Heat exchange – Flow passages for two confined fluids – Interdigitated plural first and plural second fluid passages
Reexamination Certificate
2001-09-26
2004-02-24
Leo, Leonard (Department: 3743)
Heat exchange
Flow passages for two confined fluids
Interdigitated plural first and plural second fluid passages
C165S167000
Reexamination Certificate
active
06695044
ABSTRACT:
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.
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 plates, the group comprising one or more perforated plates sandwiched between a pair of primary separator plates, each perforated plate having perforations arranged in rows across the plate in a first direction, with a land between each adjacent pair of perforations in a row and with ribs between adjacent rows, the lands forming barriers to flow in the first direction across the plate and the ribs forming barriers to flow in a second direction across the plate, the second direction being normal to the first direction, the ribs having vents through a portion of their thickness, the vents extending from one side of a rib to the other side in the second direction, whereby flow channels are provided through the vents and the flow channels lying between each adjacent pair of lands provide a flow passageway to cross the plates in the second direction, the passageways in the group of plates being separated from passageways in any adjacent group of plates by one of the separator plates.
Although the group of plates may in fact contain only one perforated plate, there may be two or more perforated plates in the group of plates and in this embodiment adjacent perforated plates are aligned whereby the perforations of a row in one plate lie in correspondence with those of adjacent plates so that the lands and ribs of the plates lie in correspondence respectively with each other.
The invention will be more particularly described below with reference to embodiments in which the, or each, group of plates contains two or more perforated plates.
It will be appreciated that the flow passages can equally be provided in the first direction instead of the second direction, i.e. the lands are effectively the ribs containing the vents and the ribs are the lands.
The separator plates may be unperforated to provide complete separation of the passageways of the respective groups of plates. Alternatively, the separator plates may contain holes positioned and sized to provide controlled mixing of the fluids in those passageways. Such a separator plate will be referred to below as a mixing plate.
As indicated above each group of perforated plates preferably 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 perforated plates separated by separator plates, each group containing two perforated plates having their perforations aligned in rows.
The passageways across the plates preferably traverse across the plates once only from a first edge to a second edge. However, in an alternative first specific embodiment, the passageways at one or both plate edges may be turned, e.g. by an appropriate passageway arrangement, through an angle whereby the passageway defined by the channels continues in a different direction through the stack, e.g. in the opposite direction so as to return from the second edge to the first edge.
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 provided 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 perforated plates of each group aligned with their perforations in rows, it will be appreciated that the solid regions (i.e. ribs and lands) of those plates between the rows of perforations and between the perforations are also aligned in rows. As the perforated plates, therefore, are stacked one above each other, the ribs and the lands are aligned through the stack and this 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 rows of ribs and of lands may run in parallel lines across the plates but this is not essential.
The perforations may be of any desired shape but are preferably elongated slots.
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 f
Calfee, Halter & Griswold, L.L.P.
Chart Heat Exchangers Limited Partnership
Leo Leonard
LandOfFree
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