Modelling and design of an efficient low temperature closed cycle dehydration system

Abstract

Research indicates that there is a growing interest for the preservation of foods due to an increasing demand as a result of a growing global population. Preservation of food commodities by means of dehydration methods and technology has been the subject of many research studies over the last 20 years, and methods to improve drying techniques are constantly pursued and investigated. Research further shows that for most foods, the retention of nutritional content and the inhibition of microbiological activity are ideally achieved during drying at low temperatures. Most widely conventional methods use heating to reduce the relative humidity levels in air to increase the moisture-absorbing capability, resulting in high energy consumption and reduced product quality. A need for a more holistic approach to drying modelling and design has been identified in which all factors, influencing drying, are to be considered in order to develop an efficient drying process. Efficient drying of products entails optimised temperature, consistency, duration, and energy consumption, which affects quality, productivity and operating costs. Most conventional drying methods can only contribute positively to a reduction in drying time due to high temperature drying, but being open cycle type systems, consistency, and energy consumption are compromised, which reduces the quality and increases the operating cost. The importance of a detailed investigation of product characteristics such as moisture diffusivity, equilibrium moisture content, geometry and dimensions, as well as the most suitable temperature at which a product can be dried without compromising on quality, is described and emphasised. As a first step in the process of drying optimisation, investigation of product characteristics is critical prior to any modelling and equipment design. It is further important to optimise the airflow rate based on the geometry, dimensions and amount of product to be dried to avoid exorbitant specific energy consumption levels. Mass transfer convection coefficients of drying air must be determined as part of the optimisation process. The surface area and the resulting effectiveness of the mass transfer process from a substance to air, determines the final saturation level and the energy required to recondition the drying air. The preferred final moisture ratio of a product should determine the rate of moisture removal by the equipment. A constant rate of moisture removal should be pursued in the design approach, which eliminates the possibility of oversized equipment. Mechanical refrigeration equipment with reheating is recommended for dehumidification and reconditioning of the air in the closed cycle drying system. In order to improve the energy efficiency of the equipment the reheating can be accomplished by means of heat recovered from the condenser of the unit instead of using electrical reheating.