GB2241249A

GB2241249A - Heat transfer surface

Info

Publication number: GB2241249A
Application number: GB9003055A
Authority: GB
Inventors: Stephen Forbes Pearson
Original assignee: Star Refrigeration Ltd
Current assignee: Star Refrigeration Ltd
Priority date: 1990-02-10
Filing date: 1990-02-10
Publication date: 1991-08-28
Also published as: GB9003055D0

Abstract

A heat transfer surface for boiling a liquid is formed by spraying a substrate (2) with metal particles (4) to form a porous layer including interstitial cavities (8). The substrate is then mechanically worked. such as by rolling, to compress the porous layer and increase the number of interstitial cavities which are re-entrant (12). The substrate is preferably in the form of a heat transfer tube. The mechanical deformation may be carried out after solidification of the particles or on the porous layer in its semi-solid state. The mechanical working may also be brought about by shot peening, broaching, burnishing, rolling or hammering. The substrate may be cupronickel, steel or copper. <IMAGE>

Description

HEAT TRANSFER SURFACE The present invention relates to an improved heat transfer surface for boiling a liquid, and a process of production thereof.

It is well known that the heat transfer co-efficient for boiling a liquid may be improved by coating the heat transfer surface of a heat exchanger with a matrix of small heat-conductive particles which produce a network of linked re-entrant cavities.

U.S. Patent 3384154 of 21st May, 1968 describes a method of coating heat transfer surfaces with small metallic particles which are subsequently sintered on to the substrate material. Such coatings result in heat transfer co-efficients which may be from two to ten times as great as the co-efficient when using a smooth tube.

European Patent Application 88307468.4 of 11th August, 1988 describes a method of coating heat transfer surfaces by spraying a particulate mixture of metal and a plastics material onto a thermally conductive surface to form a coating comprising particles of plastics material embedded in metal, and heating to a temperature sufficient to volatilise the plastics material, thereby forming pores in the metal coating.

The reasons why particular types of coating give high co-efficients of heat transfer are complex. The important parameters include: the particle size, the pore size, the range of pore sizes, the conductivity of the particles, the surface tension of the liquid being boiled and the angle of contact between liquid and particle surface in the presence of vapour.

A parameter which seems to be of particular importance is the number of cavities which are re-entrant, that is which display a small aperture at the surface of the coating which increases in diameter below the surface. Such cavities are particularly prone to trap seed bubbles within the matrix, thus giving rise to active nucleation sites.

However, the provision of re-entrant cavities by the sintering and metal spraying methods is somewhat unpredictable and irreproducible.

In essence, the present invention consists of mechanically working the surface of a sprayed coating to partially close some of the surface pores, thus producing a larger proportion of active nucleation sites.

Specifically, the present invention provides a process for the production of a heat transfer surface for boiling a liquid, which comprises; - spraying a substrate with particles to form thereon a porous layer comprising interstitial cavities; and - mechanically deforming the porous layer so as to increase the number of re-entrant interstitial cavities.

The particles are formed of a heat conductive material and are generally wholly or partially formed of metal. The particle size is usually in the range 1-100 microns, particularly 10-60 microns, and the thickness of the layer is preferably less than 250 microns. Thus, on average the layer is usually about 2 to 5 particles deep.

Usually, the mechanical deformation will be effected after solidification of the particles, according to the particular spraying process employed. Alternatively, the porous layer may be mechanically deformed in its semi-solid state.

Mechanical deformation may be brought about by mechanical working, for example, shot peening, broaching, burnishing, rolling or hammering. In a preferred embodiment, the heat transfer surface is mechanically deformed by passing between straightening rolls, which have the effect of deforming the surface and slightly spreading the porous layer.

Usually, the substrate is in the form of a heat transfer tube but may also be a plate or other heat transfer element. The substrate is usually of metal, e.g.

cupronickel, copper, steel or stainless steel.

The term "spraying" is to be interpreted broadly to include processes wherein substantially particulate solid, semi-solid or liquid material is impacted onto a substrate.

The present invention also extends to a heat transfer surface for boiling a liquid produced by the aforementioned process.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings wherein; Figure 1 shows a section through an unworked porous surface produced by spray deposition; Figure 2 shows a surface produced by the present process which has been deformed by mechanical working to produce re-entrant surface cavities; Figure 3 is an illustration on a larger scale of a row of spherical surface particles; and Figure 4 shows the row of spherical surface particles after deformation to give rise to re-entrant cavities.

Figure 1 shows a substrate 2, which is part of a heat transfer tube provided with a surface coating 4 in the form of particles which have been applied by a spraying technique so as to leave interstitial cavities 8. The cavities between adjacent particles in the surface layer 6, which are closest to the surface of the porous layer are generally concave, as indicated by reference numeral 10 in Figure 3. The object of the mechanical deformation is to reduce the number of such concave cavities 10 and to increase the number of re-entrant cavities 12 as shown in Figure 4. The extent of the mechanical working should, of course, be chosen to narrow the mouth of the cavities without substantially closing up the cavities.

In this preferred embodiment, the sprayed tube is mechanically deformed by passing through straightening rolls.

Claims

1. A process for the production of a heat transfer surface for boiling a liquid, which comprises - spraying a substrate with particles to form thereon a porous layer comprising interstitial cavities; and - mechanically deforming the porous layer so as to increase the number of re-entrant interstitial cavities.

2. A process according to claim 1 wherein the size of the sprayed particles is 10-60 microns

3. A process according to any preceding claim wherein the thickness of the porour layer is less than 250 microns.

4. A process according to any preceding claim wherein the porous layer is on average 2 to 5 particles deep.

5. A process according to any preceding claim wherein the mechanical deformation is carried out on the porous layer in its semi-solid state.

6. A process according to any preceding claim wherein the mechanical working is effected by passing the sprayed substrate between straightening rolls.

7. A process according to any preceding claim wherein the substrate is in the form of a heat transfer tube.

8. A heat transfer surface element for boiling a liquid which comprises a substrate having sprayed thereon a porous layer comprising interstitial cavities, the porous layer having been mechanically deformed to increase the number of re-entrant interstitial cavities.

9. A surface element according to claim 8 wherein substantially all the interstitial cavities in the surface layer of the porous surface are re-entrant.

10. A heat transfer surface element or a process of production thereof, substantially as described in conjunction with the drawing.