Jun 24, 2010
Would you be interested in cutting your energy costs while also gaining such additional benefits as improved drying capacity, better runnability, lower doctoring costs and less waste? What’s more, all this is available without major capital outlays, long payback times or extended shutdowns. Optimized doctoring processes can improve dryer section performance and reduce energy consumption, which are important considerations for any production line or end product.
Almost all rolls and cylinders need doctoring. The primary functions of doctors are simple: shed the sheet at sheet breaks or sheet threading and remove excess water and contaminants from the doctored roll surface. The keys to an optimal doctoring process are state-of-the-art blade materials, modern roll covers and optimal doctoring parameters. Valmet has taken big steps forward in recent years both in terms of materials technology and innovations dealing with roll covers and doctor blade composition.
Relative power consumption of doctor blade materials. Based on laboratory test in simulated paper machine environment.
The total number of doctoring positions differs between machine concepts. Modern paper machines typically employ 50 to 120 doctors. Pressing a doctor blade against a roll surface generates friction (braking effect). In other words, there are 50 to 120 mechanical brakes on a paper machine that are trying to drag down the speed generated by electric motors. Some 50 to 90% of these mechanical brakes are located at the dryer section.
The dryer section, in turn, has an effect on machine speed, the sheet’s moisture profile and break frequency, and on overall energy balance and power consumption, and its good runnability is based on trouble-free doctoring and clean cylinders. Due to the number of dryer cylinders involved, even small friction reductions between doctor blades and cylinders are multiplied severalfold along the section and can thereby produce considerable savings.
The acting coefficient of friction, which cannot be determined through theoretical calculations but has to be measured empirically, depends on a number of factors. High friction can be a real issue at the dryer section as there are several hot drying cylinders that are doctored without any lubrication. Dryer section start-up problems are often caused by the fact that the coefficient of static friction is higher than the coefficient of kinetic friction for a given material. It is not uncommon to even overload the motor, gearbox or other power transmission components under such conditions. The dryer section’s hot and humid surroundings also constantly challenge doctor blade materials.
Conventional doctor blade materials, like bronze, steel and fiberglass, have relatively high coefficients of friction and therefore high energy consumption rates (Figure 1). Despite this, they are still widely used due to their relatively lower price and deep-rooted mill practice. In other words, the dryer section constitutes a good potential source of energy cost savings at many mills.
Even when theory and calculations clearly point to potential energy savings, results are often verified in practice through comprehensive trials to determine the real savings potential of a specific machine.
Valmet has developed a simple step-by-step improvement process that enables papermakers to reduce their doctoring-related dryer section energy consumption. The process starts with a thorough study of the current status of doctoring that produces a clear project plan and recommendations for optimal doctoring parameters and blade materials. The mechanical condition of all doctoring components is checked and a list of required maintenance work and replacement parts is generated. All maintenance work and spare parts can also be included in the agreed project scope.
Valmet's intensive R&D and long-term experience in doctoring, roll covers and advanced blade materials are fully utilized in selecting the best blade materials for the customer’s process. A process-specific, or sometimes even customer-specific, low-friction doctor blade material is selected from a broad range of advanced composite materials. The results of the process are evaluated both in terms of operational aspects and clear energy consumption numbers summarizing the situation before and after.
|The performance of doctor blade materials against different roll cover materials is continuously tested and developed at Valmet’s service and technology centers|
The above real-life case examples show that energy efficiency can be enhanced also with modest investments. Improved doctoring processes help mills improve cylinder drying capacity and runnability, achieve better paper quality and higher production efficiency, reduce power consumption, and cut both doctoring and operating costs. Optimal doctoring can also have favorable safety and environmental effects.