EP1061318A1 - Finned heat exchanger tube and method of production - Google Patents

Abstract

The tube (1) e.g. of a metal such as copper, having an external diameter of some 5 - 20 mm, has a series of lengthwise inner ribs (2) measuring 0.15 - 1.5 mm high and about 1 mm apart. The ribs have an initial asymmetrical triangular cross-section with straight sides, with the tip of each rib offset to one side of its base. The ribs are subsequently curved by passing an olive (3) through the tube, so that the gaps between the tips of adjacent ribs are smaller than the gaps between the ribs' bases. The shaping of the ribs can be carried out at the same time as fins are crimped to the outside of the tube.

Description

The invention relates to a heat exchange tube, and to its manufacturing process, as well as a heat exchanger incorporating such a tube.

In the field of heat exchangers, it is known, in particular from FR-A-2 706 197, to produce a tube whose internal surface is provided with ribs making it possible to increase the exchange surface between the tube and the fluid which 'it contains and thus improves the heat exchange coefficient with the fluid, whether the latter is single-phase or two-phase. In practice, the coefficient of heat exchange in two-phase phase between a smooth metal tube, such as a copper tube, and a heat transfer fluid, such as di-fluoro-chloromethane, varies between 1500 and 3000 W / m ² / ° C as a function of the heat flux transferred, that is to say as a function of the nature of the fluid in which the tube is immersed, which may be a liquid or a gas. By using ribs internal to the tube, this coefficient can be increased to values between approximately 3,800 W / m ² / ° C and approximately 6,000 W / m ² / ° C depending on the case, which constitutes a significant increase. .

• h_conv est le coefficient d'échange thermique par convection ;
• h _nb est le coefficient d'échange thermique par ébullition et
• n est un nombre compris entre 1 et 3.

However, for at least some applications, the value of this coefficient should be further increased in order to improve the efficiency of a heat exchanger incorporating such tubes. However, the heat exchange coefficient in two-phase can be broken down into the following form h = (h conv not + h nb not ) 1 / n or

• h _conv is the heat exchange coefficient by convection;
• h _nb is the coefficient of heat exchange by boiling and
• n is a number between 1 and 3.

We have been able to show experimentally that the exchange coefficient thermal by boiling is about double the coefficient of heat exchange by convection in the same conditions. So to increase the value of the coefficient global heat exchange of a tube with a fluid, it is important basically increase the value of the exchange coefficient thermal by boiling.

The invention aims to increase the value of the coefficient heat exchange of a tube, for its internal part without as much inconveniently increase the coefficient of friction fluid on the inner surface of the tube or plate, so to limit the pressure losses induced.

In this spirit, the invention relates to an exchange tube thermal provided on its internal surface with ribs capable of be shaped, characterized in that these ribs have, before their shaping, an asymmetrical profile, the top of the profile of these ribs being offset relative to the base of this profile.

Thanks to the invention, the spaces defined between two adjacent ribs of the tube or plate appear, on one side, like a re-entrant cavity, that is to say with a partition inclined inwards, which increases substantially the heat exchange coefficient.

• Pour un flux faible, notamment inférieur à 10 kW/m²,
  • h _conv = ± 2 500 W/m²/°C
  • h _nb = ± 10 000 W/m²/°C.

Indeed, the numerical values for a copper tube, bathed by air and provided with fins or bathed by water and devoid of fins, and a heat transfer fluid of the difluoro-chloromethane type are the following:

• For a low flux, notably less than 10 kW / m ² ,
  • h _conv = ± 2,500 W / m ² / ° C
  • h _nb = ± 10,000 W / m ² / ° C.

• Pour un flux important, notamment supérieur à 20 kW/m²,
  • h _conv = 6 000 W/m²/°C
  • h _nb : 14 000 W/m²/°C

Using the above equation (1) with n equal to 2, we obtain: h = ± (2,500 2 + 10,000 2 ) 1/2 = ± 10 300 W / m 2 / ° C.

• For a large flow, in particular greater than 20 kW / m ² ,
  • h _conv = 6000 W / m ² / ° C
  • h _nb : 14,000 W / m ² / ° C

Using the above equation (1) with n equal to 2, we obtain: h = ± (6,000 2 + 14,000 2 ) 1/2 = 15,000 W / m 2 / ° C

These values are almost twice the values mentioned above.

The shaping of the ribs, which can be obtained during the crimping of external fins of the tube, is facilitated by the fact that the top of their profile is, before this setting shape, offset from its base because the effort to exert is an effort to bend or accentuate their inclination, this effort being significantly less than a crushing effort pure which should be exercised if the top was located at the level from the base of the profile.

The invention makes it possible to adapt, during the shaping of ribs, the geometry of the inner surface of the tube, so its heat exchange coefficient, to the nature of the fluid (s) with which it will be used. The fact that the top of the profile is offset from its base before this shaping allows to effectively control the final geometry obtained in modulating the effort exerted, for example using olives preselected dimensions, when crimping fins external to the tube.

By "reentry cavity" means a cavity whose edges are closed at their opening relative to their background. Such a cavity tends to "trap" the fluid it contains, which promotes the creation of bubbles steam and increased heat exchange.

• les faces latérales du profil avant mise en forme sont sensiblement rectilignes ;
• les faces latérales du profil avant mise en forme sont globalement courbes, les rayons de courbure de deux faces d'une nervure étant différents ;
• les faces latérales du profil avant mise en forme sont globalement courbes, les centres de courbure de deux faces d'une nervure étant décalés.

According to advantageous alternative embodiments of the invention, it can be provided that:

• the lateral faces of the profile before shaping are substantially rectilinear;
• the lateral faces of the profile before shaping are generally curved, the radii of curvature of two faces of a rib being different;
• the lateral faces of the profile before shaping are generally curved, the centers of curvature of two faces of a rib being offset.

Whatever the embodiment considered, one can provide that the top offset is modifiable by overwriting ribs.

According to another approach, the invention can be defined as for a heat exchange tube provided with ribs internal able to be shaped, characterized in that these ribs have, before shaping the ribs, a profile such that all the spaces defined between the ribs have an opening towards the center of the tube angularly offset from at the bottom of these spaces.

Thanks to the invention, the spaces between ribs are as many re-entrant cavities, on at least one side, which promotes all the heat exchanges and allows to reach the above-mentioned heat exchange coefficient values.

According to an advantageous aspect of the invention, the width of the opening between the ribs is less than that from their bottom. This increases the "reentry cavity" effect got.

The invention also relates to a heat exchanger comprising a tube as previously described whose performance is significantly better than that of art prior.

The invention finally relates to a method of manufacturing a tube of the aforementioned type and, more specifically, a method which consists in shaping the ribs by moving, by a radial pressure, the apex of the profile of each rib, which is offset from its base, towards a surface internal of the tube. This process creates, between the ribs, spaces with one side approaching geometry of a reentry cavity, while the intensity of the effort exercised can be precisely controlled.

Advantageously, this crushing is carried out during the crimping of fins on the outer face of the tube. The combination of the two operations gives the function additional sought without significant increase in the price of comes back from the tube.

• la figure 1 est une coupe transversale partielle d'un tube d'échange thermique conforme à l'invention, lors d'un premier stade de fabrication avant mise en forme finale de ses nervures internes ;
• la figure 2 est une coupe analogue à la figure 1 lors d'un second stade de fabrication après mise en forme des nervures ;
• la figure 3 est une coupe longitudinale du tube des figures 1 et 2 lors de l'étape de fabrication de la figure 2, on y a indiqué en I-I et II-II respectivement les plans de coupe des figures 1 et 2 ;
• la figure 4 est une vue analogue à la figure 1, pour un tube conforme à un second mode de réalisation de l'invention ;
• la figure 5 est une vue à plus grande échelle d'un profil de nervure visible à la figure 4 et
• la figure 6 est une vue analogue à la figure 2 pour le tube du second mode de réalisation représenté à la figure 4.

The invention will be better understood and other advantages thereof will appear more clearly in the light of the following description of two embodiments of a heat exchange tube conforming to its principle, given solely by way of 'example and made with reference to the accompanying drawings in which:

• Figure 1 is a partial cross section of a heat exchange tube according to the invention, during a first stage of manufacture before final shaping of its internal ribs;
• Figure 2 is a section similar to Figure 1 during a second stage of manufacture after shaping the ribs;
• Figure 3 is a longitudinal section of the tube of Figures 1 and 2 during the manufacturing step of Figure 2, there are indicated in II and II-II respectively the section planes of Figures 1 and 2;
• Figure 4 is a view similar to Figure 1, for a tube according to a second embodiment of the invention;
• FIG. 5 is an enlarged view of a rib profile visible in FIG. 4 and
• FIG. 6 is a view similar to FIG. 2 for the tube of the second embodiment shown in FIG. 4.

The tube 1 shown in Figure 1 has a generally cylindrical profile, centered on an axis XX '. We note 1 a the internal surface of the tube and 1 b its external surface. On its inner surface 1 a, the tube 1 is provided with two ribs whose profile P, that is to say the section perpendicular to the axis XX 'is substantially triangular, the side faces P ₁ and P ₂ being straight before crimping.

We denote by S the vertex of the profile P, this vertex being the trace an internal edge of the ribs 2 which may be straight or helical depending on the geometry chosen for the ribs.

We denote by R the radius connecting the axis XX 'to the lateral edge L of the base B of each rib 2 closest to the vertex S. In the configuration of Figure 1, that is to say before setting final shape of the ribs 2, the vertex S of each rib is offset from the base B of the profile P, that is to say is located opposite the base B of this profile with respect to the radius Corresponding R.

So when an olive 3 is placed inside the tube to expand it so as to crimp the fins 4, as shown in Figures 2 and 3, the ribs 2 are distorted or "crushed" in such a way that the vertex S of each rib is folded towards the surface of the tube 1, of the side of edge L of each rib, projecting from base B corresponding.

We denote by E the space defined between two contiguous ribs. On the left of each space E in FIGS. 1 and 2, the shape of the rib 2 which adjoins it is such that the space E resembles a re-entrant cavity in which the thermal contact between the fluid which circulates in the tube and the wall 1 internal 1 a of the tube is optimized, in particular during boiling in the case of use with a two-phase fluid.

We denote by d ₁ the width of the bottom F of each space E. We denote by d ₂ the width of the opening O of each space E towards the axis XX '. The opening O of each space E is angularly offset around the axis XX 'relative to the bottom F because of the inclination and the crushing of the ribs 2.

Given the geometry of the ribs 2, the width d ₂ is less than the width d ₁ , which accentuates the fact that the spaces E appear as the re-entrant cavities for the fluid which they contain when using the tube 1 .

The fact that all spaces E are re-entrant cavities increases the heat exchange coefficient of the tube with the fluid flowing through it without generating an increase sensitive to flow turbulence, which would be the case if some spaces E had other geometries, for example open.

In practice, it has been possible to determine that the turbulence of flow in a tube according to the invention results mainly from the intensive releases of vapor bubbles from the spaces E. The mechanical mixing due to the roughness of the surface 1 a n ' does not intervene or intervenes very little in the generation of this turbulence. This mixing can therefore be limited, which consequently limits the pressure drops.

In the second embodiment of the invention shown in Figures 4 to 6, elements similar to those of the embodiment of Figures 1 to 3 bear identical references. This embodiment differs from the previous one essentially in that the lateral faces P ₁ and P ₂ of the profile P before crimping are generally curved, the radii of curvature R ₁ and R ₂ of these two faces being different and the centers of curvature C ₁ and C ₂ are offset, as shown more clearly in Figure 5. as before, the crimping of fins 4 by means of the olive 3 allows to fold the profile of apex S P of the ribs 2 to the inner surface 1a of the tube 1, so that the spaces E between the ribs, in which an intense heat exchange takes place between the fluid and the tube in use, appear as reentrant cavities.

The invention is applicable to all types of tubes, they are formed "flat", that is to say shaped, rolled then welded, or internally grooved in a conventional manner. A tube produced by the invention may or may not include external cooling fins and can be used with a two-phase or single-phase fluid.

Examples

Particularly interesting results can be obtained with a tube whose external diameter varies between approximately 5 mm and approximately 20 mm and comprising numerous ribs of height ranging between 0.15 and 1.5 mm and distant from approximately 1 mm, it that is to say the distance d _{2 of which} is equal to approximately 1 mm. If such a tube is used with water, it has a relatively small number of ribs, between 10 and 20, the height of which is preferably equal to approximately 1 mm. In this case, the heat exchange coefficient obtained can be of the order of 10,000 W / m ² / ° C. Profile P is triangular, as shown in Figures 1 and 2.

For a use with a heat-transfer fluid, for example a coolant such as di-fluoro-methane, a tube is used, the height of the ribs of which remains less than 0.5 mm whereas their number is important because the surface tension of the di -fluoro-methane is weaker than that of water. As before, the heat exchange coefficient obtained can be of the order of 10,000 W / m ² / ° C. In this case, the profile P is curved, as shown in Figures 4 to 6.

A heat exchanger comprising tubes such as previously described is more effective than those of art because its overall heat exchange coefficient is superior.

Claims (10)

Tube (1) heat exchange provided on its inner surface (1 a) of ribs adapted to be shaped, characterized in that all of said ribs (2) have, before forming, a profile (P) asymmetrical, the top (S) of said profile of said ribs being offset relative to the base (B) of said profile. Tube according to claim 1, characterized in that the lateral faces (P ₁ , P ₂ ) of said profile (P) before shaping are substantially straight. Tube according to claim 1, characterized in that the lateral faces (P ₁ , P ₂ ) of said profile (P) before shaping are generally curved, the radii of curvature (R ₁ , R ₂ ) of the two faces (2) of a rib being different. Tube according to claim 1, characterized in that the lateral faces (P ₁ , P ₂ ) of said profile (P) before shaping are generally curved, the centers of curvature (C ₁ , C ₂ ) of two faces of a rib (2) being offset. Tube according to one of claims 1 to 4, characterized in that the offset of said vertex (S) is modifiable by crushing of said ribs (2). Heat exchange tube (1) provided with ribs internals capable of being shaped, characterized in that said ribs (2) have, before forming the ribs, a profile (P) such that all the spaces (E) defined between said ribs have an opening (O) towards the center (XX ') of the tube angularly offset from the bottom (F) of said spaces. Tube according to claim 6, characterized in that the width ( d ₂ ) of said opening (O) is less than the width ( d ₁ ) of said bottom (F) of said spaces (E). Heat exchanger, characterized in that it comprises at least one tube (1) according to one of the preceding claims. Method of manufacturing a heat exchange tube (1) provided with ribs capable of being shaped, characterized in that it consists in shaping said ribs (2) by moving, by means of a radial pressure , the top (S) of the profile (P) of each rib, which is offset relative to its base (B), in the direction of an internal surface (1 a ) of said tube. Method according to claim 9, characterized in that said crushing is carried out during the crimping of fins (4) on the external face ( 1b ) of said tube (1).

Images