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Risks minimized

Safety heat exchangers in polysilicon production

Published in "Chemietechnik" - May issue 2017




Barbara Schüßler,
Wieland Thermal Solutions
Jan-Christoph Funke,
Funke Wärmeaustauscher Apparatebau

The production of highly pure polysilicon for photovoltaic systems is a sensitive chemical process, through which an accident can happen if the media are mixed. To prevent this, special safety heat exchangers have been developed to ensure the required safety of the systems as process temperatures change.

The manufacture of polysilicon results in the creation of different silane compounds with extremely high chemical reactivity. Mixing these silane compounds with fluids can lead to an accident. Safety heat exchangers have metal tubes to conduct heat during process temperature changes to reduce the risk of such incidents.

The particularly good metallic contact allows efficient heat transfer without requiring fluids to transfer the heat. High thermal efficiency keeps the design of tube bundle heat exchangers compact, saving both installation space and material costs.

With Funke safety heat exchangers, the inner tube has a pyramid-shaped outer structure, allowing about 75% thermal conductivity after expansion compared to the single tube design.

Detail of the Safety Tubes

Detail of the Safety Tubes

Safety tubes with leakage gap to detect leaks for different applications
Detail of the Tubes

Detail of the Tubes

Cross-section of the compact safety heat exchanger

Safety through reliable separation of media

The double-walled safety tubes in the tube bundle have fine ducts which create a leakage gap after the two tubes have been connected.

The pressure in this gap is monitored to detect even the smallest of leaks immediately. This ensures that the media in the outer and inner tubes do not mix.

The double-tube solution from Wieland Thermal Solutions, therefore, increases the safety and availability of the system, preventing the need to shut down production immediately when leakages occur.

A defect can be rectified later as part of scheduled maintenance.

In addition to chemical process engineering, safety tubes are also used in heat recovery and domestic hot water heaters.

A wide variety of materials are used, including copper and copper alloys, carbon steel, and stainless steels.

Depending on application and processing requirements, the specific design is chosen with ribbing either on the inner or outer tube to provide the best heat transfer and processing.

Cross-section of the safety heat exchanger

Extreme conditions require special construction

Heat exchangers are used in the production of polysilicon at various points of the production process. Because the requirements vary here, the heat exchangers are designed individually in each case. In doing so, the size of the safety heat exchanger depends on the function and ranges from several kilowatt hours up to the megawatt area.

With extreme up- and downstream situations, the safety tube bundle heat exchangers must be able to withstand large support loads as well. This requires special solutions in the construction of the double-tube system, both for the connection of the tube and the tube plate as well as for each pressure chamber. An axial compensator is installed in the jacket space of the tube bundle to compensate for expansion of the construction. In addition, the safety chamber is secured with further expansion compensation in order to absorb the various stresses of the double-pipe system on the axial forces of the joint connection.

Conclusion: The double-tube solution is a reliable and robust evolution of Funke's proven safety heat exchanger. Reliability and efficiency offer prospects for the temperature control of a large number of applications with high demands for safety.

Source: Chemietechnik – Fachinformationen für Entscheider, May issue 2017

Also read this article in the may issue of the "Chemietechnik" or on the website of"Chemietechnik"-Magazine.