Drying of Paperboard Under Conventional and High-Intensity Conditions, 1992 Engineering Conference Proceedings

The web internal dynamics during paper or board drying have been studied through mathematical modeling end computer simulation. The model takes into account the various heat and mass transfer processes that can take place during drying under conventional as well as high-intensity conditions. The model equations were solved on a digital computer.

The model predictions show the development of three distinct zones within the web during drying: a dry zone, a wet zone and an intermediate zone. These zones vary in thickness as drying proceeds. The intermediate zone is a transition zone between the dry and wet zones and is characterized by flows of liquid and vapor in opposite directions, similar to that of a "heat pipe". As drying proceeds, the location of the "heat pipe" or the intermediate zone progressively moves from the hot surface towards the open surface.

Under high intensity conditions, the model predictions show that during the initial phase, liquid water can actually be pushed out of the web by a pressurized vapor zone. This together with the bulk flow of vapor can account for the high drying rates that can be achieved by high intensity drying. The model predictions also show the presence of an advancing "heat pipe" observed under conventional drying conditions. The results also show close agreement with literature data on conventional and high-intensity drying.