Through drying, Technical Information Paper TIP 0404-25 (2023)

 Through air drying (TAD) is a process in which hot air is forced through a wet, porous material by imposing a pressure differential across the material. As the air passes through the porous material, the air heats the material and supplies energy to evaporate water. The air leaves at a reduced temperature and increased water vapor concentration. It is the intimate contact between the hot air and the material, coupled with the large internal surface area of the porous material that provides high drying rates.

In the paper industry, this process has found greatest application in drying of lightweight, permeable webs, such as tissue, toweling, filter paper, and nonwovens. For these grades, through drying provides an advantage over conventional pressing because it produces a less-dense and weaker sheet. In such applications, the through dryer may be either a flat bed or rotary design (one or more drum arrangements), often followed by a steam-heated cylinder for the final drying and sheet creping. The flatbed configuration is illustrated in Fig. 1 and the rotary design in Fig. 2. In this second configuration, hot air can be forced through the web by pressurizing the porous cylinder, in which case a hood collects the exhaust air, or hot air can be drawn through the web by applying a vacuum to the porous cylinder, in which case the hood is used to distribute the hot air (as shown).

The air can be heated either indirectly through a heat exchanger or directly by mixing with products of combustion of a clean fuel (such as natural gas, LPG, or methane) in a burner or the hot air exhaust from a gas turbine or from the hood of a Yankee drying cylinder. Thermal efficiency is significantly increased by recirculating and reheating part of the through air dryer exhaust, or using the exhaust air in or from a subsequent air impingement dryer, if one is used in the process. Typical recirculation rates for stand-alone through-air dryers range from 50 to 80% of the process air. This recirculation system is shown schematically in Fig. 1 and 2. Part of through dryer exhaust is removed from the recirculation system to take out evaporated water and, if direct fired, combustion water vapor. The exhaust air must balance the fresh air intake, infiltration, and products of combustion.

Information in this paper serves as a general outline of conventional through air dryer machine parameters. Considerable progress has been made in analytical treatment of both the recirculation system and the through air drying process and several new (generally proprietary) drying processes have been developed. Drying rates, flow velocities and energy levels may be higher for new dryers than the ranges indicated in this paper. Abbreviated analytical descriptions of the through drying process are presented here to provide a basic knowledge of the process. Details on the TAD process may be found in the references listed at the end of this Technical Information Paper.