FIELD OF INVENTION
This invention relates to improved heat exchanger cores and in particular heat exchanger cores which incorporates a plurality of heat exchanger elements or modules which may be utilised in applications where heat energy is to be transferred from one fluid to another
There are many types of heat exchangers known. These can be generally divided into two main groups, the first being shell-and-tube heat exchangers and the second being plate heat exchangers. One example of a shell and tube heat exchanger comprises an array or bundle of tubes and a surrounding for external shell. Plate heat exchangers may have plate-fin arrangements such as described in U.S. Pat. No. 4,282,927 or a series of rectangular stacked plates creating flow channels from two or more fluids when the plates are sealingly stacked upon one another as described for example in U.S. Pat. No. 4,823,867.
In each of the two groups, in order to manufacture the heat exchanger, a number of parts are usually fabricated and joined together. In shell and tube heat exchangers, for example, a number of tubes are assembled into a bundle, and prior to having the ends of each tube sealed into corresponding apertures of a pair of tube sheets, baffles are installed along the length of the bundle. The bundle of tubes, baffles and tube sheets are then placed inside a pressure vessel which seals a pair of end zones around the tube sheets so that there are two fluid flow paths defined, the first being around the outside of the tubes and between the tube sheets, and the second being inside the tubes and in end chambers of the pressure vessel. In both the shell and tube configuration and the plate type heat exchanger, there is a large amount of material dedicated to defining and sealing fluid flow paths, and since the flow paths are defined by joining fabricated parts, there is always the danger of one fluid leaking into another.
In addition, fabricated heat exchangers suffer from inefficiency brought about by a plurality of boundaries constituted by welds, joins or parts interconnecting adjacent components of the heat exchangers, the boundaries providing zones of resistance to the conduction of heat to heat transmission surfaces of the exchanger. An integral part of heat exchangers formed by processes such as extrusion or casting are known as “primary structure” and must be differentiated from components welded or otherwise attached to the primary structure wherein such components including the boundaries are termed “secondary” structure. In a similar fashion surface(s) of primary structure are known as “primary” surface(s) and surface(s) of “secondary” structure known as “secondary” surface(s).
This disadvantage is particularly detrimental where liquid to air heat exchange through a plate exchanger is required. The generally low thermal capacity of the air cooling medium coupling with the inherent inefficiency of the prior art exchangers result in the requirement for very large exchangers relative to the respective liquid and cooling or heat air flows.
Reference may be made to U.S. specification 3,202,212 forming one example of prior art. This specification refers to a heat exchange element formed of extruded aluminium having a plurality of first fluid passages located in a body part thereof and a multiplicity of spines extending outwardly from opposed broad surfaces of the body part. Each spine was thin and comparatively small in cross section to thereby provide a high ratio of exposed surface to the mass of the spines for maximum thermal transfer efficiency from the body part. However each spine was spaced from each other in such a manner so as to form a number of separate rows arranged transversely across the body part so that transverse channels were formed between adjacent rows. There were also formed longitudinal channels so that while the heat exchanger was mainly formed to promote crossflow of a second fluid relative to the flow of first fluid parallel flow could also occur. This arrangement did not provide an efficient rate of heat transfer because of the crossflow of second fluid was not as efficient as a parallel flow system which enables heat to be removed and dissipated at any required rate. This occurs because crossflow heat exchange systems are not as well suited to external ducting as parallel flow systems. Also parallel flow systems achieve a more efficient arrangement in relation to being more compact and thus being more suited to stackable or modular arrangements. This has particular reference to engines which may be used in vehicles or in stationary applications and also for heat exchangers utilised in air conditioning.
Also the spines which were formed in a subsequent process to the initial production of the extrusion by cutting or gouging the spines from precursor ribs formed on the extrusion was inefficient because of the necessity of the subsequent cutting or gouging process which added materially to the cost of production of the heat exchanger.
Reference may also be made to Australian Specification 85777/75 and U.S. Specification 4,565,244 which show heat exchangers having a similar construction to U.S. Specification 3,202,212 and thus subject to the disadvantages described above.
Reference may also be made to U.S. Pat. Nos. 3,743,252; 4,352,008; 3,566,959, 4,567,074; 3,137,785 as well as U.S. Pat. No. 3,467,190 which all refer to useful background prior art in relation to this invention. These references show that formation of one piece extrusions are not new and thus U.S. Pat. No. 3,137,785 shows a one piece extrusion for use as a component in an electric heater comprising a body with longitudinally extending fins on each side of the body. Within the body are a plurality of extruded passages. However these internal passages are for receiving and containing tubular heating elements or thermally sensitive control elements and are not fluid passages.
U.S. Pat. No. 3,566,959 concerns an aluminium extrusion for use as a heat sink in semi conductor rectifiers.
U.S. Pat. No. 4,657,074 refers to a one piece aluminum extrusion tubular heat exchanger element which has a tubular body and a number of interior and exterior fins protecting integrally radially from the tubular body. However this reference is not considered relevant to this invention.
Reference may also be made to U.K. Patent 2 142 129 which describes a radiator formed from a rectangular elongate hollow body which is provided on each of two opposite sides with a plurality of spaced heat radiating fins to thus form a plurality of air channels located intermediate mutually adjacent fins. A cover plate extends across outer edges of each set of fins to form with said fins a plurality of open ended channels through which can flow air which is heated by a transfer of heat from a hot fluid flowing through the elongate hollow body.
While this reference shows a parallel flow situation and thus may overcome the problems of a cross flow aluminium extrusion discussed above in relation to U.S. Pat. No. 3,202,212 problems still occurred in operation because of the fact that such an extrusion is basically inefficient in operation because air in passing through the heat exchanger adjacent tips of the fins has substantially the same temperature and has a different temperature to air which is passing through the heat exchanger adjacent base areas of each air channel. In this regard it will be appreciated that the air passing through the heat exchanger adjacent the base areas will be substantially hotter or at a higher temperature than the air passing through the heat exchanger adjacent the tips of the fins which will be substantially cooler. This is because the tips of the fins are located substantially further away from the elongate hollow body than the base areas of each channel which are located immediately adjacent the elongate hollow body. Thus the efficiency of the heat exchanger which will require a uniform rate of heat transfer will be impaired because of the resultant heat gradient which will ensue wherein the hottest part of the exchanger through which air flow will occur is located in the base areas of each channel and the coolest part of the exchanger through which air flow will occur is located adjacent the tips of the fins. Also this problem of lack of heat exchange efficiency will be exacerbated because the volume of air passing through the heat exchanger adjacent the tips will be greater than the volume of air passing through the base areas of the air channels.
Reference may also be made to similar heat exchangers such as radiators which are described in U.S. Pat. No. 4,341,346 which have a central tubular duct for passage of hot water and a pair or webs extending on opposite sides of the central tubular duct with oppositely located fins being provided on each side of the webs and extending normal thereto. The conclusions which are expressed above in relation to U.K. Patent 2,142,129 also apply to this prior art reference. Similar conclusions also apply to U.S. Pat. No. 3,147,802, U.K. Patent 1,411,162 and U.S. Pat. No. 4,296,539.
U.S. Pat. No. 4,401,155 relates to a heat exchanger constructed from a plurality of stacked modules wherein each module has closed channels for high pressure flow and fins extending vertically up and/or down from the channels which form open channels suitable for low pressure flow when the modules are stacked parallel to each other along their length. It is specified that each fin be of uniform thickness and have an identical dimension perpendicular to the channels. Each module is generally formed from an aluminium extrusion. The heat exchanger of U.S. Pat. No. 4,401,155 is particularly directed at high pressure applications and especially where one heat exchange stream is at high pressure and another is at lows pressure. This has particular reference to cryogenic processes.
In U.S. Pat. No. 4,401,155 it is noted in regard to a first embodiment (i.e. FIGS. 1, 3 and 4) that there are provided as many channels as there are fins while in another second embodiment (i.e. in FIGS. 2, 6, 7 and 8) there are provided two channels for every fin.
In the first embodiment the heat exchanger assembly would not be efficient in relation to effective heat transfer because of the fact that the channel surface area is substantially the same as the fin surface area and in this regard it has been ascertained as described in more detail hereinafter that for efficient heat transfer to take place the surface area of the fins should be substantially greater than the surface area of the channels. This is important in a situation wherein the density of one fluid which may pass through the fins (e.g. air) is significantly different to the density of the fluid passing through the channels (eg water). In this situation air has a density much less than water and this means that air will have a much less effective heat storage capacity than water. This therefore means that an effective heat transfer from the water channels through the walls of the heat exchanger to the fins will only take place when the surface area of the fins is much greater than the surface area of the channels. This occurs because the greater surface area of the fins will facilitate effective absorption of heat to be passed through the walls of the heat exchanger from the channels to the fins whereby such heat may be transferred to the air passing through the fins. Simultaneously, the air must be able to satisfactorily absorb heat from the fins so that the heat may be transported away from the heat exchanger. In this regard the volume of air passing through the fins should be considerably in excess of the volume of water passing through the channels.
In regard to the second embodiment referred to above such embodiment does not provide an arrangement whereby uniform heat transfer in regard to a heat exchange core formed from a series of stacked modules could be obtained at least in relation to outwardly directed fins extending from peripheral modules are concerned. This is also a problem of other heat exchanger cores formed from extrusions as discussed in detail above concerning U.S. Pat. No. 3,202,212. It is relevant to note however that U.S. Pat. No. 4,401,155 describes a stacked arrangement of modules whereby outwardly directed fins from one module may extend into channels formed by adjacent fins of an adjoining module to achieve an interleaved arrangement. A similar interleaved stacked arrangement is shown in U.S. Pat. No. 3,476,179 and the same conclusions expressed herein in relation to the first embodiment of U.S. Pat. No. 4,401,155 also apply to this reference.
It is also considered that the heat exchangers shown in German patent 3 011 011 and Japanese patent 55-152 397 will also not provide efficient heat transfer for substantially the same reasons as expressed above in relation to U.S. Pat. No. 4,401,155.
It will also be appreciated that heat exchangers formed as one piece extrusions having a plurality of fins generally will also be subject to the same disadvantages as described and thus references such as U.S. Pat. No. 3,202,212, Australian Patent 85777/75, U.S. Pat. Nos. 4,565,244; 3,743,252; 4,352,008; 3,566,959; 4,567,074; 3,137,785 and 3,467,180 will also be subject to these disadvantages.
Reference may also be made to U.S. Pat. No. 5,042,247 which relates to a thermoelectric cooler wherein semiconductor materials with dissimilar characteristics are connected electrically in series and thermally in parallel so that two junctions are created. The semiconductor materials are N and P-type. In a typical thermoelectrical cooler (TEC), alternative columns of these N-type and P-type semiconductor materials have their ends connected in a serpentine fashion by electrical conductors: These electrical conductors typically are metallisations formed on insulating or ceramic plates. With the application of direct current to the TEC, heat is absorbed on the cold side ceramic, passes through the to semiconductor material and is dissipated at the hot side ceramic. A heat sink must be attached to the hot side ceramic for dissipating the heat from the TEC to the surrounding environment. Without a heat sink the TEC would overheat and fail within seconds.
In fact a heat sink is a device that is generally associated with machines or apparatus which generates substantial amounts of heat such as a TEC, or a computer so that the heat may be rapidly dissipated. Therefore a heat sink is more effective when it may dissipate heat at greater rates. U.S. Pat. No. 5,042,257 therefore describes a heat sink for a TEC which includes a pair of opposed plates which have a plurality of fins on each of opposed faces wherein one array of fins on one face may be accommodated or interleaved between another array of fins on the other face whereby one array of fins may be retained in desired position by suitable retaining means such as a pair of relatively short fins which constitute thermal surface area contacts for sufficient exchange of heat. However it will be appreciated that the relevant factors that must be taken into account when a heat sink is being designed for effective use are not relevant when one is considered the relevant parameters that are essential to design of a heat exchanger. This applies in particular to the design of a heat exchanger core comprising a stacked array of heat exchanger modules having channels or passages for two fluids which will flow in heat exchange relationship through the core. It will also be appreciated that heat sink considerations will be far removed from other heat exchanger considerations which will include relevant ducting requirements to achieve maximum heat exchange efficiency.
It therefore is an object of the invention to provide a heat exchanger assembly which will be relatively efficient and may reduce the problems of the prior art discussed above.
The heat exchanger assembly of the invention is formed from primary structure comprises a core comprising a multiplicity of primary fluid sections and a plurality of secondary fluid sections wherein a respective primary fluid section is located adjacent to a respective secondary fluid section wherein each primary fluid section is substantially parallel to each secondary fluid section characterised in that each primary fluid section is bounded by a peripheral wall and that each secondary fluid section include a first array of fins extending away from the peripheral wall of one adjoining primary fluid section and a second array of fits extending away from the peripheral wall of another adjoining fluid section wherein an individual first fin may extend between each adjacent second fin in interleaved relationship wherein the surface area of each secondary fluid section is substantially greater than the surface area of each primary fluid section.
Preferably the primary fluid sections are provided with a plurality of webs wherein each web extends from opposed faces of the peripheral wall to thereby provide a number of parallel primary fluid passages. The webs are useful for strengthening and reinforcement purposes to maintain the structural integrity of the core.
Suitably the tips of the first array of fins abut or are attached to an adjacent surface of the peripheral wall of said another adjoining fluid section. Alternatively and more preferably the tips of the first array of fins are spaced from the adjacent surface. In similar manner the tips of the second array of fins may abut or be attached to an adjacent surface of the said one adjoining fluid section but more preferably are spaced therefrom.
It will of course be appreciated that when the tips of the first and/or second array of fins abut or are attached to a mating surface of an associated peripheral wall that separate flow passages for secondary fluid may be provided. When the tips of the first and/or the second array of fins are spaced from an adjacent surface of an associated peripheral wall then each secondary fluid section may correspond to a single flow passage for secondary fluid.
It is within the scope of the invention to provide a heat exchanger core which may be formed in one piece such as an extrusion formed from aluminium or other suitable metal. However it is preferred that the core be formed from a plurality of separate modules. Suitably a particular module may include the peripheral wall which bounds the flow passages for primary fluid and both the first and second array of fins which extend away from external or outer surfaces of opposed parts of the peripheral wall respectively
It is an important feature of the present invention that the peripheral wall and the elongate fins of each module are each formed simultaneously in formation of the one piece extrusion or casting so that the heat exchanger module is formed from primary structure. In other words no secondary structure is present which can lead to disadvantages as discussed above. Also the primary fluid passages and the secondary fluid passages are substantially parallel to each other to form a parallel flow heat exchanger core which has advantages over cross flow heat exchangers of the type described previously. These advantages include the following
(i) with the secondary fluid comprising a cooling medium such as air, the parallel flow arrangement facilitates ducting of air both to and from the heat exchanger core. A fan may then be used to draw the air through and the hot air away from the heat exchanger core. This means that the heat exchange core of the invention does not have to rely upon ambient air passing through when used for example as a radiator in a motor vehicle. Therefore the heat exchanger core of the invention need not be located in front of the vehicle.
(ii) when used as a radiator, the heat exchanger core of the invention need not have a large flat surface as is the case with a conventional radiator. Therefore it can be designed in a far more compact configuration. This means that the shape of the vehicle may be changed to permit reduced wind resistance in the case of a truck or increased usage of the interior in the case of a coach or bus.
(iii) the ducting of the hot air away from the heat exchanger core of the invention also facilitates the use of the hot air for other purposes such as for the supply of heat or as a power source.
(iv) because the fluid passages are parallel this means that the heat exchanger element of the invention may be manufactured as a one piece extrusion. An extrusion could not be satisfactorily produced in the case of a crossflow arrangement. This also means that the fins form an integral part of the heat exchanger core of the invention rather than being crimped, welded or soldered to the part. This avoidance of secondary structure maximises conductivity of the heat from the fluid to be cooled to the other fluid.
(v) as a consequence of (iv) above the one piece extrusion may be cut to any desired length and may be designed to be releasably attached to other heat exchanger elements or modules to produce a core of the invention. Not only length but height and width are all flexible and may be varied to suit the particular application desired.
(vi) the heat exchanger core of the invention when compared to the prior art is far more efficient and also lighter. It also requires less power from the main power sources (e.g. an engine or motor) for operational purposes.
The core of the heat exchanger apparatus may take any form consistent with the function of providing definition for flow of both primary and secondary fluids. For example, the core may comprise a continuous cross section of indefinite length with the flow passages being disposed in the direction of the indefinite length. Such a configuration has proved to be particularly amenable to continuous casting or extrusion.
Preferably the peripheral wall of each module is substantially rectangular having a pair of opposed substantially horizontal parts in use which are joined by a pair of opposed vertical parts. The abovementioned webs of the primary fluid section formed by the peripheral wall are preferably substantially parallel to the vertical parts.
However it will also be appreciated that the peripheral wall may have any other suitable shape such as being circular in cross section or triangular in cross section or polygonal in cross section.
The primary flow passages of the primary fluid section may have any suitable shape or configuration and thus may be round, rectangular or polygonal in cross section.
It will be appreciated that the configuration of the flow passage or passages will depend upon the application of the heat exchanger and will also significantly depend upon the material of manufacture of the heat exchanger core.
It will also be appreciated that the elongate fins may have any suitable shape. Preferably however each fin is of constant height and width although this is not absolutely essential. Thus for example each elongate fin is of constant height and width although this is not absolutely essential. Thus for example each elongate fin may taper in width from one end to the other if required if the heat exchange element of the invention is produced as a casting. The elongate fins may also have projections or ribs on an outer surface to increase their surface area if required.
It is also within the scope of the invention to have internal fins located in the flow passages of the body part if such is considered appropriate.
The heat exchanger cores of the invention may be provided with manifolding means adapted to supply the primary and secondary fluids to their respective flow sections and such manifolding means may take any appropriate form. Thus an inlet manifold and an outlet manifold may be provided which enable primary fluid to communicate with the primary fluid section and secondary fluid to communicate with the secondary fluid section. Alternatively separate primary fluid and secondary fluid manifolds may be provided at inlet and outlet.
It is preferred however that the entry and exit of primary fluid and secondary fluid from the core occur at right angles to each other and thus it is desirable that suitable manifolding means be chosen so as to accomplish this objective.
In this regard therefore the flow of primary fluid and secondary fluid through their respective manifolds may also be normal to each other although this is not essential and the respective manifolds may be constructed so that the primary and/or the secondary fluid may flow at an acute angle to their respective core entry and core exit flow directions.
The primary fluid is suitably an “operating fluid” i.e. a fluid which is being processed by the heat exchanger and which is to be recirculated through a suitable flow system to which the heat exchanger of the invention is being applied. For vehicle radiators for example such a fluid is suitably water.
The secondary fluid is preferably “an active fluid” which functions as a coolant for the operating fluid and which absorbs heat from the operating fluid after passage through the heat exchanger core. Such a fluid for vehicle radiators for example may comprise air.
However it will also be appreciated that the above is not essential and the “active fluid” in accordance with this invention may become the “operating fluid” if necessary.
The heat exchange modules for use in the invention may be adapted for vertical stacking or horizontal stacking as may be required. In particular, each module may be provided with connection means for this purpose. In a preferred embodiment of the present invention the connection means may be such that interconnection of the adjacent modules defines a suitable flow passage for secondary fluid.
Preferably, each module has integrally extruded therewith connection means to enable adjacent modules of the invention to be assembled together. The connection means may be of any suitable type and thus comprise for example male members or projections engageable with female members or sockets in snap fit or interference fit-relationship. Suitably each module includes a top connection member and bottom connection member which may engage with corresponding top and bottom connection members of an adjacent
Preferably the top or bottom connection member may comprise a panel rib or flange which may engage with a corresponding ratchet rib or flange of a mating top or bottom connection member of an adjacent module.
The number of modules or heat exchange elements utilised in a horizontal or vertical stacking arrangement may then depend upon the particular application desired.
Heat exchanger cores in accordance with the present invention may be used in gas/gas, liquid/gas or liquid/liquid application and are particularly suited to gas/air and liquid/air applications using impelled ambient air as the heat transfer fluid. Alternatively, the cores can be placed in an enclosure suitable for natural or draft convection of air. Of course, heat transfer fluids other than air may be employed, such as water, ethylene glycol, ammonia, fluorocarbon compounds, silicone compounds, mineral oils and the like.
When the flow channels are manifolded in series, it is preferred to direct the flow of the first fluid counter-currently to the heat transfer fluid so as to obtain the maximum log mean temperature differential between the two fluids, and thus the most efficient heat transfer. Alternatively, where the first or heat transfer fluids are sensitive to rapid temperature changes, or have an upper or lower temperature limit, co-current flow may be utilised.
The heat exchanger cores of the present invention may be of any suitable material, the suitability of a material being generally determined with reference to the application to which it is to be put. For example, use in extreme high temperature applications may dictate that a ceramic material be used for its high temperature properties, whereas for lower temperature applications it has been found that aluminium or its alloys or other metals are suitable for their relatively high thermal conductivities, permitting thicker wall sections per unit efficiency and thus increasing mechanical strength.