DE2936406A1 - BOILER SURFACE FOR HEAT EXCHANGERS - Google Patents

Description

P.5455 / Sd / mm

Sulzer-Escher Wyss GmbH, Lindau / Bodensee (Germany)

Boiling surface for heat exchangers

The invention relates to a boiling surface for heat exchangers, consisting of a metallic base material and an applied metal layer.

As is well known, efforts are made to reduce the heat transfer from the heated surface of a heat exchanger to the boiling liquid to be improved by an appropriate surface structure.

So it is known to be made of smooth metal To apply a porous metal layer, which capillaries with outlet openings in the surface of the surface Has layer that serve as nucleation sites for the formation of vapor bubbles. Such layers are off The individual bodies connected to one another are built up on the contact surfaces, so that there are cavities in the layer located, which are connected to one another in a statistical distribution, with a capillary structure with numerous capillaries open to the surface is formed.

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The structure required to achieve a capillary structure in the boiling layer, consisting of many individual bodies, which only connected to each other at the contact surfaces, causes the heat transport within the layer only can take place via the thermal bridges formed by the contact points and is thereby significantly hindered. This leads to heat conduction resistances, which hinder the heat transfer during boiling.

In addition, the layer has a very fine capillary structure sensitive to contamination during manufacture or further processing as well as when they are used as Boiling layer against impurities in the boiling liquid.

Such layers can be produced, for example, by complex soldering or sintering processes, as described in FIG U.S. Patent 3,384,154. In this process, metallic additives or organic auxiliaries are used are used, some of which are retained in the layer.

The metallic additives are predominantly compounds with the material of the individual bodies of the layer and the base material or partly remain metallic purely back, so that the layer is built up from a large number of materials or their compounds

The organic auxiliaries required during sintering remain also in smaller quantities back in the shift.

An important field of application of heat exchangers, their boiling layers in the manner described above are made, consists in the evaporation of organic liquids that serve as working media in circular processes.

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Such equipment, which has been used for a long time, is extremely sensitive in contact with organic and also metallic foreign matter. For example, in heat pump systems and refrigeration systems as working fluids often fluorinated chlorinated hydrocarbons used, which have low admixtures contained in the lubricating oil of the refrigeration machines. Organic foreign matter still present in the boiling layer often form harmful chemical compounds with the working fluid / lubricating oil mixture, while metallic ones Foreign substances can cause the work equipment to decompose.

Another possibility for producing capillary structures in boiling layers is described in US Pat. No. 3,990,862. Then with the help of a special flame spraying process Metal particles are applied to the base material and, in the process, many of them become, for the most part, strong Deformed metal particles form a boiling layer with the desired capillary structure. An essential feature times this process consists in the partial oxidation of the metal particles in an operated with excess oxygen Flame. To ensure this oxidation, metals must be used, which in a short time a considerable Form oxide skin, the melting point of which differs significantly from that of the starting material.

For example, copper is not such a metal. The metal oxides should have sufficient strength of the boiling layer ensure with existing capillary structure. However, they are chemical compounds that cause decomposition of the work equipment, as already described at the beginning.

The object of the invention consists in the formation of a boiling surface for heat exchangers, which a very

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good heat transfer from the boiling surface to the boiling liquid ensures, has no harmful impurities and has a low thermal resistance.

According to the invention, this object is achieved in that the layer consists of partially connected, metallic individual bodies of different shapes, and that the layer is essentially free of capillaries penetrating them and of closed pores, and that the depth of the remaining free between the individual bodies and against the The boiling surface of open spaces is on average more than 40 % of the layer thickness, with the layer thickness measured over the highest elevation being 30 to 300 lan .

An advantageous method for producing the metal layer consists in that the particles forming the individual bodies with the aid of the powder flame spraying process known per se can be applied to the base material in such a way that one is heated by the combustion of two gases and by means of a cooling gas cooled and accelerated gas flow, its mass balance of all causing it Partial flows having a stoichiometric ratio of fuel gas and oxygen, the particles of the metal powder are introduced, heated by the gas flow, melted on their surface and transported to the base material. be animalized.

The layer according to the invention thus consists of a single one Material, so that the harmful influence of foreign substances that occurs with the known capillary structures ceases to exist.

By building up the layer from a multitude of

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In the case of individual bodies of various shapes, this has an enlarged area in the area of the boundary layer of the boiling liquid Surface. As a result, a large volume of liquid in the boundary layer area can be heated in a short time. the The resulting vapor bubbles lead large amounts of heated boundary layer liquid from the boiling surface away, which ensures the very good heat transfer from the boiling surface to the boiling liquid.

There are numerous notches at the transitions between the individual bodies, Undercuts and similar geometries formed, which act as parts of the bladder germ. This is an intense one Bubble formation is guaranteed, which is necessary for the removal of the heated boundary layer liquid.

For a better understanding of the mode of action achieved with the aid of the invention, reference should be made to the following facts.

The boiling process on surfaces takes place according to laws such as those used for B. in the DKV treatise no. 18.1964 "Contribution to Thermodynamics of heat transfer during boiling "by K.

Stephan are described, with bladder nuclei that are statistically distributed on the surface, small vapor bubbles arise, grow to their tear-off diameter and then tear off the surface and rise in the boiling liquid. A new vapor bubble is created and the process is repeated with the so-called bubble frequency. Such as B. by Han and Griffith in that Article "The Mechanism of Heat Transfer in Nucleate Pool Boiling" Int. J. Heat Mass Transfer, Vol. 8, 1965, pp. 887 914 and by Beer and Durst in the article "Mechanisms of heat transfer in nucleate boiling and their simulation", Chemistry-Ing.-Tech. 40 (1968), 13, pp. 632 - 638 verified

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The time described by the term "bubble frequency ¹¹ " is divided into two parts, namely the waiting time and the actual time of the bubble formation, the waiting time making up 60 to 80% of the total time After a certain temperature profile has built up in this boundary layer, the vapor bubble begins to form, grows to the bubble tear-off diameter and tears away from the boiling surface.

With this tearing off, the vapor bubble leads the surrounding, heated liquid boundary layer by "mass convection" or by their indicated "drift flow" away from the boiling surface. The area of influence corresponds to the drift flow on the boundary layer is approximately the same as the bubble detachment diameter. After the boundary-layer-mass convection, "cold liquid" flows to the boiling surface and is warmed up there again. The heat is largely removed from the boiling surface through unsteady heat conduction discharged.

As can be seen from the publications mentioned above, the following can be used to estimate the sizes Relationships are used. The bubble tear-off diameter is

d - 0.0146. Δ ° . "i / 2. σ .ν

^A 'h. (ν -

(ν "- ν ·) r

The bladder frequency can be approximated with the help of the following Relationship to be determined:

- η 1/2. _7ς r. d. = "1.75

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The boundary layer thickness when the bubble is detached is

. a . (r "

Are there

g (Τ3 ^ acceleration due to gravity

d. (m) Bubble tear-off diameter A

(o) Contact angle of the adhering vapor bubble

O (N / m) surface tension of the boiling liquid

v ¹ (m / kg) spec. Volume of the liquid ν "(m / kg) specific volume of the resulting vapor

1Of (l / s) bladder frequency

a (m / s) thermal diffusivity of the liquid

<- (s) waiting time
wz

C (s) time for bladder growth

(m) Boundary layer thickness when peeling off

On page 15, Table 1 shows the calculated values for some of the boiling liquids commonly used in refrigeration technology and usual boiling temperature ranges specified

The physical processes described so far in

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Boiling leads to the inventive finding that an improvement in the heat transfer can be achieved if the boiling surface in the area of influence of the boundary layer and the bubbles is increased considerably.

As a result, the time required for the formation of the temperature profile of the boundary layer, i. H. the wait, essential be reduced and also the volume of the boundary layer liquid, which is transported away from the boiling surface by a bubble, significantly enlarged will.

In the invention, it was recognized that to achieve the improved heat transfer the serving of the increase in surface area structure in its overall height, ie in the measured about the highest elevation film thickness, in the order of the boundary layer thickness must, but whose five times the value can not exceed lie.

The invention is described below with reference to embodiments of metal layers shown in the drawing and a device for producing the metal layers and an associated diagram.

Fig. 1 shows the achievable area enlargement in a diagram the structure depending on the different shapes of the individual bodies.

In Fig. 2, a detail of a longitudinal section of the metal layer is shown schematically.

3 and 4 show photographic recordings of surface structures made with a scanning electron microscope on a scale of 100: 1 or 500: 1.

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Fig. 5 shows in a simplified, schematic representation a flame spray device for producing a metal layer during operation.

In FIG. 6, a diagram shows measurement curves of the transferred heat outputs as a function of the excess temperature applied.

To estimate the achievable area enlargement F / F compared to a smooth base area F through fine structuring of the surface in the manner according to the invention, a calculation for individual bodies has become simpler Geometry such as cuboids, cylinders, pyramids and the like. Performed. For this purpose, the Single body assumed on the smooth base, where the ratio V is the shortest distance between adjacent Individual bodies are very small in relation to the edge length or the diameter of their base area, d. H. V ^ O, 2, is.

The area enlargements F / F that result are shown in FIG. 1 as a function of the ratio V for various geometries. The curve I applies to cuboids with a ratio of their height h to their edge length s of r- = 1; curve II for cylinders with a ratio of their height h to their diameter d of - = 1; curve III for cones with an apex angle of ^ = 30; curve IV for pyramids with an apex angle of Y = 45; the curve V for cones with an apex angle of ^ f = 45 °; curve VI for hemispheres.

As can be seen from Fig. 1, considerable surface enlargements can be achieved, this being independent of

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the absolute height of the individual bodies. Different body shapes are applied to a base material If a layer is superimposed, even more substantial surface enlargements can be achieved.

Fig. 2 shows a schematic representation Longitudinal section through a layer applied to a metallic wall 1. The individual bodies have different Formations that are mainly generated during manufacture, which will be discussed later will. As can be seen from FIG. 2, the layer is made up of almost spherical and more flattened individual bodies 4 and 5 formed. Some of the bodies adhere to the wall 1 or neighboring bodies adhere to one another at the contact surfaces 2 and 3, whereby clasps formed contact surfaces 6 arise. This creates a solid layer with good thermal conduction transitions educated. The transition from the surface of a body to the surface of the adjacent body takes place in many cases in the form of notches 7. The different body shapes result in an extremely heterogeneous structure the layer 11, whereby holes 8 up to the base material, crater 9, elongated between the individual bodies Column 10 and similar shapes result, which are excellent as bladder nucleation sites and thereby Ensure that the boiling surface is at least completely covered with vapor bubbles, even if the Excess temperature T-T of the boiling surface compared to the Saturation temperature T of the boiling liquid is very low is.

The layer thickness ο is measured between the highest elevation and the surface of the heat-emitting wall and according to the invention is from 30 to 300 yU-m. Between

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Layered bodies and their defined height remain open spaces, the depth of which can be between a few percent and 100% of the layer thickness, but is on average more than 40 % of the layer thickness. The surface structure achieved with the invention is also evident from the layer recordings according to FIGS. 3 and 4, in which the same reference numerals as in FIG. 2 are used.

As already mentioned, with the aid of the structure formed according to the invention, the metal layer can be compared to the Surface of the base material a significant increase in surface area can be achieved, for example on the order of 2 to 10.

An advantageous method for producing the layer consists in a modified type of what is known per se Powder flame spraying process, in which the metal particles of the starting material are in a partially melted or doughy state be sprayed onto the base material. The geometric shape of the metal particles before spraying can be diverse. It can e.g. B. spherical, cube-shaped, prismatic, polyhedral, dendritic and lumpy shapes are used. These metal particles are channeled into a hot gas stream and in it Gas flow evenly distributed. The gas flow serves as a means of transport for the metal particles to the base material and on the other hand as a heat source for heating the metal particles. The metal particles of their surface is heated, with the help of the temperature and the speed of the gas flow as well the flight distance of the particles the residence time of the metal particles in the gas flow and thus the degree of heating or can influence the melting in a known manner. According to the degree of melting and the

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When the metal particles hit the base material, they will deform and with the base material and clamp together at the points of contact, claw or weld, as shown in Fig.

to 4 emerges. By suitable selection of the dimensions of the metal particles, the dimensions of the bodies forming the layer can be kept in such a way that they correspond to the values given in Table 1 for the boundary layer thickness in the order of magnitude. Metal particles of spherical shape with diameters between d ■ 30 and 150 nja are advantageously used as the starting material.

A device 20 for carrying out the modified powder flame spraying method is schematically shown in FIG shown.

Fuel gas and oxygen are introduced into the device through feed lines 21 and 22. The mixture flows through a nozzle ring 23 and is then burned in the flame. The hot flame gases are through with one a feed 24 introduced into the device and through an external nozzle ring 25 into the flame injected cooling gas mixed and thereby cooled. Part of the cooling gas also acts as a means of transport for the metal powder introduced into the device through a feeder 26. This is distributed in the gas flow Powder according to the flow conditions and is transported to a wall 27 made of the base material and meets there in an almost circular area, which is to be referred to as the spray spot. This spray spot is passed over the surface of the base material and thus an areal application of the Layer achieved. The relative speed of the base material perpendicular to the spray jet is an

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flow size for the quality of the layer. If the relative speed is too low, the dwell time would be the applied The layer in the spray jet was too large and the metal particles in the layer would melt and become relatively homogeneous Form a layer without body shapes and without a sufficiently large number of bladder nuclei. When applying of the layer are the relative speed, layer thickness and the powder throughput through the spray nozzle in one certain context. The gas flows in front of the spray nozzle and the Distance between the spray nozzle and the base material, as already mentioned above, together with the Gas flow after the spray nozzle determines the residence time of the metal particles in this gas flow. There one If a metallically pure layer is sought, it is necessary that all gases flowing through the spray nozzle together result in a stoichiometric balance between fuel gas and oxygen. In this way, oxide

nen
tio- and other chemical changes in the metal parts

be avoided.

As a fuel gas, for example, acetylene or hydrogen and as a cooling gas z. B. dried and purified air or nitrogen can be used. Copper and copper alloys are preferably used as the material for the metal particles used. However, other metals, such as. B. iron and iron alloys can be used.

In an experiment, a layer such as that shown in FIGS. 3 and 4, for example, was made of copper bodies Copper pipe with an outer diameter of 18 mm is applied. The heat transfer line was installed in a suitable apparatus q measured at boiling and under conditions such as those in flooded tube bundle evaporators

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of turbo chillers, using the refrigerant CF-Cl- and at an evaporation temperature of 0 ⁰ C.

In FIG. 6, the heat transfer capacities q determined in this way are plotted as a function of the excess temperature Δ Τ (see curve W), with the heat transfer capacities also being plotted on the pipes for comparison. , with a "smooth" surface (see curve Z).

In table 2 on page 17, the throughput quantities of the gas streams, the metal powder and the associated spray distance and the relative speed of the base material perpendicular to the spray jet for a production method of a layer formed according to the invention are given in a numerical example.

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Table 1 t (R114) ( ⁰ C) f <l) 589
745 Sp. (/ Im) (R12) 60
O 72
64 569
732 40
51 CF ₂ CLCF ₂ Cl (R22) 30th
-30 72
65 672
877 50
59 CF ₂ Cl ₂ (R13B1) 20th
-40 68
59 549
655 53
64 CHF ₂ cl (R13) -20
-60 75
68 592
734 47
54 CF ₃ Br -40
-80 72
65 47
54 CF ₃ Cl

Table 2

Fuel gas:
acetylene

Powder: copper spherical

Spray distance:

area

at 1 bar 0.5 - 1.0

Oxidizing gas: Oxygen O ₃ , at 1 bar 0.6 - 1.2 Cooling gas: air, at 1 bar 3.1 - 6.2

2.5-10 30-150

5-15 preferably

Relative speed: 0.02 - 0.1

0.74 (raVh)

0.85 (πΓ / h) 5.3 (m ³ / h)

3.0 (kg / h) 40 - 80 (um) 10 (cm) 0.05 (m / s)

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Claims (4)

Claims 1> Boiling surface for heat exchangers, consisting of a metallic base material and an applied metal layer, characterized in that the layer consists of partially connected, metallic individual bodies of various shapes, and that the layer is essentially free of capillaries penetrating them and of closed pores, and that the depth of the spaces remaining free between individual bodies and open to the boiling surface is on average more than 40 % of the layer thickness, the layer thickness measured over the highest elevation being 30 to 300 μm. 2. boiling surface according to claim 1, characterized in that the grain size of the individual bodies forming starting substance between 30 and 150 microns. 3. A method for producing the metal layer according to claim 1 and 2, characterized in that the individual bodies forming particles with the aid of the powder flame spraying process known per se onto the base material in this way be applied that in a heated by combustion of two gases and cooled by means of a cooling gas and accelerated gas flow, whose mass balance of all the partial flows causing it is a stoichiometric one Has ratio of fuel gas and oxygen, the particles of the metal powder are smuggled through the gas flow is heated, melted on its surface and transported to the base material. 4. The method according to claim 3, characterized in that the metal powder made of copper particles with a grain 130011/0531 size is between 40 and 80 µm and the mass throughput of the metal powder is between 2.5 and 10 kg / h, and acetylene is used as the fuel gas with a volume flow between 0.5 and 1.0 m / h at 1 bar and pure oxygen is used as the oxidizing gas with a volume flow between 0.6 and 1.2 m / h at 1 bar, and air at ambient temperature is used as the cooling gas a volume flow between 3.1 and 6.2 m / h at 1 bar, the spray distance being between 5 and 15 cm and the relative speed between the spray spot and the base material is between 0.02 and 0.1 m / s. 13001 1/0531 ORIGINAL INSPECTED