JPS60240993A - Outside tube evaporating type heat transfer tube and manufacturing thereof - Google Patents

Abstract

PURPOSE:To exceedingly facilitate generation of air bubbles and remarkably heightening the evaporating heat transfer coefficient by forming build-up portions on the tip end sides of outer fins to narrow the interval between the fins and to form an opening interval easy for generation of air bubbles, and forming inner cavities widened toward the inner parts at the lower parts of the build-up portions to stably hold air bubble nucleuses. CONSTITUTION:Spiral fins 2 are formed around the outer periphery of a heat transfer tube material 1, and metal build up portions 3 made up of aluminum or the like are formed on the tip end sides of the fins 2. The thickness (f) on the tip end side is made larger than the thickness T on the root side [as a result, the interval (l) between the tip end sides of adjacent build-up fins is narrower than interval L on the root side]. As a result, spiral cavities widened toward the inner part, having fine spiral openings 4 are formed on the surfaces of the heat transfer tube material 1. The heat transfer tube having the above described structure is designed so that the interval (l) of the spiral opening 4 is in a range of from 20-100mum in which generation of air bubbles is easiest. Consequently, air bubbles are generated smoothly by the heat transfer tube by being stimulated by air bubble nucleuses within the inner cavities.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an extra-tube evaporative heat exchanger tube in which the extra-tube evaporation transfer coefficient is dramatically increased, and a method for manufacturing the same.

An extra-tube evaporative heat transfer tube is a tube in which a high-temperature liquid flows inside the tube and a fluid to be heated is passed around the outer circumference of the tube, and the fluid outside the tube is heated by heat exchange with the fluid inside the tube, or the latent heat of evaporation of the fluid outside the tube is used to heat the fluid inside the tube. It is used to cool fluids and is widely used in heat exchangers, evaporators, etc. By the way, from the standpoint of energy saving, new energy development, etc., demands for improved performance of heat exchanger tubes are becoming stronger. In particular, heat exchangers used for heat exchange between fluids with a relatively small temperature difference, such as exhaust heat recovery heat exchangers and ocean temperature difference heat exchangers, have extremely high evaporative heat transfer coefficients. required. In order to meet these demands, for example, there are methods to expand the effective heat transfer area by forming fins on the outer circumferential surface of the heat transfer tube, or ■ forming many fine irregularities on the outer circumferential surface of the heat transfer tube to prevent the generation of boiling nuclei. Research is being carried out on ways to increase the number of fins to smooth the generation of latent heat of vaporization and heat transfer.Specifically, methods have been proposed such as changing the shape of the fins or roughening the surface. Both have achieved certain results.

The inventors of the present invention have been conducting various research for the purpose of improving the performance of extra-tube evaporation type heat transfer tubes, but this time, we have found that if we perform a specified build-up process on so-called loaf-in type heat transfer tubes, we can increase the effective heat transfer area. We have learned that it is possible to obtain an extra-tube evaporative heat transfer tube that satisfies the two major demands of expansion and an increase in the number of bubbles generated on the outer circumferential surface and exhibits an extremely high level of evaporative heat transfer coefficient, and we hereby propose it.

In other words, the extra-tube evaporative heat exchanger tube according to the present invention has fins formed on the outer periphery of the tube body, such that the tip side of the fin is built up so that the wall thickness is larger than the wall thickness at the root side. This is the gist of the heat exchanger tube, and the method for manufacturing the heat exchanger tube involves applying metal to the surface of the fins formed on the outer periphery of the tube at an angle of 80 to 60 degrees with respect to the rising wall of the fin. The gist is that powder is plasma sprayed and built up on the side wall of the tip of the fin.

The structure and effects of the present invention will be explained in detail below, following the progress of the research.

The first basic means to increase the evaporation transfer rate is to increase the effective heat transfer area, and many techniques have been proposed in this regard, such as roughening the outer surface of the pipe and forming fins. Among these, finned heat exchanger tubes are considered to be one of those that exhibits an excellent evaporative heat transfer coefficient. On the other hand, various theoretical studies have been conducted regarding ``increasing the number of bubbles generated on the outer circumferential surface of the heat transfer tube and smoothing the generation of bubbles'', which is considered to be another basic means for improving the evaporative heat transfer coefficient. However, practical research has lagged significantly behind. The relationship between bubble generation and evaporative heat transfer coefficient will be discussed below.

As mentioned above, the evaporative heat transfer outside the tube is greatly influenced by the number of bubbles formed on the outer peripheral surface of the tube. Therefore, in order to improve the performance of evaporative heat transfer tubes, the most important issue is how many bubbles can be continuously generated on the surface of the heat transfer tube.

Easy to generate bubbles (in order to do so, ■ bubble nucleus (cavity)
■Making bubbles stable and easy to come out;
These two points are mentioned below, and these will be explained in more detail as follows.

■Bubble nuclei Many of the bubbles that are generated during boiling heat transfer occur from small depressions called cavities on the surface of the heat transfer surface, but bubbles will still occur if these cavities are too small or too large. There is an optimum opening width. If a cavity of an appropriate size is formed over the entire surface of the heat exchanger tube, many bubbles will be generated from the entire surface, and the evaporative heat transfer coefficient will be improved. Han-Gr.
Ifith means that the cavity has radius R as shown in Figure 1.
(in the figure, δ is the temperature boundary layer, Tsat is the saturation temperature of the outer fluid, and Twall is the wall temperature of the heat exchanger tube), theoretically the preferred cavity radius is expressed by the following formula: (g) It is proposed that there is a range.

δ: Temperature boundary layer thickness during natural convection (m) 02 Surface tension coefficient (Kg/m) vv: Specific volume of vapor (m37 Kg) L: Specific volume of liquid (m3/Kg) L: Latent heat of vapor (h)・m/Kg・℃) Twall: Temperature of the wall surface of the heat transfer tube (0K) Tsat: Saturation temperature of the outside fluid ('C), i.e., bubbles are generated from a cavity whose size (radius R) satisfies the relationship of the above CI) formula. He says he will come. The thickness (gradient) of the temperature boundary layer in the above equation (CI) can be determined by the following equation.

Gr-g@D3・β(Twall-Tsat)/
UI2λL: Thermal conductivity of liquid (Kcal/m@h'c
) αfc” natural convection heat transfer coefficient (Kcal/m'h'c)
D = Pipe diameter (m) β: Temperature expansion coefficient (1/'K) νL: Kinematic viscosity coefficient (m"/5) Pr Nibrandtl number Figures 2 to 5 show ammonia, Freon 22, Freon 22,
When Freon 118 and water are used, the above [■], (
10 is a graph showing the relationship between the difference between the saturation temperature and the tube wall temperature (horizontal axis) and the cavity radius (vertical axis) when the saturation temperature (saturation pressure) is used as a parameter. How to read these figures will be explained with reference to FIG. 6, which is a schematic diagram thereof. The shaded area indicates the cavity radius where bubbles are likely to occur, and the other areas indicate the cavity radius where bubbles are unlikely to occur. For example, find the difference (ΔThat) from the fluid saturation temperature (TBat) and the tube wall temperature (Twall), fix this on the horizontal axis, find the two intersections with the curve, and find the corresponding cavity radius Ra. and Rb. These are the maximum and minimum values of the cavity radius at which bubbles are likely to occur. These relationships vary depending on the physical properties of the fluid, and in these 8 figures, the region where bubbles can be generated is the widest for Freon 22, and increases as ΔTsat increases. However, it is said that the theoretical values obtained from the above-mentioned [I] and [In formulas cannot completely predict the actual values, and although the qualitative trends match, there are considerable errors.

In any case, as seen from Figures 2 to 6, in order to obtain a high-performance heat transfer tube, the cavity radius (6) must be set at 10 to 50 itm.
(2R=20-100 μm).

Regarding the stability of bubble generation, it is said that in order to stably generate bubbles, it is sufficient to make the cavity a secret, and the following relationship has been theoretically proposed. That is, as shown in Fig. 7 (4), if the relationship between the contact angle (O) between the base material and the liquid and the opening angle (φ) of the cavity is ψ<-1φ ... ([[-a), The interface (meniscus) between vapor and liquid becomes convex, and due to pressure change due to surface tension, P.< Pv(poo
: liquid temperature, pressure of liquid outside the boundary layer, Pv: pressure of vapor)
Since ToO<TV (TOo: temperature corresponding to Poo, Tv: steam temperature), the heat of the steam inside the cavity escapes into the outside liquid, and the steam gradually condenses and becomes smaller, eventually forming bubbles. It loses its function as a nucleus. On the other hand, as shown in Figure 7 (Bl), the contact angle (ψ) between the base material and the liquid
If the relationship between and the cavity opening angle (φ) is ψ〉−−φ ... cm−b+, the meniscus becomes concave, and Poo>Pvl
Since T(X) >"/, the steam always tries to grow. As a result, steam always exists inside the cavity, and a part of it is always released to the outside as bubbles, while the rest remains as bubble nuclei. The generation of bubbles occurs very smoothly and stably.

Considering this theory, it is understood that in order to obtain a high-performance heat exchanger tube, the opening diameter of the cavity should be 20 to 100 μm, and it should be a traditional method.

Taking advantage of this knowledge, we conducted various studies in an attempt to establish a technique that would allow the formation of a sophisticated cavity with an appropriate opening diameter on the surface of a heat exchanger tube, and as a result, we came up with the structure of the present invention described above. This is what I did.

That is, the structure of the heat exchanger tube according to the present invention is such that, as shown in the embodiment shown in FIG. A metal built-up part 8 made of aluminum or the like is formed on the tip side of the fin, and the wall thickness t on the tip side is larger than the wall thickness T on the base side (as a result, the distance l between the tips of adjacent build-up fins is 2), thereby forming a deep helical lumen 5 having a fine helical opening 4 on the surface side of the heat transfer tube material 1. A heat exchanger tube having such a structure is manufactured, for example, by a method as described below, and the interval l between the spiral openings 4 is set to 20 to 100 μm, which is the easiest way to generate bubbles.
It is designed to be within the range of m. Therefore, if this heat exchanger tube is used as an extra-tube evaporation type heat exchanger tube, the inner cavity 5 of Okuden will always retain the generated steam and stably hold the bubble nucleus,
As mentioned above, the openings 4 are spaced so that bubbles can easily be generated, so that bubbles are stimulated by the bubble nuclei in the inner cavity 5 and bubbles are generated very smoothly. As a result, the extremely important conditions for improving the evaporative heat transfer coefficient of ``increasing the number of bubbles generated on the outer circumferential surface of the heat transfer tube and smoothing the bubble generation'' were met, and the ``increase of the effective heat transfer area'' due to fin formation was achieved. Coupled with this effect, it comes to exhibit an outstanding steam heat transfer coefficient.

Although FIG. 8 shows an example in which the metal build-up portion 8 is formed on the tip side of the spiral fin 2, in addition, as shown in FIG. 9 (partially enlarged cross-sectional view), it is possible to It is also possible to form a vertical fin 2a in a direction substantially parallel to the axis and provide a similar metal build-up part 8 on the tip side.
It is also effective to further expand the effective heat transfer area by making cuts in the vertical fins 2a in a direction that intersects the fins.

It is thought that the metal build-up portion 8 as described above can be formed by various methods, but the inventors have conducted various experiments and found that the method of plasma spraying metal powder is the best method.
In particular, as schematically illustrated in FIG. 10, the angle from the outer periphery of the outer fin (the spiral fin 2 in the illustrated example) to the rising wall of the fin 2 (in other words, the rising angle is 80 to 60 degrees to the vertical line Y in FIG. 2). If metal powder is plasma sprayed from an inclined angle, the fin 2
It was confirmed that it was possible to build up only the tip of the tube particularly efficiently, and that the upper opening could be formed to have a narrow width while leaving a wide inner cavity 5 in the inner part. By the way, if the inclination angle of plasma spraying is less than 80 degrees, the sprayed metal will easily penetrate to the base of the fins 2, and the gaps between the fins will be filled with the sprayed metal, or the inner cavity as shown will be formed. It disappears. On the other hand, if the angle of inclination exceeds 60 degrees, the sprayed metal will scatter significantly and the loss will increase, and the top wall of the fin 2 will be overlaid too intensively, so the opening will be easily closed, and the spacing of l
adjustment becomes difficult. In addition to the plasma spraying method, there is also a thermospray method as a thermal spraying method, but in the latter method, even if the spraying angle is properly adjusted, the spaces between the fins are filled with the sprayed metal, and the inner cavity 5 cannot be formed. First, the method of the present invention is limited to plasma spraying of metal powder. The type of metal powder is not particularly limited, but aluminum powder is optimal if you consider the heat transfer characteristics and corrosion resistance required for heat transfer tubes, ease of plasma spraying, etc. In this case, aluminum powder is more than 200 mesh. If it is small, it will be oxidized and consumed in the spray flame, and an appropriate built-up portion will not be formed. Therefore, it is desirable to use an aluminum powder coarser than 200 mesh, more preferably 250 to 850 mesh. It is advantageous to roughen the surface by shot blasting or the like prior to plasma spraying, since this will improve the adhesion of the built-up portion 8.

By the way, reference photo 1 is an enlarged cross-sectional photo of the built-up part obtained by setting the plasma spray angle to 80 degrees (50 times: sprayed metal is +2
00 mesh aluminum powder), Reference Photo 2 is an enlarged cross-sectional photo of the built-up part obtained by setting the spraying angle to 60 degrees (same as above), and the upper side of the fin is intensively built up and the inner cavity is is formed. On the other hand, reference photo 8 is an enlarged cross-sectional photograph of the built-up part obtained by setting the plasma spray angle to 0 degrees (same as above), and shows that the build-up is almost uniform not only in the upper part between the fins but also in the root part. Therefore, it is not possible to narrow only the gap in the upper part. In addition, if an attempt is made to narrow the interval between the upper openings, the overlay metal will spread all the way to the base, making it impossible to form a lumen.

In this way, in the present invention, the built-up part is formed on the tip side of the outer fin to completely narrow the gap between the fins, creating an opening gap (ff) that facilitates the generation of air bubbles, and at the same time, the inner cavity of the deep floor is formed below the built-up part. The bubble core is stably held by forming a fin, which significantly increases the amount of bubbles generated and makes it extremely easy to generate bubbles, and also increases the effective heat transfer area by forming outer fins. We were able to dramatically increase the evaporative heat transfer coefficient.

By the way, Figure 11 shows (g) a smooth heat exchanger tube without surface treatment, (6) a heat exchanger tube with plasma spraying (aluminum) applied to a smooth tube, and a conventional heat exchanger tube with extra-dimensional fins (fin height: 0815w, Finpitche 80 mountains/1nch)
and (b) This shows an example of the experimental results comparing the performance of the evaporative heat exchanger tube of the present invention obtained by plasma spraying aluminum powder on the fin tip side of the trout, and shows how the effect of the present invention is. Indeed (see margin below).

[Brief explanation of drawings]

Figure 1 shows the C set by Han-Grif fi th.
I) Supplementary explanatory diagram of formula, Figures 2 to 5 are graphs showing the relationship between cavity diameter and temperature difference calculated from formula 19, Figure 6
The figure is an explanatory diagram, and Figures 7 (4) and (6) are diagrams for explaining the shape factor of the cavity with respect to the stability of the bubble nucleus.
Fig. 8.9 is an enlarged partial cross-sectional view illustrating the heat exchanger tube according to the present invention, Fig. 10 is an explanatory view illustrating the manufacturing method of the heat exchanger tube, and Fig. 11 shows the heat transfer effect of the heat exchanger tube according to the present invention. This is a graph shown in comparison with that of . 1... Main heat transfer tube, 2, 2a... Fin, 8...
・Metal overlay part. Applicant Kobe Steel, Ltd. and 1r-11 Published by

Claims (1)

[Claims] fl) An extra-tube evaporation type heat transfer tube, in which the fins formed on the outer periphery of the tube body are built up on the tip side and the wall thickness is larger than the wall thickness on the root side. 1. An extratube evaporation type heat transfer tube characterized in that it has a shape. (2) A method for manufacturing an extra-tube evaporation type heat transfer tube, in which metal powder is plasma sprayed onto the surface of fins formed on the outer periphery of the tube at an angle of inclination of 80 to 60 degrees with respect to the rising wall of the fin. . A method for manufacturing an extra-tube evaporation type heat transfer tube, characterized in that the side wall of the tip end of the fin is overlaid.