upper and lower endless outer mesh belts having respective vertically confronting lower and upper reaches extending in a direction of treatment through both an upstream zone and a downstream zone for directly engaging and compressing a material mat and advancing the compressed-material mat in the direction through the zones;

upper and lower endless inner mesh belts inside the outer mesh belts in the upstream zone only and having respective lower and upper reaches juxtaposed with the respective lower and upper reaches of the respective outer mesh belts on the mat;

respective upper and lower drives including rollers each engaging a respective one of the mesh belts for advancing same;

means in the upstream zone inside the inner mesh belts in the upstream zone for injecting steam through the inner and outer mesh belts and into the mat; and

calibration plates inside the outer mesh belts in the downstream zone.