US3421578A - Heat dissipator - Google Patents

Description

Jan. 14, 1969 L. L. MARTON 3,421,578 7} HEAT DISSIPATOR Filed Dec. 22, 1966 United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE A heat dissipator useful with electric apparatus of the type having at least one heat transfer surface. The dissipator is comprised of a first plate having first and second oppositely facing surfaces substantially uniformly spaced from one another. The plate first surface is adapted to engage said heat transfer surface. A plurality of fins arranged in substantially parallel rows are fixed to the plate second surface in heat conducting relationship therewith. Each of the fins defines a pair of oppositely facing edge surfaces and a pair of oppositely facing main surfaces. All of the fin main surfaces extend substantially parallel to one another to thereby establish a flow for a cooling medium.

The invention relates to improvements in heat dissipators to be used in electrical apparatus such as transformers, reotifiers, motors, generators, etc.

A common characteristic of electrical apparatus is that a certain loss of energy occurs in its active parts during operation causing a temperature rise, which is the main and often the only limiting factor as to the power handling capacity of the device. The temperature grandient in turn maintains a heat flow from the thermal center of the heat source toward the surfaces of the part where the heat gets dissipated by the surrounding cooling medium, such as air, oil, etc., partly by radiation, partly by convection, or by forced fiow of the cooling medium. In most cases the greatest resistance in the flow of the heat appears at the surfaces between the solid material and the gaseous or liquid cooling medium; consequently, this is the area where the largest part of the temperature grandient develops.

The object of the invention is to reduce that portion of the resistance in the flow of the heat which appears at the surfaces of the parts, and thus to reduce the temperature gradients appearing at the surfaces.

Another object of the invention is to create a compact heat dissipator with extremely high heat transfer capability per volume unit and small resistance in the flow path of the cooling medium; these features make possible a considerable extension of its applicability especially in magnetic apparatus; because of its compactness, the heat dissipator can be accommodated without difficulties on coils and cores, on their external surfaces as well as in their internal ducts.

The reduced overall temperature rise allows the designer either to reduce the size of the device by using higher specific load figures, such as current density and flux density, thereby increasing the losses, until the original temperature rise appears on the reduced device, or to lower the temperature limit. In the latter case, the part of the losses depedent on temperature, e.g., copper loss, will be reduced, and efiiciency improved; furthermore, the costs may be reduced by the use of insulating material of lower thermal class. In some cases a combination of the two approaches, i.e., a moderate reduction of both the size and the temperature dependent losses of the device along with the reduced temperature, offers the best solution. In case of parts having losses independent of the temperature, a considerable reduction Patented Jan. 14, 1969 "ice of the weight and dimensions of the heat sink, or the elimination of the forced flow of the cooling medium can be achieved by the improved heat transfer between surface and cooling medium offered by the heat dissipator according to the invention.

An essential feature of the invention is its employment of at least one transfer surface and a multiplicity of fins built with small dimensions having heat conductive connections with, and protruding from, said transfer surface into the stream of the cooling medium flowing along said transfer surface.

According to a further essential feature of the invention, the transfer surface is formed by a layer of heat conductive material and said fins are arranged on the transfer surface at substantially equal distance from each other along diagonal lines and they are oriented with their larger cross-sectional dimensions substantially parallel to the direction of the flow of the cooling medium.

The improved heat transfer of the fin arrangements built according to the invention is based on the experimentally established fact that using smooth, undivided conventional transfer surface, the cooling medium tends to form a clinging layer over the surface with growing thickness along the flow preventing good heat transfer between the transfer surface and the cooling medium. The clinging layer, however, has negligible thickness on the entering edge of the surface. If the transfer surface is provided with a multiplicity of entering edges Where no insulating layer will develop, the heat transfer will be substantially increased.

Further essential features will become apparent on the basis of the following description. Various exemplary embodiments are illustrated in the accompanying drawings.

FIG. 1 is a perspective view of a heat sink according to the invention, built mainly for use with natural convection;

FIG. 2 is a plan of an exemplary embodiment of a transfer layer according to the invention, built with punched out fins;

FIG. 3 is the plan view of another exemplary embodiment of a transfer layer according to the invention, built with separate fins attached to the transfer surface;

FIG. 4 is a plan view of two segments of an exemplary embodiment of a circular heat sink according to the invention, built mainly for forced air cooling;

. FIG. 5 is a partial cutaway perspective view of a transformer coil and core equipped with heat dissipators according to the invention;

FIG. 6 is a partial longitudinal section of a rotary type electric apparatus equipped with heat dissipators according to the invention.

Referring in detail to the drawings, it will be seen from FIG. 1 that on metal plate 1 two silicon rectifiers 2, 3 are accommodated. The front surface of plate 1 forms the transfer surface for the heat generated in rectifiers 2, 3. Pins 4, built with small dimensions, protrude from the transfer surface and are arranged in substantially equal distance from one another along diagonal lines and oriented with their larger cross-sectional dimensions H substantially parallel to the direction 5 of the flow of the cooling medium, so that the resistance for the flow of the cooling medium is small and each fin has equal exposure to the flow.

When the heat source produces heat, the temperature of the transfer surface and the fins rises gradually. causing the start and the gradual growth of the outside heat transfer, until a balance is achieved; then the passing cooling medium and the radiation dissipate equal heat to the one introduced by the heat source. The heat dissipator according to the invention introduces a substantial increase in the part of the heat transfer produced by the 3 passing cooling medium, without influencing the radiation process.

There are various ways to bring the fins into good heat conductive connections with the material forming the transfer surface. Pins 4 in FIG. 1 are built together with the transfer plate by casting or injection molding. Additional ways are illustrated in FIGS. 2 and 3.

FIG. 2 illustrates a sheet forming a transfer surface with punched out and bent up fins 6a. When two transfer surfaces are arranged facing each other for the purpose of building a duct, fins 6b protruding from the opposite wall of the duct will be accomodated in the gaps between fins 6a. The cross section of the fins 6a, 6b are modified by pressing in order to achieve an aerodynamically favorable profile offering minimum resistance in direction 7 to the flow of the cooling medium.

FIG. 3 shows a transfer surface equipped with fins 8 and 9 produced separately and joined to the surface by known emetal connecting techniques. Joints with better heat conductivity are preferable. Fins 8 are produced from strips 10 by cutting and binding up both ends. Pins 9 are produced by punching and bending up both edges of strip 11 according to the pattern shown.

When two transfer surfaces are arranged in substantailly parallel position, facing each other to build a duct, the velocity of the cooling medium can be kept even all along the length of the surfaces; the duct arrangement is very efiicient with forced flow; nevertheless, it can be used also with natural convection, especially when a chimney is added to the duct, to increase the rate of flow. Two exemplary embodiments of the duct arrangement according to the invention are illustrated on FIGS. 4 and 5.

In FIG. 4 identical segments 12, 13 of a circular heat sink occupy one sixth of the total circular area, which offers a convenient combination with a fan having the same diameter. Surfaces 14, 15 of the wedge-like body of the segments form the transfer surfaces bordering the ducts. Aerodynamically shaped spike- like fins 16, 17 protrude from both transfer surfaces 14, 15 into the duct on the left and the right :side respectively, arranged in rows at different levels, alternatively, one from surface 14, the next from surface 15 in a row, as shown on FIG. 2, for fins 6a, 6b. Heat sources 18 and 19 are attached to the base of segments 12, 13 in single, or multiple levels. Tube 20 closes the ducts in center.

FIG. 5 illustrates a portion of a transformer equipped with heat dissipators according to the invention. Those of the vertaical surfaces of core 21, which are perpendicular to the larninations, are equipped with dissipator 22, since only those surfaces have both good heat transfer inside of the core, alongside the laminations, and sufiicient heat dissippation toward the cooling medium due to their vertical position. The coil is divided by duct 23 into inside 24 and outside 25 sections, both being equipped on both vertical surfaces with heat dissipators 26. The flow of the cooling medium has vertical direction 27 with convection cooling which can be increased by forcing the flow by pumps or fans. The protruding fins in duct 23 can also serve as spacers to keep the radial dimension of the duct even all around.

FIG. 6 shows a rotary type electric apparatus equipped with heat dissipators according to the invention. The cooling air enters along arrows 28a, 28b driven by fins 29a, 29b protruding from the surface of discs 30a, 30b respectivelv. attached to rotor 31.

Fins 20a, placed tangentially along spiral curves, force part of the air through duct 32 alongside the stator laminations 35, acting as impeller blades of a turbine pump. On its way, the air flows through fins 33a protruding from ring 34a, attached to the left end of stator 35, and through fins 36, protruding from heat conductive layer 37 forming the transfer surface, and being attached to the outside surface of the stator 35. Part of the air entering along arrow 28b driven by fins 29]) attached to ring 30b fiows along the right side of the apparatus, be-

4 tween fins 33b, attached to ring 341;. All the fins, including fins 29a, 2% are oriented parallel to the relative direction of the airflow.

When rotor 31 rotates, fins 29a, 2% force the air along arrows 28a, 28b and the heat developed in rotor 31 gets dissipated in the air flow, which in turn picks up the heat generated in the stator 35 while flowing through fins 33a, 36, and 331; respectively.

When an apparatus is enclosed in a metal case involving an internal and an external cooling medium in the cooling process, the temperature gradient between the cooling mediums close to the wall of the case can be substantially reduced by applying 'heat dissipators according to the invention over the outside and/ or inside surfaces of the wall, with the result of a substantial improvement of the heat transfer between internal and external cooling medium.

All fins have three characteristic dimensions, as shown on FIGS. 1, 2 and 3: length L, average thickness W, and average height H. W and H may decrease along the length L introducing a degree of conical tendency into the shape of the fins; as the heat gets dissipated along length L, the internal heat flow decreases gradually toward the end of the fins, thus the decrease of the cross-sectional area is justifiable. All three dimensions of the fins are small; e.g., an average height of 1.5 cm. can be considered as the upper limit for height H in high economy heat dissipators according to the invention. The optimum value of these dimensions can be determined on the basis of the temperature gradients, the material constants of the materials participating in the heat transfer, and the velocity of the fiow and, further, on the basis of the specific heat of the cooling mediums; nevertheless, practical considerations, like structural strength and economy of the production and application tend to modify the results.

For example, when a heat sink is being designed, e.g., according to FIG. 1 with air convection and radiation cooling, the following effects of changes in the dimensional proportions should be considered: reducing the average height H and thickness W, the outside heat transfer per surface unit increases on the surface of the fins, because of the decrease of the thickness of the clinging layer of the cooling medium, but at the same time the resistance of the internal heat flow in the fins increases. Thus length L of the fins which can be utilized decreases, since the good outside heat transfer removes the reduced amount of heat conducted by the too slender fins within short distance from the transfer surface; any extension of the fins beyond that length is useless. Short fins in turn reduce the volume of the air involved in the transfer process. An increased number of fins per surface unit decreases the air flow because of additional friction, further reducing the volume. To improve the overall heat transfer, increased height and thickness of the fins may offer the proper solution, in spite of the decreasing heat transfer per surface unit on the fins, due to the increasing thickness of the clinging layer. An optimum in the heat transfer itself or an optimum in the overall economy of the device can be established for each condition as a compromise between variables acting against one another, by varying the number of fins per surface unit and the dimensions of the fins, and/ or the generated losses of the device and determining the heat transfer for each arrangement, as well as the cost components of the whole arrangement.

Where the resistance of the flow is not a seriously limiting factor, e.g., where forced flow is used, to increase the number of fins per surface unit may be economical. When some outside factor limits the length of the fins, e.g., fins being used inside of a coil in a duct, shorter fins may offer the best overall economy, in spite of their reduced surface.

The foregoing specification has set forth specific structures in detail for the purpose of illustrating the invention. It will be understood that such details of structure may be varied widely without departure from the scope and spirit of the invention as defined in the specification and in the following claims.

I claim:

1. A heat dissipator useful with electric apparatus of the type having at least one heat transfer surface, said dissipator comprising:

a first plate having first and second oppositely facing surfaces substantially uniformly spaced from one another, said first surface adapted to engage said heat transfer surface;

a plurality of fins each defining a pair of oppositely facing main surfaces and each being fixed to said second surface in heat conducting relationship therewith and with said fin main surfaces extending substantilally perpendicularly to said second surface;

said plurality of fins being arranged in substantially orthogonal rows and columns with each row including at least two fins and with all of said fin main surfaces extending substantially parallel to said columns and substantially perpendicular to 'said rows with adjacent fins in each column being spaced by a distance greater than the dimension of each of said fin main surfaces along said columns.

2. The heat dissipator of claim 1 wherein fins of adjacent rows are misaligned with respect to each other.

3. The heat dissipator of claim 1 wherein each of said fins including a pair of oppositely facing edge surfaces converging toward each other and spaced at a minimum distance remote from said second surface.

4. The heat dissipator of claim 1 wherein the main surfaces on each of said fins converge toward each other and are spaced at a minimum distance remote from said second surface.

5. The heat dissipator of claim 1 wherein said fins are formed integral with said plate.

6. The heat dissipator of claim 1 wherein said fins are formed integral with and punched from said plate.

7. The heat dissipator of claim 1 including a second plate substantially identical to said first plate; and

:means supporting said first and second plates in opposed relationship to form a duct therebetween with the fins on each of said plates projecting into said duct.

8. The heat dissipator of claim 3 wherein said fins are formed integral with and punched from said plate.

References Cited UNITED STATES PATENTS 2,430,631 11/1947 Eskra 165-182 3,187,812 6/1965 Staver 165185 3,220,471 11/1965 Coe 165-185 3,312,277 4/1967 Chitouras et a1 165185 3,353,591 11/1967 Zak 165185 ROBERT A. OLEARY, Primary Examiner.

CHARLES SUKALO, Assistant Examiner.

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