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π½π½ = πΌπΌ β πΏπΏπ π ππ β ππππ Eq. (5) with considering the empiric Lewis-factor πΏπΏπ π . Furthermore, properties of the drying product needs to be provided as input parameters. Property data like heat capacity for liquid water is provided by a temperature and pressure dependent object class. However, for the moist air two of these object classes are utilized to consider the special conditions in the drying chamber: One pressure and enthalpy dependent moist air object that represents the incoming dry and hot air and one pressure and temperature dependent air object that calculates the property data of the moist air directly at the surface of the drying product. It is assumed that the air on the product surface features the same temperature as the product and is considered as saturated air with a relative humidity of 100 %. Combining the quantities ouοΏ½tlined above, the mass flow rate for the water that is being removed of the product over the time can be calculated, based on film theory for mass transfer (H.D. & K., 2003): ππ ππ Eq. (6) ππΜ = π€π€ ππ π‘π‘ π€π€ π‘π‘ β β π½π½ β π΄π΄ β ππ β ( π₯π₯ β π₯π₯ ) οΏ½() πππππππ‘π‘ ππ β π π π΅π΅ ππ,π π π π π π ππ,πππππππππ π with πποΏ½ as the molar mass of water and air, π π π΅π΅ as the specific gas constant, π₯π₯π₯π₯ as the mass fraction and ππ as the activity coefficient, which considers the decelerated mass transfer at the end of a drying process when the capillary forces are limiting the water release from the product (Deans, 2001). The activity factor is characteristic for each type of product and needs to be measured. For the drying chamber model, the usual energy and mass balances are augmented with equation (7), which considers the progress of the drying: ππππ Eq. (7) π€π€ = βππΜ ππππ The modelling of the dryer cell was conducted in an object-oriented way with the potential to theoretically investigate various different drying products. That way, the standalone cell can be universally implemented in both open-loop or closed-loop drying processes. 2.2 Validation of the model The model for the drying chamber is validated against measurement data which was provided by Innotech GmbH, a German manufacturer of drying systems. The validation is conducted with two types of measurement data sets, which differ in the general scaling of the drying processes. The model was provided with measured input parameters for the inlet relative humidity, the air velocity in the drying chamber and the inlet temperature of the moist air according Table 1. Table 1: Measurement data for apple drying, provided by Innotech GmbH Mass of product Drying temperature Inlet relative humidity Outlet relative humidity Moist air volume flow Moist air velocity Drying time Removed water 3kg 55 Β°C 16% 40% 0.07 m3/h 1.3 m/s 8h - 100 kg 70 Β°C 10% - 5500 m3/h - 8h 15-20 kgH20/h Figure 2 (left) shows the successful simulation of the drying curve for the small-scaled system, with a maximum deviation of 10 % between the measurement and the model. For the large-scaledPDF Image | Design of a CO2 heat pump drier with dynamic modelling tools
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