The ventilation strategy of cleanrooms must take into consideration numerous factors that may affect the smooth operation of the serviced area. Besides controlling the temperature and the humidity of a space, cleanroom ventilation systems should strictly regulate the concentration of various air contaminants for example solid particle or gaseous contaminants. Ideally, a properly designed cleanroom ventilation system should ensure that the strict air quality criteria of the conditioned space are satisfied at the minimum possible energy consumption and operating cost.

Cleanroom indoor conditions are becoming a vital part of the production process of various industries as well as in surgery and patient isolation rooms in hospitals. Examples of industries requiring cleanroom conditions include the production of pharmaceutical and semiconductors.

Solid particle control

Solid airborne particles are produced from various sources. Small scale living or dead organisms are constantly transferred by air streams whereas industrial processes, like combustion or chemical processes, also contribute significantly in the production of solid airborne particles. Additionally, particles are generated from the human body in the form of skin cells, respiratory bacteria etc. In cleanrooms, solid contaminants can be generated from the activities and equipment within the conditioned space; they can be transferred from the outside through doors and windows or even through fresh air supply from the HVAC system. The external sources of contaminants can be very effectively controlled by over pressurizing the conditioned space in order to ensure that outside air is not allowed to infiltrate and by installing HEPA filters on all supply ducts. HEPA (High Efficiency Particulate Air) filters is a type of filter made of interlaced glass fibers with very high filtering efficiency that are essential for any space that has strict contamination control requirements. They are capable of removing 99.97% of solid particles with a size of at least 0.3 microns.

The major internal source of contaminants is the personnel working in the conditioned space, as the human body releases millions of particles every minute. This source of contaminations is mainly limited by the protective gowns worn by the personnel and by appropriately designing the ventilation system of the cleanroom to project air streams towards the personnel to drive the unwanted airborne particles away from critical areas that need to be sterile.

Ultraviolet (UV) disinfection systems are gaining popularity as a complimentary technology to the traditional HEPA filters. UV systems can provide highly effective air disinfection in ventilation ducts as they can eliminate a wide range of pathogenic bacteria and viruses without forming harmful byproducts. As a result, UV systems minimizes the spreading of airborne infections and helps provide a safer indoor environment.

Unidirectional flow

A unidirectional flow pattern, often referred as laminar flow, ideally produces parallel streamlines which can be vertical or horizontal, depending on the chosen cleanroom ventilation strategy. Regardless of the air pattern, the main aim of the ventilation system is to create uninterrupted streamlines with constant velocity that transfer the unwanted particle contaminants away from critical areas. Practically, it is impossible for the streamlines to remain uninterrupted due to the presence of objects inside the cleanroom or due to movements by humans. However, even in the presence of obstructions, laminar flow systems are still a very effective solution for cleanroom to achieve the desired indoor conditions.

In most cases, air enters a cleanroom through laminar flow diffusers with HEPA filters, that occupy the entire surface area of the ceiling of a cleanroom. Air is returned either through openings at a low level on side walls or through openings on raised floors. In horizontal flow systems, air is introduced from one of the sidewalls and is returned through an opening on the opposite wall. The main disadvantage of the horizontal flow cleanroom ventilation strategy is that air closer to the opposite wall, though which air is removed, is significantly more contaminated compared to the air next to the supply wall. As a result, this dictates the internal configuration of the cleanroom, as the most critical processes have to take place closer to the supply wall and the least critical ones closer to the return wall.

Sizing of cleanroom ventilation systems

Typically, a cleanroom ventilation system is a much more energy consuming system when compared to the ventilation system of a similar sized common space. This is mainly due to the fact that the HVAC system of a clean space, besides satisfying the heating and cooling loads of the space must also remove air contaminants at the desired rates in order to achieve the required cleanliness class.

Cleanroom design is its own area of expertise among HVAC design engineers since numerous distinctive factors need to be taken into consideration for a cleanroom ventilation system to be properly designed. If the sizing of a cleanroom ventilation system is based on traditional rule of thumb values, it is very likely that the designed system will end up being either oversized or undersized. Each case has to be treated as a unique case, taking into account the rate of production of particle contaminants, the rate of particle entry through the fresh air supply, the rate of air leakage etc. To this end, numerous sizing tools have been developed that allow system designers to have a more scientifically accurate approach when specifying the required products for a cleanroom ventilation system instead of using the traditional rules of thumb.