Patent Description:
Plate-fin heat exchangers are well known in the art to provide fluid flow passages for heat transfer between fluids. Typically, such heat exchangers include a core comprising stacked layers of corrugated fin elements separated by plate elements, also known as parting sheets. The layers of fin elements are stacked such that alternate layers, for fluid flow of the first of a hot or cold fluid, provide channels or flow paths in a first direction and a layer is provided between each of these alternate layers to provide fluid flow of the other of the hot or cold fluid in a second direction which is either parallel and opposite to, or transverse to the first direction. The stack of sheets is provided with a structurally strong top sheet and bottom sheet, and solid closure bars are provided on alternate layers at the core sides to provide a seal and direct the fluid flow through the channels. The closure bars of a conventional core design are rectangular bars that cross over at the corners of the stack defining an L-shaped corner recess at each corner. Solid corner angles are provided (e.g. brazed) at the corners of the block, in the recess, from the top sheet to the bottom sheet to provide a complete block unit that can be attached to the header parts of the heat exchanger from which the fluids are provided. The corner bars seal the layers and separate the two fluid flows, give structure to the core and may be used to provide a surface to which the header parts can be secured e.g. by welding. A plate-fin heat exchanger core according to the preamble of claim <NUM> is disclosed in document <CIT>.

Although the heat exchanger core is typically provided as a substantially rectangular block, hot air provided to the core via the header inlet port is generally provided via a narrow, usually circular cross-section inlet port, in a relatively narrow flow which impacts the core in a fairly concentrated, substantially circular, area around the centre of the fluid entry side of the core block. This results in a localised heating resulting in uneven thermal expansion in the core in some conditions. This occurs, for example, in aircraft heat exchangers during transient flight conditions.

Because the top and bottom sheets are relatively cool and also relatively heavy and solid, and so relatively resistant to bending, the concentrated heating and the uneven thermal expansion of the core elements e.g. the closure bars in the centre of the core, causes a bending distortion of the corner angles, forcing them to bend outwards. This bending load, particularly when varying due to cyclic thermal changes, causes fatigue in the corner angles and can result in cracks which propagate through the corner angles.

Because of this problem, heat exchanger cores may have to be replaced much sooner that the normal life of the heat exchanger parts. In a typical civil aircraft, for example, buffer air coolers typically require replacement or a new core at around <NUM>% of engine service life.

There is therefore a need for a heat exchanger core design that is less susceptible to damage resulting from uneven thermal stresses at the centre of the core.

According to a first aspect, there is provided a plate-fin heat exchanger core as defined by claim <NUM>.

A heat exchanger, and methods of making are also provided.

Examples of the heat exchanger core design according to the disclosure will be described by way of example only with reference to the drawings. It should be noted that variations are possible within the scope of the claims.

A typical heat exchanger will first be described, by way of background, with reference to <FIG>. The design and operation of such heat exchangers is well known and so will only be described briefly.

A typical heat exchanger of the plate-fin type has a housing <NUM> within which the heat exchanger core (described further below) is arranged. The housing includes headers having an inlet <NUM> for a first (warm or hot) fluid and an inlet <NUM> for a second (cold) fluid as well as one or more warm fluid outlets <NUM>,<NUM>. In one example, the first and second fluids are warm and cold air, but other fluids are also possible. A heat exchanger core (as described in the background above and as will be described further below) is located inside the housing, fluidly connecting the inlet and outlet ports such that the first fluid flows through channels in a first direction (here from inlet <NUM>) and the second fluid flows through the alternate channels (here from inlet <NUM>) which are arranged either in a parallel and opposite direction or in a transverse direction. Heat is exchanged between the two fluids in the heat exchanger core such that resulting warm fluid exits via an outlet (<NUM> or <NUM>).

As can be seen in <FIG>, the heat exchanger core <NUM> is arranged such that a side is in fluid communication with each of the inlets and outlets. The inlets and outlets are typically cylindrical, usually circular cylindrical ports. <FIG> shows how, typically, the warm or hot fluid provided to the heat exchanger via the first fluid inlet <NUM> is directed in a circular cylindrical stream <NUM> and impacts the side <NUM> of the heat exchanger core in a relatively concentrated central area, where it passes through the channels. This can be more clearly seen in <FIG> whish shows an example of a typical heat exchanger core <NUM>. As described in the background, such cores include layers of corrugated fins arranged to define alternating flow channels for the two different fluids - either parallel and opposite channels or transverse - i.e. cross-flow - channels. On each side of the core, the layers <NUM> of fins defining the channels for flow of the fluid entering or exiting that side are open to receive/exit the fluid and the alternate layers are closed by closure bars <NUM>. For a cross-flow arrangement, then, with reference to <FIG> and <FIG>, on a first side <NUM>, which, for example, is in fluid connection with the hot fluid inlet <NUM>, every other layer <NUM> is open to receive the hot fluid and the intermediate layers are closed by closure bars <NUM>. On a second side <NUM>, transverse to the first side in this example (in parallel flow arrangements, this would be an opposite side), the layers that were open on the first side are closed by closure bars <NUM> on the second side and the intermediate layers <NUM> (which were closed on the first side) are open to receive the cold fluid from the second fluid inlet <NUM>. The rectangular closure bars <NUM>, <NUM> overlap at their ends <NUM>, <NUM> as best shown in <FIG> to define a corner recess <NUM> at each corner of the core block <NUM>.

A solid top sheet <NUM> and bottom sheet <NUM> are located, respectively, over the top and bottom of the stacked layers. To seal the block and direct the two different fluid flows, an L-shaped corner angle <NUM> is fitted into the recess <NUM> defined by the closure bar ends and is secured to the bars and the top and bottom sheets e.g. by brazing.

Referring again to <FIG>, the concentrated flow <NUM> of hot fluid impacting the side <NUM> of the core forms a central region <NUM> on the core side <NUM> that is hotter than the remainder of the side, which results in uneven thermal expansion. This is indicated by arrows A in <FIG> where there is greater thermal expansion A1 around the middle layers than at the outer layers. The corner angles <NUM> are retained at the top and bottom by the relatively colder, heavy top and bottom sheets, but the increased thermal expansion around the middle of the core (A1) acts on the corner angle <NUM> forcing it to bend outwards (as shown by the dashed line in <FIG>). The different thermal expansion across the core results in high stresses and thermal fatigue on the components, particularly the corner angles <NUM>.

The core design of this disclosure addresses this problem as will be described further below with reference to <FIG>.

The core is of a plate-fin type essentially as described above in that it comprises a stack of layers of fins each layer providing flow channels, the channels arranged to alternate from top to bottom between a first and a second flow direction - the second flow direction being either parallel and opposite, or transverse to the first direction. Closure bars are provided to seal alternate layers where flow is to be prevented through those layers. A top sheet and a bottom sheet are provided, respectively, on the top and bottom of the stack of layers.

The core will be arranged to be provided in a heat exchanger housing having inlets and outlets as described above. The housing may be as shown in <FIG> and <FIG>, but other housing structures could also be used and the disclosure is not limited in this respect. For high temperature applications, housings are typically made of steel, but other materials may be used. The fluids, as with the known heat exchangers described above, may be hot and cold air but can also be other heat exchange fluids.

The design of this disclosure, however, modifies the closure bars as described below, with reference to <FIG>, to take into account the uneven thermal expansion due to the hot fluid stream <NUM> being concentrated mostly around the middle of the heat exchange core side on which it is incident.

According to the disclosure, rather than being straight rectangular bars as in the prior art, the closure bars <NUM> are profiled such that they have a substantially rectangular main body portion <NUM> that extends along the layer of fins to be closed
and a wider end portion <NUM> that has a width greater than the main body portion <NUM>. A radius <NUM> defines the transition between the main body portion <NUM> and the end portion <NUM>. The first edge <NUM> of the closure bar that is sealingly located with the fin layer to be closed is a continuous substantially straight edge extending along the main body portion and the end portion. The opposite edge <NUM> of the closure bar includes a main body portion edge <NUM> being a first distance from the first edge <NUM>, an end portion edge <NUM> being a second, greater distance from the first edge <NUM>, and the radius <NUM> between the main body portion edge and the end portion edge. The end portions also have an end edge <NUM> joining the end portion edge to the first edge, at each end of the closure bar. The length of the end portion edge <NUM> varies from closure bar to closure bar in the stack as shown in <FIG>. The core is formed by stacking fins layers <NUM> as is conventional. On each side of the resulting stack, closure bars <NUM> are provided on alternative layers to seal the layers to that side. The profiled closure bars <NUM> are stacked such that the closure bars adjacent the top plate <NUM> and the bottom plate <NUM> (i.e. the top-most and bottom-most closure bars) have the longest end portion edge <NUM> and that the length of the end portion edge of the other closure bars decreases towards the middle of the stack, thus forming a substantially curved end portion profile between the top and bottom plates, with the end edges of the closure bars aligned as shown in <FIG>. A corresponding structure of closure bars is formed on the adjacent side of the block (but sealing the other alternate layers). The closure bars on the two adjacent sides are stacked such that the end portions of the bars on one side overlap with the end portions of the closure bars on the other side, as shown in <FIG>. The end edges of the closure bars on one side all align with each other and also align with the end portion edges of the closure bars of the other side thus forming a solid corner section of the block between the top and bottom sheets as best seen in <FIG>.

The profile bars may be formed using laser cutting or water jet cutting for speed and precision, but other ways of shaping the bars may also be used.

The inner curved profile, C, resulting from the stacking of the profiled closure bars provides a structure that more closely matches the thermal expansion pattern described above and therefore reduces thermal loading on the structure. Furthermore, because the end portions of the closure bars all overlap to form a solid structure, there is no need for additional corner angles to be brazed to the structure and so the problem of the corner angles being damaged due to thermal stresses does not arise. Furthermore, the structure removes the need for an additional brazing step that is conventionally needed to attach the corner angles and avoids one potential leak site. The header parts of the heat exchanger can be attached to this structure e.g. by welding.

Claim 1:
A plate-fin heat exchanger core comprising:
a plurality of fin layers (<NUM>) arranged in a stack, each fin layer defining a fluid flow channel, the fin layers comprising first alternating fin layers defining a fluid flow channel in a first direction and second alternating fin layers arranged to alternate with the first fin layers in the stack, defining a fluid flow channel in a second, different direction, and a plurality of closure bars (<NUM>), each closure bar having:
a substantially rectangular main body portion (<NUM>) defined by a first edge (<NUM>) and a second edge (<NUM>) and an end portion (<NUM>) having a first end portion edge (<NUM>'), and opposite second end portion edge (<NUM>) and an end edge (<NUM>) extending between the first end portion edge and the second end portion edge, wherein the first edge of the main body portion and the first end portion edge form a continuous substantially straight first closure bar edge and wherein the second end portion edge is spaced from the first end portion edge by a distance (d1) greater than the distance (d2) between the first edge and the second edge of the main body portion, and wherein the second edge of the main body portion and the second end portion edge joined by a radius portion define a second edge of the closure bar;
the core including a first plurality of said closure bars arranged to seal the first alternating fin layers on a first side of the stack and a second plurality of said closure bars arranged to seal the second plurality of fin layers on a second side of the stack adjacent the first,
characterised in that:
the first closure bars are arranged such that their end portions (<NUM>) overlap and their end edges align, and the second closure bars are arranged such that their end portions overlap and their end edges align,
and wherein the end portions of the first and second closure bars overlap to define a solid corner (<NUM>) of the stack,
and wherein the first closure bars are stacked in an order such that topmost and bottommost closure bars have a second end portion edge of a first length and the length of the other first closure bars in the stack decrease with respect to the first length towards the middle of the stack to form a curved inner end portion profile ( C ) from top to bottom of the stack.