烘干工藝第一式：預熱
First type of drying process: preheating
預熱是大多數(shù)物料烘干前需要做的第一步，因為通常情況下，熱泵烘干的溫度要求是高于環(huán)境溫度的，特別是在氣溫低的時候或者北方地區(qū)，預熱顯得更加重要。預熱階段由于烘干房初始溫度較低，物料水分蒸發(fā)較少，因此濕度相對較低，一般只加熱不進行排濕，以保證物料迅速升溫達到物料內外溫度平衡，空氣濕度越大、熱焓值越高、導熱效果越好。
Preheating is the first step before drying most materials, because usually, the temperature of heat pump drying is higher than the ambient temperature, especially in low temperature or northern areas, preheating is more important. In the preheating stage, due to the low initial temperature of the drying room and less evaporation of material moisture, the humidity is relatively low. Generally, only heating is used without moisture removal, so as to ensure that the materials can be heated rapidly to achieve the internal and external temperature balance of materials. The greater the air humidity, the higher the enthalpy value, and the better the heat conduction effect.
預熱時間長短需要根據(jù)物料屬性、物料數(shù)量和環(huán)境溫度來確定，一般為兩到三個小時即可，預熱溫度通常不能太高，最好稍微低于物料烘干的本身溫度要求，如果溫度太高可能導致物料外觀和顏色的變化以及營養(yǎng)成分的流失，比如碎裂、變黑等等。
The preheating time shall be determined according to the material properties, material quantity and ambient temperature, generally two to three hours. The preheating temperature shall not be too high, and it is better to be slightly lower than the temperature required for drying the material. If the temperature is too high, it may lead to changes in the appearance and color of the material and the loss of nutrients, such as fragmentation, blackening, etc.
