WO1997003333A1

WO1997003333A1 - Regenerative heat exchanger

Info

Publication number: WO1997003333A1
Application number: PCT/NL1996/000284
Authority: WO
Inventors: Berend Philippus Ter Meulen
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 1995-07-12
Filing date: 1996-07-12
Publication date: 1997-01-30
Also published as: AU6472096A; NL1000786C2

Abstract

Regenerative heat exchanger comprising a first heat exchange element (1) through which a first medium to be treated (4) is to flow, and a second heat exchange element (2) through which a second medium to be treated (5) is to flow, and a transfer medium (3) for interaction with the first and the second heat exchange element in order to be in a heat exchange and mass transfer relationship with the first and the second medium, respectively, wherein the transfer medium (3), is kept separate from the first (4) and the second (5) media by means of one or more permeable, heat exchange and mass transfer barrier layers, the permeability of which is chosen to allow the intended mass transfer between the first or second medium and the transfer medium to take place, but essentially to prevent mixing of the first or second medium with said transfer medium.

Description

REGENERATIVE HEAT EXCHANGER

The invention relates to a regenerative heat exchanger of the type indi¬ cated in the preamble of the appended Claim 1. In known heat exchangers of the abovementioned type, heat exchange and mass transfer between the first or second medium respectively and the transfer medium take place on direct contact between the various media. It is known, for example, to insert a transfer medium in the form of a solid body alternately into the first and the second medium stream. Since the transfer medium constructed as a solid body is frequently porous, some of the first medium can mix, via said solid body, with the second medium, or vice versa.

The use of a solid body as transfer medium and mixing of the first and second media by the intervention of said transfer medium are disadvantage- ous.

In another known type of regenerative heat exchanger, the first and the second medium flow alternately through the heat exchange element, whilst the transfer medium constructed as a solid, preferably porous, body is arranged stationary in the heat exchanger. In addition to the fact that the use of this type of heat exchanger is characterised by the discontinuous nature of the two medium streams, there is also once again the disadvantage of the risk of mixing of the first and second medium stream by the inter¬ vention of the transfer medium, whilst the transfer medium is once again a solid body. In the two known types of regenerative heat exchanger described above, mass transfer is achieved in that, for example, vapour or gas from the first medium to be treated condenses in or on the transfer medium in order subsequently then to vaporise again and be taken up by the second medium stream. Regenerative heat exchangers are also known with which exclusively heat exchange takes place, that is to say there is no mass transfer at the same time. Exchangers of this type can be found, for example, in the cooling system of, for example, internal combustion engines, where heat from the hot combustion gases is transferred to the lubricating oil, which then releases its heat again to the cooling water circulating through the engine block. In this case the media to be treated, and the transfer medium, are kept separate from one another by substantially impermeable barrier layers.

The aim of the present invention is to provide a regenerative heat exchanger of the type referred to in the preamble, with which mixing of the first and second media to be treated is kept to the minimum possible, whilst, at the same time, mass transfer and heat exchange are possible. A further aim is to banish the use of a solid body as transfer medium. A more flexible design of the regenerative heat exchanger can be achieved as a resul . The aim of the present invention is achieved with a regenerative heat exchanger which has the characteristics summarised in the appended Claim 1. In. this context the term "barrier layer" or "barrier element" is used to denote a facility such that the various first and second media and the transfer medium are essentially kept separate from one another. In this context the term "permeable" is used to denote that the barrier element is also porous or permeable to a specific substance which it is desirable to exchange, via the intervention of the transfer medium, between the first and second media to be treated. Said permeability can be achieved by, for example, making the barrier element porous by perforating the latter, or by selecting a naturally porous material for the barrier element. It is also possible to select a material which is naturally permeable to a specific substance, but despite this is not porous. For instance, plastics are com¬ mercially available which are not porous but are permeable to, for example, water vapour but are impermeable to liquid water. In this context it is pointed out that the further operating conditions in the heat exchanger according to the present invention can partly determine the permeability of the barrier element. One example is an optional pressure difference which prevails over the barrier element and as a result of which the barrier element is, for example, permeable to a specific substance when the pres- sure difference is relatively high and is impermeable to said substance when the pressure difference is relatively low. An advantageous variant embodiment of the invention is given in the appended Claim 2. As a variant on the embodiment according to Claim 2, it would also be possible to elect for the first and second medium, respectively, which are to be treated, to flow through the hollow tubes or fibres, whilst the transfer medium flows over and around said hollow tubes or fibres. For the majority of applica¬ tions, however, with an embodiment of this type the resistance which the first and/or the second medium experiences on flowing through the hollow tubes or fibres will be too high to justify economically cost-effective use. However, with an embodiment of this type, in which the first and/or the second medium flows through the hollow tubes or fibres, it would be possible to achieve a compact construction of the complete system by, for example, arranging first and second heat exchange elements alternately one after the other within a common channel, through which channel the transfer medium then moves to flow over and around the hollow tubes or fibres.

The barrier elements ensure that the various media remain separated from one another, control the heat exchange and mass transfer and in this con- text provide a large exchange surface area.

The invention is explained in more detail below with the aid of non¬ limiting illustrative embodiments shown in the drawings. In the drawings: Figure 1 shows, diagrammatically, a view of a first embodiment of the invention; and Figure 2 shows, diagrammatically, a view of a second embodiment of the invention.

Figure 1 shows, diagrammatically, a first heat exchange element 1 and a second heat exchange element 2. Both the first and the second heat exchange elements 1 , 2 are made up of four modules arranged one after the other, as are described in EP-B 0509031. With this arrangement each module consists of a bundle of parallel hollow fibres, the respective ends of which open into collection chambers located opposite one another. The collection cham¬ bers form part of a point-symmetrical channel section, inside which said hollow fibres run. As a result of the point-symmetrical form of the channel section, the modules can be arranged one after the other in such a way that the fibres of two successive modules enclose an angle. Furthermore, the respective collection chambers of the successive modules are connected to one another. In this way the transfer medium 3 is able, through the hollow fibres, to pass successively through the respective heat exchangers 1 , 2 in the direction of the arrow A in an angularly adapted flow. Furthermore, the modules of the heat exchanger 1 and 2, respectively, which modules are arranged one after the other, define an elongated, delimited flow channel for, respectively, a first medium 4 to be treated and a second medium 5 to be treated. Said media 4, 5 to be treated flow over and around the fibre bundles of the first heat exchanger 1 and the second heat exchanger 2. Con¬ tinuous circulation of the transfer medium 3 between the first heat exchanger 1 and the second heat exchanger 2 can be provided by means of, for example, a pump 6.

Figure 2 shows, diagrammatically, a system in which the first heat exchanger 1 and the second heat exchanger 2 are integrated. Heat exchanger modules, as discussed above, are now arranged one after the other in the direction of flow of the transfer medium 3 in order to bring the first medium 4 and the second medium 5 alternately into a heat exchange and mass transfer relationship with the transfer medium 3. Each module defines a channel section for delimiting the flow of the transfer medium 3. Parallel hollow fibres run transversely to the longitudinal direction of the channel section between opposite walls, so as to open, at their respective ends, into medium collection chambers opposite one another. In each case either the first medium 4 or the second medium 5 flows through the hollow fibres of a module. Per module, the other of the two media 4, 5 to be treated flows, protected from the transfer medium 3 and the medium to be treated which is flowing through the hollow fibres, directly through to the subse- quent module, in order to flow through the hollow fibres in that module. In this way the treatment medium 3 is alternately in contact with the first medium 4 and the second medium 5 and flows over and around the hollow fibres.

The two embodiments shown in Figures 1 and 2 function as follows: Assume that the first medium 4, which flows into the first heat exchange element 1, is warm and moist air. Also assume that the second medium 5, which enters the second heat exchange element 2, is dry and cold air. Also assume that the transfer medium is a hygroscopic liquid and that the fibres through which the transfer medium 3 flows through the heat exchange elements 1 , 2 are permeable in such a way that they are permeable to water vapour but not or essentially not to the hygroscopic liquid and any liquid water from the transfer medium 3 mixed therewith. The pressure difference between the first medium 4 to be treated and the transfer medium 3, and, respectively, between the second medium 5 to be treated and the transfer medium 3 is also adjusted to the nature of the permeability of the fibres, or barrier elements, in such a way that said pressure difference also guar¬ antees that said barrier elements are permeable to water vapour but not permeable to liquid water. In the first heat exchange element 1 the hygroscopic liquid 3 will take up heat and moisture from the first medium 4. The hygroscopic liquid 3 then heated and loaded with H₂0 will subsequently release its heat and moisture in the second heat exchange element 2 to the second medium 5. The air 4 flowing through the first heat exchange element 1 will not mix either with the hygroscopic liquid 3 or with the air 5 flowing through the second heat exchange element 2. Of course, further variants outside the embodiments described and shown here are conceivable which likewise fall within the scope of the invention. For example, the direction of flow of the various media through the various heat exchange elements can be changed. It can also be elected to use a dif- ferent type of heat exchange element. What is important is that in the regenerative heat exchanger according to the invention the transfer medium is kept separate, by means of one or more barrier layers of limited permea¬ bility, from the two media to be treated, so that said two media to be treated are not able to mix with one another or with the transfer medium, but heat exchange and mass transfer can be achieved between said two media to be treated, via the intervention of the transfer medium.

Claims

1. Regenerative heat exchanger comprising a first heat exchange element through which a first medium to be treated is to flow, and a second heat exchange element through which a second medium to be treated is to flow, and a transfer medium for interaction with the first and the second heat exchange element in order to be in a heat exchange and mass transfer rela¬ tionship with the first and the second medium, respectively, characterised in that the transfer medium is kept separate from the first and second media by means of one or more permeable, heat exchange and mass transfer barrier layers, the permeability of which is chosen to allow the intended mass transfer between the first or second medium and the transfer medium to take place, but essentially to prevent mixing of the first or second medium with said transfer medium.

2. Heat exchanger according to Claim 1, wherein the transfer medium is fed through the first and/or the second heat exchange element through per¬ meable hollow tubes or fibres which function as heat exchange and mass transfer barrier layers between the first or second medium and the transfer medium, the first or second medium to flow over and around said hollow tubes or fibres.