Chiron engineers used FloVENT in addressing the effect of HEPA filter layout and exhaust grille layout on minimizing upward flow in their vial filling room.
The bulk of the cost (capital & run-time) associated with the operations of a clean environment within the pharmaceutical and health-related industries is dictated by the desire to manufacture a high quality product and the need to satisfy the regulatory bodies such as the FDA.
Mechanical ventilation design plays an extremely important role in determining the pass or fail criteria. At present the rules defining the best ventilation design practices are based on simplistic historical data that are often wrong. The leading corporations in these industries tend to have their own in-house mechanical expertise who scrutinise proposed designs, examine the final set up and verify original assumptions prior to final acceptance.
Why? Because the performance of a cleanroom is defined by a set of complex interactions between the airflow, sources of contamination and heat, position of vents, exhausts and any objects occupying the space. Consequently changes to any of these elements will affect the outcome, invalidating the built in assumptions of the empirical configuration.
In the ideal situation it is necessary to:
- predict the performance of your clean environment prior to construction or mock up;
- be able to change the layout of the room and know the effect of such changes;
- use only enough air changes per hour (energy conservation) to satisfy requirements;
- achieve the above in the shortest possible time; and
- most importantly, do so at a lower cost.
The only scientific way of achieving all the above conditions, is by solving the physics of the problem using numerical methods known as Computational Fluid Dynamics (CFD). In the mechanical ventilation design fraternity they are commonly known as airflow modelling techniques.
Case study: the vial filling room
A recent case study carried out for the Chiron Corporation, USA, demonstrates the use of CFD methods. The aim was to assess the ventilation performance in a Vial Filling Facility (PDU fill room) and its subsequent optimisation.
As one of the leaders in the field of biotechnology, Chiron’s in-house mechanical experts have had an excellent record of constructing and maintaining their facilities. Their primary objective is the manufacture of safe products. In fact, the PDU fill room in question has had over 15 years of successful performance. The use of airflow modelling reflected their visionary approach to solving what they consider to be a more efficient way of optimising the layout of new fill machines and shortening installation time.
It must also be noted that, since the study, Chiron engineers did bring FloVENT modeling in-house as part of their internal tool sets.
The example of the vial filling room addresses the effect of HEPA filter layout and exhaust grille layout on minimizing upward flow in the room. The flow visualisations are coloured according to vertical velocity. This is a measure of air flow uniformity (laminarity) and thus better ventilation design.
To provide the aseptic environment the key process areas are ventilated by HEPA filters from above. Above the fill equipment, and in the laminar flow hood, the air is drawn from the room through high-level returns. The air is then filtered and resupplied to the room through the HEPA filters. The remainder of the HEPA filters are supplied from the main plant, with the return air exhausted from the room at low level.
Other processes (eg the autoclave and oven - located on the long wall nearest the viewer) are not, however, completely surrounded by HEPA filters and so are more vulnerable to non-uniform airflow.
The design team at Chiron are upgrading the fill machine and are considering the possibility of using the opportunity to enhance the aseptic performance of the room. The chosen method was to use airflow modelling to:
- understand the current performance,;
- assess the impact of the introduction of the new equipment on the ventilation performance; and
- refine the design to minimise and contain any areas of upward flow.
The consensus of opinion is that such a design outcome would lessen the risk to the airflow’s laminarity around the vials.
This case study deals with the latter part the optimisation. The current configuration with the new fill machine can be seen to perform well within the main ‘laminar’ flow areas. However, there are some flow features which are considered less than optimal.
The figure above shows the air from the main laminar flow panels. In this case, blue represents down flow, with the colour range from blue through green and yellow to red representing vertical velocity components from -0.7 m/s (-140 fpm) to 0.7 m/s (140 fpm), so yellow and above indicate undesirable upward flows. In this view, air from the laminar flow hood can be seem to hit the floor, turn back upwards into the LAF itself and also flow across the room before flowing upwards in front of the autoclave and oven.
Less visible in this view is the upward flow behind the pillar adjacent to the fill machine, or the upward flow to the high level returns above the fill machine. Both are a potential source of turbulent diffusion against the downward flow.
The process of redesign is iterative, inspecting the features of the flow and using engineering judgement to modify the configuration to resolve the perceived problems. In this case (see right), the high-level returns are removed and the HEPA filter layout in the open part of the room is redesigned. Removing the high-level returns has, as expected, reduced the amount of upward flow, while placing the HEPA filters closer to the autoclave and oven protects them. Selective placement adjacent to the fill line has also reduced the risk of non-laminarity.
In some instances there may be a cart under the LAF. This figure (left) shows the effect of the obstruction caused when the cart is almost as large as the LAF. Low speed and slightly upward flow are created near the fill machine as well as upward flows between the LAF and fill line.
One solution is to provide local exhaust around the corner of the room below the LAF (see below). This reduces the quantity of air moving off across the floor towards the fill line, thus reducing the upward flow. This solution can be seen to have a positive effect both with and without the cart present.
The facilities engineers have identified a significantly improved configuration for the cleanroom without interfering with the normal manufacturing process. They further intend to use the technique, not only to improve the cleanliness of the environment, but also to establish whether changes to the design and layout of the equipment and protective curtains will enable operatives to work more effectively in the room.
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