HEAT EXCHANGER, METHOD FOR OPERATING THE HEAT EXCHANGER AND USE OF THE HEAT EXCHANGER IN AN AIR-CONDITIONING SYSTEM

Abstract

In a heat exchanger having a bank of capillary tubes, in which a fluid to be cooled and/or heated is conducted through capillary tubes and in which the capillary tubes are wetted, in concurrent flow with the fluid, by water or by a hygroscopic sorption solution, and in which air flows around said capillary tubes in countercurrent flow in relation to the fluid, the bank of capillary tubes is composed of at least one tube mat, the capillary tubes of which have a hydrophilic or water-dispersing surface with a contact angle of less than 20°.

Description

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.

A heat exchanger with a capillary tube register through which a fluid to be cooled or heated is guided is known from EP 0901 601 B1. The tube register is sprinkled with water in parallel flow to the fluid and subjected to a flow of air in counterflow to the fluid. The spaces between the capillary tubes are at least partially filled with foamed material, as a result of which the heat exchanger surface is enlarged. One possibility for producing this heat exchanger resides in coating the capillary tubes themselves with a foamed material coating. The foamed material layer can thereby consist of the same material as the capillary tube. However, it has been shown that uniform sprinkling of the foamed material layer is not possible. This is particularly true if sprinkling is effected with a sorption solution for dehumidifying the air instead of with water. In order to obtain satisfactory efficiency of the heat exchanger, the quantity of sorption solution should be as low as possible, if possible not more than 5% and preferably not more than 1% of the quantity of the fluid flowing through the capillary tubes. These values were however not able to be achieved for a uniform wetting of the foamed material layer.

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 tube register being wetted with water or a hygroscopic sorption solution in parallel flow to the fluid and 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 tube mat, the capillary tubes of which have a hydrophilic or water-spreading surface with a contact angle below 20°, uniform wetting of the capillary tubes takes place even with a very small quantity of water or sorption solution. Since the desired heat exchange is intended to be effected between the fluid and the air, heat absorption by the non-evaporated water or the sorption solution is disruptive since this represents a heat loss. However this heat loss is all the greater, the greater the quantity of water or of sorption solution. Therefore, the quantity ratio of water or sorption solution to fluid flowing through the capillary tubes should be no more than 5%, preferably no more than 1%, without uniform wetting of the capillary tubes being impaired.

In order to obtain a hydrophilic or water-spreading surface, the capillary tubes are preferably covered with a fleece. For uniform wetting, in particular a fleece made of glass fibres having a diameter of 0.1 to 0.5 mm has thereby proved favourable.

The plastic materials, such as e.g. polypropylene, from which the capillary tubes are produced, normally have a low solid body-surface tension and are therefore difficult to wet with water or aqueous solutions. It results therefrom that they have no or only negligibly few polar groups in their structure. Therefore, in order to achieve good wettability, they are coated advantageously with water-spreading material. Water-spreading plastic material is known for example from EP 0 149 182 B2.

In order to make possible good adhesion of the water-spreading layer on the capillary tubes, a bonding agent layer can be disposed between these. This contains polar groups in sufficient quantity and is insoluble and non-swelling in water. It can consist for example of a 2.5% solution of a mixed polymer made of 87.6% by weight of methylmethacrylate and 12.4% by weight of y-methacryloxypropyltrimethyoxysilane and have a thickness of 0.01 to 2 μm.

The capillary tube mat can advantageously be formed from capillary longitudinal and 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 in common by their ends respectively to one branch for the supply or discharge of the fluid. As a result, 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 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 transverse 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 in fact 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.

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 having longitudinal and transverse tubes 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 tube 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. 1 shows a capillary tube mat having capillary longitudinal tubes 1 and transverse tubes 2 which intersect at a right angle, have a hydrophilic or water-spreading surface 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 the 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 transverse 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 transverse tubes 2 is mutually equal, as a result of which optimum efficiency is achieved.

FIG. 6 shows the use of 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 precursor 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 have a hydrophilic or water-spreading surface with a contact angle below 20°. This 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 capillary tubes of the mats 10 are as a result wetted over their total length as uniformly as possible with the water or the sorption solution. For this purpose, a coating made of fleece or a water-spreading material is provided on the capillary tubes.

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 quantity, 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 meandering formation to different degrees and consequently the water quantity 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 downwards 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.

Claims

1. A heat exchanger comprising:

a capillary tube register, through which a fluid to be cooled and/or to be heated is led, the capillary tube register configured for being wetted with water or a hygroscopic sorption solution in parallel flow to the fluid and subjected to a flow of air in counterflow to the fluid,

wherein the capillary tube register comprises at least one capillary tube mat comprising capillary tubes, the capillary tubes comprising a hydrophilic or water-spreading surface with a contact angle below 20°.

2. The heat exchanger according to claim 1, wherein the capillary tubes are covered with a fleece.

3. The heat exchanger according to claim 2, wherein the fleece comprises glass fibres having a diameter of 0.1 to 0.5 mm.

4. The heat exchanger according to claim 1, wherein the capillary tubes are coated with a layer comprising water-spreading material.

5. The heat exchanger according to claim 4, comprising a bonding agent layer between the capillary tubes and the layer comprising water-spreading material.

6. The heat exchanger according to claim 1, wherein the capillary tube mat comprises capillary longitudinal and 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 in common by their ends respectively to one branch for the supply and discharge of the fluid.

7. The heat exchanger according to claim 6, configured, for control of the flow course of the fluid in the capillary tube mat, wherein the passage for the fluid is blocked in individual capillary longitudinal and/or transverse tubes.

8. The heat exchanger according to claim 7, wherein at least one branch of the capillary tube mat is shorter than a length of a side of the capillary tube mat parallel thereto.

9. The heat exchanger according to claim 7, wherein the capillary tube mat comprises cut-outs in the interior or at the edge.

10. The heat exchanger according to claim 7, wherein a flow course in the capillary tube mat is meandering.

11. The heat exchanger according to claim 10, wherein a degree of meandering of the flow course inside the capillary tube mat changes.

12. (Canceled)

13. The heat exchanger according to claim 6, wherein the capillary longitudinal tubes and the capillary transverse tubes respectively extend diagonally relative to the branches.

14. The heat exchanger according to claim 6, wherein the capillary longitudinal tubes and the capillary transverse tubes extend respectively at an angle of 45° relative to the branches for the supply and discharge of the fluid and are connected directly to these.

15. The heat exchanger according to claim 1, wherein the capillary tube register comprises a plurality of capillary tube mats which are disposed parallel to each other, having a common supply line for the fluid on a side and a common discharge line for the fluid on an oppositely situated side.

16. A method for operating a heat exchanger, the method comprising:

altering a humidity of air using a capillary tube register, through which a fluid to be cooled and/or to be heated is led, the capillary tube register configured for being wetted with water or a hygroscopic sorption solution in parallel flow to the fluid and subjected to a flow of air in counterflow to the fluid, wherein the capillary tube register comprises at least one capillary tube mat comprising capillary tubes, the capillary tubes comprising a hydrophilic or water-spreading surface with a contact angle below 20°;

wherein the altering the humidity of air comprises dehumidifying air or humidifying air, wherein the dehumidifying comprises wetting the surface of the capillary tubes uniformly with a hygroroscopic sorption solution and discharging condensation heat of moisture withdrawn from the air using the fluid which is lead through the capillary tube register, and wherein the humidifying comprises wetting the surface of the capillary tubes uniformly with water and delivering evaporation heat for humidifying using the fluid which is lead through the capillary tube register.

17. The method according to claim 16, wherein the altering the humidity comprises dehumidifying and wherein the sorption solution is an aqueous lithium chloride solution.

18. The method according to claim 16 wherein the altering the humidity comprises humidifying air, including wetting the surface of the capillary tubes uniformly with water and delivering the evaporation heat for humidifying the air using the fluid which is led through the capillary tube register.

19. The method of claim 16, comprising using at least two of the heat exchangers in an air conditioner, including subjecting the heat exchangers to a flow of the fluid in a closed circulation in succession, the first heat exchanger being used for cooling and dehumidifying the ingoing air and the second heat exchanger for cooling the fluid by the outgoing air.

20. The method according to claim 19, comprising cooling the outgoing air adiabatically to the dew point temperature thereof before flowing through the second heat exchanger.

21. The method according to claim 19, comprising wetting the surface of the capillary tubes of the first heat exchanger with sorption solution and that of the second heat exchanger with water.