18 VALMET


FEATURE


Pulp Paper & Logistics


3Comparing the performance of


a state-of-the-art tissue machine – such as a Valmet DCT200TS – to machines with similar characteristics reveals great improvement potential in energy performance. On-field activities have shown


how improvements can result from a correct air system setting, maintained in this way over time.


Keep the balance The air system generates hot air that is blown at high speed onto the moving paper to accomplish most of the drying. The other essential function of the air system is to guarantee the correct exhaust of humid circulation air, as well as to replenish it by conveying low-humidity make-up air. The system is in balance when


the controlled flows of the supplied air (typically make-up and combustion air for the correct operation of burners) are equal to the outgoing air (exhaust). If more air is supplied than


exhausted, then the system is in overpressure and the excess hot and humid air is blown to the machine hall through the hood edges.


On the other hand, when the


outgoing air flow is higher than that supplied, the system is in under-pressure and cold air is sucked into the hood from the machine hall.


January/February 2016


Both circumstances can lead


to abnormal and dangerous operating conditions, as well as to a performance deterioration and increase of energy consumption, therefore also of running costs. Particularly, an over-pressurised


system can cause an increase of running (energy) costs of up to €63,000/year for each kgDA/s of hot air blown into the machine hall (estimated gas cost €35/ MWh), as energy contained in this wasted air is not recovered through the heat exchanger. Further problems that may


be caused by a system in over- pressure are: • Increased risk of fires around the hood;


• Damages referable to equipment overheating nearby the hood; • Felt and profile issues; • Worsening of hall ventilation control with widespread condensation problems.


An under-pressurised system


can also result in higher in running costs (up to €57,000/year for each kgDA sucked in from the hall) because the make-up air is still cold in the hall, as it has not yet been pre-heated by the heat exchanger. But there are also additional issues that can be caused by a system in under- pressure. Such are: • Cooling and humidity peaks at the paper edges;


• Dust and dirt build-up at the hood edges, therefore increased fire risk; • Sucking of dust and dirt, and consequent build-up at the heat exchanger, relevant worse performances and risk of fire.


• Paolo Della Negra is manager of the Sales Department at Valmet SpA, Via Gregorcic, 46, Gorizia IT- 34170, Italy. Cell: 39 345 1497398. Email: paolo.dellanegra@valmet. com website: www.valmet.com


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