Measurement and analysis of heating of paper with gas-fired infrared burner

Abstract

Drying of moist paper is achieved using various modes of heat transfer namely conduction and convection. An alternative method that has found considerable application in this area is infrared (IR) radiation heating/drying. The growing application of IR radiation for paper heating/drying purposes is because of the fact that IR radiation is not only a surface-heating source, rather its energy penetrates inside porous medium, imparting volumetric heating. Radiant heating/drying is a non-contact mode of transfer of heat, which makes its application very attractive for the drying of coated surfaces. In addition, the radiation absorption of liquid water is considerably high in the near and medium IR range which leads to enhanced absorption by moist paper. Gas-fired IR heaters produce combustion on the burner surface by ignition of a pre-mixed air and fuel streams. The combustion raises the surface temperature to ranges of 800-1,100°C to emit radiation, mainly in the medium IR range, which has a relatively high absorptivity in moist paper. Experiments have been carried out on a dedicated gas fired IR unit facility in the Drying Research Center Laboratories of Texas A&M University. The focus of the experiments has been to obtain detailed real time data of paper samples during the IR radiation heating/drying process. In this aspect, samples were instrumented with fine thermocouples and dried to obtain instantaneous average moisture and internal temperature distribution data under various operating conditions. The existing paper drying theoretical model that determines the transient moisture and temperature distributions within the paper has been modified to incorporate IR heating/drying characteristics. To obtain the boundary conditions for the combined radiant convective drying process, heat flux measurements along with burner surface temperature measurements have been carried out. The penetration of IR radiation energy into the paper in the thickness direction has been characterized by an exponentially decaying function. The theoretical model predictions are found to be in good agreement with the experimental data primarily for thin papers. Based on the radiation model, theoretical depth of penetration for various levels of IR energy has been calculated. From the experimental data, the IR penetration depth is deeper than the theoretically calculated depth. Radiation efficiency for paper samples has also been determined as a function of moisture content in paper and the emitting temperature of the IR burner, and was found to lie within the range of 30 to 35%.