Experimental study of the potential use of an expander for atmospheric freeze-drying

Abstract

Atmospheric freeze-drying (AFD) is a promising technique that is increasingly popular in drying applications. In this study, a novel method to generate the cold and dry air suitable for AFD applications using an expander is proposed. The concept is simple, able to simultaneously cool and dehumidify air, control the discharged air pressure, and provides the potential for energy saving. An idealized theoretical model to simulate the process was developed. Experiments were conducted at various pressure and speed conditions. The study shows that the expander's maximum volumetric and isentropic efficiencies were 27.2% and 21.8%, respectively. Observation of the internal chamber pressures was able to explain the relatively low isentropic efficiency. The maximum power output and expander efficiency were 56 W and 42.7%, respectively. The expander was able to cool the air to below 0 °C, and the lowest measured outlet air temperature was −8.7 °C. The outlet air relative humidity was between 3.6% to 4.7%. The measured dehumidification effectiveness was between 79.8% to 93.1%. The study shows that the concept is promising, and the expander was able to generate the required air conditions suitable for AFD applications.