EP0947793A2

EP0947793A2 - Finned-tube heat exchanger

Info

Publication number: EP0947793A2
Application number: EP99200971A
Authority: EP
Inventors: Piermartino De Martino
Original assignee: ABB Combustion Engineering SpA
Current assignee: ABB Combustion Engineering SpA
Priority date: 1998-03-31
Filing date: 1999-03-30
Publication date: 1999-10-06
Also published as: EP0947793A3

Abstract

A finned-tube heat exchanger, particularly for combined-cycle systems and the like, which comprises a plurality of finned tubes (1) arranged in rows with fins (2) which comprise two parts, respectively a proximal part (3), which is solid and fixed to the surface of the tubes (1), and a segmented distal one (4); the particularity of the exchanger is that if p is the distance between two solid proximal parts (3) of two adjacent fins (2), the distal parts (4) of some fins (2) are bent away from the median plane of the fins (2) themselves, so that the distance between the distal part (4) of the bent fins (2) and the distal part (4) of a consecutive fin (2) is between 0.3 and 1.7 p.

Description

The present invention relates to a heat exchanger particularly for using in combined-cycle systems with primary generation provided by means of gas turbines and secondary generation provided by means of steam turbines powered by heat-recovery boilers.
It is known that heat exchangers used for example in a heat-recovery boiler are constituted by a plurality of finned tubes which are mutually aligned or staggered and in which a fluid to be heated or cooled, for example water or steam, flows. The sets of finned tubes are generally crossed longitudinally, or vertically in the case of vertical-type heat-recovery boilers, by a hot fluid, from which they receive heat by convection. When cooling of the fluid contained in the finned tubes is required, the fluid that flows around the tubes is at a lower temperature than the fluid in the tubes.
In both cases, the requirement of a heat exchanger is to reduce its dimensions and cost while providing an equal level of heat exchange and pressure losses.
In particular, in the case of heat-recovery boilers used in combined-cycle plants, where the fluid inside the tubes is water/steam and the external fluid is constituted by combustion gas, it is necessary to increase the heat exchange coefficient on the gas side in order to increase the efficiency of the exchanger. Finned-tube heat exchangers have been designed for this purpose.
An example of the use of finned tubes is shown in US Patent 5,337,807. This patent discloses fins which have two regions whose profile is serrated into segments and whose surface bears indentations and a third region having a linear profile and smooth surfaces.
Another example is given by US Patent 4,449,581, in which the fins have, on their surfaces, "dog-bone" corrugations so as to generate turbulence and increase heat exchange. Owing to the flow path that is generated between the finned tubes, these solutions, though increasing the thermal efficiency in the front and lateral regions of the tubes of the first row, have not turned out to be fully satisfactory as regards their rear portion. Furthermore, heat exchange is worsened already starting from the tubes of the second row, because the flow of the fluid is hindered by the presence of the tubes of the first row.
Another solution for improving heat exchange entails the use of baffles arranged between adjacent finned tubes, so as to convey the cooling or heating fluid so that it also flows over the rear part of each finned tube.
An example of the use of baffles is shown in US Patent 5,163,508. In this case also, although heat exchange in the front part of the tubes improves, satisfactory improvements are achieved in the rear region of each finned tube only with a substantial and undesirable increase in pressure losses.
The aim of the present invention is to provide a finned-tube heat exchanger, particularly for combined-cycle systems and the like, with fins which are configured so as to improve the efficiency of the heat exchange between the fluid and the finned tubes.
Within this aim, an object of the present invention is to provide a finned-tube heat exchanger whose fins have such a configuration as to increase the efficiency of the heat exchange between the fluid and the tubes, particularly in the region of the finned tubes that is normally scarcely affected by the stream of fluid that flows over said fins. Another object of the present invention is to provide a finned-tube heat exchanger, particularly for combined-cycle systems and the like, whose fins are configured so as to allow a decrease in load losses.
Another object of the present invention is to provide a heat exchanger, particularly for combined-cycle systems and the like, which is highly reliable, relatively easy to manufacture and at low costs.
The finned-tube heat exchanger according to the present invention comprises a plurality of finned tubes with fins which comprise two parts, respectively a proximal part, which is solid and fixed to the surface of the tubes, and a segmented distal one. The finned tube heat exchanger of the present invention is characterised in that, if p is the distance between two solid proximal parts of two adjacent fins, the distal parts of some fins are bent away from the median plane of said fins, so that the distance between the distal part of said bent fins and the distal part of a consecutive fin is between 0.3 and 1.7 p.
Further characteristics and advantages of the present invention will become apparent from the following detailed description of preferred but not exclusive embodiments of the finned-tube heat exchanger according to the invention, illustrated by way of non-limitative example in the accompanying drawings, wherein:

Figure 1 is a plan view of a finned tube with segmented fins used in the heat exchanger according to the invention;
Figure 2 is a longitudinal sectional view of a tube with segmented fins, used in the heat exchanger according to the invention;
Figure 3 is a plan view of the heat exchanger according to the invention, with aligned finned tubes;
Figure 4 is a plan view of the heat exchanger according to the invention, with staggered finned tubes;
Figure 5 is a partial plan view of a segmented fin used in the heat exchanger according to the invention;
Figure 6 is a partial view of the fin of Figure 5, which has grooves formed on the upper and lower surfaces of the fin, according to a preferred embodiment of the heat exchanger according to the invention;
Figure 7 is a side view of a finned tube with fins whose transverse profile is shaped according to a further preferred embodiment of the heat exchanger according to the invention;
Figure 8 is a view of the finned tube of Figure 7, with fins whose transverse profile is shaped according to another embodiment of the heat exchanger according to the invention.

With reference to the above figures, the heat exchanger according to the invention comprises a plurality of finned tubes 1 which are mutually adjacent and parallel and have fins designated by the reference numeral 2; the finned tubes 1 can be either mutually aligned or staggered, as shown in Figures 3 and 4 respectively.
In the practical execution of the finned tubes, the fins 2 wind in a helical pattern around the outer surface of the tubes 1; in particular, as shown in Figures 1 and 2, each fin 2 comprises two parts, respectively a solid proximal one 3, which is fixed to the surface of the tubes, and a segmented distal one 4, which is adjacent to the proximal part 3 and protrudes radially outward with respect to the axis of the tubes 1. The solid proximal parts 3 are fixed to the outer surface of the tubes 1 during the winding operation by means of conventional welding processes.
Advantageously, during this winding step, a suitably shaped tool, not shown in the figures, interacts with the distal parts 4 of some fins 2, bending them away from the median plane of said fins. In this manner, as shown in Figure 2, if p is the distance between the solid proximal parts 3 of two adjacent fins, the distance between the distal part of a bent fin and the distal part of an adjacent fin is between 0.3 p and 1.7 p. Some fins 2 of the tubes 1 of one row are thus placed out of the slipstream of the fins 2 of tubes 1 arranged upstream with respect to the direction of a stream conveyed externally so as to flow over the finned tubes (in the case of Figure 2, at right angles to the page and diagrammatically designated by the reference numeral 102). In this manner, the fins that are outside the slipstream are struck by a substantially fresh stream of fluid, i.e., a stream which has not exchanged heat with the tubes of the very first rows. This allows a considerable improvement in heat exchange, particularly in heat exchanger configurations in which such exchange would not be optimal because flow would be hindered by the presence of the tubes of the first row (in-line configuration).
Another advantage of the heat exchanger according to the invention is the fact that the bending of the distal parts 4 of some fins 2 generates, along a same tube 1, regions of suction and compression of the stream of fluid along the direction of its motion. Moreover, regions 100 and 101 are formed between the fins 2, where the fluid that arrives from contiguous channels is mixed continuously, leading to a considerable increase in the overall efficiency of the heat exchange between the finned tubes 1 and the fluid with modest load losses.
The percentage of fins 2 whose distal parts 4 can be bent with respect to the total number of fins 2 present in the heat exchanger can be between 10% and 90%, preferably between 20% and 80%, more preferably between 30% and 70%.
In a preferred embodiment of the heat exchanger according to the invention, along a same tube 1 there is a substantial alternation of bent distal parts 4 and straight distal parts 4, so that there is a corresponding alternation of fluid suction and compression regions and channels for the passage of substantially fresh fluid. As regards geometric dimensions, trials have shown that performance is optimised with an h/H ratio between 2 and 4 (where h is the overall radial extension of a fin 2 relative to the surface of the tube 1 and H is the radial extension of the proximal part 3 alone) and with an h/p ratio between 3 and 5. Trials were conducted with tubes 1 having an outside diameter of approximately 32 mm and a thickness of approximately 2.5 mm.
The configuration of the heat exchanger and the number of fins 2 whose distal parts 4 can be bent along a single tube and along different tubes can in any case be chosen conveniently according to the requirements and/or specific applications; thus, for example, there can be an alternating distribution along a same tube 1, as described earlier, or any random distribution on the same tube or among different tubes.
Advantageously, in another embodiment of the heat exchanger according to the invention, elements adapted to generate vortices whose axis is substantially parallel to the direction of the flow of a fluid conveyed externally so as to flow over the finned tubes, are associated with the fins.
With reference to figures 5 to 8, the heat exchanger according to this preferred embodiment comprises a plurality of finned tubes 1 which are arranged adjacent and parallel to each other and have fins which are designated by the reference numeral 2. The finned tubes 1 can also be arranged in-line or staggered.
Elements adapted to generate vortices whose axis is substantially parallel to the direction of the flow of a fluid conveyed externally so as to strike said finned tubes are associated with the fins 2. Advantageously, the generation of these vortices allows not only an improvement in the efficiency of the heat exchange between the fluid and the finned tubes but also a considerable decrease in pressure losses for an equal heat exchange.
In particular, as shown in figure 6, the vortex generation elements are constituted by grooves 6 whose longitudinal axis 10 is inclined by an angle between +/-5° and +/-45° with respect to the axis 11 of the fins 2 that lies at right angles to the axis of the tubes. Said grooves 6 can be formed on both of the surfaces of the fins 2, i.e., on the upper one 14 and on the lower one 15, or on just one of them.
The grooves 6 formed on the same surface (14 or 15) have mutually parallel longitudinal axes, designated by the reference numerals 10 and 10' respectively, and form a transverse profile of the fins whose transverse cross-section is sawtooth-shaped. Moreover, as shown in Figure 7, the sides 7 of each groove are mutually perpendicular. As an alternative, as shown in Figure 8, the transverse profile can have a trapezoidal wave-like cross-section or have any other shape adapted for the purpose.
As regards the dimensions of the grooves, if m is the absolute value of the height of the grooves and n is the distance between two consecutive vertices of said grooves, the ratio m/n is between 0.5 and 0.9. The value of m is between 0.2 and 0.4t, where t is the thickness of the fin.
In a preferred embodiment, shown in Figure 6, the grooves 6 are formed on both surfaces of the fins 2; in particular, the grooves 6 formed on the upper surface 14 have an axis 10 which is inclined in the opposite direction with respect to the axis 10' of the grooves formed on the lower surface 15; the inclination is given with reference to the axis 11 of the fins which is perpendicular to the axis of the tube.
This solution generates two series of mutually contrarotating vortices, both of which have an axis, designated by the reference numerals 10 and 10' respectively, which is substantially parallel to the direction 100 of the flow of a fluid which is conveyed externally so as to flow over the tubes 1 of the heat exchanger. The generation of these two series of contrarotating vortices allows a further improvement in the operating conditions of the exchanger.
In another embodiment, not shown, flow deflector baffles are arranged between the rows of finned tubes 1 and convey the fluid toward the finned tubes, so as to increase the heat exchange efficiency of said fins; baffles can also be arranged in front of the first row of tubes or to the rear of the last one.
The presence of the baffles allows to improve heat exchange, especially in the regions of the tubes that are normally scarcely affected by the flow of the fluid.
In practice it has been observed that the heat exchanger according to the invention fully achieves the intended aim, since it allows to convert the external flow of fluid from a substantially laminar one to a turbulent one, improving the overall efficiency of heat exchange between the fluid that flows externally over the fins of the finned tubes and the fins themselves even in the regions of the finned tubes that are normally scarcely affected by the flow of the fluid; it also allows to reduce pressure losses while maintaining an equal level of heat exchange.
The finned-tube heat exchanger thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may furthermore be replaced with other technically equivalent elements.
In practice, the materials employed, so long as they are compatible with the specific use, as well as the dimensions, may be any according to requirements and to the state of the art.
Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

A finned-tube heat exchanger, particularly for combined-cycle systems and the like, comprising a plurality of finned tubes with fins which comprise two parts, respectively a proximal part, which is solid and fixed to the surface of the tubes, and a segmented distal one, characterised in that if p is the distance between two solid proximal parts of two adjacent fins, the distal parts of some fins are bent away from the median plane of said fins, so that the distance between the distal part of said bent fins and the distal part of a consecutive fin is between 0.3 and 1.7 p.
The finned-tube heat exchanger according to claim 1, characterised in that the percentage of fins whose distal parts are bent is, with respect to the total number of fins, between 10% and 90%.
The finned-tube heat exchanger according to claim 1, characterised in that the percentage of fins whose distal parts are bent is, with respect to the total number of fins, between 20% and 80%.
The finned-tube heat exchanger according to claim 1, characterised in that the percentage of fins whose distal parts are bent is, with respect to the total number of fins, between 30% and 70%.
The finned-tube heat exchanger according to claim 1, characterised in that said fins have a ratio h/H between 2 and 4, where h and H are respectively the extension of the fin and of its proximal part in a radial direction, calculated with respect to the outer surface of the finned tube.
The finned-tube heat exchanger according to claim 5, characterised in that said fins have a h/p ratio between 3 and 5.
The finned-tube heat exchanger according to claim 1, characterised in that along a same finned tube there is an alternation of fins whose distal parts are bent and of fins whose distal parts are not bent.
The finned-tube heat exchanger according to claim 1, characterised in that at least on one of the two respectively upper and lower surfaces of the fins there are provided grooves whose longitudinal axis is inclined by an angle between +/-5° and +/-45° with respect to the axis of the fins that is perpendicular to the axis of the tube, said grooves being adapted to form vortices whose axis is substantially parallel to the direction of the flow of a fluid which is conveyed externally so as to strike the finned tubes.
The finned-tube heat exchanger according to claim 8, characterised in that said grooves are formed on the upper and lower surfaces of the fins, the grooves formed on the upper surface having an axis whose inclination is opposite with respect to the grooves on the lower surface with respect to said axis of the fins that is perpendicular to the axis of the tube, so as to generate two series of mutually contrarotating vortices.
The finned-tube heat exchanger according to claim 1, characterised in that flow deflector baffles are arranged between the rows of finned tubes.
The finned-tube heat exchanger according to claim 1, characterised in that said finned tubes are arranged in mutually aligned rows.
The finned-tube heat exchanger according to claim 1, characterised in that said finned tubes are staggered.
A finned-tube heat exchanger comprising a plurality of finned tubes with fins which comprise two parts, respectively a solid proximal one, which is fixed to the surface of the tubes, and a segmented distal one, characterised in that the distal parts of some of said segmented fins are bent away from the median plane of said fins, so as to create suction and compression regions in the direction of motion of the stream of a fluid which is conveyed externally so as to flow over the finned tubes.