Heat pump drying system helps dehumidify agricultural products

In recent years, drying technology has been widely used in the processing of agricultural products. Currently, China is the country with the highest output of agricultural products such as fruits and vegetables in the world.

Drying is a necessary processing step for all kinds of agricultural products. Its purpose is to remove excess water from agricultural products while ensuring the quality characteristics of agricultural products.

 

 

 

 

Agricultural products after drying are easy to process, store, transport and use.

 

Classification

Heat pump drying systems can be divided into three types according to the circulation mode of drying gas: closed, open and semi-open.

Among them, the closed heat pump drying system does not exchange with the outside air during the drying process, which has significant advantages in improving drying quality and energy saving.

 

Application expansion

Heat pump drying technology has developed rapidly in recent years and has been widely used in agricultural product processing, tobacco, medicinal materials, tea drying, ceramic baking and other fields.

For closed system applications, users pay particular attention to the heating and dehumidification capabilities of heat pump dryers. As for dehumidification capabilities, there are more factors that can be optimized.

 

Dehumidification capabilities

The design condition of the dehumidification capacity of the evaporator of the heat pump drying unit often only corresponds to the wet air parameters at a certain stage of the drying process. The optimal dehumidification state is achieved under this condition.

The dehumidification capacity is not optimal under other conditions, and sometimes even the moisture in the air cannot be removed.

For example, in high temperature and low humidity conditions, the evaporation temperature may exceed the dew point temperature. In addition to adjusting the throttle valve, the evaporation temperature can also be adjusted by precooling the return air temperature or changing the air volume passing through the dehumidification evaporator to ensure the dehumidification capacity.

Return air temperature pre-cooling generally adopts cross-counterflow plate heat exchanger for pre-cooling.

In the drying and dehumidification system, the principle of air pre-cooling before entering the evaporator is to reduce the temperature to the lowest dew point temperature.

 

Quality of heat exchanger

The main indicator of the quality of the heat exchanger is the heat exchange efficiency of the core. At present, the efficiency of the heat exchanger with better quality can reach more than 60%.

 

Solutions

Changing the air volume through the evaporator generally involves using variable frequency fans or duct bypass solutions.

In a closed dehumidification system, the condenser and evaporator are generally in the same duct. Using a variable frequency fan to reduce the air volume will result in a smaller condenser air volume, so duct bypass solutions are more often used.

The bypass air volume is measured by the bypass rate, which is the ratio of the air volume that does not flow through the evaporator to the air volume that flows through the evaporator.

As the bypass rate increases, the air volume passing through the evaporator decreases, and the corresponding evaporation temperature can be lower than the dew point temperature, and the amount of air to be cooled also decreases. When the cooling capacity does not change much, the dehumidification capacity increases.

 

Research results

Research results on the efficiency of heat pump drying and dehumidification show that the cooling capacity of the drying heat pump, the compressor power and other parameters, and the heat exchange of the intermediate heat exchanger are all linearly related to the air bypass rate of the system.

For open heat pump drying units and semi-open units, the evaporator dehumidification efficiency is average. As more high-temperature and high-humidity exhaust gas passes through the evaporator, the rate of water precipitation on the evaporator surface after cooling gradually increases.

In addition, during the drying process of the closed drying unit, the air bypass rate has a great influence on the dehumidification energy consumption ratio. When the air bypass rate is 0.4 and 0.6, the dehumidification energy consumption ratio is the lowest.

The dehumidification capacity is proportional to the cooling capacity and inversely proportional to the air volume and the inlet and outlet air temperature difference. The cooling capacity and power of the system are positively correlated with the evaporation air volume and the inlet and outlet air temperature.

Therefore, the bypass rate is not necessarily the higher the better. The wet air parameters at each stage correspond to an optimal air bypass rate.

At the same time, the optimal bypass rate increases the dehumidification capacity, and the system performance will also be improved to a certain extent.

 

Summary

In the study of improving the dehumidification capacity of the closed drying system, there are the following key factors:

• The optimal evaporation temperature is generally slightly lower than the dew point temperature, so that the system cooling capacity is maximized;
• The optimal bypass rate ensures the optimal balance between the difference between the evaporation temperature and the dew point temperature and the system cooling capacity under different wet air parameters;
• It is also necessary to consider the impact of parameters such as wind speed flowing through the evaporator, wind resistance in the air duct, etc. on the condensation of condensed water and air volume.