IMPINGEMENT DRYING OF PAPER

The complete drying history of paper dried under an array of multiple round jets of air was determined. The drying rate - paper moisture con tent relations, determined as a continuous function by on-line measure ments for about 80 sheets, permitted examination of the following para meters: basis weight 20 to 50 g/m(2), which covers the tissue to commu nication paper range; initial moisture content 1.5 to 3 kg water/kg fr bre; nozzle to paper spacing of 5, 7.6 and 8.5 nozzle diameters; nozzl e plate open area ratio from 1.4 to 3.1%; jet Reynolds number in the r ange of 450 to 11,100. Three methods of quantifying the complete dryin g rate curves were tested. Use of the common assumption of drying rate linear with moisture content over the falling rate Period gave inadeq uate representation. Secondly, the general method of Churchill for any transport process that has a transition between two asymptotic relati ons was applied for the first time to the drying of paper. This approa ch gives statistically valid fits, but was rejected because the values of the constant drying rate were generally too high and because the m ethod does not give a critical moisture content. A power law treatment of the falling rate period provided a good representation of the comp lete drying rate curves as well as realistic values of the constant dr ying rate and critical moisture content. To reduce subjectivity, a sta tistical program provided for each drying rate curve the best fit valu es of three parameters, the constant drying rate, critical moisture co ntent and the exponent for the power law falling rate period relations hip. With the general correlations of these three parameters for the e xperimental conditions studied, drying time may be successfully predic ted for a wide range of drying conditions.