What is the difference between primary and secondary drying in heat pump drying?

Air source heat pump drying has been successfully applied in various fields, and the properties of dried materials also vary widely. There are differences between primary and secondary drying in heat pump drying. The material properties and customer types of primary and secondary drying are different. The drying process also has certain differences. TROCKNNER has been committed to the research, development, production and promotion of heat pump drying products and has accumulated a large number of practical application cases.

One-time drying: The object is fresh products, which can be stored or sold after one-time dehydration.

Single-stage drying is aimed at removing water used only once to meet the requirements of the dried product. The primary purpose of drying materials is to remove moisture to allow for long-term storage. Items susceptible to this process include primarily fresh produce with high water concentration and a rapid evaporation rate. After drying, they reach the proper moisture for storage or direct sale. Among the most commonly treated materials are fruits, vegetables, meats, poultry, and seafood. The key is to avoid decomposition that can occur if moisture is not removed quickly, thus preventing long-term preservation. Initial drying can also set the stage for secondary drying, removing some of the water to extend shelf life and simplify future processes. For example, materials that require cutting before completion of drying need a certain level of moisture to prevent deformation or breakage in the process.

Let's take the example of fresh figs, with an initial humidity of around 78%. After about 16 hours of drying, this is reduced to 14%. By meeting the humidity standards, the figs are ready to be stored in cool, dry conditions or to be packed and sold directly. If the residual humidity is higher than necessary, there is a risk of spoilage if they are stored for a long time, which would affect their place in the market in terms of quality and price.
In conclusion, the main goal of single-stage drying is to process freshly harvested products to the desired consistency without the need for additional drying stages or post-treatments, resulting in a longer process with a high volume of water to be evaporated and a slightly less dry final product compared to successive drying procedures.

Secondary drying: the material removes water slowly and the process is relatively more complex.

Secondary drying involves drying a material for a second time, that is, it has already gone through a previous drying process. Materials that undergo this process are usually those that release water slowly and require a more careful process, such as large fruits, wood, medicinal herbs and industrial products. Some of these materials, after a first drying and a second processing, reach the degree of drying necessary to be considered finished products and can be marketed.
For example, consider the drying of wheat. After ripening, it was dried naturally in the field before being harvested. When harvested, wheat contains approximately 30% moisture. In order to store the grain in a warehouse or to process it further, its moisture content needs to be reduced to about 10% by two pre-dryings. In this process, the moisture loss is not great, the drying temperature must be moderate and the time relatively short. These precautions are essential to ensure the quality of the final product.

Furthermore, due to their characteristics, many materials have a low water evaporation rate and fail to meet the humidity requirements for drying. Even if they meet the requirements, the final quality of the products can be compromised, presenting problems such as black cores, among others. The internal humidity of fruits and tubers is difficult to eliminate. Therefore, they need to undergo a primary drying process before entering the dryer for effective drying, which constitutes a secondary drying. Wood, medicinal inputs and industrial products all have high levels of humidity that, generally, do not change in a short time. Therefore, to reduce drying time and reduce costs, it is feasible to expose them to the natural environment for a period before drying in a chamber, thus carrying out a secondary drying. Unlike initial drying, secondary drying frequently requires a higher temperature and a shorter time.