GB2069676A

GB2069676A - Evaporator tube constructions

Info

Publication number: GB2069676A
Application number: GB8103433A
Authority: GB
Original assignee: SCHMITZ KUEHLER BAIERBRUNN
Current assignee: SCHMITZ KUEHLER BAIERBRUNN
Priority date: 1980-02-15
Filing date: 1981-02-04
Publication date: 1981-08-26
Also published as: GB2069676B; FR2475918A1; DE3005751A1; FR2475918B1

Abstract

The heat transfer capacity or efficiency of an evaporator is maximised by varying the mass flow density along the evaporator tubes through which refrigerant flows in such a way that substantially uniform heat flux is achieved over the tube surface. In one embodiment of the method a series of closed tubular displacement bodies 2, 2%, 2_ with stepwise reducing cross-sections are disposed coaxially within an evaporator tube 1 thus defining a progressively increasing flow cross-section as the refrigerant changes from predominantly liquid at the inlet 7 to predominantly vapour at the outlet 8. <IMAGE>

Description

SPECIFICATION Method of increasing the heat capacity of evaporators The invention relates to a method and to an apparatus for increasing the heat transfer capacity of evaporators, in particular multiple tube evaporators.

It is known in the field of evaporators that in practice the refrigerant enters into the (or each) associated evaporator tube with a vapour content of approximately 0 to 20% with the term vapour content being understood to mean the ratio of the vapour mass flow to the total mass flow. At the end of the evaporator tube the refrigerant is completly vaporized, i.e. the vapour content equals 100%.

As evaporation increases the flow velocity along the associated tube also increases and both the pressure drop dp/dl and the heat transfer coefficient a have different values along the tube.

It is additionally known that the mean heat transfer coefficient a increases as the length of tube through which flow takes place increases and thus as the mass flow density increases. The total pressure loss Ap likewise increases pronouncedly and this reduces the mean temperature difference A6 in the evaporator. As the heat transfer is proportional to a X A# a specific optimum tube length results for each particularly application. These optimum tube lengths can, however, not be adhered to for many technical applications on constructional or manufacturing grounds.

In order to improve the heat transfer capacity it is already known to double the flow cross-secton for the refrigerant after a certain tube length by bifurcation. This measure can in many cases provide an improvement but does not however make it possible to achieve a tube optimisation or maximisation of the heat transfer capacity.

Various kinds of tube inserts are also known which extend over at least a part of the length of the tube carrying the refrigerant. These inserts are intended to influence the flow velocity of the refrigerant and thereby to improve the heat transfer capacity. The principal disadvantage of these tube inserts, which by way of example are of helical form, is that they give rise to intangible flow conditions which prevent the calculation of the heat transfer capacity of the whole evaporator.

The problem underlying the present invention is to provide a method of increasing the heat transfer capacity of evaporators which makes it possible to achieve, by design, the highest possible heat transfer capacity independently of the prevailing length of the tube.

This problem is solved in accordance with the invention in that, for the purpose of maximising the heat transfer capacity, the mass flow density along the evaporator tubes through which refrigerant flows is varied in such a way that substantially uniform heat flux is achieved over the tube surface.

The maximisation of the heat transfer capacity can in particular be achieved when the heat is transferred purely by convection from the tube wall to the refrigerant. The bubble boiling region, which in previous evaporator types occurs in regions of low vapor content and which takes up a large part of the tube length as a consequence of the relatively low heat transfer coefficients, is avoided (see sections Ha, Hb of VDI-Wärmeatlas (a publication of the Society of German Engineers)).

The mass flow density and the equivalent hydraulic tube diameter are selected, in accordance with the invention, so that a constant optimum value for the pressure gradient and the heat transfer coefficient is achieved along the tube.

The method of the invention can be realized by the use of tube inserts which extend over at least a part of the length of the tube carrying the refrigerant and indeed in such a way that the free flow cross-section of each tube is enlarged in the flow direction of the refrigerant, starting from a minimum sized region carrying substantially liquid refrigerant and going up to a maximum sized region carrying at least substantially vaporous refrigerant, by the use of at least one tube insert.

In determining the magnitude of the enlargement of the free flow cross-section the criteria of the maximisaton method of the invention are taken into account, and indeed preferably by computation because an important advantage of the invention resides in the fact that the performance of the evaporators can be computed, as a result of the simple and defined geometry and the resulting tangible flow conditions. This is not however intended to preclude the determination of the maximum for the heat transfer capacity being found empirically in any particular case by a consideration of the basic teaching of the invention.

Closed displacement bodies are preferably used for the tube inserts. However, in principle, any desired form of displacement body is suitable for realizing an arrangement constructed in accordance with the invention provided it makes it possible to define the free flow cross-section for the refrigerant along the tube in the required manner.

In practice it can be advantageous to use displacement bodies which consist of a plurality of tube sections, which directly follow one another and which are preferably positioned coaxially within the associated refrigerant tube. The diameter of the tube sections should reduce in the flow direction of the refrigerant in predeterminable steps.

One variant which should also be mentioned and which is favourable from the point of view of manufacture and assembly, is the use of individual displacement bodies having different cross-sectional areas relative to one another which can be arranged in the straight sections of a snake-like tube guided through a matrix of fins.

The invention will now be described by way of example only and with reference to the embodiments shown in the drawings in which: Figure 1 is a schematic illustration of a first embodiment of the invention and Figure 2 is a schematic illustration of a further embodiment.

As seen in Fig. 1 an evaporator tube 1 is guided in the customary snake-like manner through a pack or matrix of fins. Displacement bodies 2, 2', 2" in the form of tubes closed at their ends are inserted into the straight sections of this tube 1 and are maintained in a coaxial position relative to the tube 1 by projections 5 which are preferably realized as localised deformations of the tube. As the bends of the tube are unimportant for the transfer of heat it is not necessary to provide displacement bodies in these regions.

The displacement bodies 2, 2', 2" which are of differing size i.e. with regard to their cross-sectional areas, determine the free flow cross-sections which differ along the length of the tube for the refrigerant which enters at 7.

Thus, at the beginning of the tube 1, the displacement body produces an annular gap 3 with a comparatively small gap width which raises the mass flow density (kg/qm,s) and the flow velocity and reduces the equivalent hydraulic diameter. This assists in increasing the heat transfer coefficient and the pressure drop.

The width of the gap or the flow crosssection is thus so selected that the optimum pressure gradient dp/dl and the maximum possible mean heat transfer number cr are achieved.

In order to ensure that this optimum value of the pressure gradient is also achieved further along the tube it is necessary, having regard to the increasing evaporation, to increase the free flow cross-section. This can be achieved by reducing the respective crosssectional areas, in the flow direction of the refrigerant, of succeeding displacement bodies 2' and 2". This principle can be used successfully over relatively large tube lengths.

The use of a further displacement body can, by way of example, normally only be dispensed with when the vapor content has reached a value of approximately 90%. This is indicated in Fig. 1 by the final tube section 4 in which it is no longer necessary to arrange a displacement body and from which the practically fully vaporized refrigerant emerges at 8.

Instead of using individual displacement bodies having cross-sectional areas which differ relative to one another it is also possible, in accordance with a further embodiment of the invention, to use tube inserts which are once again formed as closed displacement bodies but which however consist of a plurality of sequential tube sections 9, 10, 11, 12 of stepped diameters. This is illustrated schematically in Fig. 2.

This embodiment also gives rise to a free flow cross-section which increases in the flow direction, with the increase of the free flow cross-section, or the reduction of the corresponding cross-sectional area of the displacement body, being determined in accordance with the size and dimensional considerations of the method of the invention.

The principle of providing specified free flow cross-sections for the refrigerant supplied to an evaporator tube can be realized not only be means of specially selected displacement bodies but also by connecting an additional refrigerant circuit in parallel with at least part sections of the evaporator tube under consideration. In an arrangement of this kind refrigerant can be made to circulate around a closed circuit part of which is defined by a section of the evaporator tube under consideration. The effect of this additional refrigerant, which can flow with or against the main flow of refrigerant through the evaporator tube, is to diminish the free flow cross-section available for the main flow of refrigerant in the section under consideration. The size of the reduction of the free flow cross-section is predetermined by the dimensions chosen for the additional refrigerant circuit.

In all practical embodiments and realizations of the method of the invention it is however necessary to ensure that the mass flow density is in each case changed so that at each point along the evaporator a substantially constant heat flux is present at the tube surface because only in this way can the desired maximisation of the heat transfer capacity be achieved in accordance with the teaching of the invention.

Claims

1. A method of increasing the heat capacity of evaporators, In particular multiple tube evaporators, by influencing the flow of refrigerant, in the sense of reducing the mass flow density thereof, over at least a part of the tube length, characterized in that, for the purpose of maximizing the heat capacity, the mass flow density along the tubes through which refrigerant flows is varied in such a way that a substantially uniform heat flux is achieved over the tube surface.

2. A method in accordance with claim 1, characterized in that the mass flow density and/or the equivalent hydraulic tube diameter is so selected that the optimum value for the pressure gradient and the heat transfer coefficient is obtained at each point along the tube thus resulting in a maximum mean heat flux.

3. A method in accordance with claim 1 and claim 2, characterized in that the mass flow density is so selected that one always lies in the range of convective heat transfer.

4. A method in accordance with claims 1 to 3, characterized in that at least a section of the tube is provided with an additional circuit for the refrigerant.

5. A method in accordance with claim 4 and characterized in that, in the flow direction of the refrigerant, sequential sections of the tube are provided with additional circuits which are respectively separated from one another and differently dimensioned.

6. Apparatus for carrying out the method of one or more of the preceding claims using tube inserts which extend over at least a part of the length of the tube carrying the refrigerant, characterized in that the free flow crosssection of each tube (1) is enlarged in the flow direction of the refrigerant, from a minimum sized region (3) carrying substantially liquid refrigerant up to a maximum sized region (4) carrying at least substantially vaporous refrigerant, by at least one tube insert (2).

7. Apparatus in accordance with claim 6 and characterized in that the increase of the free flow cross-section corresponds at least substantially to the increase of the vapour content of the changing two-phase flow.

8. Apparatus in accordance with claim 6 or claim 7 and characterized in that each tube insert (2) is formed as a closed displacement body.

9. Apparatus in accordance with claim 6 or claim 7 and characterized in that each tube insert is formed as a displacement body of star-like cross-section.

10. Apparatus in accordance with claim 8 of claim 9 and characterized in that each tube insert (2) has at least substantially the same cross-sectional area over its length and that successive tube inserts in the flow direction of the refrigerant have cross-sectional areas which reduce relative to one another.

11. Apparatus in accordance with claim 8 and characterized in that each tube insert (2) has a reducing cross-sectional area over its length.

12. Apparatus in accordance with one of the preceding claims and characterized in that each tube insert (2) is held in the tube (1) via deformed regions (5) acting as supports.

13. Apparatus in accordance with claim 6 and characterized in that each displacement body consists of a plurality of tube sections (9, 10, 11. 12)which directly follow one another, are connected closely together and coaxially held in the associated refrigerant tube (1) with the diameter of the tube sections reducing in the flow direction of the refrigerant in predeterminable steps.

14. A method substantially as herein described with reference to and as illustrated in the accompanying drawings.

15. Apparatus substantially as herein described with reference to and as illustrated in the accompanying drawings.