FR2538527A1 - HEAT EXCHANGE ELEMENT AND METHOD OF MAKING SAID ELEMENT - Google Patents

Abstract

The present invention relates on the one hand to a heat exchange element provided on the surface with a coating increasing the heat exchange coefficient in the case where there is boiling and on the other hand to a process for producing this. element. The heat exchange element according to the invention is constituted by a metal wall having on the surface a porous metal coating 2 formed by a stack of particles 21 of a structural metal bound by a brazing alloy 22. It is characterized by the fact that the particles 21 of the structural metal of this coating are substantially spherical particles and free of oxides which are obtained by atomization in a gas or by spraying of a rotating electrode. It relates in particular to tubes having a porous coating on the outside or inside. (CF DRAWING IN BOPI)

Description

Heat exchange element and method for producing said element

  The present invention relates on the one hand to an exchange element

  heat generation provided on the surface with a coating increasing the exchange coefficient in the case of boiling and on the other hand with a method of

  realization of this element It relates in particular to the tubes

  feeling on the outside or inside a porous coating.

  The heat exchange elements comprise a separate watertight wall

  the hot fluid and the cold fluid. Some of these elements

  a superficial layer to increase the exchange coefficient

heat.

  Some elements suitable for heat exchange with a

  boiling water superficially comprise a metallic coating

  porous (interconnected pores) mono or multilayer that is to say thick

  equal to or greater than the true thickness of the constituent metallic grains

  Thanks to this coating, the boiling remains of the nucleate type even if the temperature difference between the liquid and the heating wall is small and the number of cavities o are formed the vapor bubbles is high The surface of the cavities is constantly fed by a liquid film thanks to

superficial tension forces.

  Among the methods for obtaining these porous surface coatings

  some use either sintering metal particles or

  brazing metal particles with a braze alloy.

  wise. The present invention aims to provide an exchange element comprising a porous surface coating mono or multilayer having a particularly strong bond of metal particles together.

and with the support.

  The heat exchange element according to the invention consists of

  by a metal wall superficially having a metallic coating

  that porous formed by a stack of particles of a structural metal

  linked by a brazing alloy It is characterized by the fact that

  The structural metal particles of this coating are

  The manufacturing method according to the invention consists in carrying out on the metal wall a deposition 8 3 of particles of metal with a structure of metal and with no oxides which are obtained by atomization in a gas or by spraying of a rotating electrode. brazing alloy and then firing at a high temperature so as to melt the solder alloy. It is characterized by the fact that

  it consists in taking as particles of the structural metal, particles

  the substantially spherical and oxide-free which are obtained by atomic

  in a gas or by spraying a rotating electrode.

  According to one characteristic, the method consists in carrying out the de-

  deposit of the layer with a slurry, containing in a solution

  the organic metal binder, the particles of the structural metal and the brazing alloy and then smooth the layer and then dry the layer before baking. According to another characteristic, the method consists in depositing the

  layer on a tube-type element by pouring the slip on the tube.

  be and smoothing it as it rotates about its longitudinal axis

  nal, by relatively moving the casting jet relative to the tube.

  Other features and advantages of the invention are

  better from the following description which refers to the accompanying drawings

  in which: Figure 1 shows a section of an exchange element according to the invention. FIG. 2 is a diagram illustrating the various stages of the

process according to the invention.

  FIG. 3 is a diagram illustrating the filing operation which

  is carried out in the process according to the invention.

  Referring to FIG. 1, the heat element has a

  sealed metal wall 1 which separates a hot liquid and a cold liquid.

  This metal wall has for example the shape of a tube.

  The metal element has, on the surface, on the hot liquid side a porous metal coating 2 This porous coating consists of particles 21 of a structural metal connected to each other

  and at the wall 1 by a bonding coating 22 obtained by brazing with a

  brazing bonding This coating forms, between the particles 21, pores

  23 which are interconnected and open to the surface.

  The coating is formed perpendicular to the surface of the

  wall, by a stack of metal particles In the case where the coating

  is multilayered> the outermost particles are not

  walled but contiguous to other particles closer to the surface.

King.

  The particles of the structural metal of the coating are sensitive

  spherical, have regular dimensions and are practically

  oxides on their outer surface.

  obtained by a method of manufacturing powders by atomization in a gas such as described in French Patent 2,255,122 and the certificate of addition 2,366,077 or by sputtering rotating electrode as described in French Patent 2,253,591. The brazing alloy used to bond the particles of the structural metal melts at a temperature below the infusion temperature

  structural metal and metal constituting the wall.

  As an indication we can take for the brazing alloy a

  nickel-based pre-alloyed soldering powder, the composition of which is as follows:

  0.08% carbon 0.7% silicon 3.32% boron 15%

  chrome less than 1% iron nickel balance.

  The coating according to the invention has a much better

  adhesion a coating obtained with powders atomized with water.

  Referring to FIG. 2, the method according to the invention presents

the following steps.

  First, an aqueous solution of an organic binder is prepared.

  that goes into the composition of a slurry consisting of a suspen-

  sion of structural metal powder and a solder alloy powder

in said solution.

  This binder is indicative, constituted by a solution

  methylcellulose obtained by dissolving the methylcellulose in hot water This binder influences the viscosity of the slip and gives a

  cohesion in raw with the deposited layer on the wall.

  The slip is prepared by mixing the various constituents

  As an indication we can prepare for example a bartotine

  containing 64% by weight of structural metal powder of the stainless steel type

  16% of brazing alloy powder of the type defined above and 20% of aqueous methylcellulose solution. The particle size of the

  brazing is less than 110 microns and the particle size of the

  tal of structure less than 200 microns.

  The wall intended to receive the coating is previously covered by

rough and cleaned.

  A layer of slip is then deposited at once on the wall, for example by casting a slip of slip.

  slip is smoothed using a scraper that allows the thickness to be calibrated

  In the case of a tube marked 1 (FIG. 3), the latter is rotated under

2-538527

  : 4 a hopper 3 containing the slip The slip of slip that flows from the hopper forms a layer which is smoothed by the scraper 4 as and when

  The hopper and scraper are animated longitudinally with a movement

relative to the tube.

  The deposit is then hardened by a drying operation.

  The wall coated with the deposited green layer is then subjected to

  in two phases under a protected atmosphere The first phase of pre-

  heating is intended to decompose the organic agent This preheating is

  at a temperature of around 4000 C The second phase of cooking is

  its proper is performed at a higher temperature to melt the brazing alloy It is carried out at a temperature of

  1065 C in the case where the brazing alloy has the desired composition.

finished above.

  It goes without saying that we can go beyond the scope of the invention

  imagining variants and refinements of detail and similarly

  consider the use of equivalent means.

Claims (5)

  1. Heat exchange element consisting of a metal wall   only superficially presenting a porous metal coating consisting of a stack of particles of a structural metal bonded by an alloy   characterized in that the particles of the structural metal   of this coating are substantially spherical particles superficially free of oxides, obtained by atomization in a gas or by   spraying a rotating electrode.   2. A method of manufacturing the element according to claim 1, of depositing on the metal wall, a layer of particles of a structural metal and a brazing alloy and then to carry out a high temperature firing of the wall. so coated so as to melt   brazing alloy, characterized by the fact that it consists of   metal structure of substantially spherical particles and free of oxides obtained by atomization in a gas or by spraying a rotating electrode.   3. Method according to claim 2, characterized in that it consists in depositing the layer with a slip containing, in an aqueous solution of an organic binder, the particles of the structural metal and of the brazing alloy, then to dry the layer deposited before cooking.   4. Process according to any one of the preceding claims   characterized in that it consists in carrying out a first heating   fage of the wall coated with the deposited layer so as to decompose the organic binder and then to carry out a second heating of the layer deposited at   a temperature ensuring the fusion of the brazing alloy -   Method according to one of the preceding claims in the   which the exchange element is constituted by a tube, characterized in that   it consists of depositing the coating layer by casting the barb   tine on the tube and smoothing it while it rotates about its longitudinal axis, -by relatively moving the casting jet relative to the tube.   6. Process according to any one of the preceding claims   characterized in that it consists in using a powder of   structure having a particle size less than 200 microns and a solder alloy powder having a particle size less than 110 microns.