PAPER MACHINE HOODS - BACK TO BASICS

摘要

Historically low fuel costs, relatively high ambient temperatures and stable humidity in Queensland, and the luxury of a steam surplus meant the paper machine hood ventilation system at Visy Paper 8 was hardly a priority. However, a site wide energy audit suggested hood system inefficiencies were inflating steam usage by up to 3%. With energy prices rising, and the continuous push for increased paper production threatening the steam surplus, a project to optimise drying efficiency and reduce steam usage was quickly undertaken. This focus on the hood system promoted functional and operational understanding, but most importantly achieved the steam savings with minimal cost required. Investigations kicked off with benchmarking the system performance and operational practice. Measurements of supply and exhaust air flow, temperature and humidity data were compared to third party audits to develop process understanding and establish data confidence. It was also important to determine standard steam usage with constant paper grade and machine speed to justify project benefits. Data was collected across a range of paper grades and speeds to get a comprehensive picture of the hood dynamics under different operation conditions. With reasonable supply air temperatures and no major moisture leaks into the supply air, the opportunity at Visy Paper 8 lay in optimising air management across the hood. The issue lay in low supply air flow and misdirected exhaust air flow. Supply air was boosted by re-opening air distribution boxes and exhaust air was corrected by closing all doors, sealing up the hood and adjusting system ducting to capture air from across the whole hood. For minimal cost, this brought the balance between supply and exhaust air back into the design range for the enclosed-type hood. Steam usage was seen to drop by 2-3%, reducing our tonne of steam per tonne of paper requirements. Further higher cost opportunities exist, such as cleaning heat exchanger tubes and replacing steam traps but keeping control of the hood system basics is the first priority in promoting drying efficiency. This can be achieved with continual process data collection and analysis to prevent adopting an inefficient operating practice which will cost more in the long run.