GB2037974A

GB2037974A - Heat transfer tube

Info

Publication number: GB2037974A
Application number: GB7943375A
Authority: GB
Original assignee: Nikko Engineering Co Ltd
Current assignee: Nikko Engineering Co Ltd
Priority date: 1978-12-28
Filing date: 1979-12-17
Publication date: 1980-07-16
Also published as: DE2951194A1; FR2445501B1; JPS5592892A; JPS624638B2; FR2445501A1; SE7910537L

Abstract

A heat transfer tube 1 (e.g. of circular or elliptical cross-section) intended particularly for use in heat exchangers installed in the northern hemisphere, is formed on its outer surface with at least one groove 2 inclined in a left hand screw direction. In use as a surface condenser the tube is so disposed that condensate formed on the external surface of the tube flows downwardly along said groove and the arrangement is said to induce the condensate to flow down quickly or to scatter out of the groove by the centrifugal force so as to increase the heat transfer performance of the tube. The tube outer surface may have additionally, longitudinal grooves (e.g. 6, 7, Figs. 5a, 5b), which interconnect inclined grooves, and also a large number of fine grooves. <IMAGE>

Description

SPECIFICATION Heat transfer tube The present invention relates to a heat transfer tube used for various kinds of heat exchangers such as steam condensers or water recuperators.

Generally, in a power generating plant where a power is generated for circulating the operational fluid such as steam, or in a chemical plant refining substances, or in a factory where substances are recovered, condensers are used for changing steam into liquid.

Heretofore, most of the heat transfer tubes for condensers used in these plants utilize, in their horizontal tube groups, those having a smooth surface, or finned tubes formed with vertical or spiral fins in a longitudinal direction of the tube on the external surface of the circular tubes.

However, the coefficient of heat transfer of condensation on the surface of these tubes are not so large, so that a considerable large heating surface was required to obtain a desired amount of condensation.

The reason therefor is based on the following fact. A factor which affects the "flow coefficient of the condensation heat" of the heat transfer tube is generally known to be an obstructing action of the heat transfer due to a poor heat transferring property of the liquid film condensing on the heat transfer tube rather than the magnitude of the intrinsic thermal conductivity of metal material for the heat transfer tube or the condition of thermal transmission of the cooling medium. Accordingly, a variety of attempts have been proposed to improve the transmission performance by reducing the condensation film which is a main factor of obstructing the heat transmission, or to reduce the cost of equipments, or area of installation by lessening the required number of heat transfer.

For example, a fluted tube has been developed in which a large number of vertical stripes are formed in parallel along the longitudinal direction of the heat transfer tube. In the heat transfer tube of this kind, in a case where the amount of condensation is small, the performance is excellent as compared to the conventional heat transfer tube having a smooth surface or those provided with fins. However, when the amount of condensation is increased, since the condensate forms a film, particularly, since this film becomes thick at the lower part of the heat transfer tube, the intention of providing vertical stripes on the surface of the heat transfer tube is lessened, and this reduces the performance of the condensation remarkably.For this reason, when the fluted tube is utilized on a condenser which has a large amount of condensation, various kinds of contrivances are required as in the smooth tubes or finned tubes of the prior art. As the result, a large number of processes or man-hours are necessary for the manufacture.

Recently, a heat transfer tube called corrugated tube has been developed which has inclined grooves in a right-hand-screw direction on the surface. Althrough this tube, conventionally having an incl'ination angle of 6 to 90, that is, a pitch ranging from 8 to 12 mm with a tube having an outside diameter of 25.4 mm, has somewhat high efficiency when the tube is arranged horizontaliy, but when this corrugated tube is arranged vertically, no remarkable difference in efficiency is observed as compared with conventional smooth tubes, and the performance is rather lowered.

The object of the present invention is to eliminate these drawbacks found in the prior arts described above, and to provide a heat transfer tube which has a remarkably high efficiency in the case when the tube is arranged vertically.

The heat transfer tube of this invention has an inclined groove inclined in a direction of left hand screw on the externai surface of the heat transfer tube. The inclined grooves include those formed continuously over a large number of pitches. Since the inclined groove is formed in a direction of left hand screw at a predetermined inclination angle, the inclined groove is inclined right-hand downwardly as seen from any direction covering 3600 when the heat transfer tube is arranged vertically.

Next, the function of the heat transfer tube of this invention will be described. In general, when steam condenses on the outer peripheral surface of a smooth circular tube arranged vertically, the condensate increases its thickness gradually from the upper to the lower portions. The heat transfer coefficient of condensation becomes smaller as it approaches the lower portion of the heat transfer tube. It is observed that, the condensate formed at the outer peripheral surface of the heat transfer tube has a property, in the northern hemisphere, of flowing down while turning around the tube in a direction of left hand screw when the amount of condensation increases.This is considered to be caused by being subjected by a force of vector due to the effect of Coriolis' force produced by the rotation of earth when the flowing atmosphere of condensing gas or the condensed liquid on the surface of the heat transfer tube flows down naturally along the wall of the heat transfer tube after the condensation and cohesion under the effects of the surface tension or van der Waal's force possessed by the material of the heat transfer tube.Due to such a property of downward flowing condensate formed on the surface of the heat transfer tube, in the conventional heat transfer tube in which vertical stripes or inclined grooves inclined in a direction of right hand screw are formed on the surface of the tube, the flowing down of the condensate is not effected smoothly, and as the result, in many cases, the condensate would flow on the heat transfer surface in lumpform, which deteriorated the heat transfer coefficient of the condensation.

In view of this fact, the present invention has proposed a method, upon considering the fact that the films or drops of the condensate have a tendency of flowing down in a direction of left hand screw as described above, in which the heat transferring performance can be much improved by causing the condensate to flow down quickly or by scattering it with a centrifugal force caused by increasing the speed of flow rotating around the heat transfer tube.

In the accompanying drawings: Fig. lisa front view showing an example of the heat transfer tube according to the present invention; Fig. 2 is a sectional view of the heat transfer tube shown in Fig. 1 along a line XX; Fig. 3 is a front view showing another example of the heat transfer tube according to the present invention; Fig. 4 is a front view showing further example of the heat transfer tube according to the present invention; Fig. 5 (a) is a front view showing still further example of the heat transfer tube according to the present invention; Fig. 5(b) is a front view showing a state where the heat transfer tube shown in Fig. 5(a) is rotated about 900 with the axis of the tube as the center; Fig. 6 is a longitudinal section of the heat transfer tube of this invention used in the experiments on graphs in Fig. 7;; Fig. 7 is a graph showing an example of heat transfer performance of the heat transfer tube of this invention; and Fig. 8 is a front view showing another example of a heattransfertube according to this invention.

Preferred embodiments of the present invention will now be described referring to the drawings.

Firstly, a heat transfer tube of this invention shown in Figs. 1 and 2 will be described. A long, inclined groove 2 inclined in a direction of left hand screw is formed on the external surface of the heat transfer tube 1. In this example, the inclined groove 2 is formed continuously over a large number of pitches. The intervals between the inclined grooves 2 may be varied in accordance with the position of the heat transfer tube 1 as occasion demands. For example, when the heat transfer tube is arranged vertically, the intervals at the lower portion may be made prolonged.

According to experiments, a favorable result was obtained in a case of freon gas R113, when the inclination angle A with respect to the horizontal surface of the inclined groove 2 was about 320, for example, and the pitch P was set at about 50 mm when the outside diameter D of the heat transfer tube 1 was about 25.4 mm. In the case of vapour of water, it was ascertained that the inclination angle favorable for the heat transmission performance was reduced as compared,with the case of the freon gas Ru3. The depth of the inclined groove differs considerably according to the condition of the use, but a relatively favorable result was obtained when it was set at 1 to 3 mm.As to the shape of the inclined groove 2, it is preferable to select a shape having no edge portion is formed at the junction point of the heat transfer surface and the inclined groove 2 so that the condensate is readily drawn into the inclined grooves 2 below from the surface 1 a of heat transfer surface, and the shape of the cross section is preferably so selected that the condensate flown into the inclined groove 2 hardly flows out of the groove towards the lower surface 1a.

The inclined grooves inclined in a direction of left-hand screw on the surface 1 a of the heat transfer tube 1 formed as in the present invention are consistent with the nature of flowing down of condensed liquid on the surface 1 a of the heat transfer tube 1, and the condensate flows dovvn smoothly. That is to say, the liquid condensed on the surface 1 a of the heat transfer tube 1 is first flows into the inclined groove 2, then flows down smoothly along the inclined groove 2. Such a flow of condensate increases its speed as it flows downwardly, and finally scatters out of the inclined groove 2. Accordingly, the thickness of the condensed film is restrained below a constant value all along the entire length of the heat transfer tube 1.Moreover, the condensate is pulled in due to the surface tension of the condensate from the surface 1 a of the heat transfer tube 1 toward the inclined groove 2, thereby the condensation at the surface 1 a is promoted. As the result, a phenomenon similar to a drop-formed condensation is presented on the surface 1 a of the heat transfer tube 1. For this reason, the heat transfer coefficient of condensation of the heat transfer tube 1 shown in Figs. 1 and 2 is remarkably increased as compared with the conventional smooth, circular heat transfer tube.

Although not shown in the drawings, a vertical groove can be formed along the longitudinal direction of the heat transfer tube 1. By this vertical groove, the condensate flowing down around the inclined grooves is collected into the said vertical groove and flows downward rapidly.

Fig. 4 shows another embodiment of heat transfer tube according to the present invention. In this example, a smooth external surface portion 3 is provided where no inclined groove 2 is found over a predetermined length L. This example has practically the same form as that shown in Figs. 1 and 2 except the portion 3 having a smooth external surface provided over a length L, therefore the description about the form thereof is omitted. The portion 3 of smooth external surface can be utilized in a vertical type condenser, for example, when partition plates are to be disposed by providing portion 3 in the smooth external surface on the halfway of a number of heat transfer tubes arranged in parallel in a vertical direction.

Fig. 5 shows further embodiment of the heat transfer tube of this invention. In Fig. 5(b) is shown a state where the heat transfer tube shown in Fig.

5(a) is rotated about 900 with the central axis of the tube as the center. In this example, inclined grooves 4 and 5 inclined in a direction of left hand screw are formed- independently and intermittently at every half of a pitch, and these inclined grooves 4 and 5 disposed intermittently are interconnected by means of two long, vertical grooves 6 and 7 provided along the longitudinal direction of the heat transfer tube 1. When such vertical grooves 6 and 7 are provided, the condensate flowing down around the inclined grooves 4 and 5 is collected into the vertical grooves 6 and 7, and flows downwardly. Accordingly, the condensate can flow down towards the lower portion of the heat transfer tube with an extremely high efficiency.By combining these inclined grooves 4 and 5 and two vertical grooves 6 and 7, the condensing performance at the surface 1 a of the heat transfer tube 1 is further improved. In the example shown in Fig. 5, it is also possible to provide a smooth surface portion where neither inclined grooves 4 and 5 nor vertical grooves 6 and 7 are found at a predetermined position as shown in Fig. 4.

Although not shown in the drawings, if a large number of fine vertical stripes are formed along a longitudinal direction of the heat transfer tube 1 on the external surface (except the portion 3 having a length L) of the heat transfer tube 1 art a portion of said surface 1 a, that is, between the inclined grooves 2, 4 and 5, the heat transfer effect will further be improved.

Each of the heat transfer tubes 1 shown in Figs.

1, 2, 4 and 5 as examples has relatively a thick tube wall, and each of the grooves of the inclined grooves 2, 4, 5 and the vertical grooves 6, 7 are usually formed by machining. However, in the case where the wall thickness of the heat trasnfer tube 1 is small as shown in Fig. 3, the inclined groove 2' is formed by extrusion using rollers from outside of the inclined grooves 2' or by press work. In the case of the heat transfer tube 1 shown in Fig. 3, the portion of the inclined grooves 2' at the outside of the tube is formed by a protruded streak inside the tube as the result of working process, whereby an effect of heat transmission due to turbulent flow of fluid in the tube is superposed by the protrude streak.

Furthermore, it is also possible to form the inclined grooves in the heat transfer tube used in each of the afore-mentioned examples as an inclined double grooves 2" formed by two grooves closely arranged in parallel, as illustrated in Fig. 8.

By this means, in the case when the condensate flown into the upper groove of the inclined double grooves overflows from said upper groove, the overflown condensate flows into the lower groove or scatters outwardly from the boundary portion of the upper and lower grooves. Accordingly, it is prevented that the condensate flown into the inclined grooves overflows from said inclined grooves and flows out towards the lower heat transfer surface.

Although the inclination angle A of the inclined grooves 2, 4 and 5 to be formed on the external surface of the heat transfer tube 1 differs in accordance with the kind of the condensate, by setting optimum inclination angles A respectively taking the nature of flowing condensate in consideration, the flow of condensate is made smoothly, its speed is increased, and the condensate is scattered, whereby a heat transfer similar to a drop-formed condensation can be obtained.

Generally, in the heat transfer tube of a condenser for practical use, a favorable result was obtained in the one having the inclination angle A of the inclined groove in the range of from 14 to 350. Further, it is considered to be appropriate that the pitch P or interval of the inclined grooves corresponding to the said inclination angle A of 14 to 350 is ranging from 20 to 56 mm in the case that the outside diameter D of the heat transfer tube is 25.4 mm.

The heat transfer tube according to the present invention being constituted as described above, thick films of condensate are hardly produced all over the heat transfer tube, and even if they are produced, the film is maintained sufficiently thin as a whole. As the result, with the heat transfer tube according to this invention, a remarkably high heat transferring effect can be obtained as compared with the conventional heat transfer tubes with a smooth surface, or formed with a inclined grooves having a direction of right hand screw.

The performance of heat transmission of the heat transfer tube according to the invention obtained by experiments will now be described referring to graphs shown in Fig. 7. The graphs show the value of condensation obtained by experiments of a heat transfer tube of the invention shown in Fig. 6 which has a tube length of 1.6 m, using freon gas R1, outside of the tube, and water inside the tube. A favorable result was obtained that, in the case of present invention, the non dimensional heat transfer coefficient was increased by about 60 per cent as compared with smooth tubes when the flow rate of non dimensional condensate was 1000 as compared with smooth tubes. In the graphs shown in Fig. 7, (I) is a graph of values obtained by experiment with heat transfer tube of the invention described above, and (il) is a graph of values obtained by experiments with a smooth tube.

It is to be noted that the present invention is not limited to the above described embodiments.

A particularly remarkable effect is obtained when the heat transfer tube of this invention is utilized on condensers, but a sufficient effect can be obtained when it is used for other heat exchangers. The heat transfer tube of this invention shows a remarkable effect when it is used on multi-tubular condensers or heat exchangers and particularly when they are used in a vertical type. The heat transfer tube is not only limited to a circular tube, but also includes elliptical tube or others. In the case of the elliptical tube, it is preferable to form a vertical groove on the external surface along a direction of the minor axis of the ellipse.

Claims

1. A heat transfer tube used for condensers or water recuperators, characterized in that inclined grooves inclined to a direction of left hand screw are formed on the external surface of the heat transfer tube installed in northern hemisphere.

2. A heat transfer tube as claimed in Claim 1, wherein said inclined grooves inclined to a direction of left hand screw are formed continuously over a large numberof pitches.

3. A heat transfer tube as claimed in Claim 2, wherein a long vertical groove is formed along the longitudinal direction of the heat transfer tube.

4. A heat transfer tube as claimed in Claim 1, wherein said inclined grooves inclined to a direction of left hand screw are formed independently with one another and intermittently at an interval of half a pitch, and these intermittent inclined grooves are connected by two long vertical grooves along the longitudinal direction of the heat transfer tube.

5. A heat transfer tube as claimed in any of Claims 1, 2, 3 and 4, wherein said inclined grooves inclined to a direction of left hand screw are formed as double parallel grooves.

6. A heat transfer tube as claimed in any of Claims 1, 2, 3, 4 and 5, wherein a large number of vertical stripes along the longitudinal direction of the heat transfer pipe are formed on the external surface of said heat transfer tube between said inclined grooves inclined to a direction of left hand screw.

7. A heat transfer tube as claimed in Claim 1, 2, 3, 4, 5 or 6, wherein a smooth external surface on which neither said inclined grooves nor said vertical stripes are found is provided over a predetermined length.