Automotive Heat Exchanger Headers
The present invention relates to automotive heat exchanger headers.
In certain situations it is important to utilise heat exchanger headers capable of withstanding high operational pressures. For example in HVAC systems where for example CO2 is used as a refrigerant operating pressures can typically be over 100 bar with test to burst heat exchanger requirements being at a level of 300 bar or more. Aluminium brazed heat exchangers are commonly used in lower pressure applications and it would be beneficial to enable aluminium materials to be used in manufacture of high pressure automotive heat exchangers.
An improved heat exchanger header construction and automotive heat exchanger have now been devised.
According to a first aspect, the present invention provides a header for an automotive heat exchanger comprising first and second header elements bonded to one another at a peripheral interface, at least one of the header elements having a cavity defining formation arranged to define a fluid receiving cavity when the header elements are bonded to one another.
It is preferred that the peripheral interface comprises substantially flat peripheral surfaces of flange portions of the respective first and second header elements. Beneficially, the peripheral bonded interface extends around substantially the entire periphery of the fluid receiving cavity.
Preferably, both the first and second header elements have respective complementary cavity defining formations arranged to extend adjacent to one another to define the fluid receiving cavity for the header.
The peripheral interface at which the header elements are bonded preferably comprises interface surfaces extending around substantially the entire periphery of the cavity defining formations arranged to mate up in face to face relationship.
Desirably, a respective header element is coined or press formed to provide the respective cavity defining formation. Preferable both header elements are coined or press formed. Beneficially, both the header elements are press formed to define a respective cavity formation, the respective cavity portions in each of the header elements preferably being of similar volume (preferably plus or minus 20%, more preferably ±10% volume of one another).
It is preferred that the ends of the cavity are defined by end walls of the cavity formations. It is preferred that marginal flange portions are provided nrnning longitudinally of the respective cavity formations and end flanges provided adjacent to the opposed ends of the cavity formations. It is preferred that the flanges lie on a substantially common plane, which plane defines a bonded interface between the header elements.
It is preferred that the header elements are formed of aluminium plate material. Beneficially, the header elements are brazed to form a brazed join at the peripheral interface between the header elements. Preferably, one or both outer surfaces of one or both of the header elements are clad with a lower melting point material (such as a brazing filler alloy) than the core material of the header elements. At least one of the interface surfaces of the header elements is preferably clad with the lower melting point material.
Preferably, one of the header elements is provided with a series of spaced tube receiving apertures for receiving heat exchanger tubes.
In a preferred embodiment the header elements are formed to define two fluid receiving cavities extending in side-by-side relationship. Preferably, one or both of the header elements are formed to have at least one fluid communication channel extending between the side-by- side cavities, i this embodiment it is preferred that bonding flanges are provided at the longitudinal edges of the header elements and at opposed ends of the header elements, and also a bonding surface is provided between the cavities. The flanges and bonding surface preferably lie in a substantially common plane.
It is preferred that the header elements are formed to have substantially semi-cylindrical internal cavity defining wall surfaces to define a substantially cylindrical fluid receiving cavity for the header. Beneficially, the ratio of the material wall thickness (t) of the cavity wall to the internal wall radius of the cavity (r) is substantially in the range 0.6 ≤ t/r < 1.3. Advantageously, terminal portions of the fluid receiving cavity are round nosed. This enhances the pressure capability of the cavity.
According to a further aspect, the present invention provides a method of manufacturing a header for an automotive heat exchanger, the method comprising:
coining at least one header element from a relatively thick metal plate in a heavy press or die forming operation to provide a cavity defining wall portion surrounded by a peripheral flange;
mating the header element with a second header element and bonding in a fusion bonding process at an interface comprising a surface of the peripheral flange.
Beneficially, two header plates are coined from relatively thick metal plate in a heavy press or die forming operation to provide each with a cavity defining wall portion surrounded by a peripheral flange, the header elements being mated together with the cavity defining portions adjacent to define together the header cavity and fusion bonded at the interface between the contiguous flange portions of the respective header elements.
The invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which:
Figure 1 is a schematic representation of a typical heat exchanger layout;
Figure 2 is a perspective view of a pair of header elements for forming a header in accordance with the invention;
Figure 3 is a perspective reverse-side view of the header elements of Figure 2;
Figure 4 is a schematic sectional view through a header element;
Figure 5 is a schematic sectional view of an alternative embodiment of header elements;
Figure 6 is a schematic view showing the cladding scheme of an exemplary header element.
Concerns of the adverse environmental impact of automotive HVAC systems have seen many efforts by manufacturers and automotive component supplies to develop alternative, more environmentally friendly automotive HVAC systems. One of the most promising and highly developed new systems is based on the use of the natural gas CO,, as the refrigerant. A characteristic feature of the CO2 air conditioning system is that the internal operating pressures are typically an order of magnitude greater than other conventional systems.
Irrespective of system refrigerant choice however, aluminium heat exchangers offer the advantages of low cost, good corrosion durability and good heat transfer characteristics. As such aluminium is invariably the material of choice for modern automotive heat exchangers. Further to realise greatest performance the heat exchangers are brazed using the CAB process (Controlled Atmosphere Brazing). This process usually requires the use of clad aluminium sheet.
The base metal of the heat exchangers is usually clad with the brazing filler metal. This is typically AlSi alloy with a silicon content of 6.8% to 13%. The silicon reduces the melting point of the brazing metal below that of the aluminium parts. At the usual furnace operating temperatures of 590°C to 615°C the filler metal is molten, and in intimate contact with the solid aluminium base material. Capillary action draws the molten brazing metal into the joints forming a sealed unit.
Conventional brazed heat exchangers typically comprise a number of parts as identified in
Figure 1. Tubes (3) that allow the flow of refrigerant, are typically separated by airways (2) to facilitate heat exchange with the air passing across the heat exchanger. Two headers (1) locate the ends of the tubes (3), and permit the required distribution of refrigerant into
individual tubes and then back to the refrigerant circuit. The side-casings (4) add stiffness to the unit and mounting points for peripheral components within the vehicle.
Conventional heat exchanger headers are formed from rolled sheets of clad material such that they are suitable for the CAB brazing process. These types of headers are however, not suitable for high pressure systems (such as the CO2 system) due to the high operating pressures involved (over 100 bar). It is furthermore not possible to form small enough diameter tanks from rolled sheet material of the required thickness, and further problems are encountered with subsequent forming and assembly operations. Also when forming large gauge materials, forming radii are compromised i.e. restricted to large curvature radii.
To overcome these problems, prior art high-pressure heat exchanger designs have typically employed extruded header tanks, which do allow the use of small diameter tanks with large wall thickness. The disadvantage of extruded profiles is that they are constrained to one alloy, and cannot have a clad layer making them unsuitable for the CAB process. For this reason the
AlSi alloy must be applied either as a shim, or a braze paste, at the braze joints, both of which can prove problematic in a continuous manufacturing process.
A problem to be solved by the invention is therefore to provide a header tank, comprising one or more channels, that is suitable for high-pressure applications, with operating pressures of more than 100 bar, which is inexpensive and easy to adopt into the CAB brazing process.
Referring to Figures 2 to 6 there is shown a header in accordance with the present invention. In the embodiment shown in Figures 2 to 4 a top plate 10 and bottom plate 11 are formed with a pair of side-by-side wall deformations 12, 13 and 14, 15 deforming elongate cavities or channels 16 in side-by-side relationship for each header plate 10, 11. As shown in Figure 6, each plate 10, 11 comprises a core 17 of aluminium and a top and bottom surface cladding layers 18, 19 of AlSi alloy brazing filler material of lower melting point than the core aluminium layer. The top and bottom header plates 10, 11 are heavy press formed in a coining operation to deform the arcuate cavities or channels 16 without disrupting significantly the integrity of the AlSi cladding layers 18, 19.
The coining heavy press forming operation enables relatively thick plate material (typically 1-4 mm thick) to be used to form relatively narrow (small radius of curvature) cavities or channels 16 as are required for acceptable system performance, and also enables clad aluminium to be used for using advantageous brazing processes such as the CAB process.
Typical plate thickness (t) and channel/cavity radius ratios (r) will be in the range 0.5 < t r ≤1.5 typically in the range 0.6 ≤ t/r ≤ 1.3. Typically plate thickness is substantially in the range 2 mm to 4 mm.
The upper header plate 10 is provided with heat exchanger tube receiving apertures 21 , each extending across both adjacent cavities or channels 16 to receive heat exchanger tubes to be brazed in place.
The lower header plate 11 includes connecting channels 22 extending between adjacent cavities or channels 16 permitting fluid communication between the two.
The provision of plural cavities/channels 16 in side-by-side relationship enables the total header cavity/channel 16 capacity volume to be sufficient without a single over large diameter cavity to be required for the operational pressures required.
The embodiment of Figure 5 shows a single cavity or channel 16 formed of plates 110, 111 receiving a tube 130.
A peripheral flange 25, 26, 125, 126 surrounds the cavity defining wall deformed portions of the plates 12, 13, 14, 15, 112, 114. The peripheral flange defines a contiguous bonding interface for the brazed bond between the header plates 10, 11, 110, 111. During the brazing process (such as for example the CAB brazing process), the AlSi cladding brazing filler layer flows and solidifies to produce the required bond.
The cladding scheme shown in Figure 6 is exemplary only and a variety of cladding schemes may be used for example.
1. The inner surface of the top plate and outer surface of bottom could employ a filler cladding to facilitate brazing at the central seam and tube joints.
2. One or both inner surfaces could be used, relying on capillary action from inside to form the tube joints.
3. All surfaces clad to secure connectors. Connectors may be manufactured such that no clad layer is possible. In this case an outer clad layer will be preferable to avoid the use of pastes/shims.
The header described may be produced at large volume, inexpensively. They are also suitable for the CAB process meaning no new brazing technology need be employed. The headers are also of a small size and weight, making them desirable for vehicle packaging requirements.
The invention has primarily been described in relation to the manufacture of headers from aluminium material, and in particular to clad aluminium systems. The invention should however be interpreted not as being limited to such materials and systems and is believed to be of advantage using other metallic plate materials and bonding schemes.