Abstract
The design of more responsive control systems for papermachine dryer sections requires a knowledge of the coupling between web moisture content and changes in basis weight and machine speed. The interactive effect of changes in basis weight and machine speed on the measured moisture content must be known to allow proper compensation.
An estimate of the steady state coupling coefficients was made by forming an approximate mathematical model and calculating the steady state response surfaces for average web moisture content versus machine speed and basis weight. The model was a realistic representation of the partial differential equations relating heat conduction and vapour diffusion within the web. Approximate, constant values of web properties and transport coefficients and the alternating boundary conditions characteristic of conventional cylinder drying were used . Basis weight of 26-90 lb/1 000 ft2 and speeds of 800-1800 ft/min were studied.
The calculated coefficients showed that, for typical production conditions of 421b board at 1 400 ft/min, a 1 lb change in basis weight would be expected to produce a 2 percent change in web moisture content; a speed change of 100 ft/min would produce a 5 percent change in moisture content. The dependence of the coupling coefficients on the level of basis weight and speed was also demonstrated. The coupling coefficients for basis weight and for speed both increased approximately linearly with the operating speed, whereas the increases in both coefficients through basis weight changes were less significant. These results are of use in the design of non-interactive control systems for papermachine dryer sections.
Although the calculated results were restricted to the steady state gains for this system, an indication is given of the possible use of a more complex model in studying the dynamic behaviour of the drying system.
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