How contamination makes contact
How contamination makes critical contact with ICT equipment
PM enters the data centre, moves around it, and finds its way into ICT equipment, because it is transported by the airflow through the room. Outside air, sometimes referred to as makeup air, is drawn through an air handling unit (AHU), where it is conditioned by being passed through a series of air filters. The conditioned air is then supplied to the servers through the raised access floor. Fans within the servers draw the air through the equipment cabinets. Air can also enter through insufficiently sealed cable entries, or even through wall gaps or cracks.
Contaminants settle onto vulnerable equipment lying within the air flow path in three different ways:
- Gravitational settling
- Diffusion movement
- Electrostatic attraction
Gravitational settling affects large particles the most, because it is a function of particle mass. It becomes insignificant for particle smaller than approximately 1 ?m in diameter, while those greater than 10 ?m have a short residence time and are likely to settle out before reaching the ICT equipment. However, strong air currents can prolong airborne residence time, even for larger particles.
The diffusion movement of particulate contaminants is caused by random collision of air molecules against airborne particles. These collisions allow particle migration from higher to lower concentrations. Diffusion is only significant for very small particles, with minimal effect on particles greater than 0.1 ?m in diameter. Larger particles mainly deposit on horizontal surfaces, while smaller particles tend to deposit equally on both horizontal and vertical surfaces.
Electrostatic attraction relates to the force that attracts oppositely charged articles, or causes them to settle on surfaces. Clothes clinging together with static electricity demonstrate the same effect.
Particles that settle on a surface tend to remain deposited, irrespective of how they arrived, because of the cohesive forces between the particle and the surface. However, activities such as floor sweeping or movement of tiles for maintenance can cause re-suspension.
Gaseous contaminants diffuse in the air to occupy the entire room volume, but differences in air density may cause stratification. The air movement controls the movement of contaminants.