Before baking silkworm cocoons with a heat pump dryer, the cocoons shall be laid layer by layer according to the thickness of the cocoons. In general, no more than 2 and a half cocoons shall be laid horizontally. If the dryer is a hot air type with vertical penetrating airflow, then the height equals to 3 cocoons high is appropriate.
The main process of silkworm cocoon drying
The conditions which influence the drying effect are temperature, humidity, wind speed, cocoon volume and drying time. The temperature, humidity and wind speed have a great influence on the drying speed and quality of dried cocoons. It must conform to the law of water evaporation in each drying stage, for example, the temperature should gradually decrease from high to low, the humidity should gradually increase from low to high, and the wind speed should gradually decrease from large to small.
The drying process of silkworm cocoons is mainly divided into preheating, constant speed drying and deceleration drying.
(1)Preheating stage: heating the fresh cocoon, the heat energy penetrates into the cocoon cavity through the cocoon layer to form the dead auxiliary body, so that the auxiliary water enters the evaporation period. At this stage, the temperature should gradually increase.
(2) during the constant speed drying stage, the water of cocoon was evaporate in a large amount with the fastest evaporation rate. The surface evaporation rate is in an equilibrium state, the temperature of the cocoon layer are lower than the air temperature in the drying room, and the temperature of the cocoon body is lower than the temperature of the cocoon layer. This period shall continues until the water in the cocoon body diffuses gradually, which is usually last for 8 hours. lags behind the evaporation speed of the cocoon layer surface by 8 hours.
(3) During the deceleration drying stage, the moisture in the cocoon gradually decreases, and the evaporation rate decreases accordingly. During this period, the temperature of the cocoon gradually increases and close to the temperature of hot air. When the inside of the cocoon begins to evaporate, the decelerated drying is transferred from the first stage to the second stage until the drying is over.