Energy saving casting technique leads to 'greener' ships propellers
28 October 2014
A Norwegian invention has reduced by a third the energy that foundries need to manufacture large ship propeller blades.
Some of the propeller blades manufactured by Oshaug Metall AS are so big that if the complete propeller stood on end, it would span the height of a three-storey building.
In collaboration with Oshaug, Scandinavian research organisation SINTEF has developed an energy-saving casting technology which has recently successfully completed pilot trials at the foundry's manufacturing centre in Norway.
"The pilots reveal that the method can save as much as 35 percent of the electricity currently used to melt the metal we need to cast a propeller", says Arne Nordmark, a researcher at SINTEF and inventor of the technology.
The smelting furnaces at Oshaug consume as much electricity as a hundred average Norwegian households – some 1.5 million kWh annually.
"The results of the project so far indicate that we can reduce electricity consumption by half a million kilowatt hours a year, and in this way reduce both costs and negative impacts on the environment", says Stein Berg Oshaug, managing director at Oshaug Metall, who hopes that the new technology will be in operation some time next year.
Unlike water, which expands when it freezes, molten metals shrink in volume when they solidify. Foundries compensate for this during the casting process by adding a little extra molten metal from reservoirs called 'feeders'. The feeder forms part of the mould and is situated above the level of the casting. As the propeller blade solidifies and starts shrinking, additional molten metal is supplied from the feeder.
However, the metal in the feeder also solidifies. But if it is to compensate for the shrinkage in the casting, this metal must not be allowed to solidify before the propeller metal does. The feeder must retain molten metal right up until the time when the blade becomes solid. Currently, this is achieved by making big feeders – on the principle that large volumes solidify more slowly than small ones.
The new invention involves surrounding the feeder with an induction coil that supplies heat which keeps the metal in the feeder molten. This means that feeders no longer have to be so big in order to do their job. They only have to be large enough to contain the small amount of molten metal needed to compensate for the shrinkage in the casting.
Additionally, because the invention comes close to halving the amount of metal that has to be melted, Oshaug will now be able to cast more propeller blades in a given time (or indeed bigger blades) than its current capacity permits.
"The new technology enables us to increase production capacity by as much as 50 percent, and in cash terms this is even more than we save in electricity", says Stein Berg Oshaug.
After the casting process is complete and the metal has solidified, the feeder metal is cut from the casting for re-melting during the next casting operation. This cutting process is very time-consuming for feeders of large diameter. During the pilot trials of the new technology, the diameter of the feeder was reduced by 60 percent, thus reducing the cutting work.