Steam, Paper Drying, and the Bottom Line
Because energy drives the papermaking process, steam is found in many energy streams in a paper mill. Steam is used to cook fibers for improved sheet strength, heat stock for improved drainage, heat process air for ventilation, heat dryers to dry paper, and selectively heat the web for improved gloss and smoothness.
Steam is a highly usable and regularly used additive throughout the paper mill. And when it comes to the dryer section, it is the primary input used to evaporate moisture from the sheet allowing for desirable and profitable sheet characteristics to be created.
Although condensing steam to dry paper is an inherently efficient process, the drying process can be inefficient. Poor steam system design, improper control settings, and lack of control during machine upsets can send uncondensed steam to vents in the roof or to large condensers that heat water that have no use. Ventilation air temperatures and air flow rates can be set at maximum values, without regard for the drying requirements, heating tons of air that serves only to heat the environment. And high-pressure steam can be used when the process should have been set up to use only low-pressure steam, resulting in a lost opportunity to generate additional electrical power.
In addition, if machine operators do not have the tools or process insight to control steam pressures, differentials, and blow-through, process efficiency will suffer. For example, improperly controlled steam pressure in the wet end can cause the sheet to stick to the dryer surface and either release non-uniformly in the open draws or detach from the dryer fabric in the unorun and single-tier sections. Dryer cylinder steam pressures that are not properly graduated can cause the sheet to alternately bag and stretch as it passes from one dryer to the next. And if the condensed steam is not properly removed from the dryers, poor heat transfer rates and non-uniform drying can result. The end result is poor runnability, marginal product quality, and reduced productivity.
Fortunately, modern steam systems can be designed to include supervisory control via tools such as the Dryer Management System® technology to provide operators the insight and control over the drying process to meet all machine and grade demands, even upset operating conditions. New technologies and control strategies give even greater ability to manage steam to ensure optimal energy use resulting in improved drying efficiency and machine runnability.
The Bottom Line Impact
The difference between a machine that has good steam economy or usage of 2.9 million Btu/ton and one that has poor steam economy of 7.6 million Btu/ton, is 4.7 million Btu/ton, or about 4,700 lb/ton. For a machine that is producing 1,000 tons per day, with steam costs of $6 per 1000 lb of steam, this translates into a cost-savings opportunity of $28,000 per day. Nearly $10 million per year. This could mean the difference between being competitive and shutting the machine down.
Advancements in papermaking chemistry, retention and dewatering aids, environmental pulping, and bio-refining all offer significant promise to position the paper industry to compete with other forms of packaging and communication media. The way you look at steam can make a considerable difference to your process and bottom line.