On the air distribution plate the bed materials with thickness of 550-650 mm are laid respectively. Start the induced draft fan and the primary fan, gradually turn up the baffle of the air damper, slowly and smoothly increase the air flow. Generally increase 4000-4000 m3/h air volume every time, record the opening of the primary air damper and the corresponding data of air velocity, air pressure, air temperature and the air chamber air pressure, until the air volume is increased to the maximum current of the fan. And then gradually reduce the air volume and record the corresponding baffle opening, air volume, air temperature and air pressure. Use the average value of upstream and downstream data as the original data of the resistance of the cold materials layer. at this moment the measured pressure of air chamber is the sum of the materials layer resistance and resistance of air distribution plate, the measured pressure of air chamber is minus by the resistance of the air distribution plate of the corresponding air volume, then the material layer resistance is obtained. Depict them in the rectangular coordinate system of the same plane, and connect them with curve, then the relation curve of material layer resistance and air volume under different material layer thickness is obtained, namely the characteristic curve of material layer resistance. According to the cold state  characteristic curve, the cold critical air volume can be determined, and the hot state minimum operating air volume can be reckoned .