Extruded tube with twisted fins improves heat exchanger design

A small UK-based engineering company has developed a novel method of manufacturing heat exchangers which improves the efficiency and cost of manufacturing. Dean Palmer reports A small engineering firm based in Dartford has developed a novel method of extruding aluminium tubular heat exchangers, that reduces the cost of manufacture and improves the efficiency of the heat exchanger. Indeed, multi-channel extruded aluminium tube is not a new technique and has been used for many years in the construction of heat exchangers. But efficiency of the units has been limited due to the need to bond an extended surface to the tube in the form of thin, convoluted fins. The breakthrough comes from Brise Fabrications of Dartford, a four-man engineering company that has recently changed its focus from a manufacturing company that competed with cheaper products from lower labour cost countries, to a firm that now focuses on innovation and finding novel methods of manufacturing. "This is the only way we can stay ahead of Far Eastern manufacturers," explained Tim Brise, MD of the company. Brise manufactures many types of heat exchanger, mainly for the automotive market. Annual turnover is around £400k. These products are usually fabricated from aluminium using cores that are sourced globally. In order to fabricate solutions for individual customers for diverse custom applications, it is necessary to procure an exact size of core to suit the application. As Tim Brise explained: "The required size of tank, inlet/outlet tubes and mountings are then welded on to produce the finished component." The cores are normally constructed from aluminium oval, seam welded tube and thin convoluted fin. The tube/fin assembly is assembled with a header plate to the required size, then furnace-brazed to form an integrated unit. According to Tim Brise, in order to eliminate reliance on outside sources for the cores, the new twisted fin tube was developed. "The manufacturing techniques and processes required to produce a furnace-brazed core required too much investment in tooling and equipment, therefore an innovative solution was needed instead." Most heat exchanger applications require that the cooling medium and the medium to be cooled have flows that are at 90 degrees to one another. For example, in an air cooled oil radiator, the air flow moves over the tubes containing the oil flow at 90 degrees. In order to produce an extended surface during extrusion, it is only possible that the fins run in the same direction as the tube. Therefore, when the tubes are stacked together, the fins obstruct any flow at 90 degrees. To solve this problem, Brise has developed a manufacturing technique and special cutting tool that simultaneously twists and shears the axial running continuous fins on the external surface of the tube. The tool was originally a hand tool invented by Tim Brise but this has been superseded by a press tool, jointly developed by Brise and New-Tech Tools based in Swanley, Kent. This purpose-built press tool can now manufacture 900 fins per minute. Brise's 'twisted fin tube' is extruded with continuous external fins running laterally along its length. In a post-extrusion forming operation, the lateral fins are twisted and sheared into an array of fin segments. These segments allow air or liquid to flow across the tube surface at right angles to the flow of air or liquid in the internal channels. The segments are angled to promote a turbulent flow, which enhances heat transfer. As Brise explained to Eureka: "Basically, the more tortuous the route a fluid has to take through a heat exchanger, the better that unit's heat transfer characteristics will be." Most heat exchangers use adhesive to bond the fins to the tube in some form or another. With Brise's design, the twisted fins are formed from the parent material of the tube, so there is no thermal barrier caused by bonding or brazing of the extended surface. Heat transfer is therefore improved and the manufacturing costs of the unit are lower too. When the tubes are assembled into a complete heat exchanger assembly, the external fin arrays have been designed in such a way as to interlock, which produces a very complex, but thermally efficient path for fluids, while retaining a very low pressure drop. This interlocking of the fins also reduces the physical size of the assembly but maximises the tube density. The lightweight, fully-welded final assembly can cope with very high pressures and temperatures. There is no limit to the size (length) or depth of the extrusion. Brise's designs currently use around five layers of fins in the heat exchanger, but this could be increased if desired. GDM Heat Transfer, a company based in Wolverhampton, is currently looking at the development with a view to using it for industrial heat transfer applications. The development has also been given to Stafford University. According to Tim Brise, the lecturer responsible for testing the design and its heat transfer characteristics remarked that the twisted fin tube "was the most exciting development he had seen in a long time". Brise's heat exchanger is already being field tested by British Touring Cars and by the Mini Saloon Car Racing team, to give improved performance in oil/water heat exchangers, air/water turbo charge air coolers and air/oil coolers. On the Mini for example, the gearbox and engine share the same oil, so there were lots of cooling issues on the car. Brise's new unit replaces a 19-row oil cooler that is around five times larger and three times heavier than Brise's twisted fin tube. "The temperature of the oil and the water during races so far have proved to be around the same as the original system, except you get the weight and size reduction benefits now," explained Tim Brise. Many industrial applications could also benefit from the new technique. Variable speed fans, air conditioning units, air compressors and condensers could all perhaps employ similar designs for their cooling cores. Brise Fabrications is currently looking for partners to help develop new market opportunities for its patent-pending process and to help it investigate new manufacturing techniques for the twisted fin tube such as computerised plasma welding and brazing. Pointers * Brise has developed a manufacturing technique and special cutting tool that simultaneously twists and shears the axial running continuous fins on the external surface of the tube * As well as automotive applications, many other industrial applications could also benefit from the new twisted fin manufacturing technique, including variable speed fans, air conditioning units, air compressors and condensers * An industrial heat transfer company is currently investigating the technology and Stafford University is testing the efficiency of the device EUREKA SAYS: The twisted fin tube represents a genuine step change in heat exchanger technology and design. Although originally developed for automotive applications, the device could be adopted for industrial heat transfer applications where the same cooling principles apply