Heat exchanger, method for operating the heat exchanger and use of the heat exchanger in an air conditioner   

Abstract: In a heat exchanger comprising a capillary tube register in which a fluid to be cooled and/or heated is routed through capillary tubes and in which air flows around the capillary tubes in countercurrent flow relative to the fluid, the capillary tube register comprises at least one capillary tube mat formed from longitudinal and lateral capillary tubes connected together in a mesh pattern for fluid throughput. At least the longitudinal capillary tubes are each connected by way of the ends thereof to a common trunk for feeding, and to a common trunk for discharging, the fluid.

The invention relates to a heat exchanger according to the preamble of claim 1, a method for operating this heat exchanger and also a use of at least two of these heat exchangers in an air conditioner.

Capillary tubes offer good qualifications for use for example in air/water heat exchangers. They require relatively little and also economical material for production thereof and offer a relatively large outer surface for the heat exchange and hence a heat exchanger value which is higher by a multiple, for example in comparison with plate heat exchangers. In addition, they are corrosion-resistant to water and sorption solutions. Flexible plastic material tubes with an outer diameter of 0.5 to 5 mm are termed capillary tubes.

The capillary tubes are generally combined to form mats, the tubes being disposed parallel to each other at a spacing of approx. 10 to 20 mm and, at the one end, are connected to a common branch for the inflow of water or of another heating or cooling fluid and also, at the other end, to a common branch for the return flow of the water or other heating or cooling fluid. The capillary tubes are retained in their mutual position by spacers. Such a mat is shown for example in DE 196 40 514 A1. Nevertheless, these capillary tube mats still do not produce satisfactory efficiency for a heat exchanger. Also the material expenditure for production thereof is still considerable due to the use of the spacers.

It is therefore the object of the present invention to indicate a heat exchanger having a capillary tube register through which a fluid to be cooled or to be heated is led, the capillary tube register being subjected to a flow of air in counterflow to the fluid, which heat exchanger has at least a higher efficiency than previous heat exchangers using capillary tube mats.

This object is achieved according to the invention by a heat exchanger having the features of claim 1. Advantageous developments of this heat exchanger, a preferred method for operating this heat exchanger and also an expedient use of at least two heat exchangers in one air conditioner are revealed in the sub-claims.

As a result of the fact that the capillary tube register consists of at least one capillary tube mat which is formed from capillary longitudinal and capillary transverse tubes which are connected to each other in the manner of a network for the fluid passage, at least the capillary longitudinal tubes being connected respectively to one branch for the supply or discharge of the fluid, the heat exchanger surface can be significantly enlarged relative to the use of a mat consisting only of capillary longitudinal tubes, possibly can even be doubled, so that the efficiency of the heat exchanger is also correspondingly increased. Since the capillary transverse tubes ensure the mutual spacing of the capillary longitudinal tubes, the spacers are also dispensed with, it being assumed therefrom that the material expenditure for the capillary transverse tubes corresponds approximately to that for the spacers.

The configuration of the mat with capillary longitudinal and capillary transverse tubes also makes it possible to control the flow course of the fluid in the mat by blocking the passage in individual capillary longitudinal and/or transverse tubes as desired. As a result, the mat can also be provided with recesses both in the interior and at the edge or a meandering flow course can be adjusted in the mat. It is consequently possible also to configure the supply and/or discharge line for the fluid at the respective ends of the capillary tubes to be shorter than the corresponding side of the mat so that the flow of the air to be cooled or to be heated through these is much less impeded.

The capillary tubes of the mat can be disposed such that the capillary longitudinal and the capillary transverse tubes extend at a right angle relative to each other. However, it is more advantageous for the flow course if the capillary longitudinal and trans-verse tubes intersect at an angle deviating from a right angle by 5° to 20°. It is particularly advantageous in this respect if the capillary longitudinal and transverse tubes intersect at a right angle, however are inclined respectively by 45° relative to the edges of the mat and hence relative to the branches. In this case, both the capillary longitudinal and the capillary transverse tubes are connected directly to the branches.

For operation of the heat exchanger for humidifying or dehumidifying air, the outer surface of the capillary tubes can be configured to be hydrophilic or water-spreading, which surface is wetted as uniformly as possibly with water for humidifying or with a sorption solution for dehumidifying. For uniform wetting, application of a fleece or a layer of water-spreading material on the surface of the capillary tubes is recommended. The evaporation heat required in the air humidification is provided by the fluid flowing through the capillary tubes; on the other hand, the fluid must absorb the corresponding condensation heat during an air dehumidification.

Uniform wetting is required in order that the required quantity of sorption solution is as low as possible. Since the desired heat exchange is intended to be effected between the fluid and the air, heat absorption by the sorption solution is disruptive since this represents a heat loss. However this heat loss is all the greater, the greater the quantity of sorption solution used. Therefore, the quantity ratio of sorption solution to fluid flowing through the capillary tubes should be no more than 5%, preferably no more than 1%. This can however only be achieved by as uniform a wetting of the capilLary tubes as possible.

The invention is explained in more detail subsequently with reference to embodiments represented in the Figures. There are shown:

FIG. 1 a capillary tube mat with an inner cut-out,

FIG. 2 a capillary tube mat with an edge cut-out,

FIG. 3 a capillary tube mat with a shortened branch for discharge of the fluid,

FIG. 4 a capillary tube mat with a meandering flow course,

FIG. 5 a capillary tube mat having capillary longitudinal and transverse tubes extending at respectively 45° to the branches,

FIG. 6 a heat exchanger having a plurality of parallel capillary tube mats, and

FIG. 7 the schematic representation of an air conditioner,

FIG. 8 a heat exchanger with fluid distribution chambers in the surface of the capillary tube mat in plan view,

FIG. 9.1 the upper film of the capillary tube mat and the cover of the upper chamber in section, shown in FIG. 8,

FIG. 9.2 the upper chamber in FIG. 8 in section before welding of the two films of the capillary tube mat,

FIG. 9.3 the upper chamber in FIG. 8 in section after welding of the two films of the capillary tube mat,

FIG. 9.4 a horizontal section through the chamber according to FIG. 9.3,

FIG. 10 a vertical section through a chamber with a modified configuration, and

FIG. 11 the plan view on a heat exchanger having collection tubes outside the area of the capillary tube mat.

FIG. 1 shows a capillary tube mat having capillary longitudinal tubes 1 and capillary transverse tubes 2 which intersect at a right angle and the interiors of which are connected to each other at the intersection points respectively such that a fluid flowing in the one capillary tube can enter into the other capillary tube. The capillary longitudinal tubes 1 are connected in common by their upper end to a branch 3 for the supply of a fluid, preferably water, and in common by their lower end to a branch 4 for discharge of the fluid. The fluid is hence moved in the direction indicated by the arrow 5 through the mat, said fluid flowing however not only through the capillary longitudinal tubes but also through the capillary trans-verse tubes 2. Since the capillary transverse tubes 2 have the same mutual spacing as the capillary longitudinal tubes 1, their entire length is equal to that of the capillary longitudinal tubes 1 and hence the surface available for heat exchange is twice as great as in the case of a mat consisting only of capillary longitudinal tubes. Correspondingly, the efficiency is also higher. The capillary transverse tubes 2 also ensure that the mutual spacing of the capillary longitudinal tubes 1 is not altered. Hence spacers can be dispensed with.

The capillary tube mat in FIG. 1 contains an inner cut-out 6 which is free of capillary tubes. The capillary tubes opening at the cut-out 6 are configured immediately in front of these with clamps 7 so that no fluid can emerge from them but can be diverted in advance into an intersecting capillary tube.

Production of the grid-shaped capillary tube mat is relatively simple. Firstly, two half-shells are produced with respectively the contour of half capillary tubes and the two half-shells are then welded together. Clamping of the capillary tubes can be effected in the case of a finished mat in such a manner that the relevant capillary tube is pressed together and the compressed inner wall is welded by heat supply.

The capillary tube mat according to FIG. 2 corresponds to that according to FIG. 1, however the latter is not provided with an inner cut-out but with an edge cut-out 8.

In the case of the capillary tube mat according to FIG. 3, the lower branch 4 for discharge of the fluid is greatly shortened and the capillary longitudinal tubes 1 not connected to this branch are provided with clamps 7 at their lower end so that the fluid is diverted from these through the capillary transverse tubes 2 to the capillary longitudinal tubes 1 connected to the branch 4. In order that the flow paths for the fluid are extensively uniform, barriers 9 formed by clamping are provided furthermore in the capillary longitudinal tubes 1 which are connected to the branch or abut directly so that also the fluid flowing through these passes only via a diversion to the branch 4.

The capillary tube mat according to FIG. 4 comprises two barriers 9 which are obtained by clamping the capillary longitudinal tubes 1 and extend from the opposite edges of the mat respectively over half of the width thereof in the direction of the capillary transverse tubes 2. As a result, the flow path of the fluid is extended in a meandering shape. This can be sensible if the fluid/air quantity ratio is small since the flow rate of the fluid should not fall below a minimum value because otherwise the heat exchange between fluid and air drops and the flow of the fluid becomes non-uniform.

In the case of the capillary tube mat shown in FIGS. 1 to 4, having a fluid feed only into the capillary longitudinal tubes and having capillary longitudinal and transverse tubes which intersect perpendicular to each other, a diversion of the fluid by 90° is effected at the connection points. This produces sufficient throughflow even of the capillary transverse tubes, this being able however to be improved by the capillary transverse tubes extending not at a right angle but at an angle deviating from this by approximately 5° to 20°. As a result, the partial flow of the fluid passing through the capillary transverse tubes can be increased, which effects an increase in heat exchange between fluid and air.

FIG. 5 shows a particularly advantageous configuration of the capillary tube mat. The capillary longitudinal tubes 1 and the capillary transverse tubes 2 in fact likewise intersect each other at a right angle, however they extend respectively at an angle of 45° relative to the branches 3 and 4 and are also respectively connected directly to these. The fluid hence flows out of the branch 3 directly both into the capillary longitudinal tubes 1 and into the capillary transverse tubes 2 so that these are hence supplied to the same degree and only a small fluid exchange between them is effected. However, it is ensured that the heat exchange capacity of the capilLary longitudinal tubes 1 and of the capillary trans-verse tubes 2 is mutually equal, as a result of which optimum efficiency is achieved.

FIG. 6 shows the use of grid-shaped capillary tube mats, as represented for example in FIGS. 1 to 5, in an air/water heat exchanger. The capillary tube mats 10 reproduced in side view are disposed parallel to each other and vertically in one housing 11. The respective branches 3 of the individual mats are connected to a common flow line 12 for the water (fluid) and the respective branches 4 of the mats 10 are connected to a common return flow line 13. The air to be heated or to be cooled or respectively to be humidified or to be dehumidified flows parallel to the capillary tube mats 10 in counterflow to the water, i.e. from bottom to top, as is indicated by the arrows 14, 15, through the housing 11.

For the purposes of humidifying or dehumidifying the air, the capillary tubes of the mats 10 are provided on their outer surface with a hydrophilic or water-spreading coating. The latter is supplied at as high a position as possible of the respective mat 10, water in the case of humidifying and a sorption solution in the case of dehumidifying, which consists for example of an aqueous lithium chloride solution. The hydrophilic or water-spreading coating serves for the purpose of wetting the capillary tubes of the mats 10 over their total length as uniformly as possible with the water or the sorption solution. For this purpose, a fleece-like coating has proved to be particularly advantageous.

Due to gravity and also due to capillary effect, the water or the sorption solution is distributed uniformly over the length of the capillary tubes. For this purpose, the configuration of the capillary tube mat according to FIG. 5 is more suitable than that according to FIGS. 1 to 4 since all the capillary tubes are inclined to the same degree relative to the horizontal.

When flowing down the capillary tubes of the mats 10, the sorption solution absorbs moisture from the counterflowing air and is conducted with the absorbed water at the lower end of the mat 10 into a collection receptacle. It can then be regenerated and supplied again to the mats. The heat produced by the condensation of the moisture contained in the air is transferred by heat exchange to the water in the capillary tubes and discharged through the latter. Conversely, the heat required during air humidification for the evaporation of the water on the capillary tubes is brought via the water flowing in the capillary tubes.

In general, it applies for air/water heat exchangers that the highest efficiency is achieved if the so-called water number, i.e. the ratio of temperature change of the air to the temperature change of the water, is the same over the entire surface. This requirement does not present a problem in the case of dry cooling of air because the specific heat of the air, like that of the water, remains constant. In the case of simultaneous dehumidification of the air, the specific heat capacity of the air can however rise, due to the released condensation heat, to a multiple of the value of the dry air and in fact, at higher air temperatures, greater than with lower.

If now a capillary tube mat according to FIG. 4 with a meandering fluid flow is used, then the dwell time of the fluid (water) in the region of greater dehumidification can be increased by a formation which meanders to different degrees and consequently the water number for both media can be kept approximately constant.

Since the degree of dehumidification can change greatly during operation, the meandering formation is designed for the operating point at which high efficiency is particularly important.

FIG. 7 shows schematically an air conditioner in which two heat exchangers according to FIG. 6 are used. In the case of this air conditioner, extremely high heat recovery takes place, which makes additional heating or cooling of the ingoing air superfluous by a heat exchanger respectively being connected as enthalpy exchanger for the ingoing air and the outgoing air.

In summer operation, the ingoing air 16 is cooled and dehumidified in a first enthalpy exchanger 17. The cooling water flows in circulation through both heat exchangers. In the register of the first enthalpy exchanger 17, it is heated during cooling and dehumidification of the ingoing air 16. In the register of the second enthalpy exchanger 18, the cooling water is cooled again by the outgoing air 19 after this has been cooled adiabatically to the dew point temperature thereof in a preceding humidifier. The outgoing air 19 is consequently heated and humidified and subsequently discharged from the building.

In the upper part of the register of the first enthalpy exchanger 17, the coated capillary tubes are subjected to a sorption solution which diffuses down-wards inside the coating, it being enriched with water formed by condensation of air moisture.

In the same way, water in the upper part of the register of the second enthalpy exchanger 18 is supplied to the coated capillary tubes, which water is at least partly evaporated and discharged with the outgoing air 19.

FIG. 8 shows a capillary tube mat having capillary longitudinal tubes 1 and capillary transverse tubes 2 which intersect at a right angle and the interiors of which are connected to each other at the intersection points respectively such that they form a connected space. At the edges of the capillary tube mat, no connection of the interior of the capillary tubes to the exterior exists.

Within the area occupied by the capillary mat, there are situated two chambers 20 and 21, on the one hand for the supply of a fluid into the interior of the capillary tubes and, on the other hand, for discharge of the fluid out of the latter. In the chambers 20 and 21, the capillary longitudinal and transverse tubes respectively leading to the latter or leading away from the latter are interrupted, i.e. two longitudinal tubes 1 and two transverse tubes 2 in the upper chamber 20 and also three longitudinal tubes 1 and two transverse tubes 2 in the lower chamber 21. A connection exists between the interior of the interrupted capillary tubes and the interior of the respective chamber so that fluid situated in the capillary tubes can enter into the chamber and fluid situated in the chamber into the capillary tubes. The chambers 20 and 21 are provided in addition with respectively one connection for the supply or discharge of the fluid, not shown, from/to the exterior; thus for example the chamber 20 is connected via the connection to a supply line and the chamber 21 via the connection to a discharge line.

The position and the size of the chambers is chosen such that the fluid flowing through the capillary tube mat is distributed as uniformly as possible over the latter. If necessary, also a plurality of smaller chambers can be provided at various positions of the capillary tube mat respectively for the supply and/or discharge of the fluid. The size and the distribution of the chambers is essentially determined by the quantity of fluid led through the capillary tube mat and the permissible pressure loss.

The construction for example of the chamber 20 is represented in FIGS. 9.1 to 9.4. It is thereby assumed that the capillary tube mat does not consist of individual tubes but consists of two continuous plastic material films, the one of which has raised portions corresponding to the course of the capillary tubes and the other complementary concave recesses. For production of the capillary tube mat, the films are welded together over their entire surface, a raised portion and a recess respectively forming one capillary tube. At the edge of the mat, the relevant capillary tube can be pressed together and the compressed inner wall can be welded by heat supply so that the connection to the exterior is interrupted.

FIG. 9.1 shows one part only of the upper film 22 with raised portions which respectively form the upper half 1.1 of the capillary longitudinal tubes 1 and also the upper half 2.1 of the capillary trans-verse tubes 2. In the region of the chamber 20, the film 22 is cut out, the square cut-out 23 extending over two capillary longitudinal tubes 1 and, perpendicular thereto, over two capillary transverse tubes 2.

A cover 24, the side walls of which form a square and surround the cut-out 23, is placed on the film 22. The underside of the side walls is adapted to the contour of the film 22, i.e. it is flat in the regions between capillary tubes and provided with recesses corresponding to the raised portions in the region of the capillary tubes. The cover 24 preferably consists of plastic material and is welded to the film 22 in a fluid-impermeable manner.

FIG. 9.2 shows a corresponding part of the lower film 25 with recesses corresponding to the raised portions in the film 22, which recesses form respectively the lower half 1.2 of the capillary longitudinal tubes 1 and also the lower half 2.2 of the capillary trans-verse tubes 2. The lower film 25 is continuous, i.e. without a cut-out corresponding to the cut-out 23 in the upper film 22. If therefore the lower film 25, according to FIG. 9.2, is placed against the upper film 22 and welded, not only the capillary longitudinal and transverse tubes 1, 2 are formed but also the chambers 20 and 21 are closed from the bottom.

FIG. 9.3 shows the closed chamber 20 after welding of the films 22 and 25. Since the upper film 22 is cut out inside the chamber 20, a connection between the interior of the chamber 20 and the interior of the capillary longitudinal and transverse tubes opening into the chamber exists. Since the cover 24 is provided in addition with a connection, not shown, for the connection via a branch tube to a fluid source or fluid sink, the capillary tube mat can be fed with fluid via the chamber 20 or fluid can be withdrawn therefrom. The configuration of the connections can be very varied; they can be disposed in the side wall or in the cover of the chamber 20.

In FIG. 9.3, the flat regions of the films 22 and 25 which are situated between the intersecting capillary longitudinal and transverse tubes 1, 2 of the mat are cut out outside the region of the cover 24 so that only the tube network still remains. As a result, the weight of the capillary tube mat can be reduced and a saving in material is achieved.

The films 22 and 25 have a thickness of only 0.2 to 0.3 mm. The capillary tubes formed from the semicircular raised portions and recesses have an inner diameter of e.g. 1.0 mm. The tensile stress occurring in the film is proportional to the thickness of the film, to the inner diameter and to the inner pressure. The film thickness is designed for example such that, at an inner pressure of 10 bar, the maximum stress permitted for the film material which is normally polypropylene is not exceeded. This then applies for the respective inner diameter of the capillary tubes; if however the inner diameter is doubled, the film thickness must also be doubled in order that the tensile stress remains the same.

The cut-out in the film 22 in the region of the chambers in fact causes an increase in the inner diameter in the film 25 on the inner spacing of two oppositely situated side walls of the cover 24. This results in a corresponding increase in tensile stress in the film 25. Accordingly, the film 25 in the region of the chambers would have to be correspondingly thickened. Since this would lead to significant difficulties with respect to manufacture, the film 25 could also be formed to be correspondingly thicker over the entire length, which however leads to greatly increased material expenditure.

For this reason, the cover 24 in the interior, as a function of the size thereof, has at least one tie rod 26, the upper end of which is fixed in the cover 24 and the lower end of which, which protrudes down-wards beyond the side walls of the cover 24 by the thickness of the film 22, is welded to the film 25. The tie rod 26 is therefore preferably configured in one piece with the cover 24 and consists of the same material as said cover.

The plan view on a horizontal section through the chamber according to 9.4 shows that the tie rod 26 has a cross-section which corresponds to the flat region of the film 25 which is surrounded by two adjacent lower halves 1.2 of the capillary longitudinal tubes 1 and two adjacent lower halves 2.2 of the capillary transverse tubes 2. This flat region is hence connected completely to the tie rod 26. The number of tie rods 26 is based on the size of the respective chamber, i.e. the number of flat regions of the film 25 surrounded by the cover 24. The chamber 21 in FIG. 8 correspondingly has two tie rods 26.

The tie rods 26 have the effect that the tensile stress in the film 25 does not increase in the region of the chambers and that bulging of the film 25 due to the fluid pressure in the interior of the chambers is prevented.

If the capillary tube mat is produced such that firstly the cover 24 and the tie rod 26 are welded to the upper film 22 and only then is the cut-out 23 produced, then only the corresponding capillary longitudinal and transverse tubes 1, 2 are cut out, whilst the flat part of the film 22 situated between the cut-out tubes remains under the tie rod 26 and subsequently is welded to the lower film 25. The lower surface of the tie rod 26 is then situated in one plane with the lower end-faces of the side walls of the cover 24.

The upper end of the tie rods 26, as FIGS. 9.1 to 9.3 show, can be mounted on the top of the cover 24 or, as FIG. 10 shows, on a transverse web 27 supported by oppositely situated side walls. The trans-verse web 27 must not however impede the flow of the fluid between capillary tubes and the connection line.

The embodiment according to FIG. 11 can be advantageous if it is not sensible for reasons of space to use the covers which protrude out of the mat plane and also the connection lines which extend inside the mat area but likewise outside the mat plane for the fluid. This is the case for example if the mats are used in air heat exchangers in which a plurality of mats are disposed tightly in succession.

A mat is used here, in the case of which the intersecting capillary tubes extend not parallel or at right angles but respectively inclined by 45° to the sides of the mat. For differentiation thereof, the tubes extending from bottom left to top right are termed capillary longitudinal tubes 1 and the tubes extending from bottom right to top left as capillary transverse tubes 2. The capillary longitudinal tubes and the capillary transverse tubes can however also extend at angles other than 45° relative to the sides of the mat.

The intersecting capillary tubes are closed at the longitudinal sides of the mat. At the end-sides, the two respectively intersecting capillary tubes are unclosed and are led out of the mat surface as a common connection tube 28. The number of connection tubes 28 on each end-side is hence reduced to half relative to the number of capillary tubes forming the mat, i.e. the capillary longitudinal tubes 1 and the capillary transverse tubes 2.

The connection tubes 28 in turn are connected to each other via transverse tubes 29 in the most favourable manner for the respective application case, with the aim of further reducing the number of tubes. Thus the example represented in FIG. 11 shows forty mat tubes 1, 2, twenty connection tubes 28 and finally five transverse tubes 29.

The free ends of the transverse tubes 29 are guided such that they extend parallel to each other and open in through-holes in the wall of a branch tube 30 at as narrow a mutual spacing as possible. The spacing can be kept very low as a result of the fact that the tube ends are inserted into the through-holes and then the tube ends are welded inside the through-holes to the wall thereof. The through-holes for a capillary tube mat are disposed adjacently in the circumferential direction of the branch tube 30 and the through-holes for the plurality of capillary tube mats situated in succession are disposed in succession in the longitudinal direction of the branch tube 30. The branch tube 30 on the one end-side of the capillary tube mats serves for supplying the fluid to the latter and the branch tube 30 on the other end-side of the capillary tube mats serves for discharging the fluid from these.

The total arrangement comprising capillary longitudinal and transverse tubes 1, 2, the connection tubes 28 and the transverse tubes 29, as in FIGS. 9.1 to 9.4, for the capillary tube mat according to FIG. 1, can consist of two plastic material films which are welded together. It is thereby advantageous in particular for manufacturing reasons if the capillary longitudinal and transverse tubes 1, 2, the connection tubes 28 and the transverse tubes 29 have the same inner diameter. The flow speed of the fluid in these tubes is then different corresponding to the ratio of their number.

In order to be able to insert the free ends of the transverse tubes 29 in the through-holes in the branch tube 30, the flat film regions between these ends must be cut out insofar as these are inserted in the through-holes.