What are the pain points and shortcomings of heat pump drying technology?
1. Long drying time (2-5 days)
Heat pump drying is limited by the working principle, and the drying temperature is generally 45 ℃ ~ 60 ℃. Although the temperature of high-temperature drying heat pumps can reach 100 ℃ in recent years, the drying temperature is still lower than that of coal. A low drying temperature will prolong the heating time of the material and at the same time reduce the heat and mass transfer coefficient, which will inevitably lead to a decrease in the drying rate, thereby prolonging the drying time. In the middle and late stages of drying, the remaining moisture in the material is mainly bound water, and the activation energy of water molecules is low. It is difficult to remove this part of the water and requires a higher drying temperature. This also leads to the lower temperature of the heat pump drying in the later stage of drying than the combustion. Coal drying takes more time.
Traditional sun-drying methods: both shade drying and sun-drying have the disadvantage of requiring labor costs to reverse, which is time-consuming and labor-intensive. Of course, it takes longer to dry and dry in the sun (7-30 days), and it is also limited by the weather.
2. The drying scale of a single heat pump is small
The compressor power equipped in the heat pump dryer of the closed system is relatively small. The feed amount of each drying cycle is limited to the volume of the drying box. At the same time, due to the closed structure itself, the intermittent drying method is usually adopted. Therefore, the drying scale is not large, and it is difficult to achieve continuous operation and mass production. . Such as wood drying, drying with steam, the maximum drying room can reach more than 300m2, and the maximum heat pump drying is 100m2. However, this problem can be solved by multiple units and mainframes. The Institute of Physics and Chemistry of the Chinese Academy of Sciences adopted loop heat pipe technology in the northeast alpine region to design a heat pipe multi-stage series heat pump corn drying system, with an hourly corn tidal grain processing capacity of 3 148 kg , Which is 10%~15% more than coal-fired capacity.
Note: The use of multiple heat pump dryers requires a certain amount of investment in the early stage.
3. There are certain requirements for the operation level of the heat pump drying room
The heat pump working fluid used in the heat pump cycle often uses various types of refrigerants. During the working process of the heat pump system, the pressure of the system pipeline is constantly changing. Once the pipeline leaks in the high-pressure zone, the refrigerant will have an impact on the entire heat pump drying system and the surrounding environment. At the same time, the lack of refrigerant will cause the winding temperature to be higher than during normal operation, the compressor's return air pressure will decrease, the oil return will be difficult, and the exhaust temperature will rise, which will affect the life of the compressor. Therefore, there are certain requirements on the operating level of the users of the heat pump drying room operating system.
4. The drying plan of the drying room requires professional design
There may be some bacteria in the drying room. In fact, with the exception of a few heat-resistant bacteria such as molds, yeasts, etc., most microorganisms will die at a temperature of 60 ℃ to 80 ℃. Although the current heat pump drying technology belongs to the medium and low temperature zone, there is no report that the microbial content of the product after heat pump drying is higher than that of the traditional drying method. However, if the heat pump drying system has an unreasonable design, it may cause microbial contamination problems. For example, assuming that the heat pump drying system has poor dehumidification or insufficient dehumidification capacity, the entire system will be in a high-humidity environment, which is conducive to the proliferation of microorganisms. Therefore, the drying program of the drying room needs professional design, otherwise more or less defects will occur, which will affect the drying effect of the drying room.
