WO2011110772A2 - Heat exchanger - Google Patents

Abstract

The invention relates to a heat exchanger (1) for vaporising a liquid by exchanging heat with a fluid, comprising: a body including an assembly of separator plates (3) disposed parallel to a pre-determined direction (11) and defining therebetween a plurality of passages (4, 5); and a retention tank (2) for receiving the liquid, comprising a base (2a) that is substantially orthogonal to the pre-determined direction (11), said base (2a) including closure elements (9, 3) and primary distribution elements (6) pierced with holes (6a) designed to distribute the liquid from the retention tank (2) into particular passages (4). According to the invention, the base (2a) includes an extension (E) that extends orthogonally to the pre-determined direction (11) across at least 80%, and preferably the entirety, of said extension, and the invention is characterised in that each of the aforementioned primary distribution elements (6) forms a projection (6f) that extends into the retention tank (2) and each of the holes (6a) forms an opening (6b) in the projection (6f).

Description

 Heat exchanger

The present invention relates to the vaporization of a liquid by heat exchange with a second fluid by means of a heat exchanger of the vertical plate type. It applies in particular to air distillation plants.

 In double column type air distillation plants, the liquid oxygen which is in the bottom of the low pressure column is vaporized by heat exchange with the nitrogen gas taken at the top of the medium pressure column. For a given operating pressure of the low pressure column, the temperature difference inks the oxygen and the nitrogen made necessary by the structure of the heat exchanger imposes the operating pressure of the medium pressure column; it is therefore desirable that this temperature difference be as low as possible, in order to minimize the costs associated with the compression of the air to be treated injected into the medium-pressure column.

 The technology commonly used for these phase-change exchangers is that of aluminum exchangers with brazed plates and fins, which make it possible to obtain very compact members with a large exchange surface. These exchangers consist of plates between which are inserted waves or fins, thus forming a stack of "passages" vaporization and "passages" condensation. There are different types of waves, such as straight waves, perforated or "serrated" waves.

 In the case of vaporizers operating in vaporizing film down mode, a part of the apparatus is devoted to the distribution of the liquid in the vaporization passages and between the channels of the exchange wave.

 This distribution proper to each vaporizer is conventionally carried out according to the principle described in the document FR-A-2547898: the supply of vaporization passages is by the top of the condensation passages. The oxygen then passes through a row of holes that ensure its primary distribution in the vaporization passages. It then flows through a horizontal generator waveband which ensures a so-called secondary distribution (distribution of the liquid within a passage, between the channels formed by a wave).

The liquid oxygen that is vaporized contains impurities in dissolved form. The main impurities are nitrous oxide (N ₂ 0), carbon dioxide (CO 2), hydrocarbons (C2, C3, ...). Depending on the operating conditions, these impurities can be deposited in the vaporization passages, either in solid form or in the form liquid. It is important to control industrially the formation of these solid or liquid deposits to avoid any risk of explosion.

 The flow of liquid per channel, or per meter of perimeter to be wetted, is one of the important parameters for the formation of deposits. Indeed, when the liquid flow per channel is insufficient to wet the wall, there is formation of dry spray deposits.

 In this type of vaporizer (film), the distribution of liquid oxygen plays a vital role in its operation (performance and safety). It is therefore necessary to ensure in all circumstances, a good liquid distribution within each channel. For this, the liquid distribution must be sufficiently uniform between channels. A non-uniform liquid distribution may lead to poor wetting of the waves especially in the lower part of the exchanger and consequently the formation of dry spray deposits. The difficulty is to ensure an equivalent flow of liquid in all channels given the number of channels per passage and body (550 channels / passage, 55 000 channels / body).

 The quality of this liquid distribution depends on a good design and a good dimensioning of the dispenser.

 EP-A-0797065 discloses a vaporizer according to the preamble of claim 1. For the described vaporizer, the liquid enters the passages through openings in a plane and then falls on bars comprising slots to ensure that the liquid flows against the walls of the passages.

 A primary distribution of the liquid (distribution between passages) is obtained by passing it through holes located at the bottom of a pool of liquid located above the channels. A so-called secondary distribution (distribution in each passage) is generally obtained by a waveband with horizontal generator and partial shift.

 Unfortunately, the presence of impurities in the liquid creates a risk of blocking all or part of the holes for the primary distribution. This can result in significant liquid distribution defects between the passages and between the channels.

 The problem to be solved is therefore to ensure a good primary distribution between the passages, while reducing the risk of distribution defects due to solid impurities possibly present in the liquid.

To this end, the solution of the invention relates to a heat exchanger for vaporizing a liquid by heat exchange with a fluid, comprising: a body comprising an assembly of separation plates parallel to a determined direction and defining between them a multitude of passages; and

 a retention tank intended to receive said liquid having a bottom substantially orthogonal to said determined direction, said bottom comprising closure elements and primary distribution elements pierced with holes, adapted to and designed for dispensing said liquid from said tank retention in some of said passages;

 said bottom having a certain extension orthogonal to said determined direction, over at least 80% of said extension, preferably all, each of said primary distribution elements protrudes into said retention tank and each of said holes forms an orifice in said projection and on at least

80% of said extension, preferably 100%, said bottom, with the exception of said projections, develops in a plane and said orifices are situated at a distance of at least two, preferably at least three millimeters, from said plane according to said determined direction.

 Two to three millimeters represent the minimum distance that makes it possible to fully reduce the risk of clogging. There is no proper distance. This depends on the shape of the retention tank. It is necessary that the orifices are in the pool of liquid and that the primary distribution elements do not prevent the free flow of liquid in this pool. It is necessary in particular that the bath keeps the same level (no creation of "cascade" of liquid in the holding tank).

 The liquid in question can be of any kind. In a particular embodiment, it is liquid oxygen resulting from a distillation. This oxygen is usually very pure. The fluid can be of any kind. According to a particular mode, it is nitrogen derived from a distillation. It is usually gaseous before the heat exchange operation. The liquid is heated and vaporizes partially or totally. The fluid cools and condenses partially or totally.

The separating plates are generally parallel and form the armature of the body of the exchanger. They define between them many flat passages, in that they are elongate in a given direction, or longitudinal direction, and in a direction orthogonal thereto. This determined direction is that taken on average by the fluids in the passages when the exchanger is in operation. In normal operation, it is the vertical. The fluids circulate on average vertically in the passages. It is only if we imagine the exchanger oriented differently, for example to transport or store, that this direction determined is no longer the vertical, but becomes an arbitrary direction. It will be roughly horizontal if the heat exchanger is lying on its side.

 The liquid normally flows from top to bottom in some of the flat passages, which form a subset reserved for the liquid to be vaporized. The fluid flows in other passages, normally from top to bottom as well, which form another subset. In general, the passages for the liquid or the fluid alternate, so as to maximize the heat exchange. In principle, fluids circulate in all the passages.

 The liquid is first admitted into a retention tank located above the body of the exchanger (in normal operation). The bottom of this tray is approximately orthogonal to the determined direction, preferably perfectly orthogonal. The bottom is obtained by the meeting of different elements, in the manner of a pavement. These elements include holes pierced elements that serve for the primary distribution of liquid in the flat passages and closure elements, such as the bars that close the passages reserved for the fluid. Separation plates are also generally part of these closure elements; their upper part may constitute a part of the bottom of the holding tank.

 The primary liquid distribution elements are typically in the form of plates or bars and are generally located between the many separation plates. The bottom has a certain extension orthogonal to the determined direction. It is a priori that of the group of plates of separation.

 The primary distribution elements are pierced with numerous holes. These are preferably in the determined direction. They are usually cylindrical. They are usually regularly spaced on each bar. They define openings in their upper part through which the liquid enters from the retention tank towards a subset of flat passages.

At least 80% of the extension of the bottom, preferably all, the primary distribution elements form projections in the direction determined on the side of the retention tank. This makes it possible to raise (when the heat exchanger is in use position) the orifices relative to the remainder of the bottom of the holding tank. Thus, their corking is made much more difficult. The advantage of such a configuration is that any solid particles present in the liquid are less likely to obstruct a given orifice. If they accumulate, they tend to do so at the bottom of the holding tank and not at a given orifice. The solution of the invention is to raise a maximum of these orifices, at least 80%, that is to say to do it on 80%> of the extension of the bottom orthogonally to the determined direction. This extension can be likened to a surface. Preferably, it is done over the entire surface of the bottom.

 In practice, this can be done by sliding the primary distribution elements inwardly of the retention tank before brazing.

 What matters most is that an orifice is raised locally, that is to say relative to the closing elements that surround it.

 Furthermore, according to particular embodiments, the invention may include one or more of the following features:

 said primary distribution elements are plates. This shape makes it possible to slide the primary distribution elements easily before the soldering step during the manufacture of the evaporator.

 wherein the primary distribution elements are pierced with holes, each hole having at least one wall, and the one or more walls being formed in the body of the primary distribution element.

 at least 80% of said extension, preferably all, said orifices have centers located in the same plane orthogonal to said determined direction. This has an advantage during manufacture, by simplifying the arrangement of the elements forming the body of the evaporator and the liquid retention tank. In addition, this makes it possible to have the same hydrostatic liquid pressure at the inlet of the holes and allows a good distribution.

at least 80%> of said extension, preferably all, said primary distribution elements comprise an upper edge constituting a surface which, in a space reference O, Ox, Oy, Oz, where Oz is said determined direction from said body to said retention tank, can be defined by an equation of the form z = f (x, y), f being a continuous function, said surface having one or more edges and all its minimum points on said edges. In the normal operating position, the Oz axis is the vertical upward. A minimum point corresponds to a local minimum of the function f. The upper edge is then a continuous surface that has the properties of a roof vis-à-vis the rain. In the absence of a local minimum located outside the edge of the roof surface, there can be no accumulation of rain on the roof or, similarly, solid particles on the upper edge of the primary distribution element . The risk of capping, already diminished by the elevation of the orifices, is further reduced by this fact. It should be noted that, if the orifices were not raised, this property of the profile would not reduce the risk of clogging.

 at least 80% of said extension, preferably all, said upper edge has a curved profile or bell-shaped or bevel. These profiles have the property described above (form of roof construction) and are simple to achieve. They make it possible to prevent the solid particles from accumulating on the upper edges of the primary distribution elements, thus reducing the risk of clogging the holes ensuring the primary distribution of the liquid.

 - the center of the orifice is at the maximum point of the upper edge

 at least 80% of said extension, preferably all, said primary distribution elements comprise at least one notch intended to allow regular positioning, projecting inwardly of said retention tank, said primary distribution elements. This notch makes it possible to simplify the assembly of the parts, in particular by facilitating the alignment of the latter before soldering.

 at least 80% of said extension, preferably all, said primary distribution elements comprise an upper edge and two notches located on either side of said upper edge, said notches being intended to allow alignment of said distribution elements primary with said closure members, said alignment being achieved by contacting said notches and said closure elements with alignment members, preferably horizontal, so as to ensure that said upper edge protrudes a certain height by relative to said closure elements, said height being determined by the shape of said notches.

 the retention tank is arranged so that the liquid arrives directly in the orifices of the primary distribution elements, without passing through other orifices, the orifice has a circular section.

 the holes pass through the primary distribution elements and have an axis parallel to the determined direction, this axis passing in the center of the primary distribution element. said exchanger further comprises:

. secondary distribution elements comprising at least one wave, preferably horizontal generatrices and partial vertical shift, adapted to and designed to perform a distribution, on the one hand, of said liquid in each passage of said first set of passages and, on the other hand, said fluid in each passage of said second set of passages; and

 . vertical generator waves located in said passages below said secondary distribution means.

 The invention also relates to an installation for separating air by distillation, comprising at least:

 a first distillation column intended to operate under a predetermined pressure;

 a second distillation column intended to operate at a pressure lower than said determined pressure; and

 a heat exchanger adapted to and designed to at least partially vaporize the bottom liquid of said second column by heat exchange with the overhead gas of said first column or air;

characterized in that said heat exchanger is as described above.

 Other particularities and advantages will appear on reading the description below, made with reference to the figures, among which:

 Figure 1 shows a partial front view in section of an exchanger according to the invention. The section passes through the axis of one of the holes 6a of the primary distribution elements 6 of the liquid. The exchanger is shown in its operating position, that is to say vertical.

 FIG. 2 represents an enlargement of one of the elements 6 of primary distribution of the liquid present in FIG.

 In Figure 1, there is shown a vertical section of an exchanger according to the invention. Dashed lines mean that the view is partial; the exchanger continues beyond these features.

 The exchanger 1 is composed of an assembly of parallel vertical plates 3 separated by bars 6, 9 which clog the passages. The plates 3 are parallel to the determined direction 11 which is here to be vertical. They define between them a multitude of flat passages 4 reserved for liquid oxygen and passages 5 reserved for nitrogen gas. The passages of each of types 4 and 5 alternate. The whole is limited by external walls 10.

The elements 3, 6, 9 form the bottom 2a of a holding tank 2 for containing the liquid. The bars 6 are elements designed to achieve the primary distribution of the liquid in the passages 4. They have holes 6a regularly spaced. These holes 6a form orifices 6b on the upper edge of the bars 6. These orifices 6b are at a given height h of the remainder of the bottom 2a composed by the upper parts of the elements 9 and 3. This height is about three millimeters. The elements 6 protrude 6f in the holding tank 2, about three millimeters in the vertical direction.

 This is obtained by shifting the elements 6 relative to the other elements 3 and 9 constituting the bottom 2a of the holding tank 2. During manufacture, the brazing of these elements freezes in this position.

 The passages also include secondary distribution elements 7 for distributing oxygen and nitrogen within each of their respective passages. The passages also include waves 8 for creating a film of liquid on their surface.

 The elements 7 are waves of non-perforated aluminum sheet with horizontal generators (so-called "hard way" arrangement, or "difficult passage", with respect to the flow of liquid oxygen). At regular intervals, each horizontal or pseudo-horizontal facet of the waves is provided with a punctured (not illustrated) shifted upwards by a quarter of a wave pitch. The width of the punctured, measured along a generatrix of the wave, is of the same order as the distance separating each of them from two adjacent punctures located on the same facet.

 The liquid oxygen passes through the holes 6a placed above the lining 7, at a flow rate defined by the passage section of the latter and by the height of the liquid above the orifices 6b. The holes 6a thus ensure a rough predistribution, or primary distribution, of the liquid oxygen on the passages 4. The liquid oxygen thus predistributed leaves on the waves 7, which ensure a fine distribution, or secondary distribution, over the entire horizontal extension of each passage 4. The liquid oxygen thus approaches lower waves to vertical generatrices 8 by dripping in a perfectly uniform manner on all the walls of the passages which are affected to it, that is to say by forming on these walls a continuous film descending.

 At the same time, the nitrogen gas reaches the exchanger through distribution waves (not shown), then flows down the passages 5. In doing so, it progressively gives up heat to the liquid oxygen which is in the adjacent passages 4, so that the oxygen vaporizes and that, simultaneously, the nitrogen condenses.

Nitrogen also passes through elements 7 and 8 similar to those of passages 4.

In Figure 2, there is an enlargement in perspective cavaliere a primary distribution element 6 of the liquid oxygen. This is a plate, or bar, intended for partly to close the top a passage 4 located below the element 6. In fact, the element 6 is pierced with holes 6a cylindrical, vertical, regularly spaced, for passing the liquid oxygen from the retention tank to a passage 4. Two of them have been represented. The holes 6a form orifices 6b on the upper edge 6c of the element 6.

 Element 6 has an upper edge 6c having a bell-shaped profile. In the orthonormal space coordinate R, materialized by an origin O and the axes Ox, Oy and Oz, the latter being vertical upwards, the upper edge is a roof-shaped surface, which can be put into equation in the form z = f (x, y). In this case, f is for example a polynomial in y or a tangent function. The surface and the function f are continuous. The surface has no local minimum outside its edge 6g. The orifices can be considered as part or not of the surface, their edges then constituting, or not, edges of the surface. The criterion according to which the surface must not have a local minimum outside its edges remains valid in both cases.

 The element 6 has two notches 6d and 6e for aligning with the other elements included in the paving forming the bottom 2a of the retention tray 2. All these elements are based on a beam or a bulge orthogonal to the determined direction 11. The two notches form a recess of a height h equal to about three millimeters, ensuring that the element 6 makes a projection 6f of the same height in the in the retention tank 2 (in the direction Oz).

Claims

claims A heat exchanger (1) for vaporizing a liquid by heat exchange with a fluid, comprising:  - a body comprising an assembly of separating plates (3) parallel to a given direction (11) and defining between them a multitude of passages (4, 5); and  a retention tank (2) intended to receive said liquid, comprising a bottom (2a) substantially orthogonal to said determined direction (11), said bottom (2a) comprising closing elements (9, 3) and distribution elements primary (6) pierced with holes (6a), adapted to and adapted for dispensing said liquid from said holding tank (2) in some of said passages (4);  said bottom (2a) having a certain extension (E) orthogonal to said determined direction (11), over at least 80% of said extension, preferably all, each of said primary distribution elements (6) forms a projection (6f) in said holding tank (2) and each of said holes (6a) forms an orifice (6b) in said projection (6f).  characterized in that on at least 80% of said extension, preferably all, said bottom, with the exception of said projections (6f), develops in a plane and said orifices (6b) are located at a distance ( h) at least two, preferably at least three millimeters of said plane in said determined direction (11). 2. Exchanger according to claim 1, characterized in that said primary distribution elements (6) are plates. 3. Exchanger according to any one of claims 1 or 2 wherein the primary distribution elements (6) are pierced with holes, each hole having at least one wall, and the wall or walls being formed in the body of the element. primary distribution. 4. Exchanger according to any one of claims 1 to 3, characterized in that, on at least 80% of said extension, preferably all, said orifices (6b) have centers located in a same plane orthogonal to said direction determined (11). 5. Exchanger according to any one of claims 1 to 4, characterized in that, over at least 80% of said extension, preferably all, said primary distribution elements (6) comprise an upper edge (6c) constituting a surface which, in a reference (R) of space O, Ox, Oy, Oz, where Oz is said determined direction (11) oriented from said body towards said retention tank (2), can be defined by an equation of the form z = f (x, y), f being a continuous function, said surface having one or more edges and all its minimum points on said edges. 6. Exchanger according to claim 5, characterized in that, in at least 80%) of said extension, preferably all, said upper edge (6c) has a curved profile or bell-shaped or bevel. 7. Exchanger according to claim 5 or 6 wherein the center of the orifice (6b) is at the maximum point of the upper edge (6c). 8. Exchanger according to any one of claims 1 to 7, characterized in that, over at least 80%> of said extension, preferably all, said primary distribution elements (6) comprise at least one notch (6d, 6e) intended to allow a regular positioning, projecting inwardly of said retention tank (2), said primary distribution elements (6). 9. Exchanger according to claim 8, characterized in that, over at least 80%> of said extension, preferably all, said primary distribution elements (6) comprise an upper edge (6c) and two notches (6d, 6e ) located on either side of said upper edge (6c), said notches being intended to allow alignment of said primary distribution elements (6) with said closure elements (3, 9), said alignment being obtained by placing in contact said notches (6d, 6e) and said closure elements (3, 9) with alignment elements, preferably horizontal, so as to ensure that said edge upper (6c) protrude (6f) a certain height (h) from said closure elements (3, 9), said height (h) being determined by the shape of said notches (6d, 6e). 10. Exchanger according to one of the preceding claims wherein the holding tank is arranged so that the liquid arrives directly in the orifices (6b) of the primary distribution elements, without passing through other orifices. 11. Exchanger according to one of the preceding claims wherein the orifice (6b) has a circular section. 12. Exchanger according to one of the preceding claims wherein the holes pass through the primary distribution elements and have an axis parallel to the determined direction, this axis passing in the center of the primary distribution element. 13. Exchanger according to any one of claims 1 to 12, characterized in that it further comprises:  secondary distribution elements (7) comprising at least one wave, preferably with generatrices orthogonal to said determined direction (11) and partially offset in said determined direction (11), adapted to and designed to perform a distribution, a on the one hand, said liquid in some of said passage (4) and, on the other hand, said fluid in some of said passages (5); and  generating waves (8) according to said determined direction (11) located in said passages (4, 5) below said secondary distribution means (7). 14. Distillation air separation plant, comprising at least:  a first distillation column intended to operate under a predetermined pressure; a second distillation column intended to operate at a pressure lower than said determined pressure; and - a heat exchanger (1) adapted to and adapted to vaporize at least partially the vessel liquid of said second column by heat exchange with the overhead gas of said first column or air; characterized in that said heat exchanger (1) is as defined in any one of claims 1 to 13.