Plastics in the right places can improve reliability in applications from solar hot water heaters to aerospace. Tom Shelley assesses some recent developments

The lowest cost solar hot water heaters Eureka has encountered so far are made with polypropylene pipes mounted on a cellular polypropylene backing — a classic example of the benefits plastics can bring to a specific application. Similarly, new designs of rodless cylinders, still mainly made out of metal, feature polymer seals reinforced with more advanced polymers. And new coatings made by combining metals and polymers in composite form are providing enhanced wear and corrosion characteristics.

One of the main challenges to the wider use of the sun to heat hot water is the capital cost of solar collectors. Energy from the sun may be free, but the payback times of solar collectors are often such as to render the idea uneconomic in most parts of the world. So a minimal manufacturing cost is a prerequisite, and having attempted to develop such units himself in the USA, the author is particularly impressed by the designs of a Russian engineer, Igor Glushakov, brought to Eureka's attention by Yuri Neshitov, contacts and technology director of the Russian language magazine Innovations.

A small company called Sonora, which is situated in Taganrog on the coast of the Sea of Asov, is producing these new designs of collectors. The basic design consists of polypropylene pipes welded on to flat, black, cellular polypropylene panels. The internal pipes are 12mm in diameter and the external pipes are 20mm in diameter and thermally insulated. An important aspect of the design is that by using pipes and backing of the same materials, no fatigue stresses are induced by differential thermal expansion. The fronts of the collectors are 4mm thick cellular polycarbonate, somewhat cheaper than the clear sheets normally used for glazing. Supports are made from 40mm I-section aluminium alloy.

The consequences of this very simple design is that each square metre of collector weighs only 6kg dry and costs just $25 US to make. The weight increases to 10kg when filled with water, allowing collectors to be attached to existing roofs. Glushakov says that in South Russia, 10 collectors, each just over 1m square, can heat 500 litres of water per day to 60ºC. Maximum working pressure is 2bar and maximum working temperature is 90ºC. Maximum thermal efficiency is 82%. Payback time in terms of saved electricity costs is quoted as just over a year with a working life of at least ten years.

The crucial technical breakthrough, which Glushakov is keeping to himself at the present time, is his technique for welding plastic tubes to cellular plastic. But he is interested in possible licensing arrangements, especially in countries where the cost of electricity is a major. Asked whether he favoured sophisticated electronic control to further enhance performance, Glushakov says he prefers a simple approach, using either a thermal siphon or a time switch to turn a circulating pump on and off. For freezing climates, he favours a drain down system where water is pumped round when being heated but turned off to allow the water to run back into a thermally insulated reservoir. More complicated systems if required but inevitably will add to the cost and increase payback times. ( More information at www.sun-sistem.boom.ru and eurorus@ct.spb.ru )

That’s one novel application of plastics; now for something different. New 16mm bore rodless cylinders from Univer have their elastomer strip seals reinforced with Kevlar aramid fibres. This reinforcement prevents stretching and minimises the need for adjustment, which is by means of a simple clip with no special tools required for disassembly. The external thermoplastic strip seal is also reinforced with Kevlar.

These seals have to flex repeatedly by a large amount during service as they are opened and then re-sealed by rollers. But the use of Kevlar reinforcement is expected to significantly improve the efficiency and service life of this patented design of cylinder, which features an extruded aluminium alloy piston and carriage assembly and polymer guide bearings.

Our third example of a novel use of plastics is Poeton’s nickel composite coating, which combines the corrosion protection and low friction of polymers with the natural hardness of nickel. Part of the company's Apticote range, the composite coating provides superior hardness, wear resistance and chemical resistance as well as permanent lubrication when applied to components made from stainless steels and other ferrous alloys. The same coating can also substantially reduce material costs by allowing the use of low cost base materials. Parts manufactured from low carbon steel and coated with the nickel composite can be used to replace more expensive alloy components with no loss of performance. Costs are further reduced because parts can be machined or formed from more easily worked metals and alloys.

Coatings can be applied to a wide range of base materials and complex shapes, and are suitable for many applications, including those requiring USDA and FDA compliance. They achieve four times the wear resistance of electroless nickel and outperform hard chrome in dry running conditions at temperatures from -115ºC to + 250ºC.

www.sun-sistem.boom.ru
www.univer.co.uk
www.poeton.co.uk

Plastic design advice to be free at Interplas 2002 exhibition

An important feature at Interplas 2002 will be the Designing in Plastics Technical Centre sponsored by Eureka. The centre will house polymer materials companies providing advice to end-users seeking alternative materials for a specific product. The original or proposed product may have been conceived in metal or a polymer subsequently found to be unsatisfactory. Switching to plastic, or an alternative grade or blend, may enhance the performance of the product, reduce manufacturing costs, or allow it to be styled in a more interesting manner.

The centre is complete with seminar theatre and meeting rooms, so confidential matters can be discussed and visitors can learn more about the possible applications of different polymers.

Interplas 2002, which is organised by Reed Exhibitions (www.interplas-expo.co.uk) will be held at Birmingham’s National Exhibition Centre from 30 September to 4 October. A full preview of the event will appear in Eureka’s September 2002 issue.

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