Characterisation of the maximum entropy change in the process of drying materials based on a heat pump

This paper studies the maximum entropy change characteristics under different working condition parameters (e. g. inlet temperature, air volume, initial sludge temperature and sludge radius) based on the validated model. Spherical sludge blocks were used as drying materials, and heat was provided by a heat pump system. Theoretical models of mass balance, energy balance and entropy balance in the drying process are established, and the model is validated with experimental results. The results show that the change in sludge entropy during the drying process is mainly caused by irreversible moisture diffusion, heat transfer and pressure. There is a peak of entropy change when the entropy value initially climbs and then declines. The lower the peak of entropy change, the smaller the irreversible dissipation in the drying process; however, the entropy change is not the main factor of drying time. For every 5 celcius increase in inlet drying temperature, the entropy change peak increases by 5.2 similar to 18.6 % and the drying time is shortened by 27.5 %. The entropy changes the peak increase with the rise in inlet air volume, while the drying time initially shortens and then changes slightly, and the optimal air volume interval exists. For every 0.01 m decrease in sludge radius, the entropy change peak reduces by 9.4 similar to 23.8 %, and the drying time is shortened by 19.8 similar to 33.1 %. The change in the initial sludge temperature has little impact on the entropy of the drying process.

Automated summary

This paper studies the maximum entropy change characteristics under different working condition parameters and validates the model. The results show that the change in sludge entropy during the drying process is mainly affected by irreversible moisture diffusion, heat transfer, and pressure. The inlet temperature, air volume, and sludge radius have significant effects on the entropy change peak and drying time, while the initial sludge temperature has little impact.