Key Questions
The following key questions are answered in this module:
What is the purpose of a steam box?
Steam boxes are used to increase the after-press dryness of the paper by increasing the temperature of the sheet.
What is the difference between a profiling and non-profiling steam box?
Profiling steam boxes are divided into sections across the width of the machine. The steam flow to each section can be adjusted to produce a uniform CD (cross direction) moisture profile. Non-profiling steam showers apply steam evenly across the entire width of the machine, so while they do improve dewatering, they cannot address moisture streaks.
Why are steam boxes usually positioned over vacuum boxes or opposite vacuum rolls?
Vacuum helps pull the steam through the sheet so that it condenses in the sheet and not just on the surface. The goal is to have a uniform sheet temperature.
What variables affect heat transfer efficiency?
Steam box heat transfer is affected by basis weight, sheet permeability, sheet moisture, and initial sheet temperature. Heat transfer is also affected by steam flow, the distance to the sheet, vacuum level and position, and the dwell time.
What is the impact on total machine drying efficiency?
Even though steam boxes use steam, total paper machine steam consumption usually decreases. For every pound of steam used in the press section, up to 3 pounds less steam is needed in the dryer section.