EP0058628B1 - Heat exchanger with capillary structure for refrigerating machines and/or heat pumps - Google Patents

Description

The present invention relates to heat exchangers using the energy supplied during the change of liquid / vapor or vapor / liquid phase of certain fluids and, more particularly, those used in refrigeration machines and / or heat pumps.

In machines of this type, the heat exchangers are either constructed and dimensioned with a view to playing a specific predetermined role, either as an evaporator or as a condenser, or else produced so as to serve both as an evaporator and as a condenser.

If the heat exchanger is designed to be used exclusively as an evaporator, it is underutilized because it is more than half full of liquid in general and _^2/3 at most, to '' avoid "liquid blows" to the compressor.

^Z / 3 only, or even half, of the evaporator exchange surface when the fluid flow decreases, are therefore used. In addition, since the exchange takes place at reduced cold surfaces, the formation of frost is favored in the case of air evaporators, which obviously affects performance.

In the case of condensers designed essentially as such, it is necessary, for technological reasons, that the vapors arriving at the condenser are completely condensed at the outlet. This results in oversizing increasing the production costs of these exchangers.

Finally, currently, the use of reversible refrigeration machines requires the production of exchangers which are a compromise between evaporator and condenser, since they have to operate both in evaporator and in condenser, and which, therefore, in addition to the drawbacks of specific exchangers recalled above, have poor performance.

In order to improve the heat exchange yields, it has already been imagined to line the internal wall of the exchanger tubes with a capillary structure in order to improve the distribution of the liquid phase on the internal wall of the tubes.

According to FR-A 990 531, in particular, according to the preamble of claim 1, a heat exchanger for refrigeration machines and / or heat pumps, comprising a tubular heat exchange network which includes exchanger tubes in which, in service, a mixture of refrigerant and lubricating oil enters at one end and exits at the other end, a capillary structure being applied against the internal wall of at least some of these exchanger tubes by permeable means .

The purpose of the capillary structure is to completely wet the walls of the tubes, even in the most frequent case where the tubes are not full of liquid.

According to FR-A 990 531 the capillary structure is a metallic cloth applied against the internal wall of the exchanger tube by a strand of piano wire.

It has been found according to the invention that such a structure traps the lubricating oil contained in the mixture. This has the double disadvantage of depriving the compressor of this oil and of reducing the efficiency of the heat exchangers.

The purpose of the present invention is to overcome these two major drawbacks simultaneously by proposing a new capillary structure for heat exchangers allowing heat exchanges over the entire useful surface of the exchangers with a significantly increased efficiency, while not impeding the circulation of lubricating oil.

According to the invention, this object is achieved as soon as the capillary structure is constituted by a set of free and smooth fibers of suitable material, substantially rectilinear and parallel to the axis of the exchanger tubes concerned, regularly distributed in rings and plated over all the length against the wall of said tubes.

Thanks to this arrangement, the capillary effect along the wall of the tubes is maintained, but, instead of being substantially immobile, the liquid concerned by this capillarity moves longitudinally in the tube between the fibers. Displacement also concerns the oil which finds no obstacle allowing it to park. This movement is allowed on the one hand by the longitudinal arrangement of the fibers, and on the other hand by the permeability of the means applying the fibers against the internal wall of the tubes. Thanks to this permeability, the current in the central part of the tubes has a driving action in the same direction on the capillary fluid. According to FR-A 990 531, the piano wire certainly provides permeability between the capillary space and the central current, but this permeability does not allow, according to the prior document, any entrainment in the capillary space.

Certainly from US-A 4,018,269, an apparatus known as a “heat pipe” is known, in connection with which it is pointed out that the longitudinal fibers are more advantageous means of capillary action than the tissues because they allow easier flow. In a heat pipe, a refrigerant vaporizes at one end, forms in the tube a central current of vapor towards the other end, condenses and gives up heat at this other end, after which it returns to the first end by the capillary structure arranged in a peripheral manner.

Thus, the latent heat of vaporization stored by the fluid at one of the ends is returned by the latter at the other end. In this sense, a heat pipe is indeed a heat exchanger.

However, the heat exchange takes place in the longitudinal direction, that is to say from one end to the other of the tube. Between its two ends, the heat pipe is essentially adiabatic. In other words, the fibers provided according to US-A 4,018,269 purely and simply have a role in transporting the fluid and do not provide any exchange. Regarding to transport a fluid, it is quite clear that the capillary structure must offer the smallest possible resistance to the displacement of the liquid. It is therefore natural that US-A 4,018,269 recommends longitudinal fibers rather than a fabric.

Contrary to all this, the invention relates to a heat exchanger for refrigeration machines in which the fluid enters through one end of the exchanger tubes and exits through the other end instead of performing round-trip cycles in the sealed tube. of a heat pipe.

In such exchanger tubes, the general direction of the heat exchange is radial, that is to say that heat is exchanged between the interior and the exterior of the tube, and not between two exterior media by l 'through a heat pipe type element.

• - permettre au fluide de rester contre la paroi du tube;
• - malgré cela permettre que le fluide ainsi retenu contre la paroi soit en communication avec le courant central pour qu'il puisse y avoir changement de phase entre le courant central et le courant capillaire; et
• - ne pas entraîner le fluide trop rapidement de façon que l'échange avec le milieu extérieur puisse avoir lieu.

According to the invention, the fibers not only allow the circulation of the fluid but must at the same time ensure exchange, that is to say:

• - allow the fluid to remain against the wall of the tube;
• - despite this allowing the fluid thus retained against the wall to be in communication with the central current so that there can be a change of phase between the central current and the capillary current; and
• - do not drive the fluid too quickly so that the exchange with the external environment can take place.

Other features and advantages will emerge from the description below.

• - la figure 1 est une vue schématique en coupe d'un échangeur conforme à l'invention, pouvant fonctionner en évaporateur ou en condenseur;
• - La figure 2 est une vue en coupe transversale d'un élément tubulaire de l'échangeur de la Fig. 1;
• - La figure 3 représente une coupe longitudinale partielle d'un élément tubulaire de l'échangeur de la Fig. 1;
• - La figure 4 est une vue en coupe partielle d'un mode de réalisation préféré d'une structure capillaire selon l'invention;
• - La figure 5 représente une variante de réalisation du dispositif de la Fig. 4; et
• - La figure 6 est une vue en coupe axiale d'un dispositif permettant la réalisation et la mise en place d'une structure capillaire suivant la Fig. 4 ou 5 associé à une tête d'extrusion du tubes dans laquelle est intégré ledit dispositif.

Aus attached drawings, given by way of non-limiting examples:

• - Figure 1 is a schematic sectional view of an exchanger according to the invention, which can operate as an evaporator or a condenser;
• - Figure 2 is a cross-sectional view of a tubular element of the exchanger of FIG. 1;
• FIG. 3 represents a partial longitudinal section of a tubular element of the exchanger of FIG. 1;
• - Figure 4 is a partial sectional view of a preferred embodiment of a capillary structure according to the invention;
• - Figure 5 shows an alternative embodiment of the device of FIG. 4; and
• - Figure 6 is an axial sectional view of a device allowing the production and installation of a capillary structure according to FIG. 4 or 5 associated with an extrusion head of the tubes in which said device is integrated.

The exchanger of FIG. 1 comprises two manifolds 1 and 2 connected by a network of heat exchange tubes 3, parallel and identical rectilinear, made of a material which is a good thermal conductor such as copper for example.

According to the invention, all of the tubes 3 have a capillary annular structure 4 over their entire length, as does the collector of the liquid phase (collector 1 in FIG.) Of the heat transfer fluid (or refrigerant).

Figs. 2 and 3 illustrate an embodiment of said capillary annular structure 4 according to which this structure consists of a number of identical individual fibers 5, smooth, straight and of constant diameter. These fibers are free from each other while being in contact with each other and with the internal wall of the tube (1 or 3) and confined in an annular space by any suitable means. The distribution of the fibers 5 is uniform, the thickness of the annular layer being in a proportion determined relative to the diameter of the tube in order to have an appropriate circulation and flow of the fluid in the liquid phase in the conduits 1 and 3.

The fibers 5 line the internal wall of the latter over their entire useful length and are applied against the wall of the tubes, for example in known manner, by a helical element 6 (FIG. 3) forming a spring, engaged in the central part of the tubes (3, 1).

This helical element 6 could of course be replaced by any other member capable of pressing the fibers 5 against the wall such as rings for example.

The fibers 5 and the holding members 6 are made of metallic or plastic material, or the like, compatible with the nature of the fluid circulating in the exchanger.

The diameter of the fibers 5 can vary to the extent that the interstitial spaces between the fibers make it possible to obtain the capillary effect sought for the coolant or refrigerant considered.

The fibers 5 arranged in the collector 1 ensure a uniform distribution of the liquid towards the exchanger tubes 3, while the fibers 5 of the latter allow the liquid to "wet" absolutely the entire useful surface of the tubes 3 and therefore ensure maximum heat exchange between the liquid phase fluid in contact with the internal wall of the tubes 3 and the external fluid.

On the other hand, due to the geometry of the interstitial spaces both between fibers 5 and between these and the internal wall of the tubes 3 and the absence of any obstacle transverse to the axial flow of the oil of lubrication is not retained and can flow freely.

There is therefore no risk of the system compressor running out of oil.

When the working fluid arrives in the liquid phase at 1 and leaves again in the gas phase at 2, the exchanger works as an evaporator and cools the fluid (for example air) circulating in 7 between the tubes 3.

The fluid circulating in the tubes 3 is then called refrigerant.

If, on the contrary, the working fluid arrives in the gaseous phase at 2 and leaves by 1 in the liquid phase, the fluid is heat-transferable and transfers part of its calories to the fluid circulating in 7. When the latter is air, we can advantageously provide the tubes 3 externally with fins of aluminum or other material that is a good conductor of heat. The exchanger then works as a condenser.

The liquid phase is distributed over the tubes 3 as it is formed and is evacuated and the capillary structure 4 thus ensures good distribution of the temperature and therefore improves the heat exchanges.

Thus, an exchanger such as that of FIG. 1 working in an evaporator has a much higher efficiency than that of traditional evaporators, the filling of the exchange tubes in liquid phase is usually of the order of half and at most ^z / 3 while, thanks to the capillary structure 4 according to the invention in the evaporator arranged according to FIG. 1, the entire internal surface of the exchange tubes 3 is in contact with the liquid phase, uniformly, thanks to the capillary wicking effect.

It therefore therefore eliminates, in the case where the fluid flowing at 7 is air, the premature appearance of cold spots causing the formation of frost on the tubes 3. It has thus been found that with an evaporator according to In the invention, the formation of frost only appeared for a temperature of the air flowing in 7 4 to 5 ° C. lower than that at which frost usually appears on conventional evaporators.

The exchanger shown in FIG. 1 operating either as an evaporator or as a condenser, improves the coefficient of performance of reversible machines in substantial proportions (of the order of 30 to 40%).

Of course, the invention is not limited to the embodiment shown and described above, but on the contrary covers all variants, in particular those concerning the nature of the material constituting the fibers 5, their sizing, their distribution along the inner wall of the tubular exchange members and collectors of the working phase liquid phase as well as the means for pressing or containing said fibers against the inner wall of said tubular members.

The sheet of fibers 5 may comprise only a single layer of fibers more or less parallel to the axis of the tube and contiguous or not.

Fig. 4 shows a particularly interesting embodiment by its simplicity and its efficiency. In this FIG., A sheet of fibers 5 is shown, consisting of a single layer of parallel and non-contiguous fibers, said sheet being pressed against the internal wall of the tube 3 by an elastic system constituted by a sheet of wires 17 of spring steel (or material likely to have the same elasticity characteristics). The wires 17 are parallel, non-contiguous and wound in a helix.

The fibers 5 are of axis substantially parallel to the axis of the tube 3, while the wires 17 form a more or less significant acute angle with the fibers 5.

The propeller produced by the wires 17 does not comprise a single wire but several in parallel, the bundle of wires being wound in a helix. It is therefore possible to easily vary the inclination between the fibers 5 and the wires 17 while having a tight network of wires 17 in contact at numerous points with the sheet of fibers 5.

It should be noted that by appropriate dimensioning and distribution of the wires 17, the elastic sheet made up of these wires increases the capillarity effects provided by the capillary structure 5.

Fig. 5 illustrates a variant according to which the internal wall of the tube 4 is no longer smooth but striated, grooved or grooved. To this end, streaks 18 or the like are produced by any suitable means, parallel to the axis of the tube 3 and preferably with a generally flared V-shaped cross section. These recesses 18 are responsible for facilitating the correct positioning of the fibers 5, it being understood that the depth of these ridges or the like is less than the radius of the fibers 5 which are held in place by an elastic system identical to that of FIG. 4 or different.

Furthermore, the capillary structure may consist of two layers of fibers 5 with identical or different dimensional characteristics, parallel and not contiguous, the fibers of one of the layers being inclined relative to the fibers of the other layer and the assembly of this structure being pressed against the tube by an elastic system identical or not to that of FIG. 4.

It should be noted that one of the layers may comprise fibers parallel to the axis of the tube, this layer being either in contact with the internal wall of the tube, or in contact with said elastic system (vapor side).

Fig. 6 illustrates a method for producing a capillary structure according to FIG. 4 and its insertion into an aluminum or light alloy tube produced by extrusion. On a cylindrical mandrel 19 is helically wound a sheet 20 of spring wires or the like, of steel for example. The wires 21 of this sheet form contiguous turns on the mandrel 19.

The ply 20 is wrapped in a ply 22 of free smooth fibers 5 parallel to the axis of the mandrel 19. The fibers 5 are regularly distributed in a single layer around the helical ply 20.

The plies 20 and 22 at the outlet of the mandrel 19 are guided and held in shape by a cylindrical sleeve 23 in the extension of the mandrel 19 and integrated in an extrusion head 24 coaxially with the annular orifice 25 for extruding a tube 3 for example made of aluminum, said orifice 25 being delimited between the sleeve 23 and the die 26.

As the tube 3 is formed, it is automatically provided internally with the capillary ply 22 and the elastic retaining ply 20, the plies 20 and 22 being formed continuously and introduced into the tube 3 at the same speed. scrolling.

At the outlet of the sleeve 23, the plies 20 and 22 expand radially under the elastic action of the spring wires 21 and press against the internal wall of the tube 3.

The tube thus equipped conforms to what is shown in FIG. 4.

The tube 3 may internally have grooves such as 18 (Fig. 5) made during the extrusion.

The tube 3 can, of course, be obtained in another way, for example by rolling a flat plate then welding or from a strip wound helically on a mandrel, these two techniques being perfectly known.

Claims (7)

1. A heat-exchanger for refrigerating machines and/or heat pumps, comprising a heat-exchanging tubular network which comprises heat-exchanging tubes (3) in which, in use, a mixture of refrigerant fluid and lubricating oil is taken in through one end and discharged through the other end, a capillar structure (4) being applied against the inner wall of said heat-exchanging tubes (3) by permeable means (6), characterized in that said capillar structure (4) is comprised of a series of substantially rectilinear free and smooth fibers (5) of a suitable material, arranged substantially parallel to the axis of the heat-exchanging tubes (3), said fibers being regularly distributed in an annular array and applied along their whole length against the wall of said tubes (3).

2. A heat-exchanger according to claim 1, characterized in that said fibers (5) are arranged in a single layer of non-adjoining fibers.

3. A heat-exchanger according to claim 1 or 2, characterized in that the inner wall of the heat-exchanging tubes (3) is provided with longitudinal grooves or splines or the like accomodating the fibers of said capillar structure, the depth of said grooves or the like being substantially less than the radius of said fibers.

4. A heat-exchanger according to claim 3, characterized in that said grooves or the like are of spread V-shaped section.

5. A heat-exchanger according to claim 1, characterized in that said fibers (5) are arranged in a first layer of non-adjoining fibers extending substantially parallel to the axis of the heat-exchanging tubes and in a second layer of parallel non-adjoining fibers which are wound into a large pitch coil and pressed against the first layer, the inner wall of the heat-exchanging tubes being in direct contact with the first layer.

6. A heat-exchanger according to one of claims 1-5, characterized in that said means for urging the capillar structure against the inner wall of said heat-exchanging tubes are comprised of a layer of spring wires or the like which are parallel, non-adjoining and wound into a coil.

7. A heat-exchanger according to one of claims 1-6, characterized in that the heat-exchanging network comprises a plurality of parallel rectilinear tubes connected to a common header (1) for the liquid phase, said header being rectilinear and also provided therein with said capillar structure having free fibers (5), such an exchanger being operable as an evaporator and as a condenser.

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