Hydroformed heat exchangers
Original article date: October 1999
Alan Quinn visited Thermex in Redditch and discovered a completely new approach to heat exchanger design
Most proprietary shell-and-tube heat exchangers and oil coolers feature an outer cast or extruded aluminium shell, with oil inlet and outlet ports, and a tubestack comprising copper or cupronickel tubes.
Cooling is provided by water flowing through those tubes. Hot oil passes over the tubes, where its flow is turbulated (churned up!) by aluminium baffle plates, which act to increase the heat transfer.
There is a relationship of flow to heat dissipation and temperature: for a given amount of energy (kW) dissipated as heat from the oil, the temperature drop will decrease as the oil flow increases through the cooler. Accordingly, laminar flow – a straight, unmixed run of oil – fails to bring the bulk of the oil into direct contact with the heat transfer surface.
Thermex has recently introduced the hydroforming process (high pressure forming by water compaction) into fuel cooling applications. Hydroforming is a technique which uses a high pressure water-based emulsion to expand metal into the shape of a die cavity – a little like blowing up a balloon inside a milk bottle. The technique is preferable to conventional forming techniques because of the absence of welds and the lack of distortion. Also, because no metal is cut out to form the shape, there is no material wastage. Here, the process is used for forming single copper tubes with integrated baffles, to create the necessary turbulation without any baffle plates at all.
The outer tube sleeve is brazed at its ends onto the hydroformed component. Importantly, this reduces both the number of components and the man-hours expended in manually building up tubestacks and painstakingly brazing on curved baffles. What is more, the use of hydroforming is effective in overcoming the damage (fretting) problems caused by resonance.
The hydroforming process being used by Thermex was originally developed for Yanmar, the large manufacturer of marine diesel engines. The process has the additional benefit that it eliminates the need for worm-drive hose clips (or even bandclamps?) to secure tube fuel lines to the cooler unit, thereby providing much greater safety-in-use, in accord with international marine safety standards.
Hydroforming has been explored in the automotive industry as a means of being able to consolidate the number of parts required for suspension parts. The long-term potential exists that a component supplier will take the process on board to supply components on a production line basis. But the process is not fast, since the machine has to regain pressure from the ring main after each operation before carrying out the next.
- Thermex
October 1999