预热是大多数物料烘干前需要做的第一步，因为通常情况下，热泵烘干的温度要求是高于环境温度的，特别是在气温低的时候或者北方地区，预热显得更加重要。预热阶段由于烘干房初始温度较低，物料水分蒸发较少，因此湿度相对较低，一般只加热不进行排湿，以保证物料迅速升温达到物料内外温度平衡，空气湿度越大、热焓值越高、导热效果越好。
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.
预热时间长短需要根据定网站建设  伺服液位仪  防撞墙模具 唐山护栏 遮阳网 风电基础模具料属性、物料数量和环境温度来确定，一般为两到三个小时即可，预热温度通常不能太高，最好稍微低于物料烘干的本身温度要求，如果温度太高可能导致物料外观和颜色的变化以及营养成分的流失，比如碎裂、变黑等等。
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.