UNIT CONSTRUCTION PLATE-FIN HEAT EXCHANGER

Abstract: A plurality of pressure-tight unit-cell individual heat exchanger elements (10) are joined by manifold flanges (16) to form an integrated plate-fin heat exchanger. Each individual heat exchanger element (10) contains all the features of a complete counter-flow heat exchanger (40) with inlet and exit ports (14, 15), air distribution headers (16) and heat transfer fins (20, 21, 22) brazed into a single unit.

Description: UNIT CONSTRUCTION PLATE-FIN HEAT EXCHANGER BACKGROUND OF THE INVENTION This invention relates generally to plate-fin heat exchangers and more particularly to a cross-flow plate-fin heat exchanger used as a recuperator. Plate fin heat exchangers are typically monolithic structures created by brazing their many constituent pieces in a single furnace cycle. This basic construction presents several problems: The lowest quality braze joint in a typical plate fin heat exchanger dictates the net quality ofthe brazed core. This criticality creates cost in scrap, a whole core versus the offending joint, and in labor with an intensive setup procedure attempting to avoid a single poor braze among hundreds in a typical core. Dimensions of each ofthe constituent parts must be held to a close tolerance in order that differences in thickness do not compound into a gross difference in load during the braze cycle. Solid bars are used to carry load through the edges ofthe assembly to ensure that the edge joints are loaded similarly to the fin-to- parting plate joints. Four bars are typically required for each cold/hot layer ofthe heat exchanger making assembly both labor and material intensive. The monolithic construction of a typical plate-fin heat exchanger leaves little freedom for the differentially heated structure to move out-of-place to avoid strain. Gross differential thermal growths manifest as strain and adversely effect fatigue life. More specifically regarding problems three and four presented above, counterflow plate-fin heat exchangers with crossflow headers typically include a stack of headers sandwiched together to form an alternating gas/air/gas/air header pattern. Each pair of adjacent gas and air headers is separated by a relatively thin parting sheet. Additionally, conventional plate-fin heat exchangers incorporate edged bars also referred to as closure bars by those skilled in the art, to seal about the perimeters ofthe parting sheets and prevent gas or air from being flowed into the adjacent header. Inlet and Outlet manifold ducts are welded traverse to the edge bars after the headers are assembled and brazed. The edge bars create a stiff and massive structural attachment between the parting sheets. During use ofthe heat exchanger, temperature changes are experienced by the edge bars and parting sheets. Due to differences in the position and structural composition ofthe parting sheets and edge bars, the temperature changes do not affect the bars and sheets at the same rate. Since the parting sheets are structurally weaker than the edge bars, the parting sheets are strained. A second problem associated with the use of edge bars in counterflow plate-fin heat exchangers is related to the sheetmetal manifold ducts that are welded to the edge bars. The manifolds are welded to the stack of edge bars along the sides and corners ofthe core adjacent the header openings. Like the parting sheets, the manifold ducts responds quickly to changes in temperature. Since the edge bars do not respond to changes in temperature as quickly as the manifold ducts, the sheetmetal experiences a shear load at or near the weld, and is prone to damage in the heat affected zone between the weld and the base metal. Based on the foregoing, it would be beneficial to develop a means for eliminating the conventional edge bars and eliminate stresses and strains imparted on the heat exchangers during use. The foregoing illustrates limitations known to exist in present plate-fin heat exchangers. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more ofthe limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. SUMMARY OF THE INVENTION In one aspect ofthe present invention, this is accomplished by providing an individual heat exchanger element comprising: a top plate having an inlet aperture at one end thereof and an outlet aperture at the other end thereof; a bottom plate having an inlet aperture at one end thereof and an outlet aperture at the other end thereof, the peripheral edges ofthe bottom plate being joined to the peripheral edges ofthe top plate; two first finned members, one first finned member being attached to a first side ofthe top plate, the other first finned member being attached to a first side ofthe bottom plate; a second finned member being between the top plate and the bottom plate, the second finned member being attached to a second side ofthe top plate and a second side ofthe bottom plate; and means for resisting an internal pressure within the individual heat exchanger element, the means comprising fins ofthe second finned member being substantially fully attached by adhesion to the adjacent top and bottom plates. The foregoing and other aspects will become apparent from the following detailed description ofthe invention when considered in conjunction with the accompanying drawing figures. BRIEF DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a plan view of an individual heat exchanger element according to the present invention; FIG. 2 is second plan view ofthe individual heat exchanger element shown in FIG. 1 with a portion ofthe fins and top plate partially removed to show the interior details; FIG. 3 is a partial cross section of an edge ofthe individual heat exchanger element shown in FIG. 1, taken on line 3-3, showing the details of a braze reservoir; FIG. 4 is a partial cross-section ofthe inlet aperture taken on line 4-4 of FIG. 1, showing the raised flanges; FIG. 5 is an enlarged view of a portion of an internal header; FIG. 6 is a side view showing a heat exchanger containing a plurality of the individual heat exchanger elements shown in FIG. 1; and FIG. 7 is a diagram illustrating one embodiment of a manufacturing method for the individual heat exchanger element shown in FIG. 1. PETATLEP DESCRIPTION A unique aspect ofthe individual heat exchanger element 10 shown in the FIGURES is a pressure-tight unit-cell construction applied to an integrated plate-fin heat exchanger. Each unit-cell 10 contains all the features of a complete counterflow heat exchanger, with inlet and exit ports, air distribution headers and heat transfer fin brazed into a single unit, as shown in FIGS. 1 and 2 Unit-cells or individual heat exchanger elements 10 are welded sequentially to fabricate a heat exchanger matrix 40 (FIG. 6) ofthe required size for a given application. The individual heat exchanger element solves the following problems: Allows inspection, correction, rejection of a small, manageable unit rather than a full heat exchanger matrix. The result is less scrap with greater quality assurance. Avoids the risk and technical difficulty of brazing massive heat exchanger matrices with small individual heat exchanger elements. Allows for slip between layers to accommodate differential thermal strain, without the risk of leakage, to maximize durability. In the preferred embodiment, the individual heat exchanger elements 10 are assembled into a counterflow recuperator 40 used to heat combustion air for a gas combustor. The exhaust gas flows through the low-pressure side fins or gas fins 22 and the combustion air flows through the high-pressure or air fins 20. Typically, two gas fins 22 are sized at half the height required for typical plate-f n construction which uses a single segment of fin for each low- pressure cell. The gas fins 22 are bonded (preferably by brazing) to each side ofthe individual heat exchanger element 10. The individual heat exchanger element 10 is primarily formed with two plate members, a top plate 11 and a bottom plate 12, each plate having an inlet aperture 14 and outlet aperture 15. Each gas fin 22 transfers heat into (or for other applications, away from) the high pressure media within the individual heat exchanger element 10. A single layer of air fin 20 inside the individual heat exchanger element 10 is bonded (also, preferably, by brazing) to both the top and bottom plates 11, 12 to conduct heat through the plates 11, 12 and also the restrain the plates 11, 12 against the differential pressure load. Preferably, the air fin 20 restrains the plates 11, 12 against the differential load by the fin elements ofthe air fin 20 being fully bonded to the plates 1 1, 12. In addition to the air fin 20 between the plates 11, 12, header fins 21 can also be used to direct the flow of media from the inlet aperture 14 to a first edge ofthe air fin 20 and then from a second edge ofthe air fin 20 to the outlet aperture 15. For purposes ofthe preferred embodiment, the header fins may terminate at the portion of plates 11 and 12 where the plates diverge and form raised flanges 16, as shown in Figure 4. This termination configuration is shown in solid font in Figure 4. Alternatively, the header fins may be extended beyond the portion of divergence ofthe plates in the manner shown in dashed font and identified as 21'. FIG. 5 shows a preferred embodiment ofthe header fin 21. In this embodiment, a single channel 21a ofthe header fin 21 is in fluid communication with a plurality of channels 20a on the air fin 20. Also in the preferred embodiment, the header fins 21 are fully bonded to the top and bottom plates 11, 12 to provide further restraint against the differential load. A gas turning fin 24 can be provided, as shown in FIG. 1. Preferably, one gas turning fin 24 is attached to a peripheral edge of each outside surface of top and bottom plates 11, 12 on the gas inlet edge ofthe gas fin 22. In one type of heat exchanger 40, the heat exchanger is contained within a housing (not shown) where the hot gas is flowing transverse to the gas fin 22 (i.e. parallel to the gas inlet edge ofthe individual heat exchanger element 10). The gas turning fin 24 is used to turn and guide the hot gas into the gas fin 22, thereby providing more uniform distribution ofthe hot gas throughout the gas fin 22. In the preferred embodiment, the inlet and outlet apertures 14, 15 each have raised flanges 16 about the apertures (See FIG. 4). These flanges 16 are used to attached one individual heat exchanger element 10 to another by welding the flanges 16 of one individual heat exchanger element 10 to the flanges 16 of an adjacent individual heat exchanger element 10. The heat exchanger 40 is formed of a plurality of individual heat exchanger elements 10 attached to one another only at the flanges 16. The gas fins 22 of one individual heat exchanger element 10 are not attached or bonded to the gas fins 22 ofthe adjacent individual heat exchanger element 10. In this configuration, the individual heat exchanger elements 10 can grow and move separately from one another as the heat exchanger 40 temperature changes. The stacked flanges of heat exchanger 40 form a bellows structure. The bellows created by the flanges is a compliant structure and as a result, deflections produced by heat transfer are absorbed elastically by the bellows structure. The plates 11 and 12 including the flanges 16 are of substantially uniform thickness and temperature changes at the flanges are substantially the same as the temperature differences along the rest of plates 11 and 12. Thermal strain produced during operation ofthe heat exchanger are eliminated. In heat exchanger element 10, the plates 11 and 12 are sandwiched between gas fins 22 and air fin 20. The ends ofthe fins are vertically aligned. In an alternate embodiment, the ends ofthe gas fins 22 may be extended so that they are not in vertical alignment with the air fin 20. FIG 7 illustrates one method of assembling individual heat exchanger elements 10 and heat exchanger 40. The top and bottom plates 11, 12 (also known as parting plates) are formed from 0.015 inch stainless or superalloy steel sheet in roll form. The sheet is unrolled and then the plates are formed by stamping and laser trimming. Gas fins 22 and gas turning fins 24 are formed from 0.008 inch rolled stainless or superalloy steel. The metal is unrolled, the fins are folded and braze coating is sprayed onto one side ofthe gas fin 22 and the gas turning fin 24. The brazed coated gas fin 22 and gas turning fin 24 are then laser trimmed and cleaned. Instead of applying a braze coat to the gas fin 22 and gas turning fin 24, the outside surfaces of parting plates 11, 12 can be coated with the braze coating. The air fins 20 and header fins 21 are formed from 0.004 inch rolled stainless or superalloy steel. The metal is unrolled, the fins are folded and braze coating is sprayed onto both sides ofthe air fins 20 and header fins 21. The braze coated air fins 20 and header fins 24 are then laser trimmed and cleaned. Instead of applying a braze coat to the air fins 20 and header fins 24, both inside surfaces ofthe parting plates 11, 12 can be braze coated. The parting plates 1 1, 12, gas fin 22, gas turning fin 24, air fin 20 and header fins 21 are assembled to form an individual heat exchanger element 10. The individual pieces are tacked welded to temporarily hold the pieces together. In addition, the peripheral edge ofthe assembled individual heat exchanger element 10 can be laser welded. One or more assembled individual heat exchanger elements 10 are placed into a braze cell where the individual heat exchanger element 10 is heated to braze the coated surfaces to one another. Various brazing jig components can be used to load the individual heat exchanger elements 10 to minimize any distortion ofthe assembled individual heat exchanger element 10 during the brazing process. FIGS. 3 and 4 illustrate a preferred embodiment of the parting plates 11, 12 for the brazing process. A reservoir 30 is provided in top plate 1 1. This reservoir 30 holds additional braze coating which will spread in the adjacent surfaces ofthe interior of an individual heat exchanger element 10 during the brazing process. After brazing, an individual heat exchanger element 10 is pressurized to check for any leaks caused by inadequate brazing. A plurality of individual heat exchanger elements 10 are then assembled into a partial stack and the raised flanges 16 are welded together. These partial stacks are then pressure tested again. A plurality of partial stacks are then welded together to provide a heat exchanger 40. Transition pieces (not numbered) are attached to outer individual heat exchanger elements 10 to provide a place to connect the heat exchanger 40 to the inlet and outlet headers ofthe equipment the heat exchanger is a part of. A feature ofthe heat exchanger 40 described is that because ofthe full adhesion ofthe air fin 20 to the parting plates 11, 12 (which provides resistance against differential pressure load), no external pre-loading ofthe heat exchanger 40 is used.

Claims: Having described the invention, what is claimed is: 1. A method for assembling individual heat exchanger elements comprising the steps of: providing a top plate; providing a bottom plate; providing two first finned members; providing a second finned member; applying a braze coating to at least one ofthe first finned members, the second finned member, the top plate and the bottom plate; attaching one first finned member to a first side ofthe top plate; attaching a second first finned member to a first side ofthe bottom plate; assembling the top plate, the bottom plate and the second finned member forming a sandwich-like assembly with the second finned member between the top plate and the bottom plate, the second finned member between in contact with second sides ofthe top plate and the bottom plate, whereby an applied braze coating is present between any two adjacent surfaces; welding the peripheral edges ofthe top plate to the bottom plate; and brazing the sandwich-like assembly. 2. The method for assembling individual heat exchanger elements according to claim 1, wherein the steps of applying a braze coating and brazing the sandwich-like assembly are performed to provide substantially full adhesion of fins ofthe second finned member to the top and bottom plates. 3. A method for assembling a heat exchanger using a plurality of individual heat exchanger elements assembled according to claim 1, the method comprising the steps of: providing a plurality of individual heat exchanger elements, the individual heat exchanger elements having an inlet aperture with a raised flange at one end thereof and an outlet aperture with a raised flange at the other end thereof; and welding the inlet aperture raised flange on one individual heat exchanger element to the inlet aperture raised flange on an adjacent individual heat exchanger element; and welding the outlet aperture raised flange on one individual heat exchanger element to the outlet aperture raised flange on an adjacent individual heat exchanger element 4. An individual heat exchanger element comprising: a top plate having an inlet aperture at one end thereof and an outlet aperture at the other end thereof; a bottom plate having an inlet aperture at one end thereof and an outlet aperture at the other end thereof, the peripheral edges ofthe bottom plate being joined to the peripheral edges ofthe top plate; two first finned members, one first finned member being attached to a first side ofthe top plate, the other first finned member being attached to a first side ofthe bottom plate; and a second finned member being between the top plate and the bottom plate, the second finned member being attached to a second side ofthe top plate and a second side ofthe bottom plate, fins ofthe second finned member being substantially fully attached by adhesion to the adjacent top and bottom plates. 5. The individual heat exchanger element according to claim 4, wherein interior surfaces ofthe bottom plate which are in contact with interior surfaces ofthe top plate are attached to one another by adhesion. 6. The individual heat exchanger element according to claim 4, further comprising: two header finned members between the top plate and the bottom plate, each header finned member being attached to the second side ofthe top plate and the second side ofthe bottom plate, one header finned member being in fluid communication with the top and bottom plate inlet apertures and a first edge ofthe second finned member, the other header finned member being in fluid communication with the top and bottom plates outlet apertures and a second edge ofthe second finned member. 7. The individual heat exchanger element according to claim 4, further comprising: gas turning fin members attached adjacent a peripheral edge ofthe top and bottom plate members. 8. The individual heat exchanger element according to claim 4, further comprising: means for changing the flow direction of a gaseous fluid entering the first finned members. 9. The individual heat exchanger element according to claim 4, further comprising, a braze reservoir means for holding a quantity of braze, the braze reservoir means extending about a peripheral edge ofthe top and bottom plates. 10. The individual heat exchanger element according to claim 4, wherein the top plate and bottom plate inlet apertures and outlet apertures have raised flanges thereon. 1 1. A heat exchanger consisting of a plurality of individual heat exchanger element according to claim 12, wherein the inlet flange and the outlet flange of one individual heat exchanger element are attached to the inlet flange and the outlet flange of an adjacent individual heat exchanger element. 12. An individual heat exchanger element comprising: a top plate having an inlet aperture at one end thereof and an outlet aperture at the other end thereof; a bottom plate having an inlet aperture at one end thereof and an outlet aperture at the other end thereof, the peripheral edges ofthe bottom plate being joined to the peripheral edges ofthe top plate; two first finned members, one first finned member being attached to a first side ofthe top plate, the other first finned member being attached to a first side ofthe bottom plate; a second finned member being between the top plate and the bottom plate, the second finned member being attached to a second side ofthe top plate and a second side ofthe bottom plate; and means for resisting an internal pressure within the individual heat exchanger element, the means comprising fins ofthe second finned member being substantially fully attached by adhesion to the adjacent top and bottom plates.

Inventors: CHILD MALCOLM S, KESSELI JAMES B, NASH JAMES S, CHILD, MALCOLM, S., KESSELI, JAMES, B., NASH, JAMES, S.
Applicants: NORTHERN RES & ENG, NORTHERN RESEARCH & ENGINEERING COMPANY
Application year: 1997
Main classification: F28D1
IPC codes: B23K1, F28D1, F28D9, F28F3, F28F27