Improved heat transfer surface

Abstract: A heat transfer element for boiling a liquid, particularly a refrigerant, comprises a substrate (2) having a surface coating (8) formed by spraying a mixture of metal particles e.g. as a plasma, and porous carbon particles such that the carbon particles become embedded in a matrix of metal particles attached to the substrate. Preferably the carbon is active carbon and the metal is aluminium.

Description: 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 the heat exchanger with a matrix of small heat-conductive particles which produce a network of linked re-entrant cavities. US-A-3384154 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 two to ten times as great as the co-efficient when using a smooth tube. The reasons why particular types of coating give high co-efficients of heat transfer are complex. The important parameters include particle size, pore size, range of pore sizes, activity of the particles, surface tension of the liquid being boiled and the angle of contact between liquid and particle surface in the presence of vapour. Essentially, the present invention uses porous carbon particles anchored to the heat transfer surface by means of a network of metal particles. The present invention provides a heat transfer element which comprises a substrate having a surface coating comprising porous carbon particles embedded in a matrix of metal particles. Another aspect of the invention provides a process for the production of heat transfer surface for boiling a liquid, which comprises spraying a substrate with a mixture of metal particles and particles of carbon, such that the carbon particles become embedded in a matrix of metal particles attached to the substrate. 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 metal particles are preferably formed of a heat conductive material such as aluminium or other high conductivity metal or alloy known in the art. 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 will usually be about 2-5 metal particles deep, so that the carbon particles are exposed to the liquid. The porous carbon is preferably activated carbon or charcoal of a microporous nature. The particle size of the carbon particles is usually in the range 1-100 microns, particularly 10-60 microns. The activated carbon would generally comprise from 20 to 80% by volume of the matrix layer. The mixture of carbon and metal particles will usually be sprayed on under conditions which avoid degradation of the carbon particles by excessive temperature or oxidation. Usually, the mixture will be sprayed together with an inert gas such as nitrogen or argon. If necessary, the surface of the applied matrix may be mechanically worked, for example by rolling, to improve its mechanical properties and to prevent detachment of the carbon particles in use. Usually, the substrate is in the form of heat transfer tube but may also be a plate or other heat transfer element. The substrate is usually of metal, e.g. cupro-nickel, copper, steel or stainless steel. The heat transfer element of the invention allows heat exchangers for boiling liquids to be fabricated, which have high co-efficients of heat transfer. Carbon also has the advantage of having a relatively high heat conductivity. The element finds particular use in the refrigeration field. An embodiment of the invention will now be described by way of example only in conjunction with the attached drawings wherein: Figure 1 is a cutaway section through a heat transfer tube equipped with a heater for use in the test rig; Figure 2 is a test rig for comparing the heat conductivity of a tube M according to the invention with two other tubes; and Figure 3 is a graph of temperature difference (delta T) versus power P for the three tubes. A heat transfer tube according to the invention was produced as follows. A cupronickel tube (90% Cu 10%N:) was cleaned by shot-blasting and then plasma-sprayed with a mixture of particles of aluminium metal and carbon (both of size approximately 60 microns) in an inert gas or reducing atmosphere to produce a surface coating 8 of 100-200 microns thick. A single spray gun equipped with two separate feeder chambers for metal and carbon respectively was used. For testing, the heat transfer tube 2 was then fitted with a heater 4 as shown in Figure 1, and a low melting point alloy Ostalloy 158 was poured into the space 9 between the heater and the tube to provide good thermal contact. The tube was equipped with a surface thermocouple pair 6 to measure the temperature at the surface of the tube 2. Figure 2 shows a test rig for measuring temperature difference between the tube surface and a boiling liquid, in this case refrigerant R11. The surface temperature of the tube is measured by thermocouple 6 and the temperature of the refrigerant R11 measured by a series of thermocouples (not shown) placed in the bulk of the liquid. A sealed container 10 is filled with liquid R11 which is circulated to a cooler (not shown) via inlet 12 and outlet 14. The liquid is boiled by three heated tubes H, P and M each equipped with a heater and variable power supply. Each power supply consists of a watt meter 20 and varistor 22 in parallel with the heater. Heat transfer tube M is provided with an aluminium/carbon coating according to the invention, as described above. Tube P is a plain copper tube, and tube H is a high flux tube. The temperature difference between the tube surface and the R11 liquid was measured. Figure 3 shows the test results. The plain copper tube P shows a temperature difference of between 5.5°C and 8.5°C depending on the power. The temperature difference for the high flux tube H was erratic up to a power of about 60 watts and then varied from 1 to 3.5°C. The temperature difference for tube M of the invention was substantially constant at about 1.5°C irrespective of power, and thus showed the best heat transfer ability.

Claims: A heat transfer element for boiling a liquid which comprises a substrate (2) having a surface coating (8) comprising porous carbon particles embedded in a matrix of metal particles. An element according to claim 1 wherein the carbon particle size is 1-100 microns. An element according to any preceding claim wherein the carbon comprises 20 to 80% by volume of the coating. An element according to any preceding claim wherein the metal particle size is 1-100 microns. An element according to any preceding claim wherein the metal is aluminium. An element according to any preceding claim wherein the thickness of the coating is less than 250 microns. An element according to any preceding claim wherein the coating has been mechanically rolled. An element according to any preceding claim wherein the metal particles have been applied by plasma-spraying, together with the carbon particles. A process of production of a heat transfer surface for boiling a liquid, which comprises spraying a substrate with a mixture of metal particles and porous carbon particles, such that the carbon particles become embedded in a matrix of metal particles attached to the substrate.

Inventors: PEARSON STEPHEN FORBES, PEARSON, STEPHEN FORBES
Applicants: STAR REFRIGERATION, STAR REFRIGERATION LTD.
Application year: 1991
Main classification: F28F13
IPC codes: C23C4, F28F13