Drying is an important industrial process. Various temperature levels and drying principles are applied in industrial dryers. The most common dryer type is one in which air is heated with steam, gas or hot water and then circulated over the wet product. As the air picks up moisture from the wet product, its humidity increases and the energy contained in this stream may make it a useful heat source. Standard procedure is to exhaust this humid air or dehumidify it. With a heat pump, heat can be extracted from the humid air. The air is cooled down and dehumidified. The extracted heat can be increased in temperature and can be used to heat the dryer.
Thus, the use of a heat pump serves two purposes - heat the dryer and dehumidify and recirculate air. Heat pump assisted drying can give high efficiencies because of this.
The project described below is realized in practice and gives a more detailed description of the so called 'heat pump dryer'. Its evaporator both cools and dehumidifies the humid air. The extracted energy is upgraded by the heat pump to a higher temperature level that can be used to heat the dryer with its condenser.
The conventional drying process
The figure on the left shows a commonly used drying principle. The process is also drawn in the Mollier diagram below. Hot air (1), in this case 70 °C, is circulated over a product belt inside the dryer. The hot air is used to evaporate water out of the product that needs to be dried. Therefore, the temperature of the air decreases and its humidity increases. The cool, humid air (2) is than partly exhausted, however, the main part of the air is recirculated in the dryer. To compensate for the exhausted humid air, fresh dry outside air is brought into the cycle (3) and preheated (4). After preheating this air is mixed with the recirculated humid air and the mixture (5) is than heated towards the required process temperature (1).
The exhausted humid air contains a lot of energy. The temperature level of this air is low and direct reuse inside the dryer is therefore not possible. Application of a heat pump gives the possibility for waste heat recovery. With a heat pump the extracted heat from the exhaust air is upgraded to a higher temperature level and reused to heat the dryer. The principle of operation of this heat pump dryer is described below.
Heat pump dryer
On the right, a drying process is shown that is heated with the use of a heat pump. Furthermore, a Mollier diagram in which the drying cycle is highlighted is shown in order to provide more insight in this process. As compared to the conventional dryer, in the heat pump dryer the heat exchanger is replaced by the condenser (hot side) of the heat pump. Furthermore, the air exhaust duct is connected to the air supply duct. Inside this connection the evaporator (cold side) of the heat pump is installed.
Hot air (1) is circulated over a product belt inside the dryer. During this cycle the temperature of the air will decrease as its humidity increases. About 1/3 of the cool humid air (2) is circulated over the evaporator. The evaporator cools the air further down below the condensation temperature (3). Below this temperature the air will be dehumidified. The cool, dry air (4) is than mixed with circulation air from the dryer. The mixture (5) is heated to the desired process temperature inside the condenser and can be reused in the dryer cycle (1).
Theoretically, a fully closed drying process is possible with this technique, making the process independent from environmental influences. However, sometimes air refreshment is desirable. Therefore, in practice an air exhaust duct and supply air duct will always be built in a heat pump dryer.