EP0074122B1

EP0074122B1 - Heat exchanger

Info

Publication number: EP0074122B1
Application number: EP82108341A
Authority: EP
Inventors: Desmond Mulock-Bentley
Original assignee: D MULOCK-BENTLEY AND ASSOCIATES Pty Ltd
Current assignee: D MULOCK-BENTLEY AND ASSOCIATES Pty Ltd
Priority date: 1981-09-09
Filing date: 1982-09-09
Publication date: 1984-12-19
Also published as: DE3261628D1; AU556955B2; AU8816382A; EP0074122A1

Description

Field of the invention

This invention relates to heat exchangers, and in particular to a fin construction for use in heat exchangers which is suitable for use in dry cooling towers and similar industrial applications.

Background to the invention

Our DE-A-2 912 723 describes a heat exchanger comprising a series of conduits arranged adjacent each other but spaced apart, their external surfaces being formed with registering grooves. A set of fin elements in the form transversely slotted metal strips is arranged in a stack between adjacent conduits with the longitudinal edges of each fin element extending into registering grooves in the opposed conduit surfaces. Each fin element has a central longitudinal zone, a transverse end zone at each end and two substantially planar longitudinal edge zones, the central longitudinal zone being planar and the edge zones each defining an obtuse included angle with the plane of the central longitudinal zone. Both edge zones lie on the same side of the plane of the central longitudinal zone.
That these fin elements perform efficiently they have at one end a relatively wide transverse end zone and at the other end a relatively narrow transverse end zone. Alternate fin elements are displaced in the stack so that the slots in each fin are staggered relatively to those in its immediate neighbours but aligned with the slots in similarly positioned fin elements.
In the known heat exchanger, the creation of the slots from metal strip leads to a wastage of material since the metal stamped out of the slots is discarded. Also, the staggered pattern of alternate fin elements required for efficient heat exchange requires that the end zones of the fin elements must be distinguished and the elements correctly oriented during assembly. This adds to the already substantial labour costs inherent in assembling this type of heat exchanger.
United States Patent 2,703,226 describes a heat exchanger with fin elements in the form of a U-shaped channel which is slit transversely at intervals along its length in its central longitudinal zone to provide a series of fins, alternate fins being struck up into the channel of the fin element. The vertically arranged side walls of channels are provided with shoulders so that the fin elements can be stacked within each other. These stacks are fixed to the opposed surfaces of adjacent conduits in a heat exchanger. One disadvantage of the fin elements described is that the presence of the vertically arranged side walls of the channel-like fin elements requires a substantial amount of metal which is largely surplus to the requirements of good heat transfer, so that the fin elements are costlier than is desirable.
The stacks disclosed in U.S. Patent 2,703,226 further result in double vertical side walls, which adversely affects heat transfer.
An object of the present invention is to provide an improved heat exchanger which reduces the disadvantages mentioned above.

Summary of the invention

According to the invention there is provided a heat exchanger comprising a series of elongate conduits spaced apart, each pair of opposed conduit surfaces being provided with rows of grooves that register and elongate fin elements located between the conduits each fin element having a central longitudinal zone two longitudinal edge zones, and a plurality of fins each of which extends across the central zone and has a planar bridge portion at each end, with the longitudinal edge zones of the fin elements inclined at an obtuse included angle with respect to the central zone and being coplanar with and adjoining certain bridge portions, each longitudinal edge surface of each fin element extending into and contacting both surfaces of a groove in a conduit, characterized in that the fins are alternatively located in two parallel planes to form two sets of fins, the bridge portions of one set being oriented at an angle with respect to the bridge portions of the other set, the grooves being so deep relatively to the fin elements that not only the longitudinal edge zones but also the adjoining coplanar bridge portions of the set of fins are in contact with the surface of the grooves.
The spacing of the rectilinear grooves is preferably substantially twice the distance between the parallel planes of the two sets of fins of each fin element.
That the thickness of the metal in the fin element is substantially constant may comprise allowing the overall width of the fin element to reduce and compressing the metal strip at least to some extent while displacing the fins in opposite directions.

Brief description of the drawings

A fin element and heat exchanger according to the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a plan view of a fin element;
Figure 2 is a section on an enlarged scale through a portion of a heat exchanger;
Figure 3 is a perspective view on an enlarged scale of a portion of the fin element as shown in Figure 1; and
Figure 4 illustrates schematically three major steps in manufacturing the fin element from a metal strip.

Description of a preferred embodiment

The fin element shown in Figure 1 is formed from a single elongate metal strip and has two sets of fins which alternate with one another. In the drawings one set of fins is labelled with the letter A, whilst the other set is labelled with the letter B. All the fins A lie in one plane, whilst all the fins B lie in another plane which is parallel to but spaced from the plane of fins A. Both planes are equidistant from and on opposite sides of a mean plane C (Figure 2) of the fin element.
The fin element is also divided into a central longitudinal zone 10 and two edge zones 12. Each fin has a planar bridge portion 16 at each end. The three-dimensional form of the fin element can be seen from Figure 3.
To manufacture the fin element, a flat strip of metal is used as starting material. The separate manufacturing steps which will now be described although they normally take place in a single operation.
First of all, the strip is transversely slit over the central zone 10 to separate the metal which will form fins A and fins B. An even number of slits is made. Fins B are then pressed below the centre plane C, being the original plane of the strip, whilst fins A are pressed above the central plane C as shown in Figure 4(b). The longitudinal edge zones 12 are bent over as shown out of the plane C to form an obtuse included angle with the planes of the fins A, B as shown in Figure 4(c). This obtuse angle is ideally 135°.
In practice fin elements can be formed without a separate step of forming the slits. The strip is pressed selectively in interspaced opposite directions so that initial shearing of the strip is caused along the edges of the fins as the fins are pressed towards the respective parallel planes.
During the displacement of the fins the overall width of the strip is allowed to reduce as can be seen by comparing Figures 4(b) and 4(c) with Figure 4(a). By allowing this reduction to occur no appreciable thinning of the fins A and B, the bridge portions 16 or the edge zones 12 takes place. Thus there is provided a finished fin element of substantially uniform metal thickness.
Figure 2 shows a section of a heat exchanger between two conduits 14. As can be seen the conduits are corrugated, so that they have substantially rectilinear grooves 20 at regular intervals along their external surfaces. Again, as shown in Figure 2, each fin element located is in one of these grooves 20 to form an intimate fit and provide intimate planar contact with the surface of corresponding grooves. The assembly is then hot dip galvanised so as to prevent corrosion and in order to thermally bond the contacts between the intimate co-operating surfaces so as to improve heat transfer.
The mean perpendicular distance y between the fins A and B is chosen to be half the distance x between adjacent grooves 20. In this way, the fins B of one element are also spaced a mean distance y from the fins A of an adjacent element, thus producing a substantially uniform fin configuration throughout the space between the two conduits 14.
Adjacent both ends of the fin element it is seen that all fins in each case are A fins. It could be arranged that all such fins were B fins. During assembly the fin elements are superimposed one above the other. Because all the end fins are either A fins or B fins there is no need to check the longitudinal orientation of each fin element prior to insertion into the heat exchanger. The symmetry of the fin elements ensures that the set of fins A in one fin element will register exactly with corresponding fin sets A of other fin elements, and the same will apply to the fin sets B.
An important improvement provided by embodiments of the present invention is that each fin element provided retains a uniform thickness of material throughout its configuration. This is achieved as explained above by allowing the overall width of the original strip of material to reduce during pressing to form the fins A and B. In earlier proposals pressing or otherwise forming the fin elements tended to cause thinning of the material at various places. Such thinning has the effect of disrupting and reducing the heat-flowing properties of the material which adversely affects the cooling characteristics of a heat exchange assembly incorporating fin elements provided in the prior art.
It will be noted that a finished fin element having uniform thickness can also be produced according to the invention by compressing the metal strip at least to some extent during the pressing to displace and form the fins into parallel planes.
Using the fin construction described in our DE-A-2 912 723 it was necessary to insert two fin elements having alternative longitudinal orientation to provide the path for cooling air flow. The known fin element has fins in only one plane. As a result of this it can be difficult to obtain a close fin spacing between the conduits 14 without reducing the spacing between the grooves 20. It is difficult to reduce x because of the thickness of the walls of the conduits 14. Furthermore, these fins are provided by punching out slots. The punched-out material therefore becomes scrap whereas in a fin element according to the present invention, no metal is removed and there is therefore no material waste.
Fin elements generally contribute approximately 60% of the cost of a typical heat exchanger assembly prior to galvanising for a dry-type cooling tower. The use of the improved fin elements of the present invention in a heat exchanger assembly has been shown to give a saving of as much as 52% on the cost of preparing and providing the fin elements for a heat exchanger. Further savings are made and security provided during assembly because longitudinal orientation of the fin elements is the same for each fin element.

Claims

1. Heat exchanger comprising a series of elongate conduits (14) spaced apart, each pair of opposed conduit surfaces being provided with rows of grooves (20) that register, and elongate fin elements located between the conduits (14) each fin element having a central longitudinal zone (10), two longitudinal edge zones (12), and a plurality of fins (A, B) each of which extends across the central zone (10) and has a planar bridge portion (16) at each end, with the longitudinal edge zones (12) of the fin elements inclined at an obtuse included angle with respect to the central zone (10) and being coplanar with and adjoining certain bridge portions (16), each longitudinal edge surface of each fin element extending into and contacting both surfaces of a groove (20) in a conduit (14), characterized in that the fins (A, B) are alternatively located in two parallel planes to form two sets of fins (A and B, respectively), the bridge portions (16) of one set (A) being oriented at an angle with respect to the bridge portions of the other set (B), the grooves (20) being so deep relatively to the fin elements that not only the longitudinal edge zones (12) but also the adjoining coplanar bridge portions (16) of the set of fins (A) are in contact with the surface of the grooves (20).

2. Heat exchanger according to Claim 1, characterized in that the spacing of the recti- in the fin element is substantially constant throughout the fin element.

3. Heat exchanger according to Claim 1 or 2, characterizedin that the spacing of the rectilinear grooves (20) is substantially twice the distance (y) between the parallel planes of the two sets of fins (A and B) of each fin element.