JPS62796A - Heat transfer tube - Google Patents

Abstract

PURPOSE:To improve heat transfer performance by a method wherein a coating, having recesses and protrusions of substantially uniform size, is formed on the outer surface of main body of the heat transfer tube while the inner wall of the main body is provided with a plurality of annular protuberances arranged with given intervals mutually in the longitudinal direction of the main body. CONSTITUTION:The heat transfer tube is formed with the coating 11, covered by metal grains of uniform grain size and having recesses and protrusions of uniform size, on the outer surface of the main body while the inner wall of the main body of the same is provided with a plurality of annular protuberances 12 with given intervals mutually in the longitudinal direction of the main body. The coating 11 is constituted of the metal grains having the grain size of 12-60 mesh. The annular protuberance 12 is constituted of an upright surface 12a, standing orthogonally to the flow direction of fluid which flows through the heat transfer tube against the flow of the fluid, and a slant surface 12b gently slopes down from the upper edge of the upright surface 12 along the flow direction of the fluid. The height (e) of the annular protuberance 12 is designed so as to be 0.3-0.5mm while the mutual interval or the pitch P thereof is designed so as to be 30-60 times of the height (e).

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention improves the heat transfer performance in a flooded heat exchanger for, for example, an ocean temperature difference power generation plant, and further promotes the miniaturization of the heat exchanger. The present invention relates to a heat exchanger tube suitable for.

[Technical background of the invention and its problems] Utilizing the temperature difference between the surface seawater and the surrounding seawater in the ocean,
The so-called ocean temperature difference power generation, which generates electricity using the thermal energy obtained from this, is being actively researched in countries such as Japan, which is poor in energy resources, and has almost reached the point where it is technically viable.

However, it is still not enough to overcome the economic issues, and there is a need to focus research and development on these issues at an early stage. Currently, there are several problems, but more attention must be paid to the problem of extremely large heat converters, which impairs economic efficiency.

That is, in general, in an ocean temperature difference power generation plant, the temperature difference between high and low heat sources is small, and the net power transmission efficiency is as low as a few percent. Therefore, the heat load on the heat exchanger per unit power generation becomes extremely large, and there is a problem in that the heat exchanger becomes extremely large. On the other hand, the low boiling point media used here are CFCs, ammonia, etc., but these are generally inferior to water, which is the conventional working fluid, in terms of heat transfer performance, and they encourage the enlargement of heat exchangers. There is a tendency to Therefore, in order to stop the increase in size and lead in a more desirable direction, it is first necessary to improve the heat transfer performance on the working fluid side, which defines the heat transfer coefficient.

Conventionally, with the intention of improving heat transfer performance on the working fluid side, methods such as rolling, metal spraying, or powder metallurgy have been used on the heat exchanger, that is, the heat transfer tube of a flooded evaporator, from the outside. The idea that it forms irregularities and promotes boiling heat transfer has been made public. However, with such a method, it is difficult to create irregularities of uniform size over a wide area, and the economic burden becomes increasingly large if a large number of high-quality products are to be obtained.

In addition, with regard to evaporators that use Freon as the working fluid, in order to similarly promote boiling heat transfer, copper powder with excellent thermal conductivity is flame sprayed from the outside onto the heat transfer tubes to form a copper spray layer. A method is proposed. (For example, JP-A-57-
(See Publication No. 82469) However, in evaporators that use ammonia as the working fluid, ammonia is highly corrosive to copper or copper alloys, so the idea of promoting boiling heat transfer with a copper spray layer is not adopted. It doesn't reach that point.

On the other hand, even if an excellent method for promoting boiling heat transfer that does not rely on such a method is developed and the heat transfer performance of the working fluid side is improved, the transfer of heated fluid passing through the heat transfer tube, that is, the seawater side, If the thermal performance is low, the heat transfer coefficient of the evaporator is determined by the heat transfer performance of the seawater side, so consideration must be given to the heat transfer performance of the seawater side.

Conventionally, in consideration of this point, torsion plates, spiral blades, etc. were placed inside the heat exchanger tube to give rotation to the mainstream, or annular members were installed at regular intervals inside the heat exchanger tube to disturb the mainstream. In addition, methods such as providing protrusions on the heat transfer surface and thereby disturbing the boundary layer have been made public. However, all of these components intentionally create turbulent flow, so if they are used incorrectly, a large pressure loss will occur in exchange for a slight improvement in the heat transfer coefficient, which will disrupt seawater circulation. Increased pump power costs, etc.
This may cause an increase in economic burden in other ways.

[Objective of the Invention] The object of the present invention is to improve heat transfer performance by promoting boiling heat transfer on the working fluid side and creating a turbulent flow without increasing pressure loss on the seawater side. It is an object of the present invention to provide a heat exchanger tube that can further promote miniaturization of a heat exchanger.

[Summary of the Invention] One of the features of the present invention is that a covering portion having irregularities of approximately uniform size is formed on the outer surface of the main body of the heat exchanger tube. This coating is integrally constructed by spraying metal particles with a particle size of 12 to 60 mesh.

Roughly speaking, the second feature is that a plurality of annular protrusions are provided on the inner wall of the main body of the heat exchanger tube, and are arranged at regular intervals. This annular protrusion is composed of an upright surface that stands up in front when viewed from the flow direction of the fluid in the pipe, and an inclined surface that gently descends from the upper end of this upright surface along the fluid flow direction. is 0.3 to 0.5 from the inner wall surface
stand up to a height of 30 to 6 mm, while the distance between each other is 30 to 6 mm of height.
I'm trying to make it 0 times the size.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to FIGS. 1 and 2.

In FIG. 1, the heat exchanger tube of the present invention has a covering portion 11 having uniformly sized irregularities formed by covering the outer surface of the main body with metal grains of uniform grain size, and furthermore, the inner wall of the main body of the heat exchanger tube has a covering portion 11 that is formed by covering the outer surface of the main body with metal grains of uniform size. A plurality of annular protrusions 12 are provided at regular intervals.

First, the covering portion 11 will be described in detail. As the constituent material of the covering portion 11, metal grains of uniform particle size, for example, high-purity alumina (AJ2203) grains are used.

Of course, any material other than this material may be used as long as it has equivalent thermal conductivity and hardness, and the selected metal particles are sprayed onto the outer surface of the heat exchanger tube body to integrate it. At this time, just as important as choosing the metalwork is to align the grain size of the metal within a certain range. In the present invention, the particle size is finer than 12 mesh, while 6
Select and use something coarser than 0 mesh.

Next, as shown in FIG. 2, the annular protrusion 12 has an upright surface 12a that stands up at right angles to the front when viewed from the flow direction of the fluid flowing inside the heat transfer tube, and a raised surface 12a that stands up at right angles to the front when viewed from the flow direction of the fluid flowing inside the heat transfer tube. It is composed of an inclined surface 12b that slopes gently down along the flow direction. In addition, the annular protrusion 12
The height e is set to 0.3 to 0.5 mm, while the mutual spacing, that is, the pitch P, is set to be 30 to 60 times the height e.

The heat transfer performance of the heat transfer tube with the above configuration will be explained with reference to FIGS. 3 and 4 showing experimental results.

FIG. 3(a) is a characteristic diagram showing the relationship between the boiling heat transfer coefficient and the particle size of the alumina grid when ammonia is used as the working fluid in a blasting tube formed by spraying alumina. Note that the ratio of boiling heat transfer coefficients on the vertical axis is determined based on the boiling heat transfer coefficient at a particle size of 12 mesh. As is clear from this figure, when the particle size exceeds 12 mesh, the heat transfer performance gradually shows a higher value, reaches a peak as it progresses further, and then starts to decline, and when it exceeds 60 mesh, the heat transfer performance gradually becomes higher. The relationship shows that the heat transfer performance is extremely degraded. The reason for this will be explained with reference to FIG. 3(b) which schematically shows an enlarged view of the covering portion 11.
If it is less than 12 meshes, the opening radius R of the cavity of the covering part 11 is too large and does not act effectively as a bubble generation nucleus, but after 12 meshes, the opening radius R becomes an appropriate size and bubbles are generated. After a sufficient level of nucleation is obtained, as the particle size becomes finer, bubble generation becomes more active, but if the limit is exceeded, the opening radius R of the cavity becomes smaller and the depth becomes shallower. Therefore, it gradually ceases to act as a bubble generation nucleus, and it is thought that after 60 meshes, its function is almost completely eliminated.

On the other hand, in order to obtain the characteristics 1n of the heat transfer tube provided with the annular protrusion 12, the pressure loss and heat transfer coefficient were measured, and the characteristic values for the smooth tube with the pressure loss constant are shown in FIG. There is. In this case, the pressure loss can be determined by changing the flow velocity in the pipe from 0.5 to 4.0 m/s, and the heat transfer coefficient can be determined by changing the heat load while keeping the flow velocity constant (approximately 2.0 m/s). The values obtained are shown below. The horizontal axis of the figure shows the ratio P/e between the pitch P of the annular protrusion 12 and its height e, while the vertical axis shows the power (pressure loss) of the circulating water pump and the thermal load under constant constraint conditions. The ratio of the heat transfer coefficient α' of the heat transfer tube of the present invention to the heat transfer coefficient αS of the smooth tube is α'/αS. As is clear from FIG. 4, when the fluid flows in the direction of the arrow in FIG. 2 (forward direction), as the height e of the annular projection 12 increases, the ratio α'/αS of the heat transfer coefficient increases, and The height e of the protrusion 12 is 0.
When it is 5 mm, the maximum value is 1.47. When the height e of the annular protrusion 12 is further increased, the ratio of the heat transfer coefficient
α'/αS on the contrary decreases. On the other hand, when the fluid flows in the direction opposite to the arrow in FIG. 2 (reverse direction), the ratio α'/αS of the heat transfer coefficients is quite small, as shown by the dotted line.

Therefore, it is desirable to place the upright surface 12a in front when viewed from the fluid flow direction, and the height e of the annular protrusion 12 is preferably 0.2 to 0.7 degrees from a practical standpoint, and 0.3 to 0. su
m is more preferable.

Furthermore, as a general tendency, when there is a high protrusion inside the heat exchanger tube, foreign matter tends to adhere to the vicinity of the base of the protrusion, and it is also difficult to remove it. If such foreign matter were to increase the fouling coefficient of the heat transfer tube and reduce the heat transfer coefficient, there would be no point in promoting heat transfer on the fluid side within the tube.

Therefore, the height e of the annular protrusion 12 is set to 0.7 in order to improve the cleaning effect of sponge balls etc. on the inner surface of the heat exchanger tube.
Set to 1m or less.

Furthermore, as the ratio P/e between the height e of the annular protrusion 12 and its pitch P increases, the heat transfer coefficient ratio α'/αS gradually increases, and the ratio P/e increases.
When e is 45, it takes the maximum value.

Then, as the ratio P/e becomes even larger, the ratio of heat transfer coefficients gradually decreases. Therefore, the ratio P/e is 20
-80 times is preferable, and 30-60 times is more preferable.

[Effects of the Invention] As explained above, the present invention has a coating portion having irregularities of a uniform size on the outer surface of the heat exchanger tube body, and a coating portion that stands up on the inner wall of the body toward the front when viewed from the flow direction of the fluid in the tube. The heat installation performance can be improved by placing annular protrusions at regular intervals, which are made up of an upright surface and a slope that gently descends from the upper end of the upright surface along the fluid flow direction. This has the excellent effect of further promoting miniaturization.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a heat exchanger tube according to the present invention,
Fig. 2 is a developed perspective view of the annular protrusion, Fig. 3(a) is a characteristic diagram based on the boiling heat transfer coefficient of the heat transfer tube according to the present invention, and Fig. 3(b) is an enlarged view of the coating. FIG. 4 is a characteristic diagram based on the heat transfer coefficient of the heat exchanger tube according to the present invention. 11...... Covering portion 12...... Annular protrusion 12a... Upright surface 12b... Inclined

Claims (3)

[Claims] (1) A covering portion having irregularities of approximately uniform size is formed on the outer surface of the main body of the heat exchanger tube, and a plurality of annular protrusions are further provided on the inner wall of the main body at regular intervals from each other in the longitudinal direction of the main body. The annular protrusion is composed of an upright surface that stands up in the front when viewed from the flow direction of the fluid in the pipe, and an inclined surface that gently descends from the upper end of this upright surface along the fluid flow direction. A heat exchanger tube characterized by: (2) The heat exchanger tube according to claim 1, wherein the coating portion is composed of metal particles having a particle size of 12 to 60 mesh. (3) The annular protrusions stand at a height of 0.3 to 0.5 mm from the inner wall surface, and the distance between each other is 30 mm of the height of the annular protrusions.
The heat exchanger tube according to claim 1, wherein the heat exchanger tube is ~60 times larger.