Untitled record

FRENCH REPUBLIC

MINISTRY OF INDUSTRY

INDUSTRIAL PROPERTY SERVICE

PATENT

WOgATÎ212.901 International classification: F 25 h

Heat exchanger with non-uniformly arranged fins.

Company known as: "LICENCIA" TALÂLMÂNYOKAT ERTÉKESITO VÂLLALAT residing in Hungary.

Requested October 9, 1958, at 4 p.m.^h 23^m, in Paris.

Issued October 26, 1959. - Published March 28, 1960.

(Patent application filed in Hungary on March 14, 1958, in the name of Mr. Làszlô Forgô.)

There are already known heat exchangers with interrupted fins provided with fins on the side of the thermal agent of lower coefficient of heat transmission, these fins being split into narrow strips transverse to the direction of flow. By these repeated interruptions of the surface of the fins, the formation of a thick boundary layer is prevented, which makes it possible to obtain a significantly higher heat transfer coefficient than in the case of plain surfaces. Heat exchangers of this kind operate, as is known, with a high transmission coefficient even under a low flow speed of the external fluid so that, as a general rule, they are operated under a low flow speed in order to reduce the power required for ventilation.

In heat exchangers with interrupted fins of known types, by means of special manufacturing processes, fins are given which have only a length of a few millimeters in the direction of flow, i.e. dimensions very small, a given elementary geometric structure and the entire surface of the fins is obtained by repeating this elementary structure a large number of times. Besides the technical advantages already mentioned above, the heat exchangers with interrupted fins thus established also have drawbacks, however.

In case of low flow velocities, a type of flow of almost entirely laminar character is established due to the small dimensions of the fins. As a result of the repetition of the elementary structure, the fluid providing the heat, fluid which, by contact with the fins placed upstream of the flow, has already warmed or cooled to a certain point, comes into contact with fins of identical geometrical position placed downstream. But in the meantime and precisely because of the almost laminar flow, the particles whose temperature has undergone a change as a result of direct contact with the fins do not succeed in mixing sufficiently with the flowing particles at a greater distance from the fins. As a result, even if the fins are offset from each other, the heat transmission coefficient cannot reach its theoretical maximum value.

In some cases, for example in the case of hydrogen-cooled current generators or in that of gas turbines, it is necessary to operate the finned heat exchangers interrupted under a high flow speed. In such cases, the unfavorable phenomena mentioned above do not actually appear, but other properties appear which limit the possibilities of use of these exchangers.

Experience has shown that above a certain flow speed, fins of this kind enter into vibration and cause an acute whistling sound. At the same time, the resistance to flow of the exchangers also increases. The aforementioned sound phenomena can reach an intensity such that the reuse under high flow rates of the heat exchangers can, in certain cases, be greatly limited or even made impossible. Experiments have shown that the hissing which occurs in interrupted fin heat exchangers is caused by vortices which, in the case of high speed flow, periodically follow each other towards the various fin strips and cause vibration of the metal body of the whole mixer. The intensity of the sound varies with the dimensions of the heat exchanger and its height increases with the speed of flow. Since in the heat exchangers known hitherto the fin system is constituted by the repetition of a certain elementary geometric structure, if the geometric structure 9 - 41755

Price of the booklet: 1 NF [1.212.901] - 2 that elementary tends to vibrate, this multiple repetition can increase the excitation of oscillation to such an extent that finally the whole system enters into vibration.

The object of the invention is to take full advantage of the known advantages of interrupted fin heat exchangers by eliminating the aforementioned drawbacks or by improving these exchangers by increasing the transmission coefficient at low flowing speeds. and by eliminating spurious sound phenomena at high flow velocities.

In the case of heat exchangers with interrupted fins, the two aforementioned drawbacks can ultimately be attributed to the fact that the surface of the fins is constituted by the repetition of identical elementary geometric structures.

If on the other hand the fins are not produced by the reproduction of a single elementary structure but by the alternating repetition of different elementary structures these two drawbacks disappear simultaneously.

In this case, in fact, it results from the non-uniform geometrical arrangement of the set of fins a certain turbulence even at low flow speeds. But, by the same token, particles heated or cooled by fins placed upstream cannot return to contact with fins of identical geometric position over a length of several spaces of fins. As a result, even in the case of a laminar type flow, constantly renewed particles come into direct contact with the fins.

The fin surface formed by different elementary structures does not vibrate even at high flow velocities. Indeed, both the excitation frequencies and the natural frequencies of the geometrically different elements from each other are also different. The system therefore does not tend to vibrate, which also eliminates the aforementioned sound phenomena.

To better understand the object of the invention, we will now describe by way of illustration and without any limiting character several embodiments taken as examples and shown in the accompanying drawing.

In this drawing, fig. 1 shows in longitudinal section part of a heat exchanger consisting of a number of tubes 1 and fins 2. The fluid with a higher transmission coefficient circulates in the tubes 1 on which fins 2 are fixed by an assembly of good thermal conductivity. The fluid with a lower transmission coefficient flows transversely to the plane of the axes of the tubes. The plates forming the fins 2 are divided by stamping into narrow strips 3, 4, 5 transverse to the direction of flow of the fluid with a lower transmission coefficient, these strips being pushed back to the press at various distances from their position d 'origin, that is to say relative to the plane of the fin 2. These narrow bands remain connected to the plate by their two ends.

Fig. 2 shows a cross section of the same heat exchanger. The direction of flow of the external fluid is indicated there by arrows, the repelled bands, that is to say the fins, are transverse to this direction. Figs. 1 and 2 both represent an exemplary embodiment in which the repelled fins 3, 4 and 5 have different deviations from their original position while the intermediate strips have remained in their original position.

Fig. 3 also relates to the same exemplary embodiment. Arrows there indicate the direction of flow of the external fluid. The tubes are designated by the reference 1 and the fins plates by the reference 2. As shown in this section, the fins 3, 4 and 5 are variously moved from their original position.

The essential character of the invention is evident from FIGS. 1, 2 and 3. The elementary geometric structures already mentioned are each formed by two consecutive fins in the direction of flow indicated by the arrows. The first elementary structure is thus constituted by the fins 2 and 3, the second by the fins 2 and 4 and the third by the fins 2 and 5: all these structures differ from each other and the whole of the transmission surface of heat is obtained by their repetition.

Fig. 4 shows in cross section a fin plate 2 in which the consecutive fins 6, 7, 8, 9, 10 are likewise stamped at various depths.

Figs. 1 to 4 indicate exclusively the principles according to which the problem posed above can be solved. As a general rule, it is possible to form a heat transmitting surface by the alternating repetition of two geometrical structures already different from each other, with the exception, however, of the case where the two structures are symmetrical with one other as in a mirror. In the latter case, in fact, the fins which have been stamped, with a symmetry of this kind from two adjacent or juxtaposed plates give a perfectly or almost perfectly uniform arrangement of fins.

In fig. 5, the tubes are designated by the reference 1 and the fins plates by the reference 2. In this embodiment, the different geometric structures result from the fact that the dimensions of each of the fins designated in the drawing by the references 11 to 17 are different in the direction of the flow of the external fluid indicated by the arrows. Thanks to this process, any two successive fins give elementary structures different from each other even if, for example, the fins 12, 1-1 and 16 are pushed back at the same distance from their original position. A provision of this kind can of course also be combined with the repelling of the fins at various spacings.

Fig. 6 shows an embodiment in which the differences between the elementary structures result from the fact that the stamping of the various fins 18 to 24 is carried out along straight lines which are not parallel to each other and whose spacing is not identical from one to the other.

Fig. 7 shows part of a heat exchanger consisting of several rows of tubes established according to one of the embodiments hitherto described or according to any other method corresponding to the above principles; in this figure, the reference 1 designates the tubes, the reference 2 designates the fins. The strips of plates in fig. 7 can respond to the structures described above, taken individually or in combination. This embodiment shows at the same time another means of accentuating the difference of the elementary structures thanks to the arrangement of the fins. The fins 25, 26, 27, etc., coming from the stamping, do not completely cover the distance separating the two tubes 1, but in the middle of this a strip 28 is left in its original position. In this way, the strips of plates 25, 26, 27, etc., are supported on the one hand, by one of their ends, in the immediate vicinity of the tube 1, on the other hand, by their other end, on the strip 28. The support mode is therefore not identical at the two ends. In addition, the fins 29, 30 and 31 which act in very different ways from each other on the flow come juxtaposed in the part separating the two rows of tubes, so that the vibration effect of the fins disappears. Another advantage of this arrangement lies in the fact that the strip 28 allows the conduction of heat between the tubes and the ends of the fins 25, 26, 27, etc., so that the heat transmission is improved or although with equal heat transfer, the reciprocal spacing of the tubes can be increased. This results in reduced manufacturing costs.

Fig. 8 represents a heat exchanger, the exchange surface of which is formed of thin metal bars 32 transversely approached by the stream of hot fluid as indicated by the arrows in FIG. 8. In this arrangement again, each elementary structure is formed of two successive rows in the direction of flow. The structures can be made different from each other either by modifying the distribution by row or the mutual spacing of the various rows, [1.212.901] or by giving the bars various diameters or sections of various shapes (circular, oval or drops).

When the heat exchange must take place between two fluids (for example between two gases) whose heat transmission coefficients are approximately equal, it may become necessary to provide fins on both sides of the exchange surface. In this case, the above considerations apply with the appropriate arrangements, on both sides of the heat exchanger.