Pulp producers reap benefits of fan design

Fans may be able to supersede compressors in Mechanical Vapour Recompression (MVR) evaporators used in the pulp industry. The successful installation of the first such fan driven evaporator, for magnesium sulphite liquor, was at Sappi Saiccor’s Umkomaas pulp mill, near Durban, South Africa. Two other applications in Scandinavia also have had positive results on both maintenance and manufacturing operations.

MVR evaporation technology uses a mechanical device, such as a compressor or fan, for recompressing the vapour within the evaporator. Regulation of the vapour is via an inlet guide vane.

The fan manufacturer, ABB Flkt Oy, says the fan installation is up to 50% less expensive than a comparable compressor arrangement with further savings being derived from much reduced maintenance costs.

Such is the scale of these benefits that Sappi Saiccor opted for the fan evaporator, even though all other initial proposals were offering a compressor MVR. Tests arranged with an independent third party showed that guaranteed values were achieved, including: evaporation at 50 tonne per hour; 55% dry solids; and 1,060 kW of electrical consumption for both fans.

“Until now, fan technology could not deliver the large temperature differences of, say 12-15 degrees, which compressors can,” explained Gran Tallberg, export manager at ABB Flkt Oy. “As a result, fans could not compete against compressors.”

It was because of South Africa’s low electricity costs — which compensates for the amount of power needed to drive the fans — coupled to the fact that the Umkomaas mill uses sulphite liquor, which made the MVR evaporator technique an attractive possibility.

The complete turnkey evaporation plant, built by Finland’s A. Ahlstrom Corp., was part of a pulp mill upgrade designed to increase the mill’s capacity from 1,200 to 1,600 tonnes of pulp per day.

The successful implementation of the turbo fan technology at two pulp plants in Norway and Finland also demonstrated the potential of MVR designs in liquor recovery applications.

Installations at plants in Norway and Finland have shown that MVR fans are applicable to both sulphate and sulphite mills. They can be used in pre-evaporators or evaporators.

At the Kaukas pulp mill in Finland, owned by UPM Kymmene, the MVR turbo fan acts a pre-evaporator that then feeds a conventional multi-stage liquor recovery system.

Hannu Kyt, head of operations at the UPM Kymmene pulp mill at Kaukas, says that the installation of the MVR unit has helped the mill meet increasing production demands. The pulp mill has a capacity of 625,000 tonnes of pulp annually. This feeds two paper machines producing around 500,000 tonnes of LWC paper a year. The system has to be reliable with more than 8,000 hours of operation a year.

“The pre-evaporator is part of an overall investment in upgrading our recovery system,” says Kyt. “The attractive investment cost and technical solution were factors in the choice of the MVR system.”

“The fan was easier to connect to the existing evaporation system than a compressor and it has helped us remove a bottleneck in production by increasing the capacity in the recovery system,” he said. “Without this capacity, our pulp production would be limited.”

At the Borregaard sulphite pulp production plant in Norway, part of the Orkla group, which is one of that country’s largest listed companies, there were also clear benefits in adopting the fan approach.

“We think this technology is more reliable, offers better uptime, and has lower maintenance costs due to the slower rotation rates”, said Rune Hillstrm, executive vice-president at the speciality pulp plant.

“By doing this we have been able to increase capacity of the evaportion plant by 15%. It is also less sensitive to process disturbances compared to conventional turbo compressor technology.”

As a consequence of these benefits, the turbo fan design has also been used to recover the alkaline component of the bleach plant effluent to minimize emissions to water, said Hillstrm.

The main reason for the success of such fans, said Gran Tallberg, ABB Flkt marketing manager, is the simplicity of construction when compared to the complexity of traditional designs incorporating compressors. This makes maintenance easier and helps control manufacturing costs.

ABB Flkt Oy Industrial Fans, part of the ABB group, is a industrial fan manufacturer that delivers its products to more than 100 countries. For more information, contact ABB Flkt Oy Industrial Fans, P.O.Box 5, FIN-02621 Espoo, Finland, tel. +358 10 22 2000, fax +358 10 22 23480.