ES2347509A1 - Heat exchanger of channeled sheets. (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Heat exchanger formed by two sheets welded or adhered to each other, at least in one of which a channel has been stamped whose topology allows that, while defining a conduit through which the heat exchange fluid will circulate in a laminar regime with a practically homogeneous speed distribution, the surface in which said fluid is in direct contact with the sheets of the exchanger is practically 100%. In addition to being suitable for the exchange of heat from solar radiation to a heat transfer fluid in solar collectors, the proposed design allows configurations that, with minimal modifications in the manufacturing process, allows the incorporation of watertight sub-volumes in the body of the exchanger. It can be perforated to allow the formation of convective cells in another fluid with which it is intended to exchange heat by immersion. (Machine-translation by Google Translate, not legally binding)

Description

Reed heat exchanger channelled.

Description of the invention

The problem of heat exchange in situations in which one of the bodies in thermal contact is a Liquid comes in countless practical situations in the most various fields of science and technology. In particular, the transfer of heat from solar radiation to a fluid heat carrier and from this one to a second fluid in a collecting tank has received a renewed interest in recent dates due to the boom extraordinary so-called renewable energy, in particular of the so-called solar thermal energy. This is because one of the most common ways of harnessing solar energy is its photothermal conversion, in which the energy of the solar radiation with flat plate collectors, evacuated or not, (low temperature systems, generally up to about 80 ° C in non-evacuated systems and about 120ºC in evacuated systems), or through solar concentrators (medium temperature systems). This energy is subsequently communicated to a transmission fluid thermal (heat carrier or heat carrier, in low systems temperature generally an aqueous solution of ethylene glycol or variable concentration propylene glycol in non-evacuated systems), that transports it - by forced driving or by system of thermosiphon- to an exchanger in which it communicates to a circuit closed fluid, for example intended for heating, or open, for example intended to supply domestic hot water.

On the other hand, the flat plate collectors not evacuated are essentially of two types: i) collectors formed by a set of tubes, usually of copper, welded by some procedure to a continuous or split plate (fins) of the same material blackened by some procedure to increase its absorbance (eg collectors of trademarks Viesmann, Chromagen, Solahart, Reisol, etc.); and ii) those whose operation is based on the circulation of the heat transfer fluid directly in contact with two sheets, usually metallic, blackened exposed to solar radiation, a system commonly known as sandwich type (eg Nordsoll type solar panel or Solahart Multiflow system, between others). There are also systems on the market that we can call mixed, in which a tube, in the form of a coil, is embedded between the two absorber sheets (eg utility model ES241433U). The performance of sandwich equipment is, in general, significantly higher than those of the first category mentioned above, because the contact surface of the heat-carrying fluid with the surface on which the solar radiation falls is much greater than in the tube collectors However, the technical difficulties of manufacturing these sensors (welding, treatments, design and implementation of the internal thermal conduction fluid conduction circuit ...) tend to be greater than that of the tube collectors which causes that, at present, The tube systems are widely majority in the market.

On the other hand, the choice of the appropriate topology for the internal path of the heat carrier fluid between the two sheets in sandwich systems is generally a complex problem, essentially having tested two types of solutions:

i) channeled systems in which the fluid circulates in laminar regime from the inlet to the outlet of the sensor through a duct stamped on one or both of the sheets that form the collector, usually with paths in grill connecting the inlets and outlets of the plate collector, with small fluid path lengths in the inside the plate, short residence times and low contact surface fractions of the heat transfer fluid with the license plate. Among other collectors that work through this procedure we can mention, by way of example, the marketed by the Brown Boveri Company (BBC) or the corresponding to patent application US2003 / 0121842 Flynn's Thiel Boutell & Tanis PC dated 07/17/2003.

ii) Labyrinth systems, in which the sheets are welded together by spot welding, in the that the fluid follows an essentially random path between the entry and exit, forced to circumvent the highlights defined by welding points, increasing fluid residence times between the plates, but generating high speed fields inhomogeneous and turbulence in the heat carrier fluid, as well as high pressure drops between fluid inlet and outlet of the plate. In this type of collectors are framed previously cited Nordsoll or Solahart Multiflow system, in addition of those marketed under the name Natural Energy Products (NEP), among others.

Due to these inconveniences, the collectors of ducted sheets are generally more effective in their practical operation than those of the labyrinthine system. The circuits usually used in the first usually offer a series of alternative channels to the heat transfer fluid, of such so that the different fluid volume elements do not follow identical trajectories inside the collector and often they have few or no pressure drops between the inlet and the Exchanger output. However, the surface fraction occupied by the channels through which the heat transfer fluid circulates on the total surface of the absorbent sheet is usually low, which causes very significant declines in performance of these sensors, since the surface of the sheet in contact direct with the fluid is the main variable that conditions the efficiency of these collectors, as highlighted by various authors (e.g. S. A Kalogioru, Prog. Energy and Combustion Sci. 30, 231 (2004)).

For its part, as stated, the collectors labyrinthine type combine low production costs with high surface fractions in direct contact with the fluid heat carrier However, the absence of a channel that leads the flow causes the appearance of strong inhomogeneities and turbulence in the velocity field of the heat transfer fluid in the inside of the collectors. In addition, the usually very small separation distance between the sheets produces high drops of pressure between the inlet and outlet of the absorber, forcing use high power pumps to maintain fluid flow in the inside of the collector, with the consequent increase in costs of operation.

The present invention consists of a reed heat exchanger that combines the advantages of both systems, channeled and labyrinthine, namely: channeled flow homogeneous laminar, high surface fractions in contact with heat transfer fluids, small pressure drops in the circuit and low production costs. Heat exchanger proposed has a high performance derived from the fact that the proposed topology for its inner channel allows to approximate the fraction of solar radiation absorbing surface in contact Direct with the heat transfer fluid at its maximum unit value. Further, the peculiarities of the chosen topology make the costs of exchanger production are noticeably lower than those of other currently existing channeled sheet systems.

It is, in short, to use in the exchanger a circuit that maximizes the surface of contact between solar radiation absorber plate and fluid heat carrier For this purpose, an exchanger consisting of two sheets, at least the previous one exposed to solar radiation incident, high thermal conductivity metal. On the sheet later, the one not shown to solar radiation, a channel is stamped of the characteristics shown in Fig. 1. The sheet exposed to solar radiation remains unfolded to standardize the angle of incidence of solar radiation on all points of its surface while lowering production costs. The two sheets will join along the separation zones stamped on the back sheet by some procedure suitable welding or with some interior sealant, as you can observe in Figs. 2.a) and 2.b), resulting in a circuit formed by a succession of parallel channels, watertight along its entire length with the exception of one of its ends, as indicated by the flow direction arrows in Fig. 1. The length of these fluid passage areas from one channel to the next is approximately equal to the width of the longitudinal part to minimize the formation of heat transfer fluid stagnation zones and avoid excessive pressure drops The mouths of entry and exit of exchanger could be as shown in Fig. 3, which would allow easy connection to a conversion facility photothermal solar radiation when making a change between a rectangular geometry at one end and a circular one at the other.

With the proposed topology for the channel, all the surface of the exchanger except the cords of welding or sealing used to join both sheets, is in direct contact with the fluid, while achieving a flow channeled inside the exchanger body. Using conventional welding techniques, the fraction of surface that is not in contact with the liquid is minimal, less than the from any ducted duct collector on the market. Thus, for example, using laser welding systems, for which The depth / width ratio of the weld bead ranges from 4 and 10, the lost surface per weld bead will range between 0.2% and 0.5%, being possible to obtain percentages of use of incident radiation on the top sheet greater than 99.5%.

The experimental results obtained with a realization of the proposed exchanger in its configuration of solar collector connected to an accumulator in which the heat absorbed is transmitted to a body of water by means of a coil of copper tube, have revealed that the current exchanger It has a greater capacity to capture solar energy compared to other ducted sheet collectors. Thus, in Fig. 4, show the water heating curves of the accumulator (39 liters) for a BBC brand sensor of approximately 1.14 m2 of exposed surface mentioned above, together with the corresponding to the currently proposed model. As can be seen in said figure, despite the lower exposed surface of the model manufactured, and at its largest interior volume - which would explain the most Slow temperature variation at short times, and could easily corrected by adjusting print thicknesses in final manufacturing - an embodiment as the proposal for the model supplies more power than the commercial plate. In particular, over 45 minutes the slope of the curve of heating, directly related to the heat output supplied to the tank - it is higher in the proposed model than in the commercial. In addition, with the model currently proposed, detects a temperature limit for long times, but while the slope of the heating curve in the commercial model preexisting is practically nil at times greater than 80 minutes, the slope of the heating curve of the current model It is not zero even after a time of 120 minutes.

On the other hand, as the power generated in the solar collectors is usually not enough to satisfy the instant demand, accumulator systems should be used to store surplus power produced during periods Low demand These systems consist of a warehouse in which The heat transferred to the heat transfer fluid in the collector is transmitted to a second fluid by some kind of heat exchanger, usually a metallic coil by immersion. Although in their practical use the two devices, collector and accumulator, are presented together in the large most installations, to date the two exchangers of heat have configurations and fabrications completely different, which would undoubtedly increase production costs with views of eventual joint production.

The peculiarities of the present design exchanger make it susceptible to, with minimum modifications in the manufacturing procedure, be used in fluid-fluid heat exchange processes by immersion and allow the formation of convective cells in the interior of the fluid to be heated, which significantly increases its industrial interest Thus, the case that the exchanger currently proposed to be used for transmission of heat between fluids without mixing these, a slight modification the design of its internal circuit allows it to be arranged in any of the guidance settings shown in the Figs. 5 and 6, in which it can be seen that the separations between Consecutive channels are now triangular in shape (Fig. 5), with welds in all its edges, defining watertight subvolumes in the body of the exchanger, being able to present perforations in the direction perpendicular to the bases of the parallelepiped of the form indicated to favor the flow of heat receiving fluid to through the bases of the exchanger and the subsequent formation of convective cells inside the liquid to be heated. In this case, and to achieve greater energy release in the exchanger is incorporated into the inlet mouth of the exchanger a diffuser mouth to cause the passage of the fluid to cool to the turbulent regime in a certain region at the entrance of the exchanger In this use of the exchanger it is convenient that the sheets that make up the exchanger are both high metal thermal conductivity to optimize heat transmission by immersion in the accumulator fluid.

In Fig. 1 the lower face of the exchanger with a scheme of the direction of fluid flow heat carrier in each of its interior channels, from the mouth of entrance to exit. In Fig. 2 two possible forms are shown of assembling the exchanger blades. Specifically, Fig. 2.a presents an outline of the result in the event that the procedure is by welding, and Fig. 2.b represents the result of applying some type of sealant to the upper sheets and bottom of the exchanger. On the other hand, in Fig. 3 they are shown the profile and the plan of possible mouths of entry and exit of the heat transfer fluid inside the exchanger. Fig. 4 shows the results of the comparative tests performed with the proposed exchanger in use of solar collector and a commercial one of the BBC brand of approximately 1.14 m2 of exposed surface. The squares correspond to the fluid heating curve of the accumulator through the commercial panel, while the rectangles represent the heating curve of the model currently proposed. In Fig. 5 the lower face of the proposed exchanger in a configuration suitable to be used as a fluid to fluid heat exchanger, showing the underside of it with the proposed holes for the facilitation of the convection of the fluid to be heated through of the exchanger. Finally, Fig. 6 shows a configuration of the exchanger suitable for use inside cylindrical accumulators

Embodiment of the invention

A particular system configuration described consists of two 1000 series aluminum sheets, one flat and the other stamped by pressing, both 1x1 m2 of surface. The sheets were welded together by TIG welding in all its perimeter and in various points to along the 0.91 m long segments and parallel to each other corresponding to the ridges of the channels stamped by MIG welding, in such a way that they define basic parallelepipeds rectangular connected to each other by one end of its sides mediators, allowing the passage of heat transfer fluid between them, through which it circulates in alternative senses. In the exchanger two mouths of the shape and dimensions described in Fig. 3 at opposite ends of the body, by which one is driven a fluid through a drive pump. All the elements of volume of the fluid thus travel a distance of 11 m in contact direct with the exchanger blades, the surface being exposed to direct contact with the heat transfer fluid higher than 99.5% of the total sheets. Except in the changes of meaning to end of each of the channels, to the usual flows of operation of commercial pumps (approximately 2 l / min), the fluid travels this distance in laminar regime, with low energy dissipation

In use as a solar collector, the exchanger, with the flat sheet anodized in black chrome with absorption coefficient of 0.95, is introduced inside a aluminum frame box at a distance of 2-3 cm. of a high transmittance crystal due to its low content in iron on the front. Flat sheet anodizing guarantees high corrosion resistance, avoiding deterioration suffered by some paints that are sometimes used, while increasing the absorbing power of solar radiation by acting as a selective surface in its absorption. In the part later a rock wool, polyester, insulator is introduced polyurethane foam or other material, and closes with a lid aluminum.

Claims (3)

1. High-performance sheet heat exchanger, consisting of two sheets, at least one metal of high thermal conductivity, welded or bonded together in its entire perimeter, characterized in that at least one of the sheets has been stamped a channel that is extends between an inlet and an outlet and is constituted by a succession of sections in the form of parallelepipeds of approximately rectangular section and length equal to that of one of the dimensions of the sheet minus the sum of the lengths of the welds of the bases; the sections are attached to each other sharing one of its side walls and are watertight throughout its length due to the junctions of the crests of the separations with the other sheet except at one of its ends, through which it communicates with the adjoining by one of its sides allowing the continuous flow of a heat transfer fluid in alternate directions in contiguous sections along the entire length of the channel. 2. The sheet heat exchanger according to claim one, characterized in that the divisions between adjacent sections form an angle to each other in such a way that they define watertight subvolumes on the surface of the exchanger, allowing perforations in the direction perpendicular to the sheets of the exchanger body, and which can take a flat or cylindrical shape. 3. Sheet heat exchanger according to claim 2, characterized by incorporating diffuser grilles in the inlet mouths of the exchanger.