gain will now be described in greater detail with respect to the Figures. The terminology heater assembly is utilized to refer to fired heaters or boilers or other suitable heat source for heating a process stream within a refinery or petrochemical processing facility. One application of the presently disclosed subject matter is to create low level of draft utilizing an axial or centrifugal fan assembly, which can replace the existing educator and belt-driven axial fan system. Another application is utilized in the fired heater with heat pipe preheater disclosed in commonly assigned U.S. Provisional Application No. 62,352099 entitled “Fired Heater With Heat Pipe Preheater”, now U.S. patent application Ser. No. 15/606,038, the disclosure of which is incorporated herein by reference in its entirety.
A system 20 for increasing flue gas side draft in a heater assembly in accordance with one aspect of the presently disclosed subject matter is illustrated in FIG. 3. The system 20 includes an impeller assembly 21 positioned within the flue gas stream within an exhaust stack or flue gas duct 15. It is contemplated that the impeller assembly 21 may be located in either the exhaust stack of the heater assembly or a flue gas duct. The impeller assembly 21 includes an axial impeller 22 having a plurality of vanes. The axial impeller 22 is operatively coupled to a motor 23 via shaft 24. In this arrangement, both the axial impeller 22 and the motor 23 are located within the flue gas stream within the exhaust stack or flue gas duct. The motor 23 is preferably low speed below 800 rpm and high temperature motor. A bearing assembly 25 is provided for achieving smooth rotation of the axial impeller. An external lubricant supply system 26 is provided to supply lubricant to the bearing assembly. The system 26 is preferably a pressurized bearing lubrication system to enable operation in relatively high temperature environments and enhance bearing reliability. The impeller assembly 21 may further include an adjustment assembly 27 for adjusting the angle of the vanes of the axial impeller 22. Adjusting the vane angle can change the draw created by the operation of the axial impeller. Alternatively, the axial impeller speed can be adjusted by a gear box. An access panel in the stack or duct 15 may be provided to access the assembly 21 for service.
In operation, the rotation of the axial impeller 22 pulls the flue gas within the exhaust stack or flue gas duct, which results in an increase in the flue gas side draft of heater assembly. Unlike the centrifugal induced draft fan 7, the draft generated from the use of the impellers is small (e.g., typically less than 1.5 inch of water column gain). Nonetheless, this additional small draft is sufficient for increasing the firing rates of heaters and boilers or reducing the sizes of low pressure drop heat pipe air preheaters. The system 20 is relatively compact and be easily installed in the flue gas stream without major changes or additions to the heater assembly and surrounding equipment. Unlike the centrifugal induced draft fan 7, the system 20 is installed on an elevated structure (e.g. top of the heater) resulting in a reduction in the length of the flue gas connecting duct, decreasing installation costs. The system 20 provides quiet operation and eliminates the need for silencers.
A system 30 for increasing flue gas side draft in a heater assembly in accordance with another aspect of the presently disclosed subject matter is illustrated in FIG. 4. Like system 20, the system 30 utilizes an impeller assembly. It is contemplated that the impeller assembly 31 may be located in either the exhaust stack of the heater assembly, or a flue gas duct 15. The impeller assembly 31 includes an axial impeller 32 having a plurality of vanes. The axial impeller 32 is operatively coupled to a motor 33 via shaft 34. In this arrangement, only the axial impeller 32 is located within the flue gas stream within the exhaust stack or flue gas duct. The motor 33 is located outside of the flue gas stream and is preferably low speed below 800 rpm. A bearing assembly 35 is provided for achieving smooth rotation of the axial impeller. The shaft 34 extends through a seal 36 in the wall of exhaust stack or flue gas duct 15. The seal 36 prevents flue gas from escaping where the shaft 34 extend through the wall. The impeller assembly 31 may further include an adjustment assembly 37 for adjusting the angle of the vanes of the axial impeller 32. Adjusting the vane angle can change the draw created by the operation of the axial impeller. Alternatively, the axial impeller speed can be adjusted by a gear box. A bend or jog 151 in the flue gas duct 15 permits locating the motor 33 outside of the flue gas stream. The operation of the system 30 is similar to the system 20.
A system 40 for increasing flue gas side draft in a heater assembly in accordance with one aspect of the presently disclosed subject matter is illustrated in FIGS. 5 and 6. The system 40 includes an impeller assembly 41 positioned within the flue gas stream within an exhaust stack or flue gas duct 15. It is contemplated that the impeller assembly 41 may be located in either the exhaust stack of the heater assembly or a flue gas duct. The impeller assembly 41 includes a centrifugal impeller. The centrifugal impeller 41 is operatively coupled to a motor 43 via shaft 44. In this arrangement, the centrifugal impeller 41 is located within the flue gas stream within the exhaust stack or flue gas duct. The motor 43 is located outside of the flue gas stream. The presently disclosed subject matter, however, is not intended to be limited to such an arrangement, rather, it is contemplated that both the impeller 41 and the motor 43 may be located within the stream similar to the embodiment illustrated in FIG. 3. A bearing assembly 45 is provided to support and provide smooth rotation of the centrifugal impeller. The shaft 44 extends through a seal 46 in the wall of exhaust stack or by-pass duct 15. The seal 46 prevents flue gas from escaping where the shaft 44 extend through the wall. The system 40 is preferably located in a bend or jog 151 in the exhaust stack or flue gas duct 15 to permit locating the motor 43 outside of the flue gas stream. A sharp bend or jog is illustrated in FIG. 5. A more gradual bend is illustrated in FIG. 6.
Like the impeller assembly 21 and 31, the draft generated from the use of the centrifugal impeller is small (e.g., typically less than 1 inch H₂O). Unlike the induced draft fan 7, the system 40 is installed on an elevated structure (e.g. top of the heater) resulting in a reduction in the length of the flue gas connecting duct to decrease installation costs. The system 40 utilizes a self-lubrication system with a gear box to control the speed of the impeller. The system 40 provides quiet operation and eliminates the need for silencers. Nonetheless, this additional small draft is sufficient to enable an increase in firing rates beyond the ability of the exhaust stack in fired heaters and boilers or to reduce the size of the low flue gas side pressure drop heat pipe air preheaters.

Additional Embodiments

Additionally or alternately, the invention can include one or more of the following embodiments.
Embodiment 1. A system for increasing flue gas side draft in a heater assembly is disclosed. The heater assembly includes a heater, an inlet supply of combustion air, an exhaust duct and an exhaust stack. Flue gas that is produced in the heater exits the heater through the exhaust duct and stack. The system includes an impeller assembly operatively coupled to the exhaust duct or stack, wherein operation of the impeller increases flue gas draft in the heater assembly to enable an increase in firing rates beyond the ability of the exhaust stack.
Embodiment 2. The system according to Embodiment 1, wherein heater assembly further includes a low flue gas side pressure drop air preheater for preheating at least a portion of the combustion air fed to the inlet supply and a flue gas duct operatively connecting to the exhaust stack to the combustion air preheater. At least a portion of the flue gas is diverted from exhaust stack to the combustion air preheater to preheat at least a portion of the combustion air before returning the flue gas to the exhaust stack. In accordance with this Embodiment, at least a portion of the impeller assembly is located within the flue gas duct.
Embodiment 3. The system according to anyone of the preceding embodiments, wherein the impeller assembly includes a motor and one of an axial impeller and a centrifugal impeller.
Embodiment 4. The system according to anyone of the preceding embodiments, wherein the impeller assembly includes an axial impeller.
Embodiment 5. The system according to anyone of Embodiments 3 or 4, wherein the axial impeller and the motor are located within the flue gas duct. A pressurized bearing lubrication system is employed to enable operation in high temperature environments.
Embodiment 6. The system according to anyone of Embodiments 3 or 4 wherein the axial impeller is located within the flue gas duct and the motor is located externally of the flue gas duct, wherein the impeller assembly furthers include a seal located within a wall of the duct and a shaft passing through the seal, wherein the axial impeller and the motor are operatively connected via the shaft.
Embodiment 7. The system according to anyone of Embodiments 3, 4, 5 or 6, wherein the axial impeller includes a gear box or a plurality of variable vanes, such that angle of the vanes can be adjusted.
Embodiment 8. The system according to Embodiment 7, wherein adjustment of a gear box or the plurality of vanes can adjust the flue gas side draft.
Embodiment 9. The system according to anyone of Embodiments 1, 2 or 3, wherein the impeller assembly includes a centrifugal impeller.
Embodiment 10. The system according to anyone of Embodiments 1, 2, 3 or 9, wherein the centrifugal impeller is located within the flue gas duct and the motor is located externally of the duct, wherein the impeller assembly furthers include a seal located within a wall of the duct and a shaft passing through the seal, wherein the centrifugal impeller and the motor are operatively connected via the shaft.
Embodiment 11. The system according to Embodiment 1, wherein the impeller assembly is located within the flue gas duct or stack.
Embodiment 12. The system according to anyone of Embodiments 1 or 11, wherein the impeller assembly includes a motor and one of an axial impeller.
Embodiment 13. The system according to anyone of Embodiments 1, 11, or 12, wherein the axial impeller is located within the exhaust stack or flue gas duct and the motor is located externally of the exhaust stack or the flue gas duct, wherein the impeller assembly furthers include a seal located within a wall of the exhaust stack or the duct and a shaft passing through the seal, wherein the axial impeller and the motor are operatively connected via the shaft.
Embodiment 14. The system according to anyone of Embodiments 1, 11, 12 or 13, wherein the axial impeller includes a gear box or a plurality of variable vanes, such that angle of the vanes can be adjusted.
Embodiment 15. The system according to Embodiment 14, wherein adjustment of a gear box or the plurality of vanes can adjust the flue gas side draft.
Embodiment 16. The system according to anyone of the preceding Embodiments, wherein the heater is one of a fired heater and a boiler.
While the disclosed subject matter is described herein in terms of certain preferred embodiments, those skilled in the art will recognize that various modifications and improvements can be made to the disclosed subject matter without departing from the scope thereof. Moreover, although individual features of one embodiment of the disclosed subject matter can be discussed herein or shown in the drawings of the one embodiment and not in other embodiments, it should be apparent that individual features of one embodiment can be combined with one or more features of another embodiment or features from a plurality of embodiments.
In addition to the specific embodiments claimed below, the disclosed subject matter is also directed to other embodiments having any other possible combination of the dependent features claimed below and those disclosed above. As such, the particular features presented in the dependent claims and disclosed above can be combined with each other in other manners within the scope of the disclosed subject matter such that the disclosed subject matter should be recognized as also specifically directed to other embodiments having any other possible combinations. Thus, the foregoing description of specific embodiments of the disclosed subject matter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosed subject matter to those embodiments disclosed. It is contemplated that the presently disclosed subject matter may be utilized in the fired heater with heat pipe preheater disclosed in commonly assigned U.S. Provisional Application No. 62,352099 entitled “Fired Heater With Heat Pipe Preheater”, the disclosure of which is incorporated herein by reference in its entirety.
It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the disclosed subject matter without departing from the spirit or scope of the disclosed subject matter. Thus, it is intended that the disclosed subject matter include modifications and variations that are within the scope of the appended claims and their equivalents.

Claims

1. A system for increasing flue gas side draft in a heater assembly, wherein the heater assembly includes a heater, an inlet supply of combustion air, an exhaust flue gas duct and an exhaust stack, wherein flue gas produced in the heater exits the heater through the exhaust duct and stack, the system comprising:

an impeller assembly operatively coupled to one of the exhaust flue gas duct and the exhaust stack, wherein operation of the impeller assembly increases flue gas draft in the heater assembly to enable an increase in firing rate within the heater beyond the ability of the exhaust stack.

2. The system according to claim 1, wherein heater assembly further includes a low flue gas side pressure drop air preheater and associated ducting for preheating at least a portion of the combustion air, at least a portion of the flue gas and the combustion air are directed to the air preheater, wherein at least a portion of the impeller assembly is located within the exhaust flue gas duct.

3. The system according to claim 1, wherein the impeller assembly includes a motor and an axial impeller.

4. The system according to claim 3, wherein the heater is one of a fired heater and a boiler.

5. The system according to claim 3, wherein the impeller assembly includes an axial impeller.

6. The system according to claim 5, wherein the axial impeller and the motor are located within the exhaust flue gas duct.

7. The system according to claim 6, further comprising a pressurized bearing lubrication system to enable operation in high temperature environments.

8. The system according to clam 5, wherein the axial impeller is located within the flue gas duct and the motor is located externally of the exhaust flue gas duct, wherein the impeller assembly furthers include a seal located within a wall of the exhaust flue gas duct and a shaft passing through the seal, wherein the axial impeller and the motor are operatively connected via the shaft.

9. The system according to claim 5, wherein the axial impeller includes one of a gear box or a plurality of variable vanes, such that angle of the vanes can be adjusted.

10. The system according to claim 9, wherein adjustment of one of the gear box or plurality of vanes can adjust the flue gas side draft.

11. The system according to claim 1, wherein a draft created from the impeller is less than 1.5 inch of water column of pressure gain.