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 U. S. Patent Number Re 33026 and in U. S. Patent 5 122 174 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.

The use of"downflow"techniques had necessitated supplementary distribution arrangements for the exchanger, thereby increasing exchanger complexity and expense.

Accordingly in our international patent application no.

PCT/GB97/02214 filed 19th August, 1997, we provided a heat exchanger suitable for downflow oxygen boiling applications which reduces the above outlined problems.

In our aforesaid application we provide a heat exchanger comprising means designed for vaporising 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, each said region having respective weir means over which said liquid may flow into the uppermost part of an adjacent first passage and thereby pass downwardly through said first passage, the uppermost portions of each first group of passages (i. e. just below the level of the weir) incorporating 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.

We have now realised that the structure of the heat exchanger at its upper end can be strengthened while at the same time preventing any particles or other debris from above falling into the passageways of the heat exchanger.

Accordingly the present invention provides a heat exchanger of the aforesaid type of our application PCT/GB97/02214 wherein the upper end of each first passage extends above the weir and is sealed with a plug, the plug extending downwardly in the passage to a position above the level of the weir between the first passage and its adjacent second passage, the plug being brazed to the opposed walls of the vertical plates defining the first passage.

The plug prevents any particulate material or other debris from falling into the first passage where such unwanted material could obstruct the flow of liquid by blocking partially the passage. Such a blockage could cause "dry-out"in the passage with potentially dangerous consequences.

The brazing of the plug to the opposed walls of the first passageway adds strength to the structure.

The weir is preferably provided by using a vertical plate between the respective first and second passages which is shorter than the other plate defining the second passage. In this embodiment, the extension of the passage above the weir may be provided by a separate shorter length of vertical plate positioned above the level of the weir and, as indicated above, brazed to the plug.

In an alternative, albeit less preferred embodiment, the vertical plates defining the passages may all be of the same height and, therefore, extend above the level of the weir. In this embodiment the weir is provided by slots in the vertical plate between the first and second passages.

As described in PCT/GB97/02214, 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.

Finning provided in the second passages above the level of the weirs may advantageously be brazed to the opposed faces of the vertical plates defining the passage, i. e. the extension of the second passage above the weir advantageously enables further strengthening of the structure in this respect.

The distribution means may comprise perforated hardway in the form of horizontally corrugated finning. This hardway will 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 a 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.

As indicated above, 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, with the passages being continued above the weir by means of a shorter separate vertical plate.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings 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; and Figure 3 is a schematic cross-section of the upper part of the heat exchanger distribution zone.

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 (02) and nitrogen (N2) 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 alternate layer separator walls or plates 10 and 10A. The nitrogen flow is gaseous at the upper end of the heat exchanger, and is cooled such that the nitrogen is liquefied 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 and 3.

Plates 10 and l0A (sometimes called tube plates) are separated along their vertical edges by side bars 11 except near 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 and 10A arc separated between the side bars by vertically corrugated finning in contact with each plate 10 and 10A 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 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 and 10A. 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 in the nitrogen passages.

Vertically corrugated perforated or plain finning 15, or easyway, extends between alternate pairs of plates 10 and lOA above the horizontally corrugated finning 13. The uppermost portion of this easyway 15 is visible in Figure 2 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 and lOA as those between which liquid oxygen is introduced, but is separated from the liquid oxygen by divider bars 14. A nitrogen has 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 and lOA which are used to feed oxygen upwardly to the top of the heat exchanger.

The upper end of each oxygen passage is filled with a plug 1 SA which extends downwardly to the level above a weir as described in more detail below with reference to Figure 3.

Figure 3 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 alternate plates l0A between plates 10, each plate 10A stopping 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 vaporised oxygen to be vented as oxygen falls over the weir. The adjacent passage between plates 10 and l0A 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 top edge of the weir. The rate of supply of liquid oxygen is typically regulated such that a pool of liquid oxygen remains in the section of easyway finning 18. 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 I OA and located in the mid-position between the 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 are directed towards an adjacent tube plate 10 or l0A (i. e. the sets of finnings are mirror images of each other) such that there are gaps 23 between the lowermost fin and the tubeplate. 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 and lOA 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 3. 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.

In each oxygen passage, above the weir, is a plug 15A extending downwardly from the top of the assembly to a position spaced above the weir. By way of example only, the plug may be about 17 mm long and leaves a gap 25 about 8 mm above the weir.

The plugs 15A are each brazed at one side to the surface of a plate 10 and at their other side to a separate length of tubeplate 1 OB. Plate lOB is also brazed to easyway finning 15 to provide a strong structure at the upper end of the heat exchanger. Plugs 15A also conveniently prevent any possibility of debris falling from above into the oxygen passages.