High-pressure heat exchangers becoming key in Africa

Local manufacturer and maintenance service provider of utility-scale pulverised coal steam generators Steinmüller Africa is now offering turnkey solutions for both shell and tube heat exchangers – both the header type and the tube sheet type are offered.

These heat exchangers are not only integral components of power generation, petrochemical and process plants, but also of renewable- energy generation such as concentrated solar power (CSP).

Shell and tube heat exchangers are particularly prevalent in Rankine cycle power generation with regeneration, as used in most modern power stations. Steinmüller Africa pursued a technology transfer programme to develop design and fabrication methods for these heat exchangers, which are directed at meeting the challenges posed by local conditions. It worked together with the German designers of high- pressure feedwater heaters – used in most of the country’s existing power plants – and incorporated the operating experience of its clients.

Steinmüller Africa group leader of mechanical engineering Khalid Peer explains: “A shell and tube heat exchanger consists of a shell with a number of tubes mounted inside it. This is the most popular type of exchanger, owing to a combination of economics and flexibility that the designer exploits in order to cater for a wide range of parameters.”

He adds that the exchanger finds application in the petrochemicals industry, industrial processes and the power industry by way of feedwater heaters and power plant condensers.

Further, the header-type heat exchanger is essentially two thick-walled headers inside a pressure vessel shell with numerous ‘snake tubes’ connecting the two headers. The headers are very similar to those used in high-pressure water-tube boilers, components in which the company is experienced and well-equipped to design, fabricate and maintain. These header- type heat exchangers are generally only used in very high pressure applications.

Solar Power Generation 

Peer indicates that heat exchangers used in CSP plants are very effective in transferring heat gained during the day from the sun to molten salt for thermal storage. The stored energy is then used to generate electricity between sunset and sunrise.

“. . . molten salt storage enables solar thermal power plants to operate just like a conventional power plant, reliably generating electricity when it’s needed, ” he says.

With the development of CSP in the Northern Cape, Steinmüller Africa notes that its development of a local design and fabrication capability is well-timed. Process engineer Keyur Patel states: “Our heat exchangers are continuously developed to improve the design process and fabrication turnaround time. ”

Petrochemicals and Conventional Power Plants 

At petrochemicals plants, heat exchangers are required for a variety of reasons to keep the working and operating fluids within a desired temperature range. Applications are infinite; however, the main application of shell and tube heat exchangers in power generation, both fossil fuel derived and nuclear, is in the improvement of cycle efficiency by using bled steam to increase the temperature of the feedwater.

This is done in both the low-pressure and the high-pressure ranges. In view of the general age of petrochemical, process and power plants across Southern Africa, it is reasonable to expect huge interest in replacement heat exchangers for high-pressure and high-temperature applications which are locally designed and fabricated.

Manufacturing of high-pressure heat exchangers entails heavy machining, rolling, milling, drilling and welding of alloy steels. These requirements, together with unique solutions which were developed to meet the restrictions imposed by the explosive welding of the tube-to-tube sheet, as well as the replacing of large dished ends with self-sealing enclosures, necessitated manufacturing to very tight tolerances. Owing to this, a large portion of Steinmüller Africa’s manufacturing is done in-house at its 53 000 m2 workshop in Pretoria West.

Explosive Welding 

Explosive welding technology is a solid-state fusion of two metals by shock wave propagation induced by an explosive charge. The process requires extensive material interface preparation, personnel development and workshop investment. The magnitude of the shock-wave and fit-up tolerances can only be perfected by running numerous trials, mock-ups and destructive testing. A perfect sinusoidal interface without inclusions is the hallmark of an excellent explosive weld, which is far superior to traditional welding technologies in many respects.

Steinmüller Africa’s explosive welding manager Hennie Visser explains that, owing to extensive knowledge transfer through training workshops in Germany and the UK, Steinmüller Africa has developed guidelines and templates for heat exchanger designs and fabrication, incorporating explosive welding. Steinmüller Africa has developed explosive welding practices which consistently deliver superior welds.

“We can offer clients conventional welding and explosive welding. We are the only heat exchanger manufacturer offering explosive welding and explosive expansion to the tube sheet joint of the heat exchanger, which is one of the most critical parts,” he concludes.