Heat exchanger

Abstract: The invention describes a heat exchanger comprising a plurality of heat-transfer tubes arranged in parallel to each other at predetermined intervals; a plurality of fine wire fins serving as heat-transfer fins arranged between the heat-transfer tubes so as to extend across the surfaces of the heat-transfer tubes; and needle fins inserted in a heat-transfer surface formed by combining the heat-transfer tubes and the fine wire fins; said heat-transfer surface being formed by interlacing the plurality of heat-transfer tubes as warps and the plurality of fine wire fins arranged at intervals twice the diameter thereof or above as wefts in a plain weave, said needle fins being extended perpendicularly to the heat-transfer surface of a plain weave, and the needle fins being fixed at least to either the fine wire fins or the heat-transfer tubes.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a heat exchanger according to the preamble of claim 1. Such a heat exchanger is known from FR-A-2 428 224. Description of the Prior Art Fig. 1 is a perspective view of a prior art heat exchanger for an air conditioner, disclosed in Japanese Patent Laid-open (Kokai) No. 61-153388 and Fig. 2 is a sectional view of the heat exchanger of Fig. 1. Referring to Figs. 1 and 2, the heat exchanger is formed by alternately passing fine wires 2a and 2b, which serve as fins, over and under heat-transfer tubes 1 through which a heat exchanging fluid, such as a cooling medium, flows in the direction of the arrow B so that the fine wire fins 2a and 2b are in close contact with the heat-transfer tubes 1. In this heat exchanger, the thermal contact between the fine wire fins 2a and 2b, and the heat-transfer tubes 1 is secured and the positions of the heat-transfer tubes 1 and the fine wire fins 2a and 2b relative to each other are fixed by alternately passing the fine wire fins 2a and 2b over and under the heat-transfer tubes 1. The heat-transfer tubes 1 are very thin tubes having, for example, an outside diameter in the range of 1 to 2 mm and an inside diameter in the range of 0.7 to 1.7 mm. In operation, as shown in Fig. 2, a current of an external fluid, for example, air, flowing in the direction of the arrow A (Fig. 1) toward the plurality of parallel heat-transfer tubes 1 threads its way through spaces between the fine wire fins 2a and 2b, exchanging heat with the heat-transfer tubes 1 and the fine wire fins 2a and 2b. The current of the external fluids is disturbed by the fine wires 2, the current of the external fluid falling on the fine wire fins is deflected to right and left as shown by arrows in Fig. 3, and part of the external fluid flows along the fine wire fins and rises along the surfaces of the heat-transfer tubes 1, so that the external fluid is able to be in contact with the heat-transfer tubes 1 for a comparatively long contact time. When cold water is passed through the heat-transfer tubes 1 of the heat exchanger or a low-temperature cooling medium is evaporated in the heat-transfer tubes 1, air flowing past the heat-transfer tubes 1 is cooled for air cooling. If the air passing by the surfaces of the heat-transfer tubes 1 and the fine wire fins 2a and 2b is cooled to a temperature below the dew point, water droplets form over the surfaces of the heat-transfer tubes 1 and the fine wire fins 2a and 2b. The water droplets flow along the surfaces of the heat-transfer tubes 1 and the fine wire fins 2a and 2b, and drain off from the heat exchanger. In the heat exchanger of this construction, the contact area in which the fine wire fins 2a and 2b are in contact with the heat-transfer tubes 1 is very small. Therefore, the fine wire fins 2a and 2b do not reduce the contact area in which the external fluid comes into contact with the heat-transfer tubes 1 and hence heat can effectively be transferred between the external fluid and the heat-transfer tubes 1. Although the heat-transfer rate of this prior art heat exchanger is greater than that of a conventional heat exchanger for air conditioner, the heat-transfer area of the prior art heat exchanger is 1/5 or below that of a conventional heat exchanger having the same front area because the prior art heat exchanger has a very small thickness in the range of 1 to 3 mm. A heat exchanger formed by stacking a plurality of heat exchanging units of the type similar to this prior art heat exchanger may be used to secure a necessary heat exchanging quantity. However, the heat exchanger having the plurality of heat exchanging units increases the pressure loss of air, the flow of air is reduced and, consequently, it is impossible to secure a necessary heat exchanging quantity unless the power of the blower is increased. The heat exchanging ability Q of a heat exchanger is expressed by: Q = K x A x ΔT, where K is the overall heat-transfer coefficient, A is the heat-transfer area and ΔT is the temperature difference between air and the medium flowing through the heat-transfer tubes 1. Since the prior art heat exchanger is constructed by passing the fine wire fins 2a and 2b alternately over and under the heat-transfer tubes 1, it is difficult to increase the heat-transfer area per unit front area and it is difficult to increase the overall heat-transfer coefficient by enhancing the disturbance of the flow of air. After all, it has been impossible to enhance the heat exchanging quantity through the improvement of the factors that contribute to the enhancement of the heat exchanging quantity and there has been a limit to the enhancement of the heat exchanging efficiency. Such problems are remarkable particularly when the heat exchanger is used as an evaporator and vapor contained in air condenses into water droplets over the heat-transfer surface. If water droplets form over the surfaces of the heat-transfer tubes 1 and the fine wire fins 2a and 2b, the spaces between the fine wire fins 2a and 2b are clogged with the condensation to hinder the flow of air through the heat exchanger. Consequently, the flow of air is reduced due to pressure loss and the reduction of the heat exchanging efficiency results. It is generally known that, when a nonazeotropic cooling medium is used as a cooling medium which is passed through the heat-transfer tubes, the performance of the heat exchanger formed by arranging a plurality of heat exchanging units in layers is improved considerably when the nonazeotropic cooling medium is passed in a crossflow mode from the rear heat exchanging unit sequentially through the intermediate heat exchanging units toward the front heat exchanging unit so that the heat exchanger functions virtually as a crossflow type heat exchanger. The number of the heat exchanging units of a heat exchanger for the conventional air conditioner is two at the most, because an excessively large number of heat exchanging units increase the thickness of the heat exchanger and the size of the air conditioner increases accordingly. Therefore it has been very difficult to construct a crossflow type heat exchanger capable of functioning virtually as a counterflow type heat exchanger. FR-A-2 428 224 already discloses a heat exchanger comprising a plurality of heat-transfer tubes arranged in parallel to each other at predetermined intervals, a plurality of warp threads serving as heat-transfer fins arranged between the heat-transfer tubes so as to extend the surfaces of the heat-transfer tubes and weft threads inserted in a heat transfer surface formed by combining the heat transfer tubes and the warp threads, wherein this heat transfer surface is formed by interlacing the plurality of heat-transfer tubes and the plurality of warp threads arranged at intervals twice the diameter thereof or above. The warp threads and the weft threads which are arranged in parallel to the heat-transfer tubes form a plain weave in the heat transfer surface. SUMMARY OF THE INVENTION Accordingly, it is the object of the present invention to provide a heat exchanger capable of increasing heat-transfer coefficient outside a heat-transfer tube of promoting heat-transfer and of suppressing reduction in heat exchanging quantity due to reduction in the flow of air. It is also the object of the present invention to provide a heat exchanger having an increased heat-transfer area capable of exchanging heat at an increased heat exchanging quantity, and requiring a reduced space for installation. For solving this object a heat exchanger according to the present invention is characterized by the features of claim 1 respectively claim 2. Preferred embodiments of the heat exchanger according to the invention are defined in the subclaim. A heat exchanger for an air conditioner, in a preferred mode of the present invention comprises a plurality of heat-transfer tubes through which a nonazeotropic cooling medium flows, the plurality of fine wire fins, arranged in parallel to each other and being connected to each other, and wherein in operation of the heat exchanger an external fluid flows outside the heat transfer tubes with a flowing direction perpendicularly to the heat transfer tubes and along the parallel heat transfer tubes, the cooling medium is supplied so as to flow from the heat-transfer member disposed at the lowermost position with respect to the flow of the external fluid sequentially through the heat-transfer members toward the heat-transfer member disposed at the uppermost position. The heat exchanging performance of this heat exchanger is higher than that of a heat exchanger of a simple crossflow type (a heat exchanger with a single crossflow heat exchanging unit). BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary perspective view of the heat-transfer plane of a conventional heat exchanger for an air conditioner; Fig. 2 is an enlarged fragmentary sectional view of a conventional heat exchanger, showing the flow of air on the heat exchanger; Fig. 3 is a fragmentary plan view of the heat-transfer plane of a conventional heat exchanger; Fig. 4 is a fragmentary perspective view of the main part of a heat exchanger for an air conditioner in first embodiment according to the present invention; Fig. 5 is a fragmentary perspective view of the main part of a heat exchanger for an air conditioner in a second embodiment according to the present invention; Fig. 6 is a fragmentary sectional view of a heat exchanger for an air conditioner which is useful for understanding the embodiment of claim 3; Fig. 7 is a fragmentary sectional view of a heat exchanger in a modification of the heat exchanger in Fig. 6; and Fig. 8 is a graph showing the dependence of heat exchanging quantity on the number banks of the heat-transfer surfaces in the heat exchanger of Fig. 6 or Fig. 7 when a nonazen-tropic cooling medium is used. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig 4 showing a heat exchanger for an air conditioner, in a first embodiment according to the present invention in a fragmentary perspective view, heat-transfer tubes 1 are arranged at predetermined intervals to pass an internal fluid, such as a cooling medium, and fine wires are passed alternately over and under the heat-transfer tubes 1 to form fine wire fins 2a and 2b, i.e., heat-transfer fins. Intervals between the fine wire fins 2a and 2b as the weft are twice the diameter of the fine wire or above. Needle fins 6 are inserted perpendicularly to a heat-transfer surface formed by interlacing the fine wires and the heat-transfer tubes 1 in spaces defined by the fine wire fins and the heat-transfer tubes 1, and the needle fins 6 are attached to the fine wire fins 2a and 2b, or the heat-transfer tubes 1. In operation, an external fluid, such as air, threads its way through spaces between the fine wire fins 2a and 2b, needle fins 6 and the heat-transfer tubes 1, generating minute vortices. The minute vortices swirl in turbulent currents around portions on the lower side with respect to the flowing direction of the air of the fine wire fins 2a and 2b, the needle fins 6 and the heat-transfer tubes 1. Consequently, heat-transfer is promoted and the heat exchanger transfers heat at a high heat-transfer rate. Since the needle fins 6 increases the heat-transfer area along the flowing direction of the air, the heat exchanging quantity can be increased. Since the intervals between the fine wire fins 2a and 2b are increased and spaces between the fine wire fins 2a and 2b are expanded by the needle fins 6, condensed water droplets are hardly able to stay on the heat exchanger when the heat exchanger is used in a moist atmosphere where moisture contained in the atmosphere condenses in water droplets on the heat exchanger, the heat exchanger is hardly clogged with condensed water droplets, and reduction in heat exchanging quantity due to reduction in the flow of the air can be suppressed. Referring to Fig. 5 showing a heat exchanger for an air conditioner, in a second embodiment according to the present invention in a fragmentary perspective view, heat-transfer tubes 1 are arranged at perdetermined intervals to pass an internal fluid, such as cooling medium, and fine wires are passed alternately over and under the heat-transfer tubes 1 to form fine wire fins 2a and 2b, i.e., heat-transfer fins. Intervals between the fine wire fins 2a, 2b as the weft are twice the diameter of the fine wire or above. Needle fins 6 are inserted in spaces formed between the heat-transfer tubes 1 and the fine wire fins so as to extend obliquely downward, and the needle fins 6 are attached to the fine wire fins 2a and 2b, or the heat-transfer tubes 1. Although the needle fins 6 are bent in a plane perpendicular to the axes of the heat-transfer tubes 1 in this embodiment, the needle fins 6 may be bent in a plane parallel to the axes of the heat-transfer tubes 1 provided that the top of the needle fins 6 looks obliquely downward. Although the effect of this embodiment is similar to that of the first embodiment, spaces between the fine wire fins 2a and 2b in this embodiment are larger than those first embodiment. Since the needle fins 6 are bent as being obliquely downward, water droplets condensed on the heat exchanger drain off along the needle fins 6 by gravity. Therefore, the heat exchanger is hardly clogged with condensed water droplets when the same is used in a moist atmosphere where moisture contained in the atmosphere condenses in water droplets on the heat exchanger and reduction in heat exchanging quantity due to reduction in the flow of the air can be surpressed. Referring to Fig. 6 showing an embodiment of a heat exchanger for an air conditioner, which is useful for understanding the features of claims 3 in a fragmentary sectional view, there exist heat-transfer tubes 1, fine wires are passed alternately over and under the heat-transfer tubes 1 of each row to form fine wire fins 2, and headers 3. An external fluid, such as air, flows in the direction of the arrow A. The heat exchanger in this embodiment has a plurality of heat-transfer surfaces. A nonazeotropic cooling medium is used as an internal operating fluid in the heat-transfer tubes 1. The nonazeotropic cooling medium flows from the rear header 3 into the heat-transfer tubes 1 of the rear row in the direction of the arrow B, flows sequentially through the headers 3 and the heat-transfer tubes 1 of the intermediate rows toward the heat-transfer tubes 1 of the front row, and flows into the front header 3. Thus, the nonazeotropic cooling medium flows across the flow of air flowing in the direction of the arrow A in a virtually counterflow mode. Suppose that the nonazeotropic cooling medium is used as an internal operating fluid that flows through the heat-transfer tubes 1, the temperature of the nonazeotropic cooling medium, which phase change occurs in the heat exchanger, changes. In case the temperature of the nonazeotropic cooling medium at the inlet of the heat exchanger is fixed, the heat exchanging quantity of a heat exchanger provided with heat-transfer tubes arranged in a plurality of rows, in which the nonazeotropic cooling medium flows across the air flow in a virtually counterflow mode, is higher than that of a heat exchanger provided with heat-transfer tubes arranged in a single row, in which the nonazeotropic cooling medium flows simply across the air flow, which will be obvious from Fig. 8. Therefore, by the construction of the heat exchanger as shown in Fig. 6, the heat exchanging quantity of the heat exchanger becomes higher. It is also possible to bent heat-transfer surfaces without intermediate headers 3 as shown in Fig. 7.

Claims: A heat exchanger comprising: a plurality of heat-transfer tubes (1) arranged in parallel to each other at predetermined intervals; a plurality of fine wire fins (2a, 2b) serving as heat-transfer fins arranged between the heat-transfer tubes (1) so as to extend across the surfaces of the heat-transfer tubes (1); and needle fins (6) inserted in a heat-transfer surface formed by combining the heat-transfer tubes (1) and the fine wire fins (2a, 2b); said heat-transfer surface being formed by interlacing the plurality of heat-transfer tubes (1) as warps and the plurality of fine wire fins (2a, 2b) arranged at intervals twice the diameter thereof or above as wefts in a plain weave, the needle fins (6) being fixed at least to either the fine wire fins (2a, 2b) or the heat-transfer tubes (1), characterized in that the needle fins (6) extend perpendicularly to the heat-transfer surface of said plain weave. A heat exchanger comprising: a plurality of heat-transfer tubes (1) arranged in parallel to each other at predetermined intervals; a plurality of fine wire fins (2a, 2b) serving as heat-transfer fins arranged between the heat-transfer tubes (1) so as to extend across the surfaces of the heat-transfer tubes (1); and needle fins (6) inserted in a heat-transfer surface formed by combining the heat-transfer tubes (1) and the fine wire fins (2a, 2b); said heat-transfer surface being formed by interlacing the plurality of heat-transfer tubes (1) as warps and the plurality of fine wire fins (2a, 2b) arranged at intervals twice the diameter thereof or above as wefts in a plain weave, the needle fins (6) being fixed at least to either the fine wire fins (2a, 2b) or the heat-transfer tubes (1), characterized in that the needle fins (6) are bent in the shape of the inverted letter V so that the opposite ends of each needle fin (6) are arranged in such a way, that water droplets condensed on the heat exchanger drain off along the needle fins by gravity. A heat exchanger according to claim 1 or 2, wherein the heat exchanger is provided with a plurality of heat-transfer members each comprising the plurality of heat-transfer tubes (1) through which a nonazeotropic cooling medium flows and the plurality of fine wire fins (2), arranged in parallel to each other and being connected to each other, and wherein in operation of the heat exchanger an external fluid flows outside the heat-transfer tubes (1) with a flowing direction perpendicularly to the heat-transfer tubes (1) and along the parallel heat transfer tubes (1), the cooling medium flows from the heat-transfer member disposed at the lowermost position with respect to the flowing direction of the external fluid sequentially through the heat-transfer members toward the heat-transfer member disposed at the uppermost position with respect to the flowing direction of the external fluid.

Inventors: GOTOH TAKASHI, IKEJIMA KAORU, TAKESHITA MICHIMASA, YOSHIDA TAKAYUKI, YUMIKURA TSUNEO, GOTOH, TAKASHI, IKEJIMA, KAORU, TAKESHITA, MICHIMASA, YOSHIDA, TAKAYUKI, YUMIKURA, TSUNEO, Gotoh, Takashi, c/o Mitsubishi Denki K.K., Ikejima, Kaoru, c/o Mitsubishi Denki K.K., Takeshita, Michimasa, c/o Mitsubishi Denki K.K., YOSHIDA, TAKAYUKI, C/O MITSUBISHI DENKI K.K., Yumikura, Tsuneo, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1995
Main classification: F28F1
IPC codes: F28F1, B23K1