Paper Humidification & Paper Humidifiers
Humidity control in the paper industry is essential to control the moisture loss of the paper after drying.
Once produced and reeled, the paper can be exposed to ambient conditions at the end of the paper machine for considerable periods of time, maybe up to 2 hours. Exposed surfaces of the paper will change in moisture content dependent on the ambient humidity, leading to problems during the slitting and wrapping processes.
Low humidity, below 50%, causes the paper to shrink, curl and lose dimensional stability making the reel of paper unusable and in turn very costly to the manufacturer. To stop this from occurring, the typical humidity level required is 55%rH. When this level is maintained the moisture in the paper maintains not only the flex-ibility of product but also its stability during production.
Paper humidity issues
The drying end of a paper machine and the surrounding area is usuallt well ventilated so ambient humidity changes will have an affect on the paper a it is reeled up at the end of the maker.
Low humidity in the sitting area will cause moisture losses to occur, these losses will take place very quickly. However, damage to the paper may not be seen at this stage as the tension on the paper prevents any curl or dimensional change being seen.
Changes to the paper in the slitting process will show at the sheeting stage because the tension has been taken off the paper and it is now free to change its shape.
In very severe cases of moisture loss, web breaks will occur. This is where the moisture change has caused such a large dimensional change that the paper tears. Web breaks not only cause damage to the paper but also are costly to the manufacturer as it takes time to re-thread the machine.
It is important to maintain moisture in the finishing and wrapping areas to not only stop the paper from drying but also the wrappers as well. When moisture is lost from the wrapper. it will be drawn from the paper, which will shrink. These dimensional changes are permanent, once caused they will not reverse themselves so the paper will be unusable.
When humidity is below 45% static electricity will be generated. This can be a very large charge. To give an indication of the level that can build up. it takes 30,000 volts for a spark to jump 1cm, sometimes static sparks as long as 50cm can be seen in slitting machines.