The present invention relates to boiling heat transfer surfaces which are provided on a heat exchange wall surface to enhance nucleate boiling and thereby improve the heat transfer action of a boiling liquid such as a halocarbon refrigerant. It is well known that nucleate boiling in a heat transfer apparatus is improved when the heated wall portion through which heat is transmitted from a warm fluid to a boiling liquid has its surface made porous by the formation of cavities in the heated wall portion.
U.S. Pat. No. 3,384,154 teaches that the radii of the cavities provided in the heated wall portion can be controlled by the size of the sintered powder coatings utilized to make the nucleate boiling surface.
U.S. Pat. No. 3,906,604 controls the nucleate boiling cavity size by machining the metal substrate to create protrusions which are frictionally rubbed and stretched to form the cavities.
U.S. Pat. Nos. 3,696,861 and 3,768,290 utilize bent-over tube fins to create elongated nucleate boiling cavities.
German Publication No. 2,510,580 discloses heat pipe surfaces wherein the surface is first mechanically roughened by wire brushing to form close uniform scratches and then electroplated to form nucleate boiling cavities.
U.S. Pat. No. 4,018,264 discloses a boiling heat transfer surface wherein the nucleate boiling cavities are made by electrodepositing copper dendrites on a copper surface at a high current, copper plating the dendrites or nodules and then rolling the dendritic surface to partially compact the nodules.
The present invention relates to a heat transfer surface having improved cavities to promote nucleate boiling; an improved method for preparing such cavities; and an improved method of nucleate boiling which utilizes the improved cavity structures. The present method for preparing the improved nucleate boiling cavities comprises deforming the metal substrate to form indentations in the metal in the form of non-continuous pits or craters and then electrodepositing a metal on the periphery of the pits or craters at a high current density and then at a normal plating current, from a special bath, to provide honeycomb-like surfaces which optimize nucleate boiling augmentation for particular fluids, especially halocarbon refrigerants utilized in refrigerator systems.
The cavity diameters in the plated coatings are directly related to the size of the pits or craters formed on the metal substrate. In the case of sandblasting, the metal deformation is made by grits having a mesh size of 10 to 200. In the case of deforming by knurling, the cavity diameter is determined by the size and number per square inch of the projections on the knurling tool. A pre-plated surface having from 100 to 350 (10,000 to 122,500 per square inch) knurls per inch is preferred in making the present heat exchange surface. It has been found that a knurling tool having 240 knurls per square inch provides pits or craters which, when electroplated, form excellent nucleate boiling cavities in the heated wall surface of a heat exchanger using halocarbon refrigerants such as R-12.
When the heat exchange surface is deformed by sandblasting, silicon carbide of 10-200 mesh, glass beads of 10-200 mesh and ground slag of 60 mesh have been used; silicon carbide of 60 mesh size is preferred for use in R-12.