Clean CRAC Filters, Clean Data Center - Right?

You swipe your key card and step into the data center. The first thing you notice is the cool temperature, the whine of fans and the constant rush of air. The computer room air handlers (CRAH) or air conditioners (CRAC) units are hard at work keeping the air cool, but are they keeping the air clean?

Every type of air handling equipment has an air filter. Since the air in the computer room is mostly isolated from the rest of the building; and, since it is frequently passes through the air handler filters, it should be clean, right? Well, it depends what we mean by “clean”.

Data centers are designed to be cleaner than the average office space. This is accomplished by limiting access to the room, keeping out obvious contaminants (food, smoking, etc.) and isolating the air in the room. However, there are exceptions in each area: various people still work in the room; contaminants from dead skin and clothing fibers to cardboard boxes and dirty equipment are carried in; plus some amount of air from the building’s main ventilation is intentionally introduced to the room. The net effect is a continuous flow of contamination into the room.

Most of the small, light particles are initially airborne. As a result, most will come in contact with the air filters in the room. Unfortunately, most simply pass through those filters. To understand why, you need to know your MERV. MERV stands for the Minimum Efficiency Reporting Value and it is a measure of an air filter’s ability to trap particles of various sizes.

Many commercial air handling units come standard with MERV 8 rated filters. According to the ASHRAE Standard 52.2, MERV 8 filters trap 70% of particles 3.0 microns and larger. Some data centers will use MERV 11 filters—these trap up to 80% of particles 1.0 micron and larger. So, 20-30% of these relatively large particles pass through the filters along with all of the smaller particles. These airborne particles eventually settle throughout the room. Some will accumulate directly on the heat sinks and fan grills within the IT equipment. More will coat the floor and other areas until disturbed and become airborne again. Clearly, standard air filters are not enough to meet the data center definition of “clean”.

Commercial Grade HEPA VacuumsThere are more efficient filters. For example HEPA filters have a MERV rating of 17 or higher. HEPA filters remove 99% of all particles as small as 0.3 microns. Such filters are common in true Cleanroom applications, but are impractical for data centers due to their high airflow impedance. What are the options?

Fortunately, there are professional grade vacuums that incorporate HEPA grade filters. A rigorous data center cleaning program will include the use of HEPA vacuums to capture and remove particulate contaminants throughout the room. The cleaning frequency should be consistent with the rate at which contaminants are introduced.