NL8004997A - COOKING SURFACE FOR HEAT EXCHANGER. - Google Patents

Description

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t 1 J

H.O. 29,351

Boiling surface for heat exchanger

The invention relates to a cooking surface for a heat exchanger, consisting of a metallic base material and a metal layer applied to it.

It is known to strive to improve the heat transfer from the heated surface of a heat exchanger to the liquid to be evaporated by improving an appropriate surface structure.

It is thus known to apply a porous metal layer to the smooth metal surface, which has capillary channels with outlet openings in the surface of the layer, which serve as germination sites for the formation of vapor bubbles. Such layers are made up of individual bodies which are connected to each other on the contact surfaces, so that there are cavities in the layer which communicate with each other according to a statistical distribution, whereby a capillary structure with numerous open capillary channels is formed. is becoming.

The build-up of a large number of individual bodies, which are connected to each other only on the contact surfaces, for the purpose of obtaining a capillary structure in the cooking layer, means that the heat transport within the layer can only take place via the heat bridges formed by the contact surfaces. and is thereby significantly impeded. This leads to resistances in the heat conduction which impede the heat transfer during cooking or evaporation.

In addition, the fine capillary structure of the layer is very sensitive to both impurities in manufacturing and further processing, and also in its use as a cooking layer against impurities in the cooking liquid.

The manufacture of such lagoons can be accomplished, for example, by costly soldering or sintering processes, such as those described in U.S. Pat. No. 3,384,154- ". These processes employ metallic additives or additives or organic adjuvants. , which remain partly in the layer.

The metallic additives form, for the most part, compounds with the material of the individual bodies in the layer and with the base material or also partly remain in metallic pure form, so that the layer is composed of a plurality of materials or of their compounds.

The organic auxiliaries necessary for sintering also remain in layer 10 in smaller amounts.

An important field of application of heat exchangers, from which the cooking surfaces are manufactured according to the above-described method, consists of the evaporation of organic liquids which serve as working medium in circular processes. Such working media, which have been used for longer periods, are extremely sensitive to the contact with organic and also with metallic foreign substances. For example, in heat pumps and cooling installations, as working medium, fluorocarbons are frequently used, which contain small admixtures of lubricating oil from the cooling machine. Organic foreign substances still present in the cooking layer frequently form harmful chemical compounds with the mixture consisting of a working medium with the lubricating oil, while metallic foreign substances can cause a breakdown of the working medium.

Another possibility of manufacturing capillary structures in cooking surfaces is described in U.S. Patent 3,990,862. According to this publication, metal particles are applied to the basic raw material by means of a special flame-spraying process, whereby a boiling layer of the desired capillary structure is formed from a large number of, largely strongly deformed metal particles. An important feature of this method consists in the partial oxidation of the metal particles in a flame operating with an excess of oxygen. In order to ensure that this oxidation occurs, metals must be used which form a substantial oxide skin in a short time, which in their melting point is clearly distinguished from the melting point of the base material. For example, copper is not 8004997 * -ί 3 like metal. The metal oxides must guarantee a sufficient strength of the boiling layer with the capillary structure present. However, they form chemical compounds that can lead to disintegration of the working medium, as has been stated before.

The object of the invention now consists in the formation of a boiling surface for heat exchangers, which guarantees a very good heat transfer from the boiling surface to the liquid to be evaporated, shows no harmful impurities and moreover has a low heat conduction resistance.

According to the invention, this object is achieved in that the layer consists of partly coherent metallic individual bodies of different shapes, and that the layer is essentially free of capillary channels and closed pores passing through it and that the depth of the spaces which are free between the individual bodies and open to the cooking surface are on average more than 40% of the layer thickness, the layer thickness averaged over the highest point being 30 to 300 µm.

An advantageous method for the manufacture of the metal layer consists in that the particles forming individual bodies are applied in such a manner by means of the known flame-spraying process with powder on the base material that in a two-gas combustion heated and accelerated gas stream cooled and cooled by a gas, whose mass balance of all the streams building the stream exhibits a stoichiometric ratio of fuel gas and oxygen, the particles of the metal powder are enclosed, heated by the gas stream, lightly melted at their surface and the base material are transported.

The layer according to the invention thus consists of a single material, so that the harmful influence of foreign substances occurring with the known capillary structures is eliminated.

Due to the build-up of the layer from a large number of individual bodies of different shape, it has an increased surface area in the region of the boundary layer of the cooking liquid. As a result, a large liquid volume in the onn / QQ7 grens boundary layer can be heated in a short time. The resulting vapor bubbles consequently transport large amounts of heated boundary liquid from the cooking surface, which guarantees the very good heat transfer from the cooking surface 5 to the liquid to be evaporated.

Numerous notches or grooves, undercuts and similar shapes have been created at the transitions of the individual bodies which can act as seeding places for vapor bubbles. This guarantees an intensive formation of 10 bubbles, which is necessary for the discharge of the heated boundary layer liquid.

For a better understanding of the method achieved with the aid of the invention, the following situation should be pointed out:

The cooking phenomenon on surfaces takes place according to 15 laws as described, for example, in the "DKV-Abhandlung Hr. 18, 1964" Beitrag zur Thermodynamik des Warmeüberganges beim Sieden ", described by K. Stephan, in which, at bubble germination sites which on the surface are statistically distributed, small vapor bubbles are formed, which grow to their breakaway diameter and then become detached from the surface and rise in the cooking liquid. A new vapor bubble is created and the phenomenon repeats with the so-called bubble frequency. Han und Griffith in the article The Mechanism of Heat 25 Transfer in Hucleare Pool Boiling, Int. J. Heat Mass

Transfer, full. 8, 1965 »pp. 887-914" and by "Beer und Durst in the article Mechanismen der Warmeübertragung beim Blasensieden und ihre Simulation, Chemie-Ing.-Tech. 40 (1968), 13, pp. 632 - 638 ", the time described by the term" bubble frequency "falls into a two-part, namely the waiting time and the actual time of the bubble formation, the waiting time being 60 to 80% of the total period of time During the waiting period, the liquid boundary layer adhering to the cooking surface is heated by heat transport.Ha build-up of a certain temperature profile in this boundary layer begins to form the vapor bubble, grows up to the bubble's release diameter and then release from the cooking surface On release, the vapor bubble drags the surrounding heated fluid boundary layer 40 by "mas.sa convection" or by the "drift flow" it generates away from the cooking surface The area of influence of the drift flow on the boundary layer corresponds approximately to the bubble release diameter Ka the boundary layer mass convection flows "cold liquid" to the boiling surface and becomes there warmed again. The heat is thus largely dissipated from the cooking surface by unstable heat conduction.

As can be seen from the above publications, the following relationships can be used to estimate the size. The bubble release diameter is: d. - 0.0146. β 0. \ / 2 · d · τ · '· · γ "' A Vs. (V - V)

The ringing frequency can be determined approximately by means of the following formula: f. a, 1/2 = 1.75 15 The boundary layer thickness when the bubble is released is * · * · (X * + ra) - \ pip

Thereby g (^) acceleration of gravity d ^ (m) release diameter of the bubbles 20 β (o) edge angle of the still adhering vapor bubble Ö * (K / m) surface tension of the boiling liquid v '(m ^ / kg) specific volume of the liquid v "(nrVkg) specific volume of the vapor generated f (1 / s) bubble frequency 2 25 a (m / s) temperature conduction number of the liquid '* 7 ^ ζ (s) waiting time T * a (s) time required for bubble 6 ^ (m) boundary layer thickness to grow upon release

Table A shows the calculated values for some cooking liquids customary in the refrigeration technique and the usual boiling temperature ranges.

6

The physical phenomena in cooking described so far lead to the insight of the invention that an improvement of the heat transfer can be achieved if the cooking surface in the area of influence of the boundary layer and of the bubbles is considerably increased.

As a result, the time required for the formation of the temperature profile of the boundary layer, that is, the waiting time, can be considerably reduced and, moreover, the volume of the boundary layer liquid which is transported away from the cooking surface by a bubble can be considerably increased.

The invention was accompanied by the insight that in order to achieve the improved heat transfer, the structure serving to increase the surface area, in its total height, i.e. in the layer thickness measured over the highest point, is of the order of magnitude of the boundary layer thickness, while the fivefold should not be exceeded.

The invention is described below. The embodiments of metal layers shown in the drawing and an apparatus for manufacturing the metal layers, as well as an accompanying diagram, will be explained in more detail.

Fig. 1 shows in diagrammatic form the achievable surface enlargement of the structure as a function of different shapes of the individual bodies.

Fig. 2 schematically shows a detail of a longitudinal section through the metal layer.

Fig. 3 and 4 show photographic images of surface structures on a scale of 100: 1 and 500: 1 made with a scanning electron microscope 30.

Fig. 5 shows in a simplified and schematic manner a flame spraying device for applying a metal layer during operation.

FIG. 6 shows curves of the transferred heat power measured in a diagram as a function of the temperature difference.

To estimate the achievable surface enlargement F / FQ versus a smooth ground surface Fq due to a fine structuring of the surface in the manner 80 0 4 99 7 7 according to the invention, a calculation for individual bodies with simple geometry such as blocks, cylinders, pyramids and the like. For this purpose, a dense placement of the individual bodies on the smooth ground surface was assumed, the ratio 7 of the shortest distance between neighboring individual bodies with respect to the length of the sides or the diameter of their ground surface being very small, i.e. say 7 £ 0.2.

The resulting surface enlargements P / I * are shown in Figure 1 as a function of the ratio 7 for different geometries. Curve I applies to blocks with a ratio of their height h to the * u length of the side s of - = 1; curve II for cylinders 15 with a ratio of their height h to their diameter d of # = 1; curve III for cones with a vertex angle of Y = 30; curve 17 for pyramids with a vertex angle of Υ = 45 °; kormme 7 for cones with a top angle of y = 45 °; and curve 71 for hemispheres.

As can be seen from Fig. 1, significant surface enlargements can be achieved independent of the absolute height of the individual bodies. If various body shapes are applied to each other in the layer applied on a base material, even considerably larger surface enlargements can be achieved.

Fig. 2 schematically shows a longitudinal section through a layer applied to a metal wall 1. The individual bodies have different shapes which are mainly obtained in the manufacture, which will be discussed later. As shown in Figure 2, the layer is composed of approximately spherical and somewhat flattened individual bodies 4 and 5. Some of the bodies adhere to the wall 1. neighboring bodies adhere to each other at the location of the contact surfaces 2 and 3, whereby enclosing contact surfaces 6 are also formed. Thus, a solid layer with good heat transfer transitions is formed. The transition from the surface of a body to the surface of the adjoining body 40 is in many cases in the form of notches or grooves 7. The multifarious body shapes give a particularly heterogeneous structure of the layer 11, creating holes 8 up to the base material, craters 9, elongated slits 10 and the like 5 between the individual bodies, which are particularly suitable as bubble germination sites and thereby a ensure at least complete coverage of the cooking surface with vapor bubbles, also under those conditions, when the temperature difference TT of the cooking surface with respect to the saturation temperature T of the cooking liquid is very small.

δ

The layer thickness ≤ £ is measured between the highest elevation and the surface of the heat-emitting wall and according to the invention is between 30 and 300 µm. However, open spaces, the depth of which may lie between a few percent and 100% of the layer thickness, remain on average more than 40% of the layer thickness between the bodies of which the layer is built up and their defined height.

The surface structure obtained according to the invention also follows from the layer cross-sections according to Figs. 3 and 4, where the same reference numerals as in Fig. 2 are used.

As already mentioned, with the aid of the structure according to the metal layer, built up according to the invention, a considerable surface enlargement can be obtained with respect to the surface of the base material, for instance in the order of magnitude of 2 to 10.

An advantageous method for the manufacture of the layer consists of a modified manner of the per se known powder-flame spraying process, in which the metal particles 30 of the starting material are sprayed onto the base material in a melted or dough-like state. The geometric shape of the metal particles for spraying may be different. For example, spheroidal, prismatic, polyhedral, dendritic and splashed shapes can be used. These metal particles are introduced into a hot gas stream and are evenly distributed in this gas stream. The gas flow serves on the one hand as a transport medium for the metal particles to the ground material and on the other hand as a heat source for the heating of the metal-40 particles. The metal particles are thereby heated from their surface 8004997 9 '', the residence time of the metal particles in the gas flow and thus the degree of heating using the temperature and the velocity of the gas flow as well as the distance traveled by the particles. influence the melting in a known manner. According to the degree of fusion and the impact speed of the metal particles on the base material, these will deform and will clamp together, interlock or weld together with the base material at the location of the contact points, as shown in FIGS. 2, 3. and 4 turns out. The dimensions of the layer-forming bodies can be controlled by suitable selection of the dimensions of the metal particles so that they correspond to the values for the boundary layer thickness indicated in Table A in the order of magnitude. Metal particles in a ball shape with diameters between d = 30 and 150 µm are advantageously used as the starting material.

In Fig. 5, an apparatus 20 for applying the modified powder flame spraying process is schematically shown.

Combustible gas and oxygen are introduced into the device through supply lines 21 and 22. The mixture flows through a burner ring 23 and is then burned in the flame. The hot gases are introduced into the device with a cooling gas blown into the flame through an inlet pipe 24 and mixed into a flame through an outer nozzle ring 25 and thereby cooled. In addition, part of the cooling gas acts as a means of transport for the metal powder supplied to the device via a conduit 26. In the gas flow, the powder distributes according to the flow conditions and is transported to a wall 27 consisting of the base material and there it strikes this wall in an approximately circular plane, which will be indicated by spraying point. This spraying spot is passed over a surface of the base material and in this way a flat-applied layer is obtained. The relative speed of the base material perpendicular to the spray jet influences the quality of the layer. At too low a relative speed, the residence time of the commanded layer in the spray jet would become too long and the metal particles of the layer would melt too much and a relatively homogeneous layer without body shapes and without a sufficiently large number of germination sites for hells. When applying the layer, the relative speed, the layer thickness and the flow rate of powder through the spray gun are in a certain relationship to each other. For the quality of the layer, furthermore, the gas flows in front of the nozzle and the distance between the nozzle and the base material, which, as stated earlier, together with the gas flow after the nozzle determine the residence time of the metal particles in this gas flow. , of significant importance. Since it is contemplated to obtain a metallic pure layer, it is necessary that all gases flowing through the spray gun together give a stoichiometric equilibrium between fuel gas and oxygen. In this way, oxidation and other chemical changes of the metal particles can be avoided with certainty.

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

In a test, a layer, such as it is inclined, for example, in Figs. 3 and 4, is applied from copper bodies to a copper pipe with an outer diameter of 18 mm. In a suitable device, the heat transfer capacity q_ during cooking was measured under conditions such as prevail in submerged pipe bundle evaporators of turbo-cold machines, using the refrigerant and at an evaporating temperature of 0 ° C.

Fig. 6 shows the heat transfer powers q obtained in this way as a function of excess temperature ΔΤ

CL

(curve W), in which the heat transfer capacity, as measured on pipes with smooth surface (curve Z), is also given for comparison.

In Table B, in a numerical example, the flow rates of the gas flows, of the metal powder as well as of the associated spraying distance and of the relative velocity of the base material perpendicular to the spray jet for 8004997 11 are a production process according to the invention. output layer indicated.

Table A

_t (° 0) f φ dA (/ um) ^ (/ um) CFoCl.CFo01 (R114) 60 72 589 40 2 2 0 64 745 51 CFo01o (R12) 30 72 569 50 2 2 -30 65 732 59 CHF- C1 (R22) 20 68 672 53 2 -40 59 877 04 CF-jBr (E13B1) -20 75 549 47 3 -60 68 655 54 CF, C1 (R13) -40 72 592 47 5 -80 65 734 54

Table B

Area_bid preferred

Fuel gas: z

acetylene at 1 bar 0.5 - 1 »0 0.74 (π AO

Oxidation gas: 3.> oxygen O2} at 1 bar 0.6 - 1.2 0.85 (m / b)

Refrigerant gas: air, at 1 bar 3A - 6.2 5? 3 (m'V'h)

Powder: copper 2.5 - 3500 350 (kg / h) spherical 30 - 150 40 - 80 (yum)

Spray distance: 5- ^ 5 Ί0 (cm)

Relative speed: 0.02 - 0.1 0.05 (m / s) * Q Λ n L 0 0 7

Claims (4)

Cooking surface for heat exchanger, consisting of a metallic base material and a metal layer scattered on it, characterized in that the layer consists of partly coherent metallic individual bodies of different shape and in that the layer is substantially free of capillary passing through channels and of closed pores, and that the depth of the hold-open between the individual bodies and open to the cooking surface is at least more than 40% of the layer thickness on average, the layer thickness measured over the highest tops being between 30 and 300 µm . Cooking surface according to claim 1, characterized in that the grain size of the starting material forming the individual bodies is between 30 and 150 µm. Method for manufacturing the metal layer according to claims 1 and 2, characterized in that the particles 20 forming the individual bodies are applied to the base material by means of the powder-flame spraying method known per se, such that in a heated by combustion of two gases and accelerated gas flow cooled and cooled by means of a cooling gas, the mass balance of all substreams of which forms them in stoichiometric ratio of a fuel gas and oxygen, the particles of the metal powder are introduced, heated by the gas flow and fused to their surface and transported to the base material. Method according to claim 3 *, characterized in that the metal powder consists of copper particles with a grain size between 40 and 80 µm and in that the mass flow rate of the metal powder is between 2.5 and 10 kg / h, and as fuel gas acetylene is applied with a volume flow between 0.5 and 1.0 m ^ / h at 1 bar and as oxidation gas pure oxygen is used with a volume flow between 0.6 and 1.2 m ^ / h at 1 bar, and as cooling gas ambient air is applied with a volume flow between 3 »1 and 6.2 m ^ / h at 1 bar, the 40 spraying distance being between 5 and 15 cm and the relative 8004997 velocity between the spraying spot and the base material being between 0.02 and 0.1 m / s. ******* 8004997