Numerical Simulation of Turbulent Air Flows in Aseptic Clean Rooms

Abstract

Sterile air condition is an increasingly important requirement, as several manufacturing processes, such as food production, preparation of aseptic products for the pharmaceutical industry, manufacturing of microelectronics components, compact discs and photographic films, need optimal air cleaning. Aseptic clean rooms were developed to satisfy the above requirement. According to Wirtanen et al. (2002), a clean room can be defined as “a room in which the concentration of airborne particles is controlled, and which is constructed and used in a manner to minimize the introduction, generation, and retention of particles inside the room, and in which other relevant parameters, e.g. temperature, humidity, and pressure, are controlled as necessary”. The objective of clean room technology in various clean room classes in the food and beverage industry is to ensure the control of contaminants in sensitive processes. Use of this technology should be considered in processes where microbial inactivation, e.g. through thermal sterilization or deep freezing, is not feasible. If critical process risks are identified due to exposure of the product to airborne microbes during processing or if severe sedimentation of airborne microbes can occur on critical process surfaces, clean room technology can be used to solve the problems (Schicht, 1999; Whyte, 2001). In an aseptic clean room, the air flow, properly filtered, is flushed from the top of the chamber to special grids placed at the bottom of the structure. Then, it is recirculated by an air filtering unit; here, part of the air flow is ejected and replaced by external air that will undergo filtration. A main requirement of clean rooms is that they are maintained at a pressure higher than the external one, to prevent pollutants air flow from the environment. Currently accepted standards describing clean rooms are developed by the Federal Standard (Federal Standard 209 E, 1992) and adopted by ISO (2006); such standards suggest an international classification of clean rooms, based on thirteen possible classes depending on the maximum allowed number of pollutant particles in the room. Eq.1 provides an empirical relation between the aseptic class M and the diameter of the pollutant particles d [mm] in the room: