Evaporators in Vacuum Drying

Evaporators in Vacuum Drying

In the vacuum drying system, the working environment of the evaporator is under a certain degree of vacuum. This vacuum environment lowers the boiling point of water, making it easier for the water in the material to evaporate. The refrigerant in a tube and fin evaporator flows through the tubes and absorbs heat from the surrounding hot air through the tube walls and fins. The hot air then takes heat from the material being dried by convection and radiation. For example, when the material to be dried is food, the moisture inside the material turns into water vapor under the dual action of vacuum and heating, and the water vapor enters the surrounding hot air, which transfers the heat to the tube fin structure of the evaporator.

After absorbing the heat, the refrigerant undergoes a phase change from liquid to gas. The gaseous refrigerant is then transported by the compressor and other equipment to the condenser for cooling, changing back to the liquid state, completing a refrigeration cycle. In this process, the multi-row fin structure of the evaporator acts as a highly efficient heat exchanger, continuously absorbing heat from the vacuum drying chamber to maintain the temperature and heat balance required for the drying process.

Tube-fin evaporators are mainly composed of tube bundles and fins. The tube bundle is usually made up of many parallel metal tubes (such as copper or aluminum tubes) that serve as channels for refrigerant flow. The fins, on the other hand, are tightly attached to the outer surface of the tubes and are typically aluminum. The fins serve to greatly increase the heat transfer area, as heat needs to be efficiently transferred from the heat source to the material being dried during the vacuum drying process. For example, fins can increase the heat transfer area by several or even a dozen times, enabling the evaporator to achieve a higher heat transfer efficiency in a smaller volume.

The tubes are arranged in a variety of ways, with the common ones being smooth rows and staggered rows. In this kind of evaporator with a large number of rows, the downstream arrangement may make the flow of airflow between the tube bundles more regular, and the pressure loss is relatively small; while the staggered arrangement will increase the pressure loss, but it can enhance the perturbation of the fluid and improve the heat transfer efficiency.

Advantages of multi-row design

An evaporator with many rows can increase the residence time of the refrigerant in the evaporator so that it can fully absorb heat, thus improving the heat transfer efficiency of the evaporator. For example, in vacuum drying applications, each row of tubes allows heat exchange with the hot air surrounding the material being dried. As the number of rows increases, the total heat exchange area and heat exchange time are increased, which is more conducive to the evaporation of moisture from the material. And the multi-row design can make the evaporator adapt to different drying process requirements, by adjusting the number of rows can flexibly control the heat exchange capacity.

Performance influencing factors

Influence of fin spacing and tube spacing

Fin spacing is an important factor affecting the performance of the evaporator. If the fin spacing is too small, although it can increase the number of fins per unit volume, thereby increasing the heat transfer area, but it will lead to increased resistance to air flow, affecting the circulation of air between the fins, reducing the heat transfer efficiency. On the contrary, the fin spacing is too large, although the air flow is smooth, but the heat transfer area will be reduced. Tube spacing has a similar situation, tube spacing is too small will increase the refrigerant flow resistance in the tube, but also not conducive to the flow of air between the tube bundles; tube spacing is too large will reduce the number of tubes per unit volume, reducing the heat transfer area.

The relationship between the number of rows and pressure loss

As the number of rows increases, the flow path of air in the evaporator becomes longer, and the pressure loss will gradually increase. This is because the air needs to overcome the resistance of more tube bundles and fins to pass through the evaporator. Excessive pressure loss leads to a reduction in air flow, which affects the heat transfer efficiency of the evaporator. Therefore, when designing this kind of evaporator with a large number of rows, it is necessary to consider the relationship between the number of rows, the air flow rate and the pressure loss, and to reduce the pressure loss by optimizing the arrangement of the tube bundles and the fin structure and so on, and at the same time to ensure sufficient heat exchange efficiency.