Open unidirectional airflow benches

The purpose of open unidirectional airflow benches is to protect products from particulate contaminants by creating a controlled environment. These benches are used, for example, in electronic, biological, pharmaceutical, and food industries. It should be mentioned that within pharmaceutical production, aseptic sterile processes must be carried out in a Class 100 environment (U.S. Federal Standard 209 E, Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones). To avoid particle contamination in the bench, horizontal or vertical airflow with high-efficiency particulate air (HEPA)-filtered air is used. The air velocity is normally 0.4-0.5 ra s . Some examples of typical arrangements of open unidirectional airflow benches are shown in Fig. 10.51. 

FIGURE 10.5 I Examples of typical arrangements of open unidirectional airflow benches. 

In order to maintain a high level of air qualit), it is necessary to test airflow velocities and HEPA filters for integrity. These tests are described in a variety of standards and recommended practices depending on the use of the airflow bench. 

The bench should be supplied with HEPA-filtered unidirectional airflow, having a velocity sufficient to sweep particulate matter away from the working area. Normally a velocity of 0.45 m s plus or minus 20% is adequate. It is important to monitor the air velocity at suitable intervals because significant reduction in velocity or uniformity in velocity can increase the risk of contamination. 

In order to achieve a high level of product safety it is well known that good work practices in the bench are necessary, and having a clean environment and proper work clothing are of vital importance. Knowledge about the interaction between air movements and the dispersion of contaminants plays an important role. Wake regions and vortex streets can easily be formed behind obstacles. 

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It has also been shown, using visual illustrative methods, that accumulation can occur in the wake of people or objects, provided that the contaminants are emitted in the vortex region. Special consideration must be taken with instabilities and vortices generated by the working person. Vortices can also occur in empty open unidirectional airflow benches. 

When open unidirectional airflow benches are being used in production sensitive to contamination, a thorough function check should always he carried out before the start of production. Studies of air movements with visual illustrative methods give both quick and valuable information. 

In vertical unidirectional airflow benches, the area along the vertical wall in front of the operator is usually entirely or partially open. When the other side walls reach down to the working surface in the bench a stagnation flow with stationary vortices is usually created as shown in Fig. 10.53. 

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