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What Do Air Filters In My Data Center HVAC And CRAC Units Do?

May 23, 2013

Air filters in my Data center HVAC and CRAC: Why are they needed?

 Looking under the covers

So far, we’ve discussed the two most important aspects of air filters: particle removal efficiency and resistance to air flow (pressure drop). These factors generally oppose each other. More efficient filters remove more particles but offer more resistance. Low pressure drop filters are generally less effective in removing particles, especially smaller particles. But why do the HVAC and CRAC units need air filters at all? If they slow down the air, they make the fan work harder which uses more energy (electricity) and costs more money. But there’s not a lot of dust in the data center air, right? Unlike our homes, data centers and IT server rooms do not have fabric covered furniture, pets, or children that generate a lot of dust. So what’s all the fuss? Why is this an important issue to discuss? After all, maintenance personnel or a service company can clean up dust bunnies when they clean the room, and isn’t that sufficient?

To be clear, HVAC and CRAC unit manufacturers don’t install particle filters in their units to help keep the data center "clean" in the traditional sense. If that were the case, they would save on the expense of the filter, the added cost of additional fan capacity, and the electrical usage needed to overcome the filter’s pressure drop. Specific to data centers, the filters are there to protect the cooling coils from accumulating microscopic dust particles. These particles can attach themselves to the heat exchanger and will form a thin blanket of insultation over the surface. This dust film will make it harder for the air molecules to come into contact with the heat exchanger, thereby reducing the effectiveness of the unit. If the issue is severe enough, the unit would not be able to supply the rated amount of cooled air, meaning that the electrical equipment served by the HVAC and CRAC units would be operating at higher temperatures than desired. Over time, this could lead to poor performance and even heat related failures.

 

Cooling Coils (Source 1)

More importantly, particles can affect the performance of electronics. Similar to heat exchangers, the particles can form thin films of dust on circuit components, negatively affecting their ability to dissipate heat. Some particles can have chemical or physical reactions with exposed electronics, causing problems which will be covered in a later piece.


Air Filter Array (Source 2)

Changing the CRAC air filters when they become dirty is important to their proper operation, and ultimately saves energy. Dirty filters have more pressure drop than clean filters, and thus require more fan energy to operate at the desired flow. Very dirty filters may overcome the fan’s capacity, resulting in less air than is typically required for proper cooling. In a raised floor design, this will likely result in less cooled air at the end of the aisle (most distant from the CRAC unit) and the electronics there will be exposed to higher operating temperatures as a result. (A side thought: Do you have any experience on the energy usage, operational issues, or failures at the end of a cooled aisle vs. the center and end near the CRAC unit that you would be willing to share?).

And by the way, banging the filters to remove the dust or even vacuuming them is not an effective way to keep them clean. Many particles are deeply lodged in the filter matrix and are difficult to remove. Even worse, if these particles are displaced, they can eventually work their way through the filter and cause a number of issues (as outlined above).

Air filters are needed for proper operation of HVAC and CRAC units, and beyond this implementation, these filters must be changed when recommended (by the manufacturer) to help to maintain efficient operation. Also, routine maintenance will insure valuable electronics and the mission critical data that may be contained within. The next piece in this series will discuss filter ratings and why they are important for your Data Center.

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