Open Unidirectional Air Flow Benches

10.3.4.1 General

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 m s_1. Some examples of typical arrangements of open unidirectional airflow benches are shown in Fig. 10.51.

Open Unidirectional Air Flow Benches

J

HEPA

Filter

Open Unidirectional Air Flow Benches

HEPA filter

подпись: hepa filterVertical airflow

FIGURE 10.51 Examples of typical arrangements of open unidirectional airflow benches.

In order to maintain a high level of air quality, 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-1 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.

Integrity testing of HEPA filters should be performed by using an aerosol challenge (dioctylphthalate (DOP) or similar). Initial testing, when the benches are installed, to detect leaks around the sealing gaskets, through the frames, or through the filter media is essential. Thereafter, integrity tests should be performed at suitable intervals. For example, in pharmaceutical production it is common to perform such testing twice a year. When the leak­age through the HEPA filter(s) is less than 0.01% of the upstream aerosol challenge level, acceptable conditions are considered to exist.

10.3.4.2 Principle

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.

Dispersion of Airborne Contaminants

Ljungqvist and Reinmiiller11 have described contamination risks in clean environments and have estimated the “critical contamination region” in un-

Point

Source

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1 4

Uniform

Velocity

45cm/s

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подпись: i“-*..30 cm

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Working surface

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FIGURE 10.52 Critical contamination region in a uniform parallel flow field. Qualitative solution of the diffusion equation in a velocity field with the velocity equal to 0,45 m s’1, diffusion coefficient Equal

Disturbed and disturbed parallel flow fields with dispersion from a fixed con­tamination source. A qualitative solution in an undisturbed parallel flow is given in Fig. 10.52, when the velocity is 0.45 m s_1 and the distance between the working surface and the point of source is 30 cm.

In a vortex (rigid-body rotation), Ljungqvist12 has shown that the mean value of the concentration over the entire region inside the streamline where the point of emission is situated is considerably higher than that of the outside. This allows us to use the concept of contamination accumulation in the context of vortices.

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.

10.3.4.3 Air Movements

Air Movements in Empty Benches

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.

To avoid such vortices, the side walls must be designed with openings. At a long bench designed with equally large openings on the longitudinal and opposite wall, the flow can be considered as two-dimensional. If it is further assumed that the flow

Open Unidirectional Air Flow Benches

May be regarded as turbulence free, it is possible to find an exact solution for the Navier-Stokes’ equations. The solution was first given in a thesis by Hiemenz.1

This plane flow leads to a stagnation point in the middle of the bench on the working surface. Furthermore, it is worth noting that the boundary layer is propor­tional to the square root of the kinematic viscosity, and that the thickness of the boundary layer does not vary along the working surface. In reality, a stagnation re­gion often arises where the height and appearance can vary as shown in Fig. 10.54.

In the stagnation region shown in Fig. 10.54, contaminants will move in an irregular and often unpredictable manner. This means that production with exposed products in stagnation regions gives rise to contamination risks.

In the unidirectional air flow of an open bench, a vortex street is easily created behind small obstacles. Such an obstacle can be as insignificant as a small lamp or a fixture connecting HEPA filters. Ljungqvist, et al.14 have, with the help of isothermal smoke, visually depicted the air movements behind such a horizontal 30 mm wide fixture at an air velocity of 0.45 m s_1. The observed flow pattern is schematically shown in Fig. 10.55.

The flow pattern in Fig. 10.55 has a violent turbulent region, character­ized by a vortex street and two free vortices rotating in opposite directions

Open Unidirectional Air Flow Benches

I

Open Unidirectional Air Flow Benches

(long side).

Close to the working surface. These observed vortices are known as an irrota — tional or free vortex and sometimes as a potential vortex. This vortex is char­acterized by the fact that the velocity varies only with the radius and increases toward the center.

By using a smoke photography technique one of these free vortices (irrota — tional) in Fig. 10.55 has been visualized (see Fig. 10.56)

Open Unidirectional Air Flow Benches

Open Unidirectional Air Flow Benches

Air Movements in Benches with Obstacles

The air movements outside the vortex region in the bench shown in Fig. 10.53 is depicted visually with smoke at an air velocity of 0.45 m s_1 In Fig. 10.57.

If a bottle is placed in the bench, a wake is easily created in the region with horizontal flow. This is depicted visually with smoke in Fig. 10.58. If the bottle is situated close to the opening of the unit, ambient air will be entrained into the clean zone in the bench. The length of the reversed region can be esti­mated to two to three times the diameter of the bottle, and can reach twice this length when the bottle is situated just beside the side wall.

The Reynolds number, which is directly proportional to the air velocity and the size of the obstacle, is a critical quantity. According to photographs presented elsewhere,15,16 a regular Karman vortex street in the wake of a cyl­inder is observed only in the range of Reynolds numbers from about 60 to 5000. At lower Reynolds numbers, the wake is laminar, and at higher Rey­nolds numbers, there is a complete turbulent mixing.

In the case shown in Fig. 10.58, the Reynolds number is approximately 2700. In the situation shown in Fig. 10.55, where the arrangement between two filter modules forms a flow obstacle, the Reynolds number is approximately 900.

Open Unidirectional Air Flow Benches

Open Unidirectional Air Flow Benches

However, one should be cautious when comparing the Reynolds number from regular Karman vortex streets with the Reynolds number calculated from factual situations in clean benches as the airflow from behind an obstacle is usually not the typically formed Karman vortex street predicted for an in­definitely long circular cylinder. The wake situations during actual conditions often seem to have a three-dimensional structure.

If a hand is placed over the smoke source in Fig. 10.57, a wake region is created in the vertical flow field, as shown in Fig. 10.59.

It is possible that ambient room air can reach the critical region in the bench due to created wakes. Such a wake region is depicted in Fig. 10.60 for a hori­zontal flow bench with small bottles and an air velocity of 0.45 m s“1.

Essentially, vortices caused by people are of two kinds. Relatively stable and stationary wakes are created by the body. Unstable and nonstationary vortices arise

FIGURE 10.5?

 

Dispersion of smoke behind a hand (arm) placed in the bench shown in Fig. 10.57.

 

Open Unidirectional Air Flow Benches

Open Unidirectional Air Flow Benches

Open Unidirectional Air Flow Benches

FIGURE 10.61 Dispersion of smoke In a horizontal unidirectional flow bench when operator’s hands are in the upper position (undisturbed smoke dispersion).

As a consequence of the movements of the body. In this respect, n is obvious that the movements of the hands and arms play a significant role in creating unstable situations. Work situations in a clean air bench with a horizontal unidirectional air­flow are demonstrated by using a smoke technique in Figs. 10.61 and 10.62.

10.3.4.4 Conclusion

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

In order to reduce the influence of unfavorable stagnation regions and vor­tex structures with their risk for accumulation of contaminants, tests should be carried out to characterize the functioning of the bench. In connection with these tests, induction tests should also be performed. Here smoke (particles) generated outside the bench and the probe of a particle counter placed inside the bench in the critical regions can give valuable information.

Investigations should also be carried out with equipment in place and with test persons performing activities according to a standardized plan of movements.

The above approach, with visualization of air movements and particle challenge tests together with calculation of a risk factor, presents a method, the LR method (Limitation of Risks), to evaluate the risk of human interfer­ence in the critical zone of the bench. It also gives valuable information con ­cerning potential weak links. A more thorough description of the LR method is given in Ljungqvist and Reinnniller.I1-37

Open Unidirectional Air Flow Benches

Hands are In motion.

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