The following factors determine the speed of the drying process surrounding temperature, air speed, relative humidity, surface area and characteristics of the material. If both the surface area and characteristics of the material are fixed the only way to speed up the drying process is to either increase the temperature increase the air speed or reduce the relative humidity. When comparing the effects on the overall process of drying air speed is more important than temperature while relative humidity is more important than both. For example, if you leave wet clothes outside on a cold but dry winter day, they can finish drying within several hours. However, if you did the same thing on a warm and humid spring day they will still be damp even after several days. Therefore the best way to speed up the drying process is first to reduce the relative humidity in the air then to increase the air speed and finally to raise the temperature. While it is very easy to increase the air temperature it is very difficult to reduce the relative humidity.

Currently, there is a great and urgent demand for more advanced energy-saving drying technology and equipment. At the present, the most advanced energy-saving drying technology is heat pump drying which is divided into open-loop and closed-loop drying. The open-loop heat pump dring method has the same drying concept as the open-loop drying method with only one exception: The heating structure is replaced with a heat pump which saves one to two times more electricity than the simple electricity heating machines. The heat pump dryer is composed of an external structure and an internal structure. The external structure takes in energy from the air and sends it into the drying house through the internal structure. Open-loop heat pump drying also has similar problems as the traditional open-loop drying method. In addition, it may not achieve normal working conditions when the temperature of the environment reaches below 5 degrees Celsius.