D* H* T ( mm) Surface area ( m² / m³ ) Free volume ( % ) Number per m³ Bulk density ( Kg/ m³ ) 25* 25* 0.6 250 93 59200 420 38* 38* 0.6 138 94.7 14500 350 50* 50* 0.8 121 95 7000 300 76* 76* 1.0 75 95 1950 280