GB2424471A - Rotary heat exchanger with a sector plate featuring suction ducts - Google Patents

Abstract

A rotary heat exchanger comprising a rotor extending along an axis of rotation and having, at opposite axial ends of the rotor, respective first and second axial end surfaces substantially perpendicular to the axis and surrounding the axis, a first sector plate having a sealing surface adjacent the first axial end surface, the sealing surface extending across the first axial end surface so as to divide the first axial end surface into two halves, wherein the first sector plate defines suction ducts extending through the first sector plate from the sealing surface, and the rotary heat exchanger further includes a pressure device arranged to draw gas through the suction ducts from between the sealing surface and the first axial end surface. The pressure device may be a suction device. There may be provided a second sector plate extending across the second axial end surface arranged opposite to the first sector plate. There may be a gas inlet duct and a gas outlet duct provided at each axial end of the rotor. There may be a support frame provided that includes sector plate ribs.

Description

A Rotary Heat Exchanger and a Method of Reducing Leakage in a Rotary Heat

Exchanger The present invention relates to rotary heat exchangers.

The Invention is particularly concerned with rotary regenerative heat exchangers of the rotating matrix format as typically disclosed in patent US 4,124,063 with a later improvement on that patent being detailed in US 5,443,113. These rotating matrix heat exchangers comprise a frame, a housing carried by said frame, a rotor rotatable within said housing about an axis, a multiplicity of heat exchange elements mounted in said rotor, first and second sector plates mounted at the first and second axial ends of said rotor, said sector plates each extending along a diameter of said rotor, gas inlet and outlet ducts at said first and second axial ends respectively and arranged on the same radial side of said sector plate and air outlet and inlet ducts at said first and second axial ends respectively and arranged on the opposite radial side of said sector plates from said gas inlet and outlet ducts While reference has been made herein to air inlet and outlet ducts, heat exchangers of this type are also used for heat exchange between two different gases so that instead of a gas/air heat exchanger one has a gas/gas exchanger. These heat exchangers are commonly used to recover heat for example from exhaust gases allocations such as power stations and where air is the other medium, this air is significantly heated and such preheated air can than be fed as combustion air to the burners of the power station.

As indicated, however, these heat exchangers can also be used in a gas/gas mode, for example in the purification of the exhaust gases to remove NOx and SOx gases. These rotary heat exchangers are conventionally of a massive structure weighing several hundred tons. It is possible to mount the heat exchangers so that the axis of the rotor is horizontal but it is most common to mount the rotor with its axis vertical. The first and second sector plates will then be upper and lower sector plates.

With heat exchangers of this general type, there is a significant problem of thermal deflection of the rotor during use. Conventional sector plates have had an inherent rigidity and this has meant that very often both the upper and lower sector plates have been provided with a hinged structure to allow the sector plates to be pre-set or adjusted in a particular position to adapt the shape of the sector plate to that of the rotor after thermal movement. Resulting gaps can exist between the radial seals at the rotor and the sector plates and this can give rise to unsatisfactory leakage.

US 5,443,113 specified a means of overcoming the problem of thermal distortion according by both detailing how to fabricate and form the cold end second sector plate from a generally flat, plate material combined with a set of least two longitudinally extending sector plate ribs in a manner whereby the surface of the second sector plate, when cold, is convex and is complementary to the concavity of the cold end of the rotor when the rotor is at its operating temperature. While that patent emphasised the importance of achieving the concave nature of the bottom sector plate to best match the deflected shape of the heater rotor during operation, it also indicated similar, static sealing plate arrangements at the hot end of the rotor. The nature of these static sealing plate structures both produce lower maintenance requirements and provide the facility for the further developments as described in this invention.

Figure 6 of the accompanying drawings depicts a previous rotor. The pressure of the inlet air, Pal, is greater than the pressure of the outlet air, Pao, which is greater than the pressure of inlet gas, Pgi, which is greater than the gas outlet pressure, Pg0 (Pai > Pao > Pgi >Pgo). There will therefore be direct leakage between the air outlet and the gas inlet. This results in a less efficient rotary heat exchanger.

It is an object of the present invention to reduce direct leakage.

According to the present invention, there is provided a rotary heat exchanger including: a rotor extending along an axis of rotation and having, at opposite axial ends of the rotor, respective first and second axial end surfaces substantially perpendicular to the axis and surrounding the axis; a first sector plate having a sealing surface adjacent the first axial end surface, the sealing surface extending across the first axial end surface so as to divide the first axial end surface into two halves; wherein the first sector plate defines suction ducts extending through the first sector plate from the sealing surface; and the rotary heat exchanger further includes a pressure device arranged to draw gas through the suction ducts from between the sealing surface and the first axial end surface.

By causing gas to flow through ducts in the sealing surface of the first sector plate direct leakage between the air outlet and the gas inlet is reduced. The pressure device is preferably a suction fan arranged the opposite side of the sector plate from the rotor. The suction provided should be less than the main boiler fans. By thus reducing both the air and gas flow rates that must be handled by both the forced draft and induced draft fans, the use of such a system will either allow the boiler load to be increased or higher pressure drops to be accommodated across the air or gas paths on either side of the heater, without the need to upgiade or replace these much larger and more expensive fans.

The sector plate is preferably arranged to extend through the axis of the rotor and across the diameter of the rotor, dividing the first axial end surface into two equal halves. The ducts in the sector plate are preferably slots through which gas can be drawn.

The gas can be fed into the rotor by a first gas inlet duct arranged at the first axial end of the rotor and then output through a first gas outlet duct at the second axial end of the rotor. The first gas inlet duct and the first gas outlet duct can be arranged on the same radial side of the first sector plate. A second gas inlet duct and a second gas outlet duct can be arranged at the first and second axial ends of the rotor respectively and on the opposite radial side of the first sector plate on the first gas inlet and first gas outlet duct. The gas outlets described here can be used for any gas, including for example, air. Discharge from the pressure device and gas from the second outlet duct may conveniently be fed to a common outlet.

There may be a second sector plate adjacent the second axial end of the rotor which extends across the second axial end surface and which is arranged directly opposite the first sector plate.

The sector plates preferably comprise at least two longitudinally extending sector plate ribs which extend from the first sector plate in a direction away from the rotor which are welded to support structure ribs which are welded to a frame of the rotary heat exchanger. The sector plate is preferably formed from a generally flat, plate material. This arrangement of the sector plate provides a convenient surface through which the gas can be drawn.

Rotor walls are preferably provided extending from the first axial end surface through to the second axial end surface define a plurality of heat exchange spaces arranged in a circumferential array. The sealing surface then preferably extends sufficiently that, at any given time, at least one complete heat exchange space on either side of the axis of the rotor is in its entirety directly and entirely opposite a respective part of the sealing surface. This ensures the maximum efficiency for the rotary heat exchanger. This can be accomplished by the sealing surface having an extent at least equal to the cross-sectional area of two adjacent heat exchanger spaces. Each heat exchange space preferably extends substantially along an entire radius of the rotor.

With the first sector plate shaped as a sector of angle X, the angle between the radially extending walls defining the heat exchange spaces should be less than X/2.

According to a further aspect of the invention there is provided a method of reducing leakage in a rotary heat exchanger including driving, preferably by sucking, gas from the space between the sector plate and the rotor through suction ducts extending through the sector plate.

In order that the present invention may more readily be understood, the following description is given merely by way of examples reference being made to the accompanying drawings in which: Figure 1 is a schematic perspective view of a rotary regenerative heat exchanger; Figure 2 is a three-dimensional, transparent, space-frame view of rotary regenerative heater showing the various seal gaps through which direct leakage of air to gas occurs due to the pressure differential between the two streams in a conventional rotary heat exchanger; Figure 3 is a three-dimensional, transparent space-frame view of rotary regenerative heater showing the additional leakage that occurs from the air-side to gas side of the air preheater due to air entrainment in the rotor chambers in a conventional rotary heat exchanger; Figure 4 is a schematic perspective view of a conventional top or bottom sector plate of a heat exchanger as shown in Figure 1; Figure 5 is a view similar to Figure 4 of both the top and bottom sector plates according to the development described in US 5,443, 113; Figure 6 is a cross-sectional view of the both the fixed sector plates shown in Figure 5 and the rotor, radial plates and radial seal plates as they pass beneath the sector plate. This arrangement shows the most common arrangement on old air preheaters of single sealing.

Figure 7 is a plan view of the double sealing arrangement that was quoted as prior art in US 5,915,339.

Figure 8 is a cross-sectional view of the sealing arrangement similar to that in Figure 6 but showing another view of the double sealing arrangement in Figure 7.

Figure 9 is a cross-sectional view of one of the forms of this invention showing in this case the addition of a suction-sealing fan.

Figure 10 is a typical sketch of a fossil fuel fired boiler showing how the additional sealing fan flow arrangement relates to the wider view of the complete air and gas circuit through the unit.

Figure 11 is a typical sketch of a fossil fuel fired boiler showing how the additional sealing fan flow arrangement might relate to the wider overall plant arrangement, if a boosted over-fire air fan has been is added as a combustion modification measure to reduce NOx..

Figure 12 is a typical sketch of another arrangement covered by this invention, showing how the low leakage fan sealing system would be applied to minimise the leakage in a gas/gas heater as commonly used to reheat the cold, saturated flue gas exiting from a conventional wet flue gas desulphurisation plant.

Referring first to Figure 1, there is illustrated therein a rotating matrix type heat exchanger which includes a frame 10 upon which is mounted a housing 12 within which is rotatable, about a vertical axis 14, a rotor assembly 16 including a peripheral wall 18 connected to the hub 15 surrounding the axis 14 by a large number of radial seal plates 20. Located within the spaces between the seal plates and spacer plates 22 are a multiplicity a heat exchange elements 24. The rotor is rotatable relatively slowly, usually about one revolution per minute, about the axis 14 by a rotor drive shown schematically at 26.

The rotor drive is mounted on a diametrally extending top structure 38 to the lower surface of which is mounted a first upper sector plate, Similar in structure to. 35 the second lower sector plate 28 described below, but which is not visible in the drawing of Figure 1.

The lower or second sector plate 28 is positioned below the rotor and is in closely adjacent relationship to the lower axial end of the rotor. An end pillar 30 can be seen on which are mounted axial seal plates engaging the outer surface of the rotor 18. Secured to the frame is a bottom structure 24 upon which the lower sector plate 28 is mounted, Reference numeral 36 indicates supports which can be used for mounting the whole assembly in any suitable location.

Ducting, portions of which are shown at 40, 42, are provided for feeding hot gas to the far side, as viewed in Figure 1 of the sector plates and through the rotor and for withdrawing the gas- cooled by the heat exchanger respectively. Further ducts 44, 46 are shown on the right of the seal plates and are used for conducting air into and out of the heat exchanger: Such a structure is fairly standard and the concept is that hot combustion gases, for example from the furnace of a power station, are fed downwardly via the hot gas inlet duct 40 to contact the heat exchange elements 24 which are thereby heated. The somewhat cooled products of combustion exit via the outlet duct 42. At the same time cool air to be used as combustion air for the furnace) is fed in through the duct 44 and exits via the duct 46. It passes over the heat exchange plates which are being rotated by the rotor 19 and these heal exchange plates give up their heat to the incoming air the temperature of which therefore rises. The hot air is then used as the combustion air for the furnace, this significantly improving the thermo- dynamic performance of the whole arrangement. Thus the upper end at which the first sector plate (not shown) is positioned, will be the hot end and the lower end at which the second sector plate 28 is positioned, will be the cold end of the rotor.

In order to better understand the detailed nature of this invention, that is designed to reduce the leakage level of air to gas through the heater, it is first important to appreciate the different nature and sources of the different components of leakage through the heater. The two different types of leakage are shown on Figures 2 and 3, respectively. - Referring first to Figure 2, the broad, light shaded arrows show the extent of the various paths for direct leakage of air to gas through the various and inevitable, residual gaps between the various seals and the corresponding sealing surfaces. Note that the main leakage paths totally encompass the heater rotor and include radial seal leakage (at both hot and cold ends of the heater) and axial seal leakage which occurs across the vertical seals placed at both ends of diameter and hub region leakage. While the description of this invention concentrates upon reducing the leakage across the radial seals, because these generally represent the highest leakage component, the principles described herein equally apply to both the axial an hub seal leakage paths.

A final, slightly more tortuous direct leakage path shown in Figure 2 is the air bypassing that occurs across the circumferential seals at the cold end of the rotor on the air side before passing onwards though the axial seals.

Turning to Figure 3, while this retains the arrows of Figure 2 showing the direct leakage path, the large rotational arrow illustrates the component of transfer and hence leakage of air into gas that occurs due to entrainment of air within the rotor sectors as it slowly rotates from air to gas side of the rotor. Note, however, that this only occurs on one side of the rotor hub of as, on the other side of the hub, gas is entrained into the air side. As entrained leakage typically represents the next highest leakage component to radial seal leakage, several cases of prior art have been directed towards reducing or eliminating this entrainment. Various forms of this prior art are described in US 3,315, 729, US 3,338,300, US 3,587,723 and US 4,427,054 are all directed towards different methods of redirecting flows to flush the entrained air or gas from the rotor before it rotates from one side to the other. However, none of these methods are directed specifically towards the reduction of the direct leakage component towards which this invention is directed.

If reference is now made to Figure 4, a conventional lower sector plate 28 is illustrated. This comprises an upper sector plate member 41 to which are welded a plurality of strengthening plates 43. These are secured by independent and adjustable vertical posts 33 passing through apertures 35 in the bottom structure 24, the posts being welded the bottom of the strengthening plates 43 and are terminated at individual adjusting mechanisms. By so doing, the sector plate structure can be adjusted at any time. To prevent air of gas escaping, a seal is provided where the rod passes through the bottom structure with metallic bellows 37 utilised to provide freedom of movement. As the only means of support to the sector plate is at discrete points, the sector must rely on its box like structure to provide the necessary stiffs ness. The resulting upper surface of the plate 41 is therefore not usually very flat and it has to be machined to a flat state. The identical arrangement is repeated on the top sector plate.

With the structure showing in Figure 5, according to US 5,443,113, the top plate 41 has welded to its lower surface at least two longitudinally extending sector plate ribs 45. The bottom structure 34 has similar upwardly extending bottom structure ribs 47. It has been found that the ribs 45 can satisfactorily be welded to the sector plate 41 without any significant distortion thereof. With the unit thus assembled, the ribs 45 are telescoped over the ribs 47 and are welded thereto. During this mounting of the plate 41 relative to the bottom structure 34 the actual position of the plate 41 can be accurately determined before the ribs 45, 47 are welded together.

Such a structure thereafter needs essentially no adjustment and no machining of the top surface of the plate 41. Similar methods of construction are applied to both the hot and cold end sector plates at top and bottom of the heater. With regard to the present invention, the simple box structure formed between the various plates also forms an ideal, gas-tight conduit through which to transport gas All of the above structures have been described as having the first, hot end of the rotor at the top and the second, cold end at the bottom. It will be appreciated that the first, hot end could equally be at the bottom. Similarly, the rotor could be mounted with the axis other than vertical. e.g. it could be horizontal, with the sector plates then being located to one side and the other, re- spectively.

Considering next Figure 6, this shows a combination of the prior art for the sector plate designs as shown in Figure 5 together with the most common use of a single set of radial seals passing under the sector plates at any one time. For illustrative purposes, the pressure of both the inlet air to the air preheater, Pai, and the outlet air from the air preheater, Pao, have been indicated on the figure together with the corresponding gas inlet and outlet pressures, Pgi and Pgo, respectively.

Note that, as in all air preheater applications Pai > Pao > Pgi > Pgo, it is self apparent that direct air leakage will be forced through the residual seal gaps from air to gas side of the rotor as shown by the direction of the arrows.

Figure 7 shows one of the measures taken to reduce this direct leakage by doubling the number of radial plates in the rotor to ensure that there are always two radial plates and seals simultaneously under the sector sealing plates.

The mechanism by which this approach operates is better illustrated in Figure 8, where it can be seen that the resultant intermediate gas pressures, Pi, between the seals are typically half way between the air and gas pressures at each end of the rotor. In this manner, double sealing acts as a labyrinth seal, which both reduces the pressure drop across the individual seals and correspondingly reduces the amount of direct leakage. While this leakage can be reduced still further by ensuring there are even more seals under the sector plates at any time, however, depending upon how these additional modifications are implemented, they may result in higher heater pressure drops and increased ID or FD fan power.

Figure 9 shows one implementation of this invention whereby a suctionsealing fan has been added, which draws gas from the intermediate space between the double seals shown on Figure 8. This suction sealing flow is drawn through a series of slots of varying dimensions that are distributed along the radial length of the upper sector plate as shown on Figure 9. Thereafter, the rectangular flow channel mentioned earlier produces an ideal flow chamber though which to lead the suction sealing flow before it passes through the outer surface of the rotor top structure and onwards to the suction fan inlet. Thereafter, in this arrangement of the invention, the outlet flow from the suction sealing fan is directed back into the main duct carrying the outlet air flow from the air preheater to the boiler.

It can be appreciated that, by progressively drawing increasing quantities of suction sealing flow through the slots, the leakage air flow rate across the radial seal nearest to the air-side will increase, compared to its original value without suction sealing. Equally, however, this increased pressure drop reduces the intermediate pressure, Pi, between the seals such that this intermediate pressure slowly approaches the gas side inlet pressure, Pgi. Hence, as the suction sealing flow is increased, the residual pressure drop across the radial seal closer to the gas side of the heater, which can be seen to be Pi - Pgi, steadily decreases. Indeed, when sufficient suction sealing flow is applied, the intermediate pressure, Pi, can be induced to become less that the gas inlet pressure (i.e. Pi<Pgi). In this case, it can be seen that the direct leakage across the radial seal gaps can actually be seen to reverse such that it is theoretically possible to totally eliminate all radial seal leakage.

Considering, now Figure 10, which shows how the additional sealing fan flow arrangement relates to the wider view of the complete air and gas circuit through the unit. Note that the major benefit of applying this suction sealing as described above is to reduce and ultimately eliminate the direct leakage of air to gas across both the hot and cold end radial sector plates of the heater.

Consequently, as this leakage has been noticeably reduced, for the same overall flow rates of air and gas passing to and from the boiler at the hot end of the heater, the flow rates that must be delivered by both the forced draft and induced draft fans are correspondingly reduced. In many cases, the maximum generating capacity of the unit may be limited or reduced by the maximum air or gas handling capacities of either or both of these fans. This may be further exacerbated if the pressure drop or flow requirements are increased on either the air or gas side of the rotor as might be caused by either heater fouling or the addition of further plant items such as modified combustion system or the retrofit of a selective catalytic reactor (SCR) for NOx reduction.

Hence, the use of the additional suction sealing fans described in this invention will recover useable fan operating margins, thereby minimising the need for load reduction, while preventing having to make major changes or even complete replacement to the much larger and more expensive induced draft or forced draft fans.

Turning next to Figure 11, this shows one of several possible installation arrangements of a suction sealing fan when the unit is installed or retrofitted with an over-fire air firing system as a means of reducing the generation of nitrogen oxides (NOx). In other applications, the suction sealing fan may equally be chosen to direct its output elsewhere - such as to the primary air inlet of the coal pulversing mills.

Finally, considering Figure 12, this shows a representative sketch of alternative arrangement of this patent as applied to gas/gas heaters that are commonly used to reheat the flue gas after conventional wet flue gas desuiphurisation systems. Note, that in this case, the low leakage fan has been arranged to draw its inlet gas flow from the reheated flue gas flow rate after it flows from the gas gas heater and passes onwards to the stack. In this case, therefore, this low leakage fan flow rate is blown rather than drawn through the slots in the sector plates as shown in Figure 9, thereby reversing the direction of the leakage flows shown in that figure.

By increasing the low leakage fan flow rate, the pressure of the gas between the double seals will be slowly increased compared to its natural value without the low leakage fan. As the low leakage fan flow rate is increased, therefore, this inter-seal pressure (Pi) approaches the untreated gas inlet pressure (Pugi) thereby slowly reducing the amount of direct leakage of untreated to treated gas across the seals. As with the suction sealing described above, when the low leakage fan flow rate is increased sufficiently the inter-seal pressure can be raised above the gas inlet pressure, thereby eliminating virtually all of the direct leakage across the rotor.

Finally, considering Figure 12, note that this is a schematic diagram of a GGH arrangement simply showing the typical flow arrangement in a conventional GGH. However, depending on the duct arrangement, the hot end of the gas gas heater may either be at the bottom of the heater as indicated on the figure or at the top of the heater if more extensive ductwork is installed.

Equally, while shown as the normally preferred option, this invention does not preclude the possible introduction of the low leakage fan flow through slots in the cold end sector plate.

Claims (14)

1. A rotary heat exchanger including: a rotor extending along an axis of rotation and having, at opposite axial ends of the rotor, respective first and second axial end surfaces substantially perpendicular to the axis and surrounding the axis; a first sector plate having a sealing surface adjacent the first axial end surface, the sealing surface extending across the first axial end surface so as to divide the first axial end surface into two halves; wherein the first sector plate defines suction ducts extending through the first sector plate from the sealing surface; and the rotary heat exchanger further includes a pressure device arranged to draw gas through the suction ducts from between the sealing surface and the first axial end surface.

2. A rotary heat exchanger according to claim I wherein said pressure device is a suction device.

3. A rotary heat exchanger according to either claim I or claim 2 further including a second sector plate adjacent the second axial end of said rotor, said second sector plate extending across the second axial end surface and arranged directly opposite said first sector plate.

4. A rotary heat exchanger according to any one of the preceding claims further including: a first gas inlet duct and a first gas outlet duct at said first and second axial ends of said rotor respectively and arranged on one radial side of said first sector plate; and a second gas inlet duct and a second gas outlet duct at said first and second axial ends of said rotor respectively and arranged on the opposite radial side of said first sector plate from said first gas inlet and outlet ducts.

5. A rotary heat exchanger according to claim 4 wherein discharge from said pressure device and gas from said second outlet duct are fed to a common outlet.

6. A rotary heat exchanger according to any one of the preceding claims further including frame, at least two longitudinally extending sector plate ribs which extend from said first sector plate in a direction away from said rotor and support structure ribs, said first sector plate being formed from a generally flat, plate material to which are connected said at least two longitudinally extending sector plate ribs, said support structure ribs being directly connected to said frame, and said support structure ribs being connected to said sector plate ribs.

7. A rotary heat exchanger according to claim 6 wherein said first sector plate is welded to said at least two longitudinally extending sector plate ribs, said support structure ribs and said sector plate ribs being welded to each other and said support structure ribs being welded to said frame.

8. A rotary heat exchanger according to any one of the preceding claims further including frame, at least two longitudinally extending sector plate ribs which extend from said second sector plate in a direction away from said rotor and support structure ribs, said second sector plate being formed from a generally flat, plate material to which are connected said at least two longitudinally extending sector plate ribs, said support structure ribs being directly connected to said frame, and said support structure ribs being connected to said sector plate ribs.

9. A rotary heat exchanger according to claim 8 wherein said first sector plate is welded to said at least two longitudinally extending sector plate ribs, said support structure ribs and said sector plate ribs being welded to each other and said support structure ribs being welded to said frame.

10. A rotary heat exchanger according to any one of the preceding claims wherein the rotor includes, at the first axial end surface, walls defining a plurality of adjacent heat exchange spaces connecting with respective heat exchange passageways through to the second axial end surface and arranged in a circumferential array such that, with relative rotation of the rotor and the first sector plate, respective heat exchange spaces move past the sealing surface one after the other; and wherein the sealing surface has an extent such that, at any one time, at least one complete heat exchange space on either side of the axis is in its entirety directly and entirely opposite a respective part of said sealing surface.

11. A rotary heat exchanger according to claim 10 wherein, on either side of the axis, the sealing surface has an extent at least equal to the cross sectional area of two adjacent heat exchange spaces.

12. A rotary heat exchanger according to claim 10 or 11 wherein each heat exchange space extends substantially along an entire radius of the rotor.

13. A rotary heat exchange according to any one of the preceding claims wherein said first sector plate is shaped as a sector of angle x, said rotor including radially extending walls, the angle between said walls being less than x12.

14. A method of reducing leakage in a rotary heat exchanger including driving gas from the space between the sector plate and the rotor through suction ducts extending through the sector plate.