Our environment contains millions of extremely small-diameter particles (< 1 µm) such as viruses, bacteria, fine dust or soot. Most of these particles are of no significance for research and industry. However, special high-tech production processes or chemical analyses demand a much higher level of protection. Here we describe how production, packaging or research activities can be conducted in the almost complete absence of particles on the basis of an example. Most of these particles are invisible to the naked eye. One exception is the Tyndall effect which occurs when macroparticles become perceivable in sunlight due to light scattering phenomena. The individual particles cover a wide range of diameters which can be removed from the ambient air.
It is, however, relatively complicated to combine different methods in order to remove all the particles from the ambient air. At the Stuttgart-based Fraunhofer Institute for Manufacturing Engineering and Automation (IPA), researchers investigated the effectiveness of conventional walk-in clean rooms and came to the conclusion that operators introduce a large proportion of the particles themselves and spread these through their movements.
The following factors may influence equipment or samples in clean rooms:
At the same time, the following factors have an impact on clean room contamination:
That is why, as far as possible, it is necessary to attempt to completely enclose equipment without any human intervention. However, walk-in clean rooms are necessary in many cases. To compare their effectiveness, a clean room classification system has been developed and certified worldwide (Table 1).
The standard also presents a method for calculating the number of residual particles in a clean room of a given class. The clean room classes are based on the particle diameter and the number of particles per m³. To illustrate this, let us take the following example: ISO class 1 means that only a single golf ball (Ø 3.5 cm) relative to the volume of the Earth (Ø 12,760 km) is permitted, something that is almost inconceivable.
To reduce the number of particles from several million per cubic meter to one or less, internationally standardized high-performance filters are used. (Fig. 1: The Spetec Laminar Flow Module)
To create clean rooms suitable for the production of laser optics or microchips, Spetec constructed another series of products in the form of its Laminar Flow Boxes (Fig. 2: The Laminar Flow Box is available in six different sizes). Using this solution, a laminar flow of filtered clean air is introduced into the working area. This air is then able to escape again through a perforated floor. A low-turbulence flow is ideal in order to remove and replace the air in corners with poor access.
This type of air flow is defined in terms of its grade of turbulence (GoT):
(Standard deviation of velocity distribution) divided by (Mean velocity)
A clean air flow of 0.45 m/s was calculated for the Spetec Laminar Flow Box in order to meet the requirement for low turbulence. A special fan is used to generate the air flow, which is pre-cleaned by passing through a pre-filter. For example, the H14 filter, which is the most frequently used, guarantees particle collection efficiency of 99.995%. This means that all particles with diameters > 0.5 µm and 99.5% of particles with a diameter of 0.3 µm are filtered out. The isolation factor – that is to say, the number of particles in the ambient air relevant to the number of particles inside the clean room – is therefore 1 million. In the years following this initial development, Spetec has supplied many different variants of the flow box series in sizes and performance levels adapted to individual customer requirements. Initially, these were intended for use in chemical-analytical laboratories in order to permit the continued use or introduction of standard solutions under clean-room conditions.
The filter modules can also be combined with a side-mounted strip curtain to create clean working areas for electronic measurement equipment, for example (Fig. 3 Modules with side mounted strip curtain).
Alongside applications in the optical and electronics industries, these clean room boxes are also used in the foodstuffs industry, the medical sector and, more generally, for sterile packaging.
