Abstract:
A rotary air preheater has a rotor, at least one seal disposed proximate to the rotor, a drive for reciprocally driving a portion of the seal between a first position adjacent the rotor and a second position spaced from the first position, and a control system for activating the drive. The control system includes a boiler load power sensor, means for storing a first stored boiler load having a value between the minimum boiler load and the maximum boiler load, and logic for comparing the sensed boiler load to the first stored boiler load. The logic provides a first activation signal to the drive when the sensed boiler load rises above the first stored boiler load and a second activation signal to the drive when the sensed boiler load falls below the first stored boiler load. The first activation signal activates the drive to drive the portion of the seal to the first position and the second activation signal activates the drive to drive the portion of the seal to the second position.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to rotary regenerative air preheaters having sector plates dividing the air preheater into a gas sector and one or more air sectors and having radial seals which cooperate with the sector plates to reduce the leakage between the air preheater sectors. More particularly, the invention relates to control systems for sector plate drive means for adjusting the position of the sector plates relative to the radial seals. 
     A rotary regenerative air preheater transfers sensible heat from the flue gas leaving a boiler to the entering combustion air through regenerative heat transfer surface in a rotor which turns continuously through the gas and air streams. The rotor, which is packed with the heat transfer surface, is divided into compartments by a number of radially extending plates referred to as diaphragms. These compartments are adapted to hold modular baskets in which the heat transfer surface is contained. 
     The air preheater is divided into a flue gas side or sector and one or more combustion air sides or sectors by sector plates. Flexible radial seals on the rotor, usually mounted on the top and bottom edges of the diaphragms, are in close proximity to these sector plates and minimize leakage of gas and air between sectors. In a typical rotary regenerative heat exchanger, the hot flue gas and the combustion air enter the rotor shell from opposite ends and pass in opposite directions over the heat exchange material housed within the rotor. Consequently, the cold air inlet and the cooled gas outlet are at one end of the heat exchanger, usually the bottom and referred to as the cold end, and the hot gas inlet and the heated air outlet are at the opposite end of the heat exchanger, usually the top and referred to as the hot end. As a result, an axial temperature gradient exists from the hot end of the rotor to the cold end of the rotor. In response to this temperature gradient, the rotor tends to distort and to assume a shape similar to that of an inverted dish (commonly referred to as rotor turndown). As a result, the radial seals mounted on the top edges of the diaphragms are pulled away from the sector plates with the greater separation occurring at the outer radius of the rotor. This opens a gap permitting the undesired intermingling of the gas and the air. 
     Various schemes have been developed to maintain the seal between the radial seals mounted on the diaphragms and the sector plates during this temperature change. It is well known that rotary regenerative air preheaters utilize a flexible sealing member that extends across the gap between the diaphragms and the sector plates. As the rotor transitions from a cold, non-operating condition to a hot, operating condition, the temperature gradient along the rotor increases, and the gap between the hot end diaphragms and the sector plates increases. These flexible sealing members are designed to maintain contact with the sector plate. 
     However, these seals are subject to a number of problems. It has been experienced that the continuous contact between the sealing member and the sector plates results in wear to both the sealing member and the sector plates. Special liners are sometimes utilized to reduce sealing surface wear. However, use of such liners results in higher capital and labor costs. Further, those seals are subject to premature failure due to flexure and edge fracturing. Finally, the amount of gap that may be closed by these seals is limited. 
     SUMMARY OF THE INVENTION 
     The present invention provides a control system in an air preheater for maintaining a controlled gap between the rotor and the sector plates or axial seal to reduce leakage. The rotary preheater of the present invention includes at least one seal means (the sector plates or the axial seal) disposed proximate to the rotor and cooperating with the rotor to retard leakage around the rotor. The preheater also includes drive means activatable for reciprocally driving at least a portion of the seal means between a first position adjacent the rotor and a second position spaced from the first position. The control system includes power sensing means for sensing the boiler load, storage means for storing a first stored boiler load having a value between the minimum boiler load and the maximum boiler load, and logic means for comparing the sensed boiler load to the first stored boiler load. The logic means provides a first activation signal to the drive means when the sensed boiler load rises above the first stored boiler load and a second activation signal to the drive means when the sensed boiler load falls below the first stored boiler load. The first activation signal activates the drive means to drive the portion of each of the seal means to the first position and the second activation signal activates the drive means to drive the portion of the seal means to the second position. 
     For example, the hot end seals which extend from the diaphragm plates of the rotor move away from the hot end sector plates by an amount increasing from the inboard end to the outboard end as the air preheater progresses from a cold condition to a hot condition. In accordance with the subject invention, the first activation signal causes the drive means to drive the outboard end portion of the sector plates toward the hot end seals of the rotor to close this increasing gap when the sensed boiler load reaches a predetermined value. 
     It is an object of the invention to provide a new and improved control system for active leakage control in an air preheater. 
     It is also an object of the invention to provide a control system to control leakage around the rotor of an air preheater that does not require sensors within the air preheater. 
     Other objects and advantages of the invention will become apparent from the drawings and specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general perspective view of a conventional rotary regenerative air preheater. 
     FIG. 2 is a simplified top view of a trisector air preheater showing the rotor in the housing and showing the sector plates. 
     FIG. 3 is a diagrammatic representation of a rotary regenerative heat air preheater in the cold state. 
     FIG. 4 is a diagrammatic representation similar to FIG. 3 showing rotor turndown with the sector plates in the fully up position. 
     FIG. 5 is a further diagrammatic representation similar to FIG. 4 but with the sector plates in the fully down position. 
     FIG. 6 is a top view of one embodiment of the sector plate drive system. 
     FIG. 7 is an end view of the sector plate drive assembly of FIG.  6 . 
     FIG. 8 is a schematic view of a control system in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 of the drawings is a partially cut-away perspective view of a typical bi-sector air preheater  10  showing a housing  12  in which the rotor  14  is mounted on a drive shaft or post  16 . The housing is divided by means of the flow impervious sector plates  18  and  20  into a flue gas sector  24  and an air sector  26 . Corresponding sector plates are also located on the bottom of the unit. The hot flue gases enter the air preheater  10  as indicated by the arrow  28 , flow through the flue gas sector  24  where heat is transferred to the heat transfer surface in the rotor  14 . As this hot heat transfer surface then rotates through the air sector  26 , the heat is transferred to the air flowing through the rotor from the bottom as indicated by the arrow  30 . Consequently, the cold air inlet and the cooled gas outlet define a cold bottom end and the hot gas inlet and the heated air outlet define a hot top end. Located on the top end of the housing is the hot end center section  32  with a corresponding cold end center section  34  on the bottom end. The rotor  14  has a shell  36  and is divided into a plurality of pie-shaped compartments  38  by the diaphragm plates  40  with each compartment containing a plurality of heat exchange basket modules  42 . 
     FIG. 2 is a plan view representation of a trisector air preheater comprising the rotor housing  12  and the rotor  14 . The housing is divided in this case into three sectors by the sector plates  18 ,  20  and  22 . The sectors are the flue gas sector  24  and the air sector  26  which is divided into the primary air sector  44  and the secondary air sector  46 . This figure illustrates the sector plates in cross-section for purposes of clarity. The rotor  14  is composed of the shell  36  and the diaphragm plates  40  dividing the rotor into compartments  38 . Attached to the top and bottom edges of these diaphragm plates  40  are the radial seals. 
     When the air preheater  10  is put into service, the rotor  14  is cold and the relationship of the rotor  14  to the sector plates  18 ,  20  is depicted in FIG.  3 . This figure shows the rotor housing  12 , the rotor  14  composed of the rotor shell  36  and the diaphragm plates  40  and the sector plates  18 ,  20 . On top of the diaphragm plates  40  are the radial seals  48  which will be shown in some more detail in later figures but which may be of any conventional radial seal design. As can be seen in this FIG. 3, the radial seals  48  are engaging or in close proximity to the sector plates  18 ,  20  to form the desired seal. 
     As the moving rotor  14  progresses from a cold non-operating condition to a hot operating condition, an axial temperature gradient develops from the hot top end of the rotor to the cold bottom end of the rotor as a result of heat exchange between the counter-flowing air and gas streams. This axial temperature gradient causes the rotor  14  to distort such as shown in FIG.  4 . This is referred to as rotor turndown. As a result, the radial seals  48  mounted on the hot end of the diaphragm plates  40  are pulled away from the sector plates  18 ,  20  with the greater separation occurring at the periphery of the rotor  14 . This opens a gap  50  between the radial seals  48  and the sector plates  18 ,  20  which would allow flow between sectors, resulting in an undesired intermingling of the gas and air. By contrast, the sector plates  52  and  54  on the bottom cold end are shaped and mounted to start out in the cold condition with a preadjusted gap  56  at the periphery between the sector plates  52 ,  54  and the radial seals  58  as seen in FIG.  3 . As the rotor is heated and rotor turndown occurs, this gap  56  is closed as shown in FIG.  4 . 
     To close the gap  50  created by rotor heating and turndown, the hot end sector plates  18 ,  20  are driven so that the outboard end portions  60  of the sector plates  18 ,  20  move toward the rotor  14 . This is illustrated in FIG. 5 where it can be seen that the outboard portions  60  of the sector plates  18 ,  20  have been forced downwardly back into contact with the radial seals  48 . A sector plate drive system  62  is used to reciprocate the sector plates  18 ,  20  up or down and thereby close gap  50 . 
     In a first embodiment, the sector plate drive system  62  includes one or two electric motors  64  which drive two mechanical screw actuators  66  via a speed reducer  68  and a first 3-way mitre box  70 . If two electric motors  64  are utilized, each of the motor drives is coupled to the speed reducer  68  by a second 3-way mitre box  72 . The electric motors  64 , 3-way mitre boxes  70 ,  72 , speed reducer  68 , and mechanical screw actuators  66  are all mounted to a welded steel base  74  which is mounted to the hot end center section  32 . The drive of each mechanical screw actuator  66  is connected to a sector plate  18 ,  20  by a linkage rod  76 . A sealing/guiding assembly  78  maintains linkage alignment and prevents leakage from occurring outward around the rod  76 . 
     Another embodiment of the sector plate drive system is disclosed in U.S. patent application Ser. No. 09/339,250, filed Sep. 20, 1999 and assigned to the assignee of the subject application and incorporated herein by reference. This application discloses a sector plate drive assembly which utilizes a drive shaft, dual drive belts, dual drive cylinders, and dual eccentric disk devices to reciprocate the sector plates  18 ,  20 . 
     In conventional sector plate drive systems, the drive means (the electric motor(s) or drive shaft) is operated by a control system in response to either changes in seal clearance or the temperature differential between the hot and cold ends of the rotor. 
     The seal clearance is detected by rotor position sensors attached to each hot end sector plate which are actuated upon contact by the rotor T-bar located on the outer circumference of the rotor. When the system is in the “auto ” mode, the drive means moves the outboard ends of the sector plates toward the rotor. Upon contact with the rotor T-bar, the sensor signals the drive to stop and retract the sector plate a small distance from the rotor, leaving a nominal clearance at the periphery. It should be appreciated that the rotor position sensors are subject to wear, high temperatures, and vibration due to the rotary movement of the air preheater. In addition, transients in the operation of the air preheater can cause mechanical shock damage to these sensors. 
     The temperature differential is sensed by sensors mounted in the gas inlet and gas outlet ducts. The control system operates the drive means to close the gap when the desired temperatures are reached. It should be appreciated that the sensors in the gas inlet are also exposed to high temperatures and both temperature sensors are also subject to vibration and mechanical shock. 
     In an air preheater leakage control system  80  in accordance with the present invention, the position of the sector plates  18 ,  20  is controlled on the basis of the boiler power output or boiler load. It has been discovered that the actual temperatures of the gas and air flow in a power boiler stabilize within a rather narrow range of full load temperature at a rather low percentage of full load operation. Only the flow volumes change significantly. This permits a very close approximation of air preheater rotor turndown and seal clearance based simply on the boiler load. By measuring the boiler load and controlling the sector plate drive system  62  on the basis of the boiler load, the need for the sector plate position sensors or the differential temperature sensors is eliminated. 
     In a preferred embodiment, the subject invention comprises a two position system where the outboard portion  60  of sector plates  18 ,  20  may be driven from a fully up position  82  (FIG. 4) to a fully down position,  84  (FIG.  5 ), with the length of the sector plate travel based on a percentage of calculated full load rotor turndown. The subject air preheater leakage control system  80  utilizes a first predetermined percentage of boiler load, stored in a memory section  86  of the control system  80 , to activate the sector plate drive system  62 . The value is based on the load demands of the boiler. For example, the value of the selected initiation boiler load may be as low as 35% of boiler load, where approximately 75% of the full rotor turndown will have occurred. Based on duty cycle or other customer requirements, a percentage of boiler load closer to full load operation may be desired. 
     The subject air preheater leakage control system  80  utilizes a boiler load sensor  88  which is part of the boiler control system  90  for sensing the boiler load. A signal proportional to the boiler load is transmitted by the boiler load sensor  88  and received in a receiver portion  92  of the control system  80 . When the sensed operating load rises above the stored first boiler load value, the logic section  94  of the control system  80  transmits an activation signal via a transmitter portion  96  of the control system  80 . The activation signal activates the sector plate drive system  62  to drive the outboard portions  60  of the sector plates  18 ,  20  downward from the fully up position  82  to the fully down position  84  and thereby close the gap  50 . If the sensed boiler load falls below the first boiler load value, the subject control system  80  activates the sector plate drive system  62  to withdraw the outboard portions  60  of the sector plates  18 ,  20  from the fully down position  84  to the fully up position  82 . 
     If the boiler load is anticipated to be above the first boiler load value for the greatest percentage of the time, the control system logic  94  cycles the sector plate drive system  62  from the fully down position  84 , to the fully up position  82 , and back to the fully down position  84 , at a periodic interval to assure the sealing surface remains free to travel. 
     Because there is no need to protect a sensing device from wear, there is no need to back the sector plate  18 ,  20  away from the rotor  14  as is required with a rotor sensing system when contact is made with the rotor. If the initiation point is set at levels of operating load approaching or at full boiler load, sealing effectiveness may actually exceed the more complicated control system for this reason. 
     The above described control system  80  is a two position systems, where the outboard portions  60  of the sector plates  18 ,  20  are moved between fully up and fully down positions  82 ,  84 . Alternatively, the control system  80 ′ may position the outboard portions  60  of the sector plates  18 ,  20  at three or more positions, where the additional positions are intermediate positions  98  (FIG. 4) located between the fully up and fully down positions  82 ,  84 . In a three position system, the control system  80 ′ initiates moving outboard portions  60  from the fully up position  82  to a single intermediate position  98  when the sensed boiler load rises above a predetermined intermediate boiler load value, the intermediate boiler load value being between the minimum, or zero power, boiler load value and the first boiler load value. 
     The control system  80 ′ maintains outboard portions  60  in the intermediate position  98  until the sensed boiler load either rises above the first boiler load value or drops below the intermediate boiler load value. If the sensed boiler load drops below the intermediate boiler load value, the control system  80 ′ returns the outboard portions  60  to the fully up position  82 . If the sensed boiler load rises above the first boiler load value, the control system  80 ′ moves the outboard portions  60  to the fully down position  84 . The control system  80 ′ maintains the outboard portions  60  in the fully down position  84  so long as the sensed boiler load is greater than the first boiler load value. If the boiler load drops below the first boiler load value, the control system  80 ′ initiates movement of outboard portions  60  to the intermediate position  98 . As previously described, the control system  80 ′ maintains outboard portions  60  in the intermediate position  98  until the sensed boiler load either rises above the first boiler load value or drops below the intermediate boiler load value. 
     It is possible for the subject control system to move the outboard portions of the sector plates  18 ,  20  over a continuous range of positions between the fully up position  82  and the fully down position  84 , depending on the value of the boiler load. However, the control band is so narrow (approximately 75% of rotor turndown occurs at 35% of boiler load and approximately 90% of rotor turndown occurs at 70% of boiler load) that the control system is effectively limited to a relatively small number of positions. 
     The subject control system  80 ,  80 ′, then, achieves a close sealing relationship with the rotor  14  over the greatest majority of operating conditions while eliminating any need to sense actual position of the air preheater rotor or temperatures of the gas or air flows. 
     The two position control system  80  may also be applied to the axial seal plates where applicable, for example in installations where separate primary preheaters are employed, as the simplicity and lower cost of the subject control system allows such use to be cost effective. Conventional systems are sufficiently complex and costly so as to have precluded axial sealing applications. 
     Although the invention has been described with respect to an air preheater with the hot end on the top and with the drive assembly mounted on the top, the invention is equally applicable to an air preheater with the hot end and the drive assembly on the bottom. Likewise, the invention may be applied to a horizontal flow air preheater with a horizontal shaft and with the drive assembly mounted on the hot vertical end.