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If the end of the drying process is defined with 70 kg of water that has been evaporated from the initial 100 kg of apples, then the following times are needed to finish the drying: "Open-loop Benchmark" 11.7 h, "No Bypass" 13.6 h, "20 % Bypass" 14.5 h and "50 % Bypass" 16.8 h, as illustrated by figure 6 (right) The required drying time is the shortest for the open-loop benchmark system, as relatively dry air is heated and fed to the drying chamber. The circulation of air within a closed-loop setup leads to a higher relative humidity and thus extended drying times (figure 7). Relatively, the utilization of heat pumps leads to an increase in time by 13 – 43 % regarding the setup. The COP for all three h𝑃𝑃ea𝑄𝑄ṫ pump-assisted systems is approximately 3.2, with a gas cooler to compressor load ratio of 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 [kW/kW] = 29/9 for "No Bypass", 25/8 for "20 % Bypass" and 18.5/6 for "50 % Bypass". The heat supply of the "Open-Loop Burner" was 56 kW. The process efficiency was evaluated through the specific moisture evaporation rate (SMER) for the drying process, which states how much water can be removed from the product with the utilization of 1 kWh. The SMER is defined by 𝑆𝑆𝑀𝑀𝑆𝑆𝑅𝑅 = 𝑎𝑎𝑚𝑚𝑎𝑎𝑁𝑁𝑎𝑎𝑑𝑑 𝑎𝑎𝑜𝑜 𝑅𝑅𝑒𝑒𝑎𝑎𝑝𝑝𝑎𝑎𝑃𝑃𝑎𝑎𝑑𝑑𝑅𝑅𝑑𝑑 𝑤𝑤𝑎𝑎𝑑𝑑𝑅𝑅𝑃𝑃 (𝑘𝑘𝑘𝑘) Eq. (8) 𝑅𝑅𝑎𝑎𝑅𝑅𝑃𝑃𝑘𝑘𝑒𝑒 𝑁𝑁𝑢𝑢𝑅𝑅𝑑𝑑 (𝑘𝑘𝑘𝑘h) Figure 8: Simulation results for four investigated systems, comparison of the specific moisture evaporation rate (SMER) In the examined temperature range, the latent heat of water evaporation is 0.63 kWh (2250 kJ). This amount of energy is required to evaporate 1 kg of water, leading to a reciprocal value of 1.6 kg/kWh. Therefore, figure 8 shows that the SMER for the open-loop benchmark system is quite low in comparison to typical values for the SMER than can be found in literature (Chou et al. 2006). This indicates rather inefficient drying conditions and a product that is difficult to dry (in this case apples, with an initial moisture content of approximately 80 %). The implementation of a heat pump has the potential for energy related savings of 80 % in comparison to the examined benchmark process, which can be even increased with the usage of a bypass. Since the open-loop benchmark system was operated without a bypass, the evaluation of the bypass cycles should be referred to the "No Bypass"-system, to highlight the impact of the bypass utilization towards the same general system setup. Therefore, in comparison to the "No Bypass"- system, the "50 % Bypass"-system offers a decrease in energy consumption by 28 %, but also an increase of the drying time by 24 %. It should be noted that the investigated bypass systems were not optimally controlled in this study, leading to a constant recirculation of moist air and thus, to a non-optimal utilization of the bypass. An adequate control strategy to address the varying heat demand during a typical drying process provides potential for further energy and time savings.PDF Image | Design of a CO2 heat pump drier with dynamic modelling tools
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