FR2483591A1 - Gas and liquid heat exchanger - has liquid tubes with external spiral fins either smooth or non continuous - Google Patents

Abstract

The heat exchanger works between two fluids, one of them being a gas. It has an assembly of tubes (2) joining two collectors and the first fluid flows inside them. The tube assembly is placed in the gas flow and the tubes have external spiral fins (1). The fins are smooth and slotted having a mixture of both. In the direction of gas flow the first rows of tubes have non continuous fins and the later ones are smooth.

Description

 The present invention relates to a heat exchanger for transferring heat between two fluids, at least one of which is gaseous and more particularly, for example, an air cooler.

 From the usual baleen in an air cooler, the fluid to be cooled, generally liquid, is circulated inside the tubes of a tubular bundle connecting an inlet manifold and an outlet manifold.

On the other hand, a gaseous cooling current, generally air, is generated which circulates around the hot tubes. To increase the heat exchange surfaces, the tubes are always provided with metal fins in direct contact with the actual wall of the tubes.

 There are many techniques for flaskage and the establishment of such fins, themselves of very diverse shapes. We could thus recall, for example, the fins simply cut and force-fitted at regular intervals onto the tube, or else the fins obtained by winding a continuous bacon film3-supported on the wall of the tube or set in a groove helical exterior. Extruded fins are also produced by pushing the outer part of the tube by means of a form wheel, or more generally a more malleable metal tube previously placed in an outer sheath around the liquid circulation tube. mode of realization or installation of the fins, we are led to distinguish two main categories for these.

 The first is constituted by fins often called "smooth", that is to say whose exchange surface is continuous, whether they are stacked or helical fins. Such fins have the advantage of being relatively inexpensive to produce, and of creating only low pressure losses opposing the circulation of air. However, their heat exchange coefficient is relatively low, due in particular to the easy formation of boundary layers upon their immediate contact.

The increase in their efficiency then requires increasing the number of tubes or fins, or increasing the speed of air circulation, which leads at the same time to an increase in cost and an increase in the energy required. to ensure the circulation of the cooling air.

Another solution consisting in increasing the surface area of the fins has, on the other hand, the drawback of facilitating the appearance of limited layers, the development of which is easier on large surfaces.

 The fins of the other category are those sometimes called "turbulence" or whose exchange surface is divided and discontinuous. This will be the case for example for fins extruded, then split radially, with alternating longitudinal offset of the portions thus formed. I1 can also be, in this same category, fins stacked but sensing one or more stamped parts with partial cutting stecarting more or less from the initial plane of the fin. We can also put in this category of fins in wire, simply constituted by loops of wire locally welded to the wall of the tube. Such fins have the advantage of a good thermal efficiency3 because the boundary layers can hardly be formed on reduced unit surfaces, and on the other hand the turbulence caused by surface "accidents" blow out the boundary layers which could have formed locally. On the other hand, the tubes thus equipped are more expensive because of their preparation and the special devices with which they are provided.

In addition, because of these devices, they offer greater resistance to the passage of the refrigerant. In order not to increase operating costs too much, and in particular the energy necessary for the circulation of air, it is then necessary to limit the speed of the refrigerating fluid, which leads to not making maximum use of the thermal possibilities of such devices.

 The present invention allows the production of finned tube bundle heat exchangers which have heat exchange coefficients much higher than those of smooth fin heat exchangers3 without completely introducing the faults of high turbulence devices.

 The invention therefore applies to a heat exchanger between two fluids, comprising a bundle of tubes connecting two collectors and internally traversed by the first fluid, the bundle of tubes being disposed in the gas stream of the second fluid which circulates around the tubes provided of external fins. According to the invention, the bundle comprises - both tubes provided with smooth fins each having a continuous exchange surface, and tubes provided with fins each carrying discontinuous exchange surfaces.

 Otherwise, a preferred embodiment of the invention, in the direction of the gas stream, the tubes are arranged in alternating groups of each type, the first group preferably being of the type with discontinuous fins.

 The invention will now be described in more detail with reference to a particular embodiment given by way of example and shown in the accompanying drawings.

 Figure 1 recalls, in a partially exterior view and partially in axial section, the known structure of uCtube-provided with smooth fins.

 Figures 2 and 3, respectively in section along II-II of Figure 3 and along III-III of Figure 2, show a tube with fins with discontinuous exchange surfaces, also of known type.

 FIG. 4 shows in a simplified manner the structure of an air cooler produced according to the invention.

 Found in Figure 1 a finned tube produced by helical winding of a strip 1 in a groove 2 formed on the / outer wall of a tube for circulation of the fluid to be refrigerated.

 In the production of a fin tube recalled in FIGS. 2 and 3, the fluid circulation tube 5 is surrounded by an adjoining tube 6 made of more malleable metal, which has then been pushed back with the thumb wheel to form transverse fins 7 Each fin was then split by radial grooves 8, and the lobes thus formed are alternately spaced apart on either side of the transverse mean plane of the initial fin.

One thus realizes, in the direction of a transverse air flow, an irregular surface with abrupt discontinuities which generate a certain turbulence of the air flow around the fins.

 The air cooler shown diagrammatically in FIG. 4 is constituted in the usual way by a bundle 10 of tubes passing through the walls of two capacities 11 and 12 at each of their ends, with tight assembly of any usual type known at the ends. tubes on the walls.

We can assume that the capacity 11 is the distributor of a liquid to be cooled, which then flows in the direction of the arrows 13 towards the collector 12. Of course, it is only to simplify the drawing that we have only shown 'a layer of tubes, because the number of stages of tubes 10 is unrelated to the nature of the invention. The layer of tubes 10 represented here alternately comprises tubes 14, 16 with split fins of the type of FIGS. 2 and 3, and two tubes 15, 17 with smooth fins of the type of FIG. 1.

 It will be noted that, for a flow direction represented by the arrows 19, the stream of cooling air first meets a tube 14 with split fins. We know that this type of tube has a good calorific transmission agent thanks to the turbulence it generates, at the cost of a relatively large pressure drop imposed on the air circulation. The next tube 15 with smooth fins would tend, in a laminar current, to allow the formation of boundary layers unfavorable to heat exchange. But it takes advantage here of the turbulence created by the tube 14, and which has the effect of destroying the boundary layers; moreover ~ the tube 14 introduced into the air stream a low pressure drop specific to its structure. We will find the same phenomenon with the tubes 16 and 17, while also observing that the air arriving on the tube 16 will have - e regulated by the fins of the tube 15, so that the turbulence generated by the split fins of the tube 16 will not be excessive, as it could have been the case in the case of two identical successive tubes, with the excess pressure drop that could result.

 We see that under these conditions we can manage to use the faults specific to one type of tube to reduce the harmful effect of faults of the other type.

 Of course, the invention is not strictly limited to the embodiment which has been described by way of example, but it also covers the embodiments which would differ from it only in details, in variant embodiments, or in the use of equivalent means. Thus, according to the data specific to each embodiment, it will be possible to determine different combinations of tubes of each type, these combinations being able to be taken tube by tube or by group of a few identical tubes.

De morne if it can be considered that it is generally preferable that the first tube or tubes encountered by the air flow are of the type with discontinuous fins with high turbulence, one could also consider the reverse in particular cases, and for example if the the air current already exhibited a certain turbulence when it arrived on the beam.

Claims (3)

 l.- Heat exchanger between two fluids comprising a bundle of tubes connecting two collectors and internally traversed by the first fluid, the bundle of tubes being disposed in the gas stream of the second fluid which circulates around the tubes provided with external fins, characterized by the fact that the bundle comprises both tubes provided with smooth fins each having a continuous exchange surface, and tubes provided with fins each having discontinuous exchange surfaces.  2.- Heat exchanger according to claim 1, characterized in that in the direction of the gas stream, the first row or rows of tubes are of the discontinuous fin type.  3.- Heat exchanger according to claim 2, characterized in that in the direction of the gas flow the tubes are arranged in alternating groups of each type, the last group being of the type with smooth fins.