HEAT EXCHANGERS

Abstract: A down-flow reboiler heat exchanger for use in nitrogen liquefaction. A liquid oxygen distributor permits liquid oxygen to flow over the lip of a weir (17) onto hardway distribution elements (19) such that the oxygen flows downwards in wetting contact with thetube plates (10) which separate the oxygen and the nitrogen being cooled.

Description: Heat Exchangers Technical Field The present invention relates to heat exchangers and more particularly but not exclusively to heat exchangers used in so-called oxygen boiling processes to liquify nitrogen. Such boiling processes result in the vaporisation of a liquid (e.g. liquid oxygen) in one group of passages of a heat exchanger, whilst in an adjacent group of passages a second fluid (e.g. nitrogen) is cooled sufficiently to liquefy it. Background Art In the past the most common technique or process for nitrogen liquification has arranged for liquid oxygen to boil in an upward flowing direction such that vapour and liquid passes upwards in a first set of layers of the exchanger. Gaseous nitrogen is presented in a second set of layers of the exchanger such that it is condensed to provide a downward flow of liquid nitrogen at the bottom of these second layers. The boiling temperature of oxygen at the bottom of the exchanger is elevated slightly by the increased pressure placed upon it by the weight of liquid above it. This increase in boiling temperature reduces the efficiency of liquification particularly with oxygen and nitrogen as their respective boiling and dew points are so similar. The oxygen liquid enters at the bottom of the exchanger into alternate adjacent passages to the nitrogen and then flows vertically upward boiling and so absorbing heat from the nitrogen before exiting at the top as a vapour-liquid mixture. The pressure of the oxygen at the bottom of the exchanger must be greater than the pressure at the top to force the flow through. The difference in these two pressures is the sum of the frictional pressure drop through the oxygen passages, and the static pressure due to the weight of the vapour-liquid mixture in the oxygen passages. The effect of the higher pressure at the bottom of the exchanger is to increase the boiling temperature at that point. This increase in boiling temperature reduces the temperature difference between the condensing nitrogen and the boiling oxygen streams, resulting in the size of heat exchanger presently found in air separation plants. More recently and as described in US Patent Number Re 33026 and in US Patent 5122174 a technique or process called 'downflow oxygen boiling' has been used. This technique arranges that the oxygen flow is downwards and so has the advantage that the liquid does not increase the pressure of the oxygen and so boiling temperature at the inlet. At present, the use of 'downflow' techniques has necessitated supplementary distribution arrangements for the exchanger, thereby increasing exchanger complexity and expense. It is an objective of the present invention to provide a heat exchanger suitable for downflow oxygen boiling applications which reduces the above outlined problems. Disclosure of Invention We provide a heat exchanger comprising means designed for vaporizing a liquid by heat exchange with a second fluid while maintaining no more than a small temperature difference between said liquid and said second fluid, said exchanger having side walls enclosing an assembly of parallel vertical plates having walls defining therebetween a multitude of flat passages, said passages comprising a first group of said passages, and a second group of passages available for the flow of said second fluid, constituting the remainder of said passages defined by said walls of said plates, said second group of passages extending upwardly to a height less than the height of said first group of passages so as to provide regions of said group of first passages above the second group of passages. A header tank located laterally adjacent the uppermost portion of said assembly for supplying said liquid into said regions, and each said region has respective weir means over which said liquid may flow into the uppermost part of an adjacent first set of passages and thereby pass downwardly through said first set of passages. The uppermost portions of each first group of passages incorporate distribution means extending at least partly along their depth, said distribution means being adapted to spread said liquid in a horizontal direction and to permit it to percolate downwardly. The first group of passages will typically alternate with the second group of passages. The distribution means may include means for preferentially feeding the liquid onto the walls of said vertical plates as the liquid leaves the distribution means. Each said region may incorporate horizontally corrugated finning, typically perforated, extending across the depth of the region, such finning acting to distribute the liquid along the depth of each region. There may be vertically corrugated finning above the horizontally corrugated finning or hardway on either side of the weir. Such vertically corrugated finning may be perforated. The distribution means may comprise perforated hardway in the form of horizontally corrugated finning. This hardway may extend across the full width of each first passage (i.e. between the walls of the vertical plates) over only a portion of its height. The lowermost fin of the hardway may terminate short of the wall towards which it is directed, leaving a gap between the wall and the edge of the fin. The hardway may comprise a plurality of corrugations between each pair of plates providing a first passage. There may for example be a pair of corrugations each extending on opposite sides of an intermediate hardway plate which runs parallel to the vertical heat exchanger plates and which may be positioned midway between them. The lowermost fin of one or each corrugation may be shorter than the remaining fins. The lowermost fin may be directed towards a heat exchanger plate rather than a hardway plate, such that the lowermost fin terminates leaving a gap between the edge of the fin and the wall of the heat exchanger plate. The gap so provided may be of the order of around half the height of the fin. The weir means may be provided by the uppermost extent of a number of the heat exchanger plates which are of restricted height i.e. do not extend to the full height of the assembly. Brief Description of Drawings An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawing in which :- Figure 1 is a schematic diagram indicating typical gas/liquid flows in a heat exchanger; Figure 2 is a schematic perspective of a distribution zone of the exchanger; Figure 3 is a schematic cross-section of the upper part of the heat exchanger distribution zone; and Figure 4 is a schematic cross-section of an alternative construction of the upper part of the heat exchanger. Best Mode for Carrying Out the Invention In the embodiment illustrated, liquid oxygen is to be vaporised and nitrogen gas is to be thereby condensed. Referring to Figure 1, the direction of oxygen (0 2 ) and nitrogen (N 2 ) flows in the body of the heat exchanger are indicated by the arrowheads. Both flows are downward in order to reduce pressure problems inducing increased boiling temperatures. Figure 1 only illustrates two oxygen layers and one nitrogen however it will be appreciated in a practical heat exchanger there will be many more layers. In order to liquify the nitrogen gas, heat is exchanged through layer separator walls or plates 10. The nitrogen flow is gaseous at the upper end of the heat exchanger, and is cooled such that the nitrogen is liquified by the time it reaches the lower end of the heat exchanger. In order to ensure concurrent downward flow of both nitrogen and oxygen it is necessary to introduce both at the top of the exchanger. This is seen more clearly in Figures 2, 3 and 4. Plates 10 (sometimes called tube plates) are separated along their vertical edges by side bars 1 1 except at the top and bottom of the heat exchanger where gaps in side bars 11 permit the entry and exit of nitrogen and entry of oxygen. Over the main heat exchange zone of the heat exchanger plates 10 are separated between the side bars by vertically corrugated finning in contact with each plate 10 which acts as a secondary heat exchange surface. This is a known feature of such heat exchangers and is not illustrated herein. Liquid oxygen enters the heat exchanger via a header tank 12 adjacent the uppermost part of the assembly and gaps in alternate side bars 11 (see Figure 2). The oxygen is distributed across the depth of the assembly along horizontally corrugated perforated finning 13, or hardway, provided between alternate pairs of plates 10. The liquid oxygen is prevented from travelling downwardly within the heat exchanger by means of divider bars 14 located beneath the horizontally corrugated finning 13 adjacent the lower rim of header tank 12. Divider bars 14 have the same width as side bars 11 and extend across the depth of the heat exchanger. Vertically corrugated perforated or plain finning 15, or easyway, extends between alternate pairs of plates 10 above the horizontally corrugated finning 13. The uppermost portion of this easyway 15 is visible in Figure 3 although it would in practice be under a header tank. Liquid oxygen being pumped under pressure into header tank 12, rises upwardly through the easyway 15 across the depth of the heat exchanger. Nitrogen gas is introduced into the heat exchanger between the same alternate pairs of plates 10 as those between which liquid oxygen is introduced, but is separated from the liquid oxygen by divider bars 14. A nitrogen gas header tank (not shown, but similar to the oxygen manifold 12 and positioned below it) feeds nitrogen into horizontally corrugated perforated finning 16 which distributes the nitrogen gas across the depth of the heat exchanger before it passes downwardly to be condensed. It can be seen that the uppermost portions of the nitrogen containing passages form regions between plates 10 which are used to feed oxygen upwardly to the top of the heat exchanger. Figure 3 (and alternative Figure 4) illustrates the passage of liquid oxygen in two identical pairs of layers in the uppermost portion of the heat exchanger. Liquid oxygen rising through easyway finning 15 passes over a weir 17 provided by plate 10, alternate ones which stop short of the very top of the heat exchanger. The configuration associated with the weir allows even distribution to each layer and also permits any vapourised oxygen to be vented as oxygen falls over the weir. The adjacent passage between plates 10 to that which is used to supply liquid oxygen to weir 17, has a short vertical section of perforated easyway finning 18 starting at the edge of the weir located above perforated hardway finning 19. The rate of supply of liquid oxygen is typically regulated such that a pool of liquid oxygen remains above hardway finning 19, which acts to distribute the liquid oxygen across the depth of the heat exchanger. The lowermost fin of the corrugated hardway 19 may stop short of the plate 10 to which it is directed leaving a gap 20 which serves to preferentially feed liquid oxygen onto the wall of that plate 10. Liquid oxygen then moves down the surface of the plate 10 under gravity, which as a result of improved wetting improves the heat exchange characteristics as compared with previous distribution configurations. An alternative liquid oxygen distribution arrangement is shown in Figure 4. Liquid oxygen rises, as in the previous example, from the region above each nitrogen layer after horizontal distribution via hardway finning may be a mitred distributor 13 and passes through the easyway finning 15. It then falls over the weir 17 to form a liquid pool in a section of easyway finning 18 starting at the top edge of the weir. Beneath the easyway finning 18 are two sets of perforated corrugated hardway finning 21 of the same fin height separated by a hardway plate (or secondary tube plate) 22 extending parallel to plates 10 and located in the mid-position between two plates. Both the uppermost and the lowermost fin of each set of finning 21 may be shorter than the remaining fins. The lowermost fins of each of the two sets of finning is directed towards an adjacent tube plate 10 (i.e. the sets of finning are mirror images of each other) such that there are gaps 23 between the lowermost fin and the tube plate. These gaps are typically of the order of half the height of the finning 21 e.g. if the finning height is 3 mm, the gap may be 1.5 mm. It is desirably such that sufficient liquid oxygen is preferentially fed to the surface of each plate 10 so as to ensure a liquid curtain over the oxygen-side wall of each plate over the majority of its height. An example of an uppermost gap is identified by arrow 24 in Figure 4. There may also be a similar gap at the uppermost fin of the distributor 13. Below the hardway finning 21 , the usual easyway finning (not shown) extends downwardly through the oxygen layer.

Claims: CLAIMS: 1. A heat exchanger comprising means designed for vaporizing a liquid by heat exchange with a second fluid while maintaining no more than a small temperature difference between said liquid and said second fluid, said exchanger having side walls enclosing an assembly of parallel vertical plates (10) having walls defining therebetween a multitude of flat passages, said passages comprising a first group of said passages, and a second group of passages available for the flow of said second fluid constituting the remainder of said passages defined by said walls of said plates (10), characterised in that said second group of passages extends upwardly to a height less than the height of said first group of passages so as to provide regions of said group of first passages above the second group of passages, a header tank (12) is located laterally adjacent the uppermost portion of said assembly for supplying said liquid into said regions, each said region having respective weir means ( 17) over which said liquid may flow into the uppermost part of an adjacent first passage and thereby pass downwardly through said first passage, and the uppermost portions of each first group of passages incorporate distribution means (19, 21) extending at least partly along their depth, said distribution means (19, 21) being adapted to spread said liquid in a horizontal direction and to permit it to percolate downwardly. 2. A heat exchanger as claimed in claim 1 characterised in that the first group of passages alternates with the second group of passages. 3. A heat exchanger as claimed in claim 1 or claim 2 characterised in that the distribution means (19, 21) includes means (20, 23) for preferentially feeding the liquid onto the walls of said vertical plates as the liquid leaves the distribution means. 4. A heat exchanger as claimed in claim 1 characterised in that each said region incorporates horizontally corrugated finning (13), typically perforated, extending across the depth of the region, such finning (13) acting to distribute the liquid along the depth of each region. 5. A heat exchanger as claimed in claim 4 characterised in that there is provided vertically corrugated finning ( 15) above the horizontally corrugated finning (13). 6. A heat exchanger as claimed in claim 3 characterised in that the means for preferentially feeding the liquid includes horizontally corrugated finning with a plurality of corrugations between each pair of plates, with the lowermost corrugation terminating short of the plate (10) to which it is directed, thereby leaving a gap (20, 23) between the plate wall and the edge of the fin (19, 21). 7. A heat exchanger as claimed in claim 1 characterised in that the weir means (17) is provided by the uppermost extent of a number of heat exchanger plates (10) which do not extend to the full height of the heat exchanger assembly. 8. A heat exchanger as claimed in claim 6 characterised in that the liquid feeding means includes a plurality of side-by-side horizontally corrugated fins (21) separated by intermediate plates (22). 9. A heat exchanger as claimed in claims 6 or 8 characterised in that there is provided vertically corrugated finning (18) above the horizontally corrugated finning (19, 21). AMENDED CLAIMS [received by the International Bureau on 28 January 1998 (28.01 .98) ; original claim 1 amended ; remaining claims unchanged ( 1 page) ] 1. A heat exchanger comprising means designed for vaporizing a liquid by heat exchange with a second fluid while maintaining no more than a small temperature difference between said liquid and said second fluid, said exchanger having side walls enclosing an assembly of parallel vertical plates (10) having walls defining therebetween a multitude of flat passages, said passages comprising a first group of said passages, and a second group of passages available for the flow of said second fluid constituting the remainder of said passages defined by said walls of said plates (10), said second group of passages extending upwardly to a height less than the height of said first group of passages so as to provide regions of said group of first passages above the second group of passages, and a header tank (12) is located laterally adjacent the uppeπnost portion of said assembly for supplying said liquid into said regions, characterised in that each said region has respective weir means (17) over which said liquid may flow into the uppermost part of an adjacent first passage and thereby pass downwardly through said first passage, and the uppermost portions of each first group of passages incorporate distribution means (19, 21) extending at least partly along their depth, said distribution means (19), 21) being adapted to spread said liquid in a horizontal direction and to permit it to percolate downwardly. 2. A heat exchanger as claimed in Claim 1 characterised in that the first group of passages alternates with the second group of passages. STATEMENT UNDER ARTICLE 19 With reference to the International Search Report mailed 4 December 1997. the applicants have amended Claim 1 and wish to make the following comments. Firstly, it is pointed out that the cited EP 0 738 862 was published on 23 October 1996, whereas Claim 1 of the present application, it is submitted, is fairly based on the priority application GB 9617413.1 filed 20 August 1996. See in particular the paragraph from page 2, line 21 to page 3, line 17 of the priority document. Under EPC practice at least, therefore, EP 0 738 862 is not a valid prior publication but merely a prior pending application whose teaching cannot be combined with that of the cited US RE 33026. It is also submitted that none of the art cited in the International Search Report discloses or suggests the novel features of construction of the present invention as now claimed including that in which a liquid, e.g. liquid oxygen, passes from regions above a second group of passageways over a weir into a first group of passageways and thereby travels downwardly through the first group of passageways. Claim 1 has been amended accordingly and a new page 10 of the claims is enclosed accordingly in triplicate.

Inventors: CHATWIN CHRISTOPHER JOHN, CLINTON RICHARD ANTHONY, CHATWIN, CHRISTOPHER, JOHN, CLINTON, RICHARD, ANTHONY
Applicants: CHATWIN CHRISTOPHER JOHN, CLINTON RICHARD ANTHONY, IMI MARSTON LTD, CHATWIN, CHRISTOPHER, JOHN, CLINTON, RICHARD, ANTHONY, IMI MARSTON LIMITED
Application year: 1997
Main classification: F25J3
IPC codes: F25J3