FEATURE

FIELD OF THE INVENTION

[0001] This invention relates to a heat transfer sheets assembly for rotary

regenerative air preheaters for transfer of heat from a flue gas stream to a combustion air stream and more particularly relates to heat transfer sheets assemblies having two disparate profiles which cooperate to facilitate the use of higher soot blowing pressures than the norm whilst maintaining structural rigidity and heat transfer characteristics.

BACKGROUND OF THE INVENTION

[0002] Rotary regenerative air preheaters are typically used to transfer heat from a flue gas stream exiting a furnace, to an incoming combustion air stream to improve the efficiency of the furnace. Conventional preheaters include a heat transfer sheet assembly that includes a plurality of heat transfer sheets stacked upon one another in a basket. The heat transfer sheets absorb heat from the flue gas stream and transfer this heat to the combustion air stream. The preheater further includes a rotor having radial partitions or diaphragms defining compartments which house a respective heat transfer sheet assembly. The preheater includes sector plates that extend across upper and lower faces of the preheater to divide the preheater into one or more gas and air sectors. The hot flue gas stream and combustion air stream are simultaneously directed through respective sectors. The rotor rotates the flue gas and combustion air sectors in and out of the flue gas stream and combustion air stream to heat and then to cool the heat transfer sheets thereby heating the combustion air stream and cooling the flue gas stream.

[0003] Conventional heat transfer sheets for such preheaters are typically made by form-pressing or roll-pressing a sheet of a steel material. Typical heat transfer sheets include sheet spacing features formed therein to position adjacent sheets apart from one another and to provide structural integrity of the assembly of the plurality of heat transfer sheets in the basket. One example is PCT Publication WO 01/13055 (Brown), which has a plurality of widely scattered spacing feet. Another earlier example is illustrated in Figure 3 of US Patent 2,596,642 (Boestad). Unlike in Brown, Boestad has adjacent pairs of sheet spacing features forming channels for the flue gas or combustion air to flow through. The provision of flow channels is now standard in the art. Advantageously, to promote controlled flow these channels may be closed-sided as illustrated in Figure 2 of US Patent 4,396,058 (Kurschner) and as improved in US Patent 4,744,410 (Groves). Some heat transfer sheets include undulation patterns between the sheet spacing features to impede flow in a portion of the channel and thereby causing turbulent flow which increases heat transfer efficiency. Boestad is an example of this in a heat transfer sheet assembly having what may be termed open-sided channels in which undulations traverse the sheet spacing features permitting transverse gaseous flow between adjacent channels. US Patent 5,836,379 (Counterman) discloses a heat transfer sheet assembly having closed-sided channels with undulations in which the channels are formed by spacing notches of a first element sheet contacting spacing flats of an identical second element sheet (as can be seen in Counterman Figure6).

[0004] It will be understood that the size, position and configuration of the sheet spacing features in combination with the sheet material thickness, stacking pressure in the basket and thermal cycling experienced in use, contribute to the structural rigidity of the walls of the channels.

[0005] Typical sheet spacing features are of a configuration that allows the flue gas or combustion air to flow through open sided sub-channels formed by the sheet spacing features, uninterrupted at high velocities and with little or no turbulence. As a consequence of the uninterrupted high velocity flow, heat transfer from the flue gas or combustion air to the sheet spacing features is minimal. It is generally known that causing turbulent flow through the plurality of heat transfer sheets such as through the channels defined by and between adjacent sheet spacing features increases pressure drop across the preheater. In addition, it has been found that abrupt changes in direction of flow caused by abrupt contour changes in the heat transfer sheets increases pressure drop and creates flow stagnation areas or zones that tend to cause an accumulation of particles (e.g., ash) in the flow stagnation areas. This further increases pressure drop across the preheater. Such increased pressure drop reduces overall efficiency of the preheater due to increased fan power required to force the combustion air through the preheater. The efficiency of the preheater also reduces with increasing weight of the assembly of heat transfer sheets in the baskets due to the increased power required to rotate the flue gas and combustion air sectors in and out of the flue gas and combustion air streams.

[0006] Therefore, it will be understood that there is a trade-off between material composition, structural stability and operational efficiency. In long term operation it has proved problematic if too little packing pressure is used particularly if sheets upon thermal expansion of the baskets are able to rattle against one another causing mechanical and/or fatigue damage. An obvious solution is to make the channels smaller, i.e. structurally more rigid but this has a negative impact on both operational efficiency and cleanability. The latter issue is critical particularly in cold-end elements, i.e. those at the cold side of the preheater, because here the accumulation of soot and predisposition to clogging with popcorn ash is greater than at hot-end elements.

[0007] There exists a need for improved light weight heat transfer sheets that form a closed channel sheet element assembly that is able to survive higher soot blowing pressures or more soot blowing cycles than hitherto without materially effecting thermal performance and mechanical/structural stability.

SUMMARY

[0008] According to an aspect of the present disclosure, a heat transfer sheet assembly for a rotary regenerative heat exchanger includes: a first sheet element having a first profile comprising a plurality of parallel and elongate first and second sheet spacing features extending longitudinally in a gaseous flow direction; and a second sheet element of equivalent length having a second profile comprising a complementary plurality of wide gauged, parallel and elongate third and fourth sheet spacing features. The first sheet is packed against the second sheet with the first and third spacing features of respective matched pairs of the plurality of first and second sheet spacing features and the plurality of third and fourth sheet spacing features seating against one another and the second and fourth features of the respective matched pairs seating against one another to define for each matched pair a generally close sided elongate channel for gaseous flow therethrough. The first element has lobular heat transfer undulations extending laterally and uninterrupted in between each of the first and second spacing features. The second sheet element further includes a respective elongate fifth sheet spacing feature extending longitudinally along at least half a length of the second sheet element, intermediate the third and fourth sheet spacing features of each matched pair. Each fifth sheet spacing feature includes a lobe contacting the lobular heat transfer undulations between the first and second sheet spacing features of a respective matched pair. The fifth feature lobe has an amplitude less than or equal to a spacing provided by the seated first and third sheet spacing features and the seated second and fourth sheet spacing features.

[0009] The first sheet spacing feature could include a lobe extending away from the nominal plane of the first sheet element and the third sheet spacing feature could include a flat in the nominal plane of the second sheet element.

[0010] In an embodiment, the second sheet spacing feature includes a lobe extending away from the nominal plane of the first sheet element and the fourth feature includes a flat in the nominal plane of the second sheet element.

[0011] The fifth sheet spacing feature could be a notch configuration that includes a notch extending the length of the second sheet element and having the lobe extending away from the nominal plane of the second sheet element toward the said first sheet element and a second lobe extending in the opposite direction away from the first sheet element with the two lobes connected by a flat sheet section.

[0012] In another embodiment, the fifth sheet spacing feature is an alternating notch configuration extending the length of the second sheet element and including at least one first elongate section having a lobe or notch extending away from the central plane of the second sheet toward the first sheet element adjacent at least one second elongate section. Opposing ends of the first and second elongate sections are connected to one another.

[0013] The second elongate section could include a lobe extending away from the central plane of the second sheet element oppositely to the first elongate section lobe.

[0014] The second sheet element could include lobular heat transfer undulations extending laterally and uninterrupted respectively between the third and fifth sheet spacing features and the fifth and fourth sheet spacing features.

[0015] In an embodiment, the undulations of the first sheet element run oblique to the undulations of the second sheet element. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic perspective view of a rotary regenerative preheater;

[0017] Figure 2 is a photograph of portions of two heat transfer sheets of a heat transfer sheet assembly in accordance with a preferred embodiment of the invention with the sheets offset longitudinally solely for illustrative purposes;

[0018] Figure 3 is a perspective view of a portion of a first heat transfer sheet element of the assembly illustrated in Figure 2;

[0019] Figure 4 is a perspective view of a portion of a second heat transfer sheet element of the assembly illustrated in Figure 2;

[0020] Figure 5 is a partial cross-sectional view of the assembly illustrated in Figure

2, and

[0021] Figure 6 is a planar view of the second heat transfer sheet element in the direction A- A shown in Figure 5. DETAILED DESCRIPTION OF THE INVENTION

[0022] As shown in Figure 1, a rotary regenerative air preheater is generally designated by the numeral 1. The preheater 1 includes a rotor assembly 2 rotatably mounted on a rotor post 3. The rotor assembly 2 is positioned in and rotates relative to a housing 4. For example, the rotor assembly 2 is rotatable about an axis A of the rotor post 3 in the direction indicated by the arrow R. The rotor assembly 2 includes partitions 5 (e.g., diaphragms) extending radially from the rotor post 3 to an outer periphery of the rotor assembly 2. Adjacent pairs of the partitions 5 define respective compartments 6 for receiving a heat transfer sheet assembly 7. Each of the heat transfer sheet assemblies 7 include a plurality of heat transfer sheets 8 and/or 9 (see, for example, Figure 2) stacked against one another.

[0023] As shown in Figure 1, the housing 4 includes a flue gas inlet duct 10 and a flue gas outlet duct 11 for the flow of heated flue gases through the preheater 1. The housing 4 further includes an air inlet duct 12 and an air outlet duct 13 for the flow of combustion air through the preheater 1. The preheater 1 includes an upper sector plate 14 extending across the housing 4 adjacent to an upper face of the rotor assembly 2. The preheater 1 includes a lower sector plate 15 extending across the housing 4 adjacent to lower face of the rotor assembly 2. The upper sector plate 14 extends between and is joined to the flue gas inlet duct 10 and the air outlet duct 13. The lower sector plate 15 extends between and is joined to the flue gas outlet duct 11 and the air inlet duct 12. The upper and lower sector plates 14,15 respectively, are joined to one another by a circumferential plate 16. The upper sector plate 14 and the lower sector plate 15 divide the preheater 1 into an air sector 17 and a gas sector 18.

[0024] As illustrated in Figure 1, the arrows marked 'A' indicate the direction of a flue gas stream 19 through the gas sector 18 of the rotor assembly 2. The arrows marked 'B' indicate the direction of a combustion air stream 20 through the air sector 17 of the rotor assembly 2. The flue gas stream 19 enters through the flue gas inlet duct 10 and transfers heat to the assemblies 7 mounted in the compartments 6. The heated assemblies 7 are rotated into the air sector 17 of the preheater 1. Heat stored in the assemblies 7 is then transferred to the combustion air stream 20 entering through the air inlet duct 12. Thus, the heat absorbed from the hot flue gas stream 19 entering into the preheater 1 is utilized for heating the assemblies 7, which in turn heats the combustion air stream 20.

[0025] In a preferred embodiment, as shown in Figure 2, the heat transfer assembly 7 comprises a laminate of a multiplicity of heat transfer sheet elements 8 and 9 closely packed one after another under pressure and with sheets 8,9 having distinct profiles. First sheet element 8, also illustrated in Figure 3, is of a first profile or configuration comprising parallel wide gauged first 21 and second 22 sheet spacing features called notches which in this embodiment are in lateral cross-section of lobular form with oppositely extending lobes preferentially connected by a flat sheet material section operatively effective accurately to space apart adjacent elements. The lobular sheet spacing features 21, 22 extend parallel to the direction of intended gaseous flow from one end of the sheet to the other.

[0026] Second sheet element 9, also illustrated in Figure 4, is of a second profile or configuration comprising parallel wide gauged third 23 and fourth 24 sheeting spacing features which in this embodiment are flats generally in the nominal plane of element 9 which seat respectively against the features 21 and 22 of elements 8 (i.e. one sheet 8 under element 9 as shown in Figure 2 and another element 8 over, not illustrated). The features of element 8 are also illustrated on Figure 6 and the same features together with those of element 9 are illustrated on the cross-section of Figure 5. From this cross-section it can be seen that the wide gauges of features 21,22 and of 23, 24 are equivalent so that features 21,23 and 22,24 respectively cooperate by seating one against the other to define an elongate close- sided gaseous flow channel 25 extending from one end of the assembly to the other.

[0027] Thus a plurality of close-sided gaseous flow channels 25 are created within the heat transfer sheet assembly 7 having the sizing, heat transfer, gaseous flow, pressure drop, cleanability and structural characteristics not atypical to that already known in the prior art. It will be understood that whilst in the preferred embodiment element 8 is provided with lobes 21,22 and sheet 9 with flats 23,24, it remains conceivable that in another embodiment of the invention element 8 is provided with flats and element 9 with lobes. In yet another embodiment element 8 could be provided with a mixture of lobes and flats whilst element 9 has a corresponding and associated plurality of flats and lobes. In a further embodiment it could be that both elements 8 and 9 are provided with lobes or other crimped or stamped structures that facilitate the spacing apart of the sheets so as to define a plurality of close- sided gaseous flow channels 25 within the assembly 7.

[0028] Heat transfer sheet element 9 additionally in this and other embodiments of the invention comprises a fifth intermediate elongate sheet spacing feature 26 typically parallel and equidistant from its features 23 and 24. Element 9 further comprises a plurality of heat transfer undulations 27,28 of lesser amplitude than feature 26 extending laterally of the element 9 obliquely to the gaseous flow direction. Undulations 27 extend typically obliquely between the third 23 and fifth 26 sheet spacing features and similarly undulations 28 extend between the fifth 26 and fourth 24 sheet spacing features. Also, element 8 further comprises a plurality of heat transfer undulations 29 of lesser amplitude than the sheet spacing features 21,22 extending laterally of element 8 therebetween and obliquely to the direction of gaseous flow and as illustrated in this preferred embodiment also obliquely of the undulations 27, 28 of the second sheet 9.

[0029] Advantageously, in this preferred embodiment the undulations 27,28,29 are of similar cross-section and undulations 27,28 are oblique to undulations 29. The intermediate sheet spacing feature 26 of element 9 is in contact with the undulations 29 of element 8 and provides additional structural rigidity to the gas channel 25. Typically sheet spacing feature 26 is of similar shape to the features 21,22 albeit with lower amplitude and suitably extends the length or substantially the length of the element 8 in any event more than half of the length. Within the channel 25 can be found sub-channels 30, 31 with the intermediate sheet spacing feature 26 further defining a shared longitudinal side wall therebetween. As illustrated in Figure 5, sub-channel 30 is defined between immediately adjacent pairs of sheet spacing features 21, 23 providing a closed side-wall and adjacent intermediate sheet spacing feature 26 contacting opposing peaks of undulations 29 providing a side-wall perforated by troughs of the undulations 29. Similarly, sub-channel 31 is defined between immediately adjacent pairs of sheet spacing features 22, 24 providing a closed side-wall and the adjacent intermediate sheet spacing feature 26 contacting opposing peaks of undulations 29 providing a side-wall perforated by troughs of the undulations 29.

[0030] Although the side-wall provided by the cooperation of feature 26 and undulations 29 is perforated, it is postulated that this might well mimic or perhaps closely approximate a closed sidewall facilitating high speed gaseous flow either side thereof whilst still permitting some limited transverse gaseous flow, i.e. effectively dividing each gaseous flow channel into two corresponding quasi-independent flow channels albeit interconnected for some transverse gas passage therebetween.

[0031] Additionally, intermediate sheet spacing feature 26 in mechanical contact with undulations 29 should inhibit drumming and/or vibration of the elements 8 and 9 between the respective spacing features 21, 22 and 23, 24. This is believed to help reduce the incidence of mechanical and fatigue damage.

[0032] In an alternative embodiment (not shown) the elongate intermediate sheet spacing feature 26 is replaced by an alternating notch configuration similar to the alternating notch disclosed in Applicant's US Patent Application No. 14/877,451 filed on 7 ^th October 2015.

[0033] It will be readily understood that the provision of intermediate sheet spacing feature 26 provides increased structural rigidity to the gaseous flow channel 25 defined between cooperating features 21, 23 and 23, 24. Because this feature 26 provides a perforated side-wall to sub-channels 30, 31 it has been found surprisingly not to impact efficiency negatively in the manner anticipated in paragraph 6 on page 2 for an assembly having smaller gaseous flow channels. It is believed this is because turbulent flow of gas transverse of the gaseous flow channel is facilitated by gaseous flow through valleys of undulations 29 of element 8 traversing the feature 26.