Humidity control and process steam

In pulp mills, the solutions relevant to Texco relate to controlling excess humidity in areas with high moisture loads and to generating process steam for the processes that use it. Sizing starts from the working temperature, the moisture load, the air flow rate and the steam requirement. If the mill has a source of waste heat, using it to generate steam can also be assessed.

Areas close to the paper machine and to wet processes, such as coating, can operate at high humidity and produce condensation on metal components, window frames and ceiling structures. Controlling condensation in these areas means coordinating ventilation, vapour extraction and, where the moisture load requires it, dehumidification. This can reduce the risk of corrosion and damage to finishes.

In mills where waste heat is available from other processes, using it to generate process steam can be assessed. Feasibility depends on the temperature, flow and availability of the heat source, as well as on the mill’s steam requirement. Separately, sizing the generator to the actual load and consumption profile influences fuel consumption and operating costs.

Temperature influences the moisture exchange between paper and air and must be assessed together with relative humidity. Maintaining stable conditions in storage and production helps preserve the dimensional properties of the material.

Paper and board reels stored in unstable humidity conditions can suffer distortion and curl. In many paper and board applications, a level of around 45–55% RH helps limit dimensional variation and the problems caused by overly dry air. The exact target value is set according to grammage, fibre furnish and the requirements of the downstream processes.

In corrugated board manufacturing, steam is used to heat and condition the paper, making flute formation and bonding easier. The requirement is calculated in kg of steam per hour, based on the consumption of the connected equipment, the pressure the process requires, the degree of simultaneous use and the peak loads on the converting lines.

In operations where steam comes into direct contact with paper or board, the quality and dryness fraction of the steam must be matched to the process requirements. Condensate or impurities carried by the steam can affect the material and the process, depending on the application. The steam source must be sized and maintained according to the requirements of these operations.

Paper expands and contracts as its moisture content varies. In multicolour printing, these variations cause register problems between colours. In corrugated board packaging, the same variations can affect sheet flatness and warp, as well as creasing accuracy. Maintaining constant humidity in the printing and converting hall reduces the rejects arising from these causes.

The high unwinding speed of the paper web generates electrostatic charge, especially at low humidity. The accumulated charge can cause sheets to stick together, attract dust to the surface, lead to incorrect feeding of the material and cause machine stoppages. Controlled humidity complements the earthing and ionisation measures already in place in the installation.