Energy optimization of the Yankee-Hood dryer

The Yankee-Hood dryer is the crucial section of the paper machine due to the consumption of energy. As a consequence of continuously increasing energy costs, determination of the proper operating conditions is essential to operate the tissue machine in the most efficient and economic means while ensuring paper quality. The objective of this study is to evaluate the Yankee-Hood dryer in Ipekkagit' tissue paper factory, set the mass and energy balances and make optimization studies in order to achieve the desired production rate whilst keeping the drying parameters within limits at the minimum energy costs. The approach to solve the problem is to develop a steady state analytical model. The complicated drying system is simplified as the overall and sub systems. Material and energy balances are set up for each system. Data of machine parameters at several paper grades are collected at the factory during production. Material and energy balance equations are solved with the available data for the unknown parameters and for the determination of the drying parameters most effective on energy consumption. The critical drying parameters are air supply velocity, wet- and dry-hood temperatures, steam pressure in cylinder and exhaust humidity. Efficiency, defined as the ratio of energy required to evaporate the water to energy input through the system boundaries, is calculated as 28-30% for different grades of tissue paper production., Steady state rate of heat transfer to the wet sheet is analyzed considering the two-way mechanism, from steam through the cast iron shell and from heated air. The transfer of heat is eased in both mechanisms by the lightweight tissue paper that has a comparatively lower internal thermal resistance with respect to other types of paper products. Steam side is modeled considering different thermal resistances in series: Condensate layer, shell, shell-sheet contact, and paper sheet. These resistances are to the same order of magnitude, cast iron shell being the largest and paper sheet the smallest in value. Sheet-shell contact heat transfer coefficient is strongly dependent on sheet moisture content that is assumed to vary almost linearly due to the angular position on the Yankee. Air side heat transfer is mostly affected by the temperature, humidity and velocity of the air that is blown on to the sheet. Calculations resulted out that rate ofheat transfer from air side is 55-65% while that from steam side is 35-45% of their total indicating the dominance of air side on drying.