Air distribution in class 7 and 8 cleanrooms

Stewart Horne, from Krantz Systems, highlights the benefits of using induction radial diffusers by comparing them with the tangential flow louvre face diffusers

The cleanroom industry, and in particular the pharmaceutical sector, has traditionally used louvre faced diffusers. This allowed the flexibility of arranging the air flow with the objective of providing good room scavenging.

This proved effective up to a point, but was not without certain restrictions particularly with reference to the spacing and achievable room air change rates. Figure 1 shows a typical high induction radial system. The system offers the following advantages:

The radial diffuser operation, shown in Figure 2, indicates the radial air distribution and features. Its operation involves multiple air jets at high velocity and at low momentum. The room air is induced between air jets and the rapid mixing slows the air jets and quickly dilutes dt.

A feature of terminals is the temperature gradient. This, and the uniform temperatures achieved in the occupied zone, are clearly demonstrated in Figure 3.

The tangential louvre face system, as illustrated in Figure 4, shows that the room air patterns, position and induction ratio all fall below the standards of the high induction system. The air velocity at the diffuser is 2 – 5m/s, with a vertical air projection. There is minimal entrainment of the room air and the cleanest air below the diffusers is ideal for localised cleaning conditions. The very low velocities between the diffusers are stagnant areas and, therefore, high contamination areas.

To further emphasise the spacing and air change rate advantages of the high induction system, a comparison of the rates is shown in table 1. The air velocity at the diffuser is 2 – 5m/s. There is room air entrainment and the coander effect maintains a high velocity. There are high velocities along the wall surfaces and high room velocities below the collision points. The diffuser positions are, therefore, critical to avoid high room velocities.

high induction terminal devices

There is a significant increase in the use of high induction terminal devices in current cleanroom design. All the features design engineers and the cleanroom personnel are constantly striving to achieve, can now be met by the use of high induction radial outlets.

The benefits of high induction terminal devices can be summarised as follows:

A final benefit is the visible and practical effect. The radial diffuser is pleasing on the eye, it attracts the attention of the architects during the early planning stages. Because of the flexibility on the spacing, the fight for ceiling space with the electrical consultant becomes less of an issue.

There is also the benefit of it being very end user friendly. It is far easier to ensure the necessary cleaning due to the curved surface design and it is free of the many crevices created by the welded louvre face. High induction type diffusers provide both the designer and the client with a better engineered solution.