CROSS-REFERENCE TO RELATED APPLICATION

[0001] This Patent Cooperation Treaty application claims the benefit of U.S. Provisional Application No. 61/375,003, filed on August 18, 2010, entitled "Heat Exchanger Perimeter Seal" and assigned to the assignee of the present application. The disclosure of this application is hereby incorporated herein by reference in its entirety.

BACKGROUND

[0002] The present invention relates to heat exchangers and, more particularly, to sealing elements that reduce leakage between hot air conduits and cold air conduits of regenerative heat exchangers.

[0003] Heat exchangers transfer heat from a hot flow conduit to a cold flow conduit. For example, regenerative heat exchangers are used to provide preheated air to heavy machinery, such as fuel burning power plants, chemical processors, refineries, mills, and the like. These examples of heavy machinery typically will exhaust hot gas and will operate more efficiently when supplied with preheated air. For example, the boiler of a power plant will typically produce hot flue gases having a temperature of approximately 700 degrees Fahrenheit (370° C). A regenerative heat exchanger can preheat incoming air to a temperature of approximately 600 degrees Fahrenheit (315° C).

[0004] Generally, in a regenerative heat exchanger, two fluid flow passages extend through the heat exchanger, one passage comprising a hot gas conduit that communicates with a hot exhaust outlet of the heavy machinery, and the other passage comprising a cold air conduit that communicates with a cool air intake passage. Hot exhaust gases flow from the hot exhaust outlet into the hot gas conduit of the heat exchanger, and cool air flows from the cool air intake passage into the cold air conduit. In the heat exchanger, heat is extracted from the hot exhaust gas flow and is transferred to the cool air flow.

[0005] A rotary regenerative air heat exchanger performs the heat transfer using a rotor that turns continuously within a housing through the hot gas flow and cool air flow. The rotor comprises a heat exchanging body that includes multiple thermal transfer surfaces comprising radially extending plates or sheets that are attached to a central cylinder. A group of the plates or sheets may be segregated into sectors called diaphragms or baskets. The hot gas flow and cool air flow typically enter the housing from opposite ends of the heat exchanger and pass over the thermal transfer surfaces in opposite flow directions. During operation, the rotor rotates at a relatively slow speed (1-2 rpm) and thereby moves the thermal transfer surfaces within the housing from the hot gas flow where the surfaces absorb heat into the cool air flow where the surfaces give off heat into the cool air flow. The well-known configuration by Frederik

Ljungstrom dating from 1920 is an example of a rotary regenerative air heat exchanger. The description herein relates to a rotor that rotates about a vertical axis, but it should be understood that the considerations in this description can also relate to a horizontal axis rotor. [0006] Figure 1 is an illustration of a rotary heat exchanger 100 showing a rotor 102 that rotates within the heat exchanger housing 104. Figure 1 shows the thermal transfer plates 106 of the rotor divided into sectors 108. The rotor rotates on a shaft 110. For heat transfer efficiency, the hot gas flow 112 and cool air flow 114 must be kept sealed from each other within the heat exchanger housing. The two flows are kept separated by sealing elements mounted at the junctions between the rotor and the heat exchanger housing. The sealing elements comprise radial seals 118 mounted on the radially extending edges of the upper and lower surfaces of the rotor sectors, and perimeter seals 120 at the free ends of the rotor sectors. Sector plates 124, 126 block the flow of gas and air so as to form a hot gas side 128 and a cool air side 130. The perimeter seals may be mounted to the rotor sectors, in which case they are generally referred to as circumferential seals, or they may be mounted to the internal walls of the housing itself, in which case they are generally referred to as bypass seals. Some configurations may have both circumferential seals and bypass seals.

[0007] Because the hot exhaust gas flow 114 enters the rotary heat exchanger housing at one end and the cool air flow 114 enters at the other end, the hot exhaust inlet end is generally referred to as the hot end, and the cool air inlet end is generally referred to as the cold end. This results in an axial temperature gradient from the hot end to the cold end. As noted above, the temperature gradient of the air flow between the hot end and cold end during operation can be approximately 100 degrees Fahrenheit (about 40° C). The temperature gradient differential can be even greater when the heat exchanger is being brought from a non-operational condition (such as after installation or maintenance) up to an operating condition. The temperature gradient can expand and distort the rotor, and create sealing problems between the hot gas flow and the cool air flow. For large power plant boilers, a rotary heat exchanger can have a diameter of approximately 30 feet to 60 feet (about 10-20 meters). On that large scale, operating efficiency can be drastically effected by the rotor distortion.

[0008] The rotor distortion due to the temperature gradient is commonly referred to as rotor turndown, and causes the radial edge of the rotor sectors to assume a shape similar to that of an inverted bowl or dish. Rotor turndown can also cause the outer edge (peripheral edge or circumferential edge) of the rotor sectors to distort into a curved edge. As a result of the rotor turndown, the radial seals mounted on the hot end of the rotor are pulled away from the sector plates of the housing with the greater separation occurring at the outer radius of the rotor. This opens a gap which allows flow and results in an undesired intermingling of the hot gas and the cool air.

[0009] Figure 2 is a side view of the Figure 1 heat exchanger 100 in an initial cold operating condition. Figure 3 is a side view of the Figure 1 heat exchanger 100 at operating temperature. Figure 2 and Figure 3 illustrate the phenomenon of rotor turndown. The radial seals 118 are shown in Figure 2 forming an effective seal with the sector plates 124, 126 at the top end in the cold condition, whereas there is a gap 204 at the bottom end between the lower radial seals 218 and the bottom sector plates 224, 226. Conversely, Figure 3 shows that a gap 302 is created between the upper radial seals 118 and sector plates 124, 126 at the heat exchanger upper end in the heated operational condition, but the gap is closed at the bottom end between the lower radial seals 218 and the bottom sector plates 224, 226. [0010] It is important to maintain the seal between the movable heat exchanging body and the heat exchanger housing to thereby maintain the separate hot gas and cool air flows. A flexible and resilient construction of the seals helps the seals to maintain a seal over the operational temperature gradient. Unfortunately, the seals are often subjected to severe demands and harsh operating conditions. In a power plant, for example, the seals are typically exposed to corrosive fly ash and soot that attack the sealing surfaces. As the heat exchanging body moves with respect to the housing or vice versa, the seals are also exposed to mechanical wear because the seals are positioned to maintain sliding contact with the stationary surfaces. Consequently, the seals can wear down relatively quickly and require periodic maintenance and cleaning.

[0011] From the discussion above, it should be apparent that there is a need for a resilient sealing element that maintains an effective seal over a temperature gradient and that resists corrosion, mechanical wear, and thermal distortion. The present invention satisfies this need. SUMMARY

[0012] Embodiments of the present invention provide a heat exchanger sealing element that comprises a perimeter seal having two elongated sealing elements, one laid on top of the other. Both sealing elements have spaced apart slots that define tabs. The slots are spaced apart such that the tabs of one sealing element overlap the slots of the other sealing element. In addition, the tabs of one sealing element include a crease or bend such that a ridge is formed that can rest in a slot of the other sealing element in a matching engagement. The tabs of one sealing element are not fastened or connected to the tabs of the other sealing element, but the matching engagement permits the two sealing elements to fit closely together for effective sealing while also allowing for relative movement and expansion in response to temperature gradients. The accommodation of relative movement permits the sealing element to be configured in a heat exchanger with zero seal tolerance, meaning that the sealing element can be adjusted to zero gap upon installation, while still maintaining a seal after thermal expansion has occurred due to temperature gradients experienced during operation. The tabs and slots provide the resiliency needed for maintaining an effective seal across a broad temperature range. The sealing elements can be constructed of materials configured to be resistant to corrosion, mechanical wear, and thermal distortion.

[0013] Other features and advantages of the present invention should be apparent from the following description of exemplary embodiments, which illustrate, by way of example, aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a perspective view of a prior art rotary heat exchanger. [0015] Figure 2 is a cut-away view of the Figure 1 heat exchanger in an initial cold operating condition.

[0016] Figure 3 is a depiction of the Figure 2 representation illustrating rotor turndown.

[0017] Figure 4 is a perspective view of a rotary heat exchanger perimeter sealing element constructed in accordance with the present invention, showing the sealing element members separated apart from each other. [0018] Figure 5 is a plan view of the slotted and creased sealing member illustrated in Figure 4.

[0019] Figure 6 is a plan view of the slotted sealing member illustrated in Figure 4.

[0020] Figure 7 is a perspective view of the assembled sealing members illustrated in Figures 4-6, showing the creased sealing member on top.

[0021] Figure 8 is a perspective view of the sealing members of Figure 7 separated and positioned adjacent each other.

[0022] Figure 9 is a perspective view of the assembled sealing members illustrated in Figures 4-6, showing the slotted sealing member on top. [0023] Figure 10 is a detail perspective view of the assembled sealing members of Figure 7 showing the outer surface of the creased sealing element.

[0024] Figure 11 is a detail perspective view of the assembled sealing members of Figure 7 showing the outer surface of the non-creased sealing element.

DETAILED DESCRIPTION

[0025] Figure 4 shows a heat exchanger sealing assembly comprising a perimeter seal 400 having two elongated sealing elements 402, 404, one sealing element laid on top of the other. Both sealing elements have spaced-apart slots 406 that define tabs 408. The slots are spaced apart such that the tabs of one sealing element overlap the slots of the other sealing element. One of the sealing elements is referred to as a creased sealing element 402 and has a ridge 410 formed by a crease or bend in its tabs such that the ridge is configured to rest in a corresponding slot between two adjacent tabs of the other sealing element 404, in a matching engagement. The other sealing element has no creases in the tabs, and is referred to as a slotted sealing element or non-creased sealing element 404. That is, the tab ridge 410 of the creased sealing element 402 is configured to substantially occupy the slot space between two adjacent tabs of the non-creased sealing element 404. The tabs of one sealing element are not fastened or connected to the tabs of the other sealing element, but the matching engagement between the ridges and the slots of two different sealing elements permits the two sealing elements to fit closely together in a matching engagement fit for more effective sealing, while also allowing for relative movement and expansion between the sealing elements 402, 404 in response to temperature gradients. The greater sealing efficiency, coupled with resiliency throughout the temperature gradient, provides improved performance.

[0026] With the illustrated configuration, it is possible to adjust the cold condition gap at the top of the heat exchanger (as illustrated in Figure 2) so there is zero clearance, and it is also possible to adjust the cold condition gap at the bottom of the heat exchanger (as illustrated in Figure 3) so there is zero clearance. Once the heat exchanger is operating, the sealing elements 402, 404 have sufficient resiliency to maintain the zero gap dimension at the top and bottom ends of the heat exchanger. The resiliency can be achieved in part because the tabs 408 of the two sealing elements 402, 404 are not interlocking, and therefore the sealing elements can

accommodate a greater amount of thermal expansion and relative movement through the temperature gradient. This greater resiliency offers superior sealing performance over the operational envelope of the heat exchanger. The greater resiliency that follows from the configuration of the sealing elements 402, 404 permits the selection of a broader range of materials for the sealing elements as compared to conventional seal materials. Thus, the seals can be constructed from materials that are more resistant to corrosion, mechanical wear, and thermal distortion.

[0027] Figures 4 through 8 show that a portion of the sealing elements comprises an elongated strip that is bent away from the slots 406 and tabs 408 at an angle. The elongated strip comprises a mounting surface portion 414 for the sealing elements. For example, a bolt, screw, rivet, or other fastening mechanism can be used to attach the two sealing elements 402, 404 to the rotor sectors, comprising circumferential seals, or they may be mounted to the internal walls of the housing itself, comprising bypass seals. In this way, fastening the sealing elements 402, 404 to the rotor or housing will hold the sealing elements with sufficient force to maintain the sealing function while still achieving the desired resiliency, and thereby obtain the benefits described herein. Alternatively, the two sealing elements could be fastened together along the mounting surface portion 414 and then the fastened assembly can be mounted to the rotor or housing.

[0028] Figure 5 shows the regular spacing of the slots 406 and tabs 408 of the creased sealing element 402. The advantages described herein can best be achieved with the majority of the tabs having the ridge-forming crease, with a minimal configuration having a creased tab at each end of the rotor, so that at least two of the creased sealing element tabs are configured with the ridge. Figure 5 shows that one or more tabs 408 of the creased sealing element 402 may be provided without a crease forming a ridge 410, depending on considerations for mounting the sealing element, sealing efficiency at the rotor ends, and the like. For example, the sealing element 402 of Figure 5 has end tabs that are free of any crease or ridge, comprising a substantially flat tab, whereas all the other tabs 408 have a crease running at mid-tab. [0029] Figure 5 also shows that, in the illustrated embodiment, the ridges have a height of approximately 0.36 inches (about 9 mm), which is substantially sufficient for the ridges to fit into the slots of the slotted sealing element 404 and have the tabs of the creased sealing element 402 rest substantially in contact with the corresponding tabs of the non-creased sealing element 404, in minimal separation. Although the tabs and slots of the two sealing elements 402, 404 are shown in uniform spacing at regular intervals of approximately three inches (about 70 mm), it should be understood that other spacing intervals, including irregular spacing intervals, can be utilized in accordance with the teachings herein. It is sufficient to obtain the benefits described herein if the tabs of one sealing element overlap the tabs of the other sealing element so as to close any gaps between the slots, and for the ridges of the creased sealing element to be received into the slots of the slotted sealing element, to facilitate the two sealing elements lying substantially flush with each other in matching engagement.

[0030] Figure 6 is a detail perspective view of the non-creased sealing element 404, showing that the tab-to-tab spacing of the non-creased sealing element 404 matches the tab-to-tab spacing of the creased sealing element 402. As noted above, although the slot spacing is shown at regular intervals along the entire length of the sealing elements, other slot spacing intervals can be provided, including irregular intervals. Variation in spacing can be accommodated while still achieving the benefits described herein. Most effective sealing is obtained with the slots of the slotted sealing element 404 being configured to accept the ridges 410 in the tabs of the creased sealing element 402, thereby permitting the minimal separation and matching engagement between the two sealing elements.

[0031] Figure 7 and Figure 8 illustrate the minimal separation of the two sealing elements 402, 404 when they are arranged in matching engagement as described above. Figure 7 is a perspective view of the assembled sealing elements 402, 404 illustrated in Figures 4-6, showing the assembled perimeter seal 400 with the sealing element 402 on top in the drawing. Figure 8 shows the two sealing elements 402, 404 separated and aligned in a manner that illustrates how the tab ridges 410 of the creased sealing element 402 can be received into the slots 406 of the non-creased sealing element 404. The two end tabs 802, 804 on the creased sealing element 402 in Figure 8 have no crease, but it should be understood that the two end tabs could also be provided with a crease, so that all of the creased sealing element tabs have a ridge that is received into a corresponding slot of the non-creased sealing element 404. [0032] Figure 9 is a perspective view of the assembled sealing elements 402, 404 illustrated in Figures 4-6. Figure 9 shows the non-creased sealing element 404 on top in the drawing, so that the ridges 410 of the creased sealing element 402 are visible through the slots 406 of the non- creased sealing element 404.

[0033] Figure 10 is a detail perspective view of the assembled sealing members of Figure 7 showing the outer surface 1002 of the creased sealing element 402. More particularly, the outer surface 1002 of the creased sealing element faces away from the non-creased sealing element 404, and the inner surface of the creased sealing element is adjacent the non-creased sealing element when the two elements are assembled into a perimeter seal.

[0034] Figure 11 is a detail perspective view of the assembled sealing members of Figure 7 showing the outer surface 1102 of the non-creased sealing element 404. More particularly, the outer surface 1102 of the non-creased sealing element faces away from the creased sealing element 402, and the inner surface of the non-creased sealing element is adjacent the non-creased sealing element when the two elements are assembled into a perimeter seal.

[0035] While certain exemplary embodiments have been described in detail and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not intended to be restrictive of the broad invention, and that this invention is not to be limited to the specific arrangements and constructions shown and described, since various other modifications may occur to those with ordinary skill in the art.

[0036] In the drawings, particular examples of structures are identified by reference numerals without identifying all such structures in each drawing, for simplicity of illustration. It is to be understood that like descriptions apply to like structures in the drawings.

[0037] As used herein, the use of "a", "an" or "the" is intended to mean "at least one", unless specifically indicated to the contrary.