Calculating And Understanding Particulate Contamination Risk - Pharmaceutical Technology

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Calculating And Understanding Particulate Contamination Risk
The author presents a method to calculate the relationship between supply air volume flow and airborne particle concentrations. These methods and approaches facilitate the overall understanding of airborne contaminants and provide valuable information when designing facilities and processes for sterile manufacturing.

Pharmaceutical Technology Europe
Volume 23, Issue 3

Contamination theories — airborne particle dispersion

Particles spread within a volume because they are affected by an external force, such as air volume mass flow (diffusion and or convective spreading, i.e., through vortices or turbulence), gravitational deposition, electrostatic forces or thermal differences.

Gravitation deposition

Gravity creates a downward motion on particles based on mass, and there is a ratio between the mass weight of the particle and the settling time. Free-floating particles within an air volume will also have a gravity effect against each other; however this force is very small and, in a pharmaceutical environment, it can be neglected.

Electrostatic forces

Particles and surfaces can be positively or negatively charged via static electricity. Particles with a positive charge will be drawn to a particle or surface with a negative charge. The potential for static charge also depends on the material; for example, glass has fewer tendencies to be charged than plastic materials. A material's tendency to develop static electrical charge is an important issue to consider when it comes to using them in a cleanroom. A surface with a positive charged material can attract negatively charged particles, which will accumulate on the surface. If a surface gets grounded, particles may also come loose and spread to the surroundings.

Convective airflow

Figure 1: Convective deposition of particles.
The energy in airflow influences particles to move parallel with the airflow vector, i.e., particles are spread in the same direction as the air movement (Figure 1). This knowledge is commonly used in the pharmaceutical industry to create clean areas and air barriers. Unidirectional airflow (UDF) units have a parallel airflow such that airborne particles are transported away in the flow vector direction. The flow in a UDF unit is either laminar or turbulent. Ljungqvist and Reinmüller tell us that "it is assumed in a parallel flow field that, next to surfaces along the main flow direction, there is a thin sub layer (boundary layer) in which the transfer of momentum is dominated by viscous forces, and the effect of weak turbulent fluctuations can be neglected. The situation is quite different for particle diffusion. In this case, even weak fluctuations in the viscous sub layer contribute significantly to transport".3


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