Heat exchanger and/or fluid mixing means

Details Heat exchanger and/or fluid mixing means Heat exchanger and/or fluid mixing means

2002-12-04

2004-05-18

Atkinson, Christopher (Department: 3753)

Heat exchange

Flow passages for two confined fluids

Interdigitated plural first and plural second fluid passages

C165S167000, C165S174000, C165S178000

Reexamination Certificate

active

06736201

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a compact heat exchanger and/or fluid mixing means which incorporates a series of plates having apertures which define a plurality of passages through which fluid may flow.
BACKGROUND AND SUMMARY
Compact heat exchangers are characterised by their high “area density” which means that they have a high ratio of heat transfer surface to heat exchanger volume. Area density is typically greater than 300 m
2
/m
3
. and may be more than 700 m
2
/m
3
. Such heat exchangers are typically used to cool (or heat) process fluids.
One well known but expensive to manufacture type of heat exchanger is the so-called tube and shell heat exchanger. Essentially such heat exchangers consist of an exterior tubular shell through which run a number of longitudinally-extending smaller diameter tubes carrying one or more fluids. Other fluids, with which heat is to be exchanged, typically pass transversely across the heat exchanger such that heat is exchanged through the tube walls. A large number of tubes may be needed and they each have to be individually and accurately fixed/secured into a header plate at each end of the shell. In each case holes need to be drilled in the header plates very accurately to locate the tubes. High quality tested tubing then needs to be assembled into the plates and brazed or welded or mechanically-expanded into position. As the tubes are reduced in diameter to increase surfaces available for heat transfer and hence performance/compactness, the more difficult and expensive such configurations become to manufacture.
A second known type of heat exchanger is the so-called primary plate/secondary plate type exchanger in which a stack of plates is assembled; the stack having primary plates which directly separate two different fluid streams and secondary plates between adjacent primary plates. The secondary plates act as fins which add to the strength of structure and may be provided with perforations to provide additional flow paths for the fluids. The plates are usually bonded together by brazing but this may have the disadvantage of affecting the physical properties of the plates in the brazed regions or may introduce into the system, by means of the braze material, a potentially less satisfactory structure in terms of strength and corrosion resistance. It has been proposed to bond the plates together by diffusion bonding but a satisfactory construction that can withstand the high pressures involved has not been achieved and the fins may buckle during the bonding process.
It is an object of the present invention to provide an improved construction of this second type of heat exchanger which can be satisfactorily made by, for example, diffusion bonding or by brazing. It also aims to provide a heat exchanger construction which can also be readily adapted for use as a fluid mixing means, e.g. it can be used as a chemical reactor in which fluids which are to react together are mixed. Thus, where a reaction is exothermic, the invention may provide a means whereby the exothermic heat of reaction may be removed efficiently or, alternatively, it may be used to supply heat to an endothermic reaction. The products of the invention are also useful as fuel reformers and gas clean-up units associated with fuel cell technology.
Accordingly, the present invention provides a heat exchanger or fluid mixing means comprising a bonded stack of plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates and the passageway(s) in the group of main perforated plates, being separated from passageway(s) in any adjacent group of perforated plates by an intervening plate.
The intervening plate may be unperforated to provide complete separation of the passageways of the respective groups of plates. Such an intervening plate will be referred to below as a “separator plate”. Alternatively, as is described in more detail below, the intervening plate may contain holes positioned and sized to provide controlled mixing of the fluids in those passageways. Such an intervening plate will be referred to below as a mixing plate.
Each group of main perforated plates comprises at least two perforated plates but may contain three or more adjacent perforated plates as desired. A stack may, for example, comprise two or more groups of main perforated plates separated by intervening plates, each group containing two perforated plates having their perforations aligned in rows.
The passageways formed by the rows of discrete channels across the plates may simply traverse across the plates one from one side to the other. However, in a first specific embodiment, the perforations at one or both ends of each row are shaped to turn their respective channels through an angle whereby the passageway defined by the channels continues in a different direction through the stack.
In a second specific embodiment two or more separate passageways are provided across a group of plates whereby streams of different fluids may flow parallel to each other in the same layer prolided by said group of plates. This embodiment can provide improved temperature profiles across the plates and reduced thermal stress.
Because the plates are stacked with the main perforated plates of each group aligned with their perforations in parallel rows, it will be appreciated that the solid regions (i.e. ribs) of those plates between the rows of perforations are also aligned in parallel rows. As the perforated plates, therefore, are stacked one above each others the parallel ribs are aligned through the stack and hence this not only provides the discrete channels referred to above, it provides strength through the assembled stack whereby the pressures generated in the bonding process can be withstood. The invention, therefore, provides a stack structure that can be bonded without the risk of the fins of the secondary plates collapsing under the pressures generated. The fins also provide the means of withstanding internal pressures in the operating streams.
The perforations may be of any desired shape but are preferably elongated slots. In the aforementioned first embodiment the slots at the end of a row are preferably “L” or “V” shaped with the angle of the “V” being determined by the desired change of direction of the passageway.
The plates may be rectangular, square or circular for example or of any other preferred shape.
Where the plates are square or rectangular, each row of slots may extend from a first edge of the plate parallel to a second edge of the plate and for substantially the whole length of that second edge. It will be appreciated that a substantially unperforated edge or border will normally be required around the perimeter of the major faces of the plate to enable the plates of the stack to be bonded together and to provide pressure containment for the stream or streams. However, a completely unperforated border is not essential and slots in the border may be required for inlet and outlet means, for example. A plurality of rows of slots may, therefore, extend across the plate from the first edge towards the opposite, third, edge. In respect of the first embodiment described above, adjacent that opposite third edge the slots at the end of the row may be “L” shaped whereby each row then extends at right angles to its original direction, i.e. extends parallel to the third edge. A second right angle turn may then be arranged whereby the rows of slots then extend back across the plate parallel to the first

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