Fit-for-purpose vacuum system and efficient doctoring together with controllable dewatering solutions for forming and press sections are the fundamental base for good energy efficiency and low-cost pulp, paper, board and tissue production.
By Jukka Lehto, General Manager, Jussi Lahtinen, Sales & Marketing Director & Patrik Högl, Technical Sales Manager, Runtech Systems Ltd.
Vacuum in a paper machine is used for a variety of reasons. Dewatering of the paper sheet is obvious, holding and controlling the route the sheet takes through the press section and conditioning the press felts are other key uses of vacuum in a paper machine. Vacuum is also used to hold and control the dewatering jet at a suction roll.
Early paper machine designs did not use vacuum at all, but relied on gravity and press nips to dewater paper. This we refer to as nip-dewatering. Machine speeds were then much slower than what we today are accustomed to. Today it is hard to imagine a paper machine without a vacuum system, and felt-dewatering is today still the most common way to dewater the paper in a press section.
The vacuum levels at a paper machine are typically between 5 and 70 kPa, which is relatively low compared to other vacuum applications in, for instance, the chemical industry. The air flows, though, are very high due to the large open areas we see in a paper machine.
Vacuum, which is a result of air being pulled through a restriction, is an expensive tool; it is one of the top three energy consumers in a paper mill. Historically it has been made available in abundance, and often used in excess. Today we face increasing cost of energy. Paper mills are demanding reduced operational costs to stay competitive. Therefore, it makes sense to review air flow demands and, when suitable, apply old school thinking, i.e. nip-dewatering rather than felt dewatering.
Each vacuum element in the machine, such as suction box and suction roll, need a certain amount of air flow to operate at an ideal vacuum level. The needed capacity is depending on felt life and type, paper grades, basis weights, machine speed etc. Traditional liquid ring pumps and fixed speed blowers cannot cater for these dynamic vacuum and flow needs, resulting in, often significant, waste of energy.
EP turbo blowers are designed to operate efficiently across a wide range of vacuum levels and air flows to allow paper mills to optimize vacuum levels while taking advantage of the higher efficiency a blower has over a traditional liquid ring pump device. High speed motors developed driven by frequency converters allows a typical RunEco blower to provide paper machine vacuum levels between 30 and 70 kPa. Wide range impeller designs allow high efficient levels across the operating range. This means that the amount and the usage of throttling valves can be reduced to a very minimum. The result is 30 to 70% energy savings compared with traditional vacuum systems.
Further, EP turbo blowers operate without the need for sealant liquid (water) so contribute to water savings in a paper mill.