EP2379977B1 - Heat exchanger comprising tubes with grooved fins - Google Patents

Description

The invention relates to a tube heat exchanger comprising finned tubes, in which the tubes extend in a certain axial direction and are provided with heat exchange fins, each fin having a heat exchange surface. surrounding a tube which extends in a certain radial direction relative to the tube and which is structured in relief to form grooves spaced relative to each other in the radial direction. Such an exchanger is known from the document US 2008/0023180 A1 , see in particular the Figures 5 and 6 . Such an exchanger corresponds to the preamble of claim 1.

The invention applies more particularly to a tube heat exchanger using air as a secondary exchange fluid such as aero-refrigerant, aero-condenser, aero-heater or aero-evaporator type equipment, used respectively for cooling, condensing, reheating and evaporation of a fluid, in particular in refining processes, gas treatment and compression plants, gas liquefaction units, coal synthesis units and gas, power generation facilities, regasification units, or any other fluid treatment facility.

In general, such equipment comprises a main heat exchanger equipped with a bundle of external finned tubes in which circulates the fluid to be cooled, condensed, heated or evaporated, as well as distribution and distribution manifolds. fluid between the tubes. In particular, the fluid is cooled in the outer finned tubes by heat exchange with a second fluid circulating around the external tubes and fins, including ambient air. For this, a circulation or forced ventilation of ambient air is provided by fans positioned either below (what is called a forced draft) or above (so-called induced draft) tubes exchanger .

In general, the ambient air is pulsed through the finned tube bundle at a relatively low frontal speed of between 1.5 and 4 meters per second (m / s). At such speeds and for the geometric configurations considered (in particular air passage sections, space between two fins or two consecutive tubes), the flow regime of the ambient air is generally laminar with some local turbulence. , which is characterized by heat exchanges with the relatively small external fins. The areas of the heat exchanger where heat exchange is most important are the leading edges of the fins and tubes in the direction of the flow of air. Thus, because of the structure of the flow and the exchanger, the zones of the tubes located at the rear of the tubes in the direction of the flow of air are almost unexploited for heat exchange. These so-called recirculation zones of the exchanger are characterized by recirculation of the air which generates losses of charge and which do not allow good cooling of the fin.

Patent document is known US-2008023180 a fin for aero-refrigerant tube which has on the surface a relief with cavities or grooves formed by mechanical deformation of the fins. Such cavities or grooves make it possible to increase the heat exchange between the air and the fin thanks to creating turbulence while increasing pressure losses. In particular, concentric grooves 42 of semi-cylindrical section are formed on each fin.

Patent document is also known WO 2007/147754 a fin for a heat exchanger tube equipped with airflow deflectors in the form of projecting surfaces which modify the structure of the air flow in order to improve heat exchanges between the air and the fin . These surfaces are in the form of rectangular or triangular cutouts in the fin. However, the heat exchangers being mostly outside and the ambient air not being filtered, the cuts made in the fin can be sources of fouling due to dust, insects, etc ... that come to obstruct the cuts.

The object of the invention is to provide a grooved fin structure for a heat exchanger tube which makes it possible to obtain an increase in the heat exchange between the air and the fluid circulating in the tube, without deteriorating the loss of heat. charge.

For this purpose, the subject of the invention is a tube heat exchanger comprising finned tubes, in which the tubes extend in a certain axial direction and are provided with heat exchange fins, each fin having a heat exchange surface surrounding a tube which extends in a certain radial direction relative to the tube and which is structured in relief to form grooves spaced from each other in the radial direction, and wherein the grooves of a fin have dimensions different that decrease as one moves away from the tube in the radial direction so as to provide a guide for a fluid around the tube.

The main advantage of such a staggered conformation of the relief of the fins is that it makes it possible to better guide the flow of air at the rear of the tubes in the radial direction of the tubes (according to the direction of the flow which arrives on the tubes ). By using tubes with external fins according to the invention, it is thus possible to greatly reduce an air recirculation zone at the rear of the tubes in the direction of the flow of air, which is normally important when uses flat-profiled (flat-profile) finned tubes. Thus, the stepped relief surface guiding the air at the rear of the tubes reduces the areas of recirculation where the heat exchange is bad and therefore to take better advantage of the surface of the fins. In this way with a fin according to the invention, the gain obtained in terms of thermal performance can be very important.

According to certain features of the exchanger according to the invention, the grooves of a fin may have different depths and widths which decrease as one moves away from the tube in said radial direction. Each fin may have a thickness which decreases as one moves away from the tube in said radial direction. The grooves of a fin may be spaced from each other in a concentric shape pattern or in an elliptical shape pattern. The grooves of a fin may be very close to one another, that is to say joined. The grooves may be disposed on both sides of the fin. Each fin can be rolled up helically around the tube or where the fins may be in the form of disc.

• la figure 1 montre schématiquement en coupe un échangeur de chaleur ;
• la figure 2 est une vue en plan d'une ailette selon l'invention ;
• la figure 3 est une vue en coupe partielle radiale selon l'axe III-III de la figure 2 d'un tube avec deux ailettes selon l'invention;
• la figure 4 est une vue en coupe partielle radiale selon l'axe III-III de la figure 2 d'un tube avec deux ailettes selon l'invention dans un autre mode de réalisation ;
• la figure 5 est une vue en plan d'une ailette selon l'invention dans encore un autre mode de réalisation ;
• la figure 6 est une vue en coupe radiale selon l'axe III-III de la figure 2 d'un tube muni de plusieurs ailettes selon l'invention ;
• la figure 7 est une vue en coupe radiale d'un ensemble de tubes avec ailettes à profil plat montrant des lignes de courant dans un plan entre deux ailettes obtenues par simulation numérique ;
• la figure 8 est une vue en coupe radiale d'un ensemble de tubes avec ailettes selon l'invention montrant des lignes de courant obtenus par simulation numérique ;
• la figure 9 représente schématiquement un graphique représentatif de la perte de charge en fonction de la vitesse frontale de l'air arrivant sur une ailette selon l'invention et sur une ailette à profil plat;
• la figure 10 représente schématiquement un graphique représentatif de la puissance échangée en fonction de la vitesse frontale de l'air arrivant sur une ailette selon l'invention et sur une ailette à profil plat.

The present invention will be better understood and other advantages will appear on reading the detailed description of some embodiments taken as non-limiting examples and illustrated by the appended drawings in which:

• the figure 1 schematically shows in section a heat exchanger;
• the figure 2 is a plan view of a fin according to the invention;
• the figure 3 is a radial partial sectional view along the axis III-III of the figure 2 a tube with two fins according to the invention;
• the figure 4 is a radial partial sectional view along the axis III-III of the figure 2 a tube with two fins according to the invention in another embodiment;
• the figure 5 is a plan view of a fin according to the invention in yet another embodiment;
• the figure 6 is a radial sectional view along the axis III-III of the figure 2 a tube provided with several fins according to the invention;
• the figure 7 is a radial sectional view of a set of tubes with flat profile fins showing current lines in a plane between two fins obtained by numerical simulation;
• the figure 8 is a radial sectional view of a set of tubes with fins according to the invention showing current lines obtained by digital simulation;
• the figure 9 schematically represents a representative graph of the pressure drop as a function of the frontal velocity of the air arriving on a fin according to the invention and on a flat-profile fin;
• the figure 10 schematically represents a representative graph of the power exchanged as a function of the frontal speed of the air arriving on a fin according to the invention and on a flat profile fin.

On the figure 1 , there is shown a heat exchanger 1 comprising a tube bundle 2 with finned circular section disposed in several substantially parallel superimposed rows extending in an axial direction A in which circulates a fluid to be cooled between an inlet B and an outlet C fluid, and around which circulates a flow of pulsed ambient air from the bottom upwards in the direction indicated by the arrows D, transversely to the tubes 2, by fans 3 positioned above the heat exchanger 1. The flow of fluid is here divided into three passage sections or passes 2a, 2b, 2c successive schematically represented on the figure 1 to improve the cooling of the fluid. A heat exchanger 1 thus generally comprises between three and eight rows of superposed tubes 2 arranged staggered or aligned with respect to the fluid flow direction in the tubes 2 as indicated by the arrows F.

The tubes 2 are provided with outer radial annular fins 4 substantially perpendicular to the tube 2 and substantially parallel to each other, favoring the heat exchange between the ambient air and the fluid, as well as guiding the flow of air towards the rear of the tubes. tubes 2 in the axial direction, as this will be described below. In general, the outer fins 4 make it possible to increase the external heat exchange area by a factor of between 15 and 25 with respect to the surface of a similar tube 2 without fins. Such a surface increase makes it possible to increase the heat exchange, but also generates losses which are in particular compensated by the use of efficient fans.

For the sake of clarity, we have shown on the figure 1 a few fins 4 spaced from each other on a tube 2, it is obvious that fins 4 are preferably arranged all along all the tubes 2 of the exchanger 1. Moreover, the shape and size of the outer fins 4 may vary from one tube to another of the bundle of tubes 2. The configurations of tubes 2 outer fins 4 are not necessarily homogeneous within a bundle of tubes 2, in particular the diameters of the tubes 2 may vary.

We have shown on the figure 2 around a tube 2, a fin 4 according to the invention with a radial surface structured in relief 5 to form grooves 5a, 5b, 5c spaced from each other in a certain radial direction E by a fin portion 8 substantially flat annular. The grooves 5a, 5b, 5c of the fin 4 have different dimensions which decrease as one moves away from the tube 2 in such a way as to guide the flow of ambient air around the tube 2 in the axial direction A. More precisely, the grooves 5a, 5b, 5c of a fin 4 have respective respective depths p1, p2, p3 in the axial direction A and respective widths 11, 12, 13 in the radial direction E, respectively. width and depth grooves decreasing as one moves away from the tube 2, from an inner edge 4b of the fin 4 attached to the tube 2 to an outer peripheral edge 4a free of the fin 4. As can be seen better on the figure 3 the innermost groove 5a is the uppermost and widest of the grooves, the outermost groove 5c is the lowest and the smallest and the middle groove 5b is of intermediate height and width.

Preferably, the number of grooves 5a, 5b, 5c on a fin 4 is between two and four, but other grooves may be added depending on the application. On the figure 3 the raised surface 5 consists of three circular grooves 5a, 5b, 5c arranged in a concentric pattern and centered around the tube 2. Adjacent fins 4 may have concentric grooves 5a, 5b, 5c which are respectively in alignment axial (the fins 4 have the same raised surface 5 and thus a groove 5a, 5b, 5c of a fin 4 is in axial alignment with the corresponding groove of the other fins 4 on the tube 2). On the figure 3 , the concentric adjacent grooves 5a, 5b, 5c of a fin 4 are separated (disjoint) radially from each other by annular planar flat portions 8. These annular portions 8 may have in the radial direction E the same width d1, d2 or different widths d1, d2 according to a variable diagram, d1, d2 being for example between 1 and 5 mm. For example, the portion widths decreases going from the tube 2 to the outer peripheral edge 4A or vice versa. It is also possible to provide adjacent grooves which are contiguous and in this case the width of the separation portions 8 is very small (less than 1 mm).

For simplicity of manufacture, a tube 2 has fins 4 of the same configuration over its entire length. But in a heat exchanger 1, one can provide tubes 2 with different configurations of fins 4. For example, one can have a tube 2 in which the fins 4 have adjacent grooves 5a, 5b, 5c whose widths d1, d2 of separation portion 8 are increasing towards the outer peripheral edge 4A, and an adjacent tube 2 in which the fins 4 have adjacent grooves 5a, 5b, 5c whose widths d1, d2 of the separation portion 8 go inversely decreasing towards the outer peripheral edge 4A.

On the fin 4 of the figure 3 , the grooves 5a, 5b, 5c are formed on the same face 4c of the fin 4, that is to say oriented in the same direction relative to the fin 4. The figure 4 shows another embodiment of a fin 4 according to the invention in which grooves 5d, 5e, 5f are oriented on either side of the fin 4, that is to say they are arranged alternately on two faces 4c, 4d of the blade 4 opposite, which can confer a better mechanical strength with respect to the grooves 5a, 5b, 5c.

We have shown on the figure 5 another embodiment of a fin 4 according to the invention in which the concentric grooves 5a, 5b, 5c have been replaced by grooves 6a, 6b, 6c arranged according to an elliptical shape diagram 4. Such grooves 6a, 6b 6c elliptical make it possible to take advantage of the phenomenon of guiding the air through the grooves while limiting the increase in the associated pressure drop. The advantage of this solution is an increase in performance gain for similar conditions of use, ie iso speed and even pressure drop.

The outer fins 4 can be made from an aluminum strip 7, or even another heat-conducting material, helically wound in the axial direction A around each tube 2, as shown schematically in FIG. figure 6 . Note that the fins 4 are here very slightly inclined relative to the tube 2 and the direction A, as indicated by the arrow 4e, this inclination being small because the fins 4 are very close to each other, so that we can consider that the fins 4 are almost perpendicular to the tube 2. It is also possible to make a tube 2 with fins 4 more inclined relative to the axial direction A of the tube 2. Another embodiment of an outer fin 4 is formed by means of a series of rotating disks. The attachment between the fin 4 and the tube 2 may be made by embedding the fin 4, for example in a groove previously made on the periphery of the tube 2 (not shown), or by winding the fin 4 to the base of which a folding is achieved and crimping on the tube 2 for example knurled. The fin 4 can also be obtained by forming or deformation of an aluminum tube attached to the tube 2. The fin 4 can also be made using stacked disks.

As visible on the figure 3 , the fin 4 has a thickness which decreases as one moves away from the tube from the inner edge 4b the fin 4 towards its outer edge 4a. Advantageously, the thickness e1 of the fin 4 at its outer edge 4a may be between about 0.15 and 0.4 millimeters (mm) and the thickness e2 of the fin 4 at its inner edge 4b can be between about 0.4 and 1 mm.

The grooves 5a, 5b, 5c have respective depths p1, p2, p3 of between approximately 0.4 and 1.5 mm, as well as respective widths 11, 12, 13 at the base of the groove of between approximately 1 and 4. mm, the grooves 5a, 5b, 5c having different heights and widths so as to obtain the stepped relief decreasing away from the tube 2 such that p1> p2> p3 and 11> 12> 13.

The fin 4 according to the invention has a length H of between approximately 10 and 20 mm and preferably between approximately 12 and 18 mm. The pitch P between two consecutive fins along the tube 2 is between about 2.2 and 3.5 mm and preferably between about 2.5 and 3.2 mm, or generally less than the conventional spacing between two blades profiled consecutive dish.

Generally, a heat exchanger 1 comprises a bundle of tubes 2 resting on a steel structure (not shown) and formed of about 50 to 300 tubes 2 of diameter between about 15 millimeters and 55 millimeters, the width of the exchanger heat 1 being between 0.3 meters and 5 meters, and its length between 8 meters and 18 meters.

The tubes 2 may be composed of steel, for example stainless steel or carbon steel or a high-alloy steel, such as incoloy, the choice of the material of the tubes 2 being a function of the transported fluid which may be aggressive. , and operating conditions. The outer fins 4 are generally made of aluminum, but may also be stainless steel, or any other heat conducting material.

The Figures 7 and 8 present current lines (obtained by numerical simulation) of the ambient air flowing in the direction D around several tubes 2 of the heat exchanger 1 in a plane M substantially perpendicular to the tubes 2 and located centrally between two fins 4 consecutive as indicated on the figure 1 and on the figure 3 . More specifically, the figure 7 presents the case of a flat-profile fin and the figure 8 presents the case of a fin 4 according to the invention having concentric grooves 5a, 5b, 5c. As can be seen on the figure 7 , a fluid recirculation zone Z1 is located at the rear of the tubes 2 in the direction of the flow D of the air in which the heat exchange is bad. On the other hand, as visible on the figure 8 , there is a very large decrease in the recirculation of the fluid in a zone Z2 located at the rear of the tubes 2 in the direction of the flow D of the air. This is due to the grooves 5a, 5b, 5c of the fins 4 which guide the flow of air towards the rear of the tubes 2 in the axial direction, which makes it possible to reduce the areas where the heat exchange is bad and thus to draw a better profit of the fins 4.

On the figure 9 the pressure drop is represented as a function of the frontal air speed on the tubes 2 for flat-profile tubes 2 (curve 9A) and for tubes 2 with fins 4 according to the invention with grooves 5a , 5b, 5c concentric (curve 9B). In general, there is an increase in the pressure drop that is caused by the raised surface 5 or the grooves 5a, 5b, 5c of the fins 4. This increase in the pressure drop can be compensated by spacing the fins 4 each other along the tube 2. For these calculations and the following, the pitch P between two consecutive fins is different according to whether the surface of the fins is raised or not: 2.54 mm in the case of the flat profile vane and 3 mm for the fin 4 according to the invention with grooves 5a, 5b, 5c concentric. In this way, as can be seen on the figure 9 the increase in pressure drop caused by the raised surface 5 remains very low.

We have shown on the figure 10 the power exchanged as a function of the frontal velocity of the air on the tubes 2 for flat-profile tubes 2 with fins (curve 10A) and for tubes 2 with fins 4 according to the invention with concentric grooves 5a, 5b, 5c (curve 10B) and for pitch P between fins as defined above. The exchanged power of the heat exchanger 1, that is to say the gain obtained, increases by about 10 to 25% depending on the frontal air speed, which corresponds to an increase in performance per unit of heat. exchanger length of between 2 and 10%.

In addition, the spacing of the fins 4 along the tube 2 reduces the amount of material used to make the fins, which compensates for the increase in material caused by the realization of the raised surface 5 on the fin 4 by modifying the surface of the fin, and decreases the amount of material used to achieve a saving of the order of 3 to 6% per meter.

Claims (9)

1. Heat exchanger (1) with tube (2) comprising finned tubes, wherein the tubes extend according to a certain axial direction and are provided with heat exchange fins (4), each fin having a heat exchange surface surrounding a tube that extends according to a certain radial direction (E) relative to the tube and which is relief structured to form grooves (8a, 5b, 5c, 6a, 6b, 6c) spaced apart from one another following said radial direction, characterized in that the grooves of a fin have different dimensions that decrease on moving away from the tube following said radial direction so as to form a guide for a fluid around the tube.
2. Heat exchanger according to claim 1, wherein the grooves of a fin have different depths and widths that decrease on moving away from the tube according to said radial direction.
3. Heat exchanger according to claims 1 or 2, wherein each fin has a thickness that decreases on moving away from the tube according to said radial direction.
4. Heat exchanger according to one of the claims 1 to 3, wherein the grooves of a fin are spaced apart from each other according to a concentric shaped pattern.
5. Heat exchanger according to one of the claims 1 to 3, wherein the grooves of a fin are spaced apart from each other according to an elliptical shaped pattern.
6. Heat exchanger according to one of the claims 1 to 5, wherein the grooves of a fin are spaced apart from each other in a joined manner.
7. Heat exchanger according to one of the claims 1 to 6, wherein each fin has two opposite faces (4c, 4d) acting as heat exchange surface, said grooves being arranged on the two faces of the fin.
8. Tube heat exchanger according to one of the claims 1 to 7 wherein the tubes are each fitted with a heat exchange fin helicoidally wound around the tube.
9. Heat exchanger according to one of the claims 1 to 7, wherein the tubes are each fitted with a disc shape fin.

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