The invention and application of the shoe press was something of a revolution in papermaking when introduced by Beloit in the early 1980s. But since then that revolution has transformed to become an evolution, particularly for tissue machines as producers go in search of the perfect result. Andritz has played a firm role in this evolution over the last 14 years, the latest result being its PrimePress XT EVO shoe press which it says brings a long line of improvements and advancements to tissue making. PPI spoke to Andreas Anzel, deputy director, Technology and R&D, at Andritz about ongoing developments of the remarkable shoe press, and what tissue producers can expect in the way of improvements to both quality and efficiency.
PPI: Can you tell us about the history of the shoe press, and why it is technically so much better than a conventional press roll?
Anzel: Back in 1980 the first shoe press was introduced in conventional papermaking by the Beloit Corporation after extensive development. The idea was to improve dewatering by providing a bigger press impulse, involving a longer nip residence time and higher nip load. So shoe presses have been around for decades in conventional paper making.
Their application for tissue production was introduced in the late 1990s. The former standard in tissue, the conventional press roll, has only an “average” maximum nip pressure and notable rewetting due to the symmetrical pressure curve. The idea for tissue was to improve dewatering by providing a high maximum nip pressure, together with an abrupt pressure relief at the end of the nip, thereby allowing only minimal rewetting effects. And if focus is on maximum bulk, the maximum nip pressure can be kept low, still benefitting from minimal rewetting. Andritz has been one of the pioneers in this demanding environment of the tissue segment.
What has been the main focus at Andritz on the evolution of the shoe press for tissue?
Strong focus has been put on increasing the perceived customer value. Therefore the targets for the development of the new PrimePress XT EVO were: reducing costs, and offering affordable technology with a higher customer return, achieving even higher post-press dryness, superior dewatering and less thermal drying. Also implementing a low flow, low pressure loading system, which means reduced energy consumption and of course lower operating costs.
Higher runability and operational efficiency have also been targeted by introducing a new more flexible shoe design, reducing the bending stiffness of the shoe by 60%, as well as improving the pressing in-machine cross direction. The result of this is higher operational efficiency. We have also introduced a means for much greater edge control.
Do you have any figures that can reveal how much difference the addition of the shoe press makes to energy consumption and speed?
Among papermakers there is a rule of thumb: “an increase by 1% dryness after the press, will result in thermal energy savings of roughly 4%”. This rule holds for constant production, that is unchanged machine speed. In terms of kWh/tonne, an increase by 1% dryness after the press will roughly give you thermal energy savings of 60kWh/tonne. A shoe press may give you an increased dryness after the press of up to 4%, whereby the savings in thermal energy are correspondingly higher.
What do you think is the ROI on a shoe press?
This must be evaluated individually for every customer, as the boundary conditions always differ. Customers pay tremendously different prices for steam, gas and electricity – this also varies strongly geographically. In addition, machine setups may vary strongly from steam hood to high temperature im- pingement hood, small cast Yankee to large diameter high performance steel Yankee, availability of heat recovery systems such as the re-evaporation system, or energy production via cogeneration systems. Last but not least there is the machine width: wide machines show a better ROI than small ones. Having said all this, the ROI for a shoe press could be 1.5 to 2 years, with up and down potential strongly depending on the individual customer.
Can the PrimePress XT EVO be installed on any existing tissue machines?
There are no issues at all for new machines. Rebuilding shoe presses to existing machines must be evaluated from case to case, accounting for the existing machine design.
The biggest proportion of shoe press installations come from new tissue machines, which is ideal, as all systems are correctly sized and tuned from the beginning. This includes engineering of the vacuum system, setting up the drive system, an optimized arrangement of the system components to generate maximum sheet wrap on the Yankee (i.e. suction turning roll, shoe press roll and Yankee), implementing the hydraulic system and embedding the shoe press in the control system. If this is done right from the beginning, the effort and additional costs are straightforward, giving an affordable technology for the customer.
When considering the addition of a shoe press into an existing tissue machine, which was done in the past, this involves checking and maybe also adjusting at least some of the above systems. So the rebuild investment is far higher than the add-on costs for a shoe press for a new machine. In the past Andritz offered a tissue shoe press with an especially small diameter, which is ideal when thinking of rebuilding an existing machine, but in reality the market was mainly focused on new machines.
The application of shoe presses for tissue is special, as the forming of the sheet is performed on a water-saturated felt – based on the Crescent Former concept – and therefore requires an appropriate vacuum conditioning of the felt upstream of the press nip. Furthermore, as the Yankee is not a perfect counter-roll press member, the shoe press must also compensate for the non-ideal Yankee surface, by means of shoe flexibility and edge control capability. In order to operate efficiently, the whole system must be well set up, including the vacuum system, the felt and belt selection as well as the shoe press design.
Is there any extra automation involved?
All automation adaptions are done as part of the shoe press erection. The control and logics for the shoe press will be completely integrated to the machine DCS. during production there is no extra effort.
Does Andritz undertake the whole project, from installation, set up and optimizing the addition of the shoe press?
Andritz is clearly a project and service oriented paper machine supplier. Our customers look for a certain benefit and we will provide the suitable technology. In terms of shoe press technology, the customer will be completely served, including engineering, erection, start up and optimization.