EP0654647A1

EP0654647A1 - A finned tube for a heat exchanger device

Info

Publication number: EP0654647A1
Application number: EP94203360A
Authority: EP
Inventors: Jan Hubertus Deckers; Hendrikus Louis Gerardus Marie Giesen
Original assignee: Dejatech Bv
Current assignee: Dejatech Bv
Priority date: 1993-11-18
Filing date: 1994-11-17
Publication date: 1995-05-24
Also published as: NL9301995A

Abstract

A finned tube, suitable in particular for use in a heat exchange device, which finned tube is manufactured from metal and comprises a tubular body (1) which defines a channel (2), connecting means (4, 5) and fin-shaped projections (3, 12) extending from the body, wherein the finned tube is manufactured by die casting in a die. In further elaboration of the invention, the cross section of the tube (1) and the fins (3; 12) can be shaped through the die casting operation in such a manner that during circumfusion of the tube by gas substantially perpendicularly to the channel (2), the intensity of the contact between the finned tube and the circumfluent gas is optimal.

Description

The invention relates to a finned tube, suitable in particular for use in a heat exchange device, which finned tube is manfactured from metal and comprises a tubular body which defines a channel, connecting means and fin-shaped projections extending from the body.
In the heating of liquids by heat exchange, use is made of tubes, and a highly heated fluid is passed through a channel or on the outside around the tube. The tubes are thereby heated and can deliver the thus absorbed heat to a liquid to be heated. In order to obtain a good heat transfer between the heated fluid and the liquid to be heated, these tubes are typically provided with uniformly spaced-apart fins extending from the outside of the tube. Such a tube is referred to as a finned tube and has the advantage that the contact surface is relatively large by virtue of the fins.
Finned tubes are manufactured by providing an extruded tubular profile with at least a series of fins by means of finishing operations, such as turning or milling, or, for instance, by welding-on or by cold deformation. Further, in the known finned tubes, the ends are provided in comparable manner with, for instance, grooves for fitting packing rings or like sealing and connecting means.
An important disadvantage of the finishing operation on the extruded tubes is that it renders these finned tubes relatively expensive to produce. Moreover, the finishing operation entails substantial loss of material. Furthermore, these finishing operations give rise to stresses in the material, which may for instance lead to stress ruptures during use. The known finned tubes which are finished by turning are relatively large and have a regular rotation-symmetrical shape, so that heat exchange devices comprising such finned tubes are voluminous and have an efficiency which is not optimal.
Accordingly, the object of the invention is to provide a finned tube of the type described in the preamble which does not have the above-mentioned disadvantages and enables a good heat transfer. To that end, the finned tube according to the invention is characterized in that it is manufactured by die casting in a die.
By manufacturing the finned tube according to the invention by die casting in a die, the advantage gained is that the fins and connecting means can be formed directly on the body without necessitating any further shaping finishing operations after the finned tube has been withdrawn. In this manufacturing method only a minimum of material is lost and it allows great freedom in designing inter alia the fins and the channel, so that optimum heat transfer is made possible.
The finned tube generally used in CH boilers is circumfused by flue gases on the outside thereof, in a direction perpendicular to the channel. With finned tubes having a circular cross section, during operation the outside of the tube facing in the downstream direction of the flue gases is significantly cooler than the outside of the tube facing in the upstream direction of the flue gases due to the fact that the side of the finned tube facing in the downstream direction is much less intensively in contact with the hot flue gases.
The finned tube according to the invention provides the possibility of giving both the body and the fins of the finned tubes a cross section other than a circular cross section. By virtue of the finned tube being manufactured by die casting, the cross section of the tube and the design of the fins can be made in such a manner that during circumfusion of the tube by gas perpendicularly to the channel, the intensity of the contact between the tube and the circumfluent gas is optimal. Thus, for instance, the surface of the side of the finned tube facing in the downstream direction of the flue gases can be kept to a minimum. Thus a considerably better thermal efficiency of the entire boiler can be obtained.
By virtue of the finned tube according to the invention being manufactured by die casting in a die, a further advantage is gained in that fins can be obtained which are relatively long with respect to the diameter of the body, and which may be relatively thin as well as relatively thick. Thus the thermal efficiency can be increased even further. Moreover, the fins can be provided both internally and externally.
In a particularly advantageous embodiment of the finned tube according to the invention, it has a substantially oval, preferably elliptical, cross section. As a consequence, a relatively large number of finned tubes can be arranged in a narrow space, so that with a relatively small volume of a heat exchange device a large capacity can be obtained. Moreover, when the finned tubes are positioned in such a manner that the long axis of the elliptical cross section extends in the direction of flow of the flue gas, the resultant achievement is that the greater part of the surface is in intensive contact with the hot flue gases since the surface of the outside of the finned tube facing in the downsteam direction of the flue gases is then minimal.
The fins may be provided with recesses and/or openings, so that the finned tube can be secured in a heat exchanger device in a simple manner, for instance against rods or ribs of the heat exchanger device which extend in the recesses of the respective fins. Moreover, with them, contact between juxtaposed finned tubes and/or the housing of a heat exchange device can be effected or, conversely, be prevented.
According to a further elaboration of the invention, for the purpose of enlarging the heating surface, the internal surface of the finned tube may be provided with ribs extending parallel to the channel.
In a further elaboration of the finned tube according to the invention, the body of the finned tube, viewed in the longitudinal direction, has an irregular wall thickness and the channel has a varying cross section. Thus, the flow velocity through the channel of the finned tubes, the extent of heat delivery to the tube by the heated, flowing fluid, and the extent of heat delivery to the liquid to be heated can be controlled in optimum manner, so that the efficiency of the heat exchange device comprising a number of finned tubes according to the invention can be increased considerably compared with a heat exchange device comprising a number of known finned tubes.
In an alternative embodiment of the finned tube according to the invention, the fins are provided on the body with different interspaces, inclined relative to the channel, and have a curved surface. By virtue of the fins being provided on the body at non-uniform interspaces, the contact surface varies at different positions along the finned tube, so that the heat transfer can be further optimized. Due to the fins being inclined relative to the channel and having a curved surface, the fluid flowing along the finned tube during use can be made to whirl, yielding a better contact between the fluid and the finned tube, and hence a better heat transfer. This embodiment moreover has the advantage that the finned tube has a relatively small overall width in combination with a relatively large contact surface.
In a further variant embodiment of the finned tube according to the invention, the fins are hollow and open towards the channel. As a result, during use the fluid flowing through the channel can also flow through the fins, so that the contact surface between the liquid and the inside of the finned tube is appreciably enlarged.
It will be clear that the finned tube according to the invention can be used in heat exchange devices working according to different principles of operation. Thus, for instance, a heated liquid or gas can be passed through the channel with the finned tube surrounded by stationary or fluent liquid to be heated, or a liquid to be heated can be passed through the channel with the finned tube surrounded by a heated liquid or gas. The finned tube can therefore be heated directly or indirectly.
To clarify the invention, a number of exemplary embodiments of the finned tube will now be described with reference to the drawing.

Fig. 1 shows a finned tube according to the invention, in a partly sectioned side elevation;
Fig, 2 shows a cross-sectional elevation of the finned tube taken on line II-II of Fig. 1;
Fig. 3 shows a cross-sectional elevation of an alternative embodiment of a finned tube according to the invention;
Fig. 4 shows an alternative embodiment of the finned tube according to the invention in a cross-sectional view; and
Fig. 5 shows, in top plan view, a finned tube having rectangular and internal fins.

The finned tube as shown in Fig. 1 comprises a tubular body 1 comprising a channel 2 which is connectible to an inlet channel and a discharge channel not shown in the drawing. Extending from the body 1 are a series of fins 3a-3z mutually separated by interspaces 8a-8z, the fins 3 each residing in a plane approximately perpendicular to the longitudinal direction of the channel 2. The fin 3a and the fin 3z, being, respectively, the lowermost and the uppermost fin as viewed in the drawing, are each located at some distance from the adjacent ends 4 and 5, respectively. A wall 6 extends on two diametrically located sides of the body 1 between the lowermost fin 3a and the uppermost fin 3z. Through the channel 2 of the finned tube flows a fluid, for instance a liquid to be heated, such as CH water. Flowing around the finned tube, generally in a direction extending in a plane perpendicular to the channel 2, is a fluid. The fluid can for instance be a hot gas, such as hot flue gas coming from a burner.
As appears from Fig. 2, the fins and the channel 2 according to Fig. 1 have an elliptical cross section, the two walls 6 being located on the short axis K of the ellipse. The elliptical cross section leads to an improved heat exchange between the finned tube and the fluid, such as for instance hot flue gases, flowing around the finned tube in a direction 10 perpendicular to the channel 2. Moreover, ellipse-shaped tubes enable better packing of the space, such as for instance a flue duct, through which flows the circumfluent fluid 10 than in the case where this space is filled with finned tubes having a circular cross section. The channel 2 of the finned tube has a cross section whose area increases from the middle towards the end, while the wall thickness 7 decreases in the same direction. This is particularly advantageous if it must be possible for the (pressure) die cast finned tube to be withdrawn in order that the mandrils for forming the channel 2 can be removed from this channel 2. Optionally, as shown in Figs. 1-3, the internal surface of the channel 2 may be provided with ribs 11 extending in the longitudinal direction. These ribs 11 enlarge the heating surface on the inside of the tube, which effects an improvement of the efficiency of the finned tube.
In an alternative embodiment of the finned tube as shown in Fig. 3, the fins 3 have an elliptical cross section, while the channel 2 has a circular cross-section. The advantage of this embodiment is that the fins 3 enable optimum heat exchange while the finned tube can yet be connected to conventional circular connecting openings, which permits the use of generally available sealing means, such as O-rings, for the sealing thereof. In the embodiment shown in Fig. 3, the channel 2 is placed in the center of the elliptical fins 3, but it is, of course, also possible to place the center of the cross section of the channel 2 closer to one of the edges of the ellipse-shaped or optionally differently shaped fins 3, so that on one side of the finned tube a greater heat transferring surface is obtained than on the opposite side.
The interspace 8 between the fins 3 can be smaller in the middle than at the ends. The less good heat exchange in the middle of the finned tube, resulting from the greater flow velocity in situ which is due to the smaller throughflow area in the middle of the finned tube, is thus compensated by the larger number of fins 3 per unit length.
The fin height, i.e., the extent to which the fins 3 extend outside the body 1, can be relatively large with respect to the diameter of the body 1. Thus, fins 3 having a height of, for instance, 60 mm or more can be provided on a body 1 with a maximum outside dimension of, for instance, 23 mm. Further, the fins 3 may then be thin, for instance 3 mm or less, as well as thick, for instance 10 mm or more, and the fins 3 can be distributed over the body 1 with any desired interspace, for instance at intervals of between 3 mm and 10 mm. Incidentally, the body 1 can basically have any cross section. A typical finned tube according to the invention is, for instance, 450 mm long, has a body of a diameter of 23 mm, and fins of a height of 60 mm, a thickness of between 3 and 10 mm and spaced apart a distance of between 3 and 10 mm. The surface of the fins is preferably within a square of about 150 x 150 mm.
Fig. 4 is a sectional view of a different embodiment of a finned tube according to the invention. The exemplary embodiment shown is characterized by a large variety in shape of the fins 12 of the finned tube. The fins 12 are even partly hollow, which positively affects the heat exchanging properties of the finned tube. Moreover, the fins 12 are arranged at a slight inclination relative to the channel 2, which can cause the circumfluent fluid 10 to whirl, so that the intensity of the contact between the circumfluent fluid and the finned tube is favorably affected and hence the heat exchange efficiency of the finned tube is improved. It is clear that in the manufacture of such a finned tube a degradable core has to be used because the channel 2, owing to the fins 12 provided with cavities 13, has a non-clearing shape.
Fig. 5 shows a top plan view of a finned tube having rectangular, and more specifically square, fins 3 of, for instance, 150 x 150 mm. The body 1 has a circular cross section having a 30 mm outer diameter. In the channel 2 eight internal fins 11 are provided, which may for instance be finger-shaped, rib-shaped or identical in shape to the outer fins 3. The wall thickness of the body 1 is, for instance, some millimeters and optionally varies along the length of the finned tube. The fins 3 may have the same thickness throughout their surface, preferably of between 3 and 10 mm.
The internal fins 11 provide for an additionally intensive contact between the body 1 and the medium flowing through the channel 2, in particular when the internal fins 11 are finger-shaped, because this gives rise to turbulence.
The fins 3 are each provided, on diametrically opposite sides thereof, with a recess 20 in which, for instance, a rib (not shown) of a housing of a heat exchanger device can be snugly received for support.
It is observed that the invention is not limited to the exemplary embodiments shown but that various modifications are possible without departing from the scope of the invention.

Claims

A finned tube, suitable in particular for use in a heat exchange device, which finned tube is manufactured from metal and comprises a tubular body (1) which defines a channel (2), connecting means (4, 5) and fin-shaped projections (3, 12) extending from the body, characterized in that the finned tube is manufactured by die casting in a die.
A finned tube according to claim 1, characterized in that the cross section of the tube (1) and the design of the fins (3; 12) are shaped through the die casting operation in such a manner that during circumfusion of the tube by gas substantially perpendicularly to the channel (2), the intensity of the contact between the finned tube and the circumfluent gas is optimal.
A finned tube according to claim 2, characterized in that the finned tube has a substantially oval, and preferably elliptical, cross section.
A finned tube according to claim 2, characterized in that the channel (2) has a substantially circular cross section.
A finned tube according to any one of the preceding claims, characterized in that the inside surface of the finned tube comprises ribs (11) extending parallel to the channel (2).
A finned tube according to any one of the preceding claims, characterized in that the body (1) has an irregular wall thickness (7).
A finned tube according to any one of the preceding claims, characterized in that the channel (2), viewed in the longitudinal direction, has a varying cross section.
A finned tube according to any one of the preceding claims, characterized in that the fins (3; 12) are provided on the body (1) with different interspaces.
A finned tube according to any one of the preceding claims, characterized in that the fins (12) are inclined with respect to the channel (2).
A finned tube according to any one of the preceding claims, characterized in that the fins (12) have a curved surface.
A finned tube according to any one of the preceding claims, characterized in that the fins (12) are designed to be hollow (13) and open towards the channel (2).
A finned tube according to any one of the preceding claims, characterized in that the connecting means (4, 5) comprise a receiving space, formed on the finned tube, for a packing.