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Particle Filter Series: What You Can't Remove

Jun 27, 2013

Particle Filter Series: What You Can't Remove

Particle filters may not be enough to protect IT equipment in all environments.

In this series, we’ve compared data center particle filters to those found in a home HVAC system, mainly to highlight the similarities for filter consideration. In either application, needless to say, particle filters remove particles. Some remove more particles or smaller particles, some less. The more particles a filter removes, the less efficiency-robbing dust there will be on heat exchangers, fans, and electronic circuitry. Unless we use HEPA or ULPA filters (as is done in semiconductor manufacturing to remove very small size particles), some will get through. What is needed, is a balance of cost, efficiency, pressure drop, an understanding of the cleanliness of the local environment, and the OEM’s requirements.

But particle filters cannot and do not remove gases such as oxygen and nitrogen. Of concern here is the fact that particle filters do not remove corrosive gases, those that can corrode exposed metal in electronic assemblies. Equipment manufacturers often take precautions to prevent corrosion. Coatings are sometimes used, as well as materials that specifically inhibit corrosion. But since the implementation of RoHS lead-free initiatives, the materials used for electronics are now less resistant to the effects of corrosive gases than they were before the regulation updates. Nobody wants to go back to using lead and other environmentally hazardous materials, so additional precautions may be needed.

What are the corrosive gases of concern? The most corrosive are two byproducts of combustion: sulfur dioxide (SO2) from automobile exhausts and home heating systems and hydrogen sulfide (H2S) from burning coal, pulp mills, landfills, or waste treatment plants. Needless to say, large urban areas known for poor air quality, especially communities that are situated "downwind" of coal-fired electrical generators. In these areas, the air can have elevated concentrations of these corrosive gases.  Other corrosive agents are the list of chemicals containing chlorine or chlorides. As seaside residents know, rust is an issue on metallic surfaces. Facilities near coastal areas, as well as those near water treatment plants or pulp and paper mills, are also areas of concern for chloride-based gases.

The effects of corrosive gases are everywhere: remember your grandmother’s silverware? What needs to be understood is the rate of corrosion.  To help with guidelines, ASHRAE’s Technical Committee 9.9 has issued a report to help IT professionals with the issue of corrosive gases. (Link to ASHRAE guidelines)

Table 2 from the ASHRAE report titled 2011 Gaseous and Particulate Contamination Guidelines For Data Centers shows a reference to the International Automation Society’s standard for electronic materials corrosion, ISA-71.04 (1985). The table describes copper corrosion activity per month and classifies it into four categories; the greater the amount of corrosive gases, the greater the amount of corrosion per unit time. Table 4 describes ASHRAE’s latest recommendations for Acceptable Limits of Gaseous Contamination. The ASHRAE guidelines add silver corrosion activity to the guidelines. This is due to the use of silver as a replacement for lead in RoHS compliant electronics, and the relatively high reactivity of silver to sulfur containing gases. ISA is considering adding silver to the ISA-71.04 standard. Electronic device manufacturers often reference these documents in their equipment warranty policies.


Too Small?

Particle filters, no matter how efficient they may be, cannot filter out these gaseous “particles”. They are simply too small (meaning the size of air molecules), and therefore cannot be removed with this type of filtration.

Several companies specialize in determining the level of corrosion on copper and silver strips of metal in data centers and other environments. Typically they employ metal strips referred to as “coupons” that are placed in the IT space for thirty days and sent to be analyzed. A report describing the level of corrosion per month as well as the types of gases present is the result.  In many locations, the levels of corrosive gases in the data center are low enough to be considered safe and within OEM specification.

       particle resized 600        

Figures 3a, 3b: Corrosion Classification Coupons, also known as Reactivity Monitoring Coupons from two suppliers for use in Data Centers based on methods describes in ASHRAE TC 9.9 and ISA-71.04 guidelines.  

(Link to Image)














Large urban centers in Asia and South Asia can experience high levels of corrosive gases in the local atmosphere. Keep in mind, urban centers in Europe and North America have also experienced high levels, especially during home heating season months. Figures 4a and 4b show results from the US EPA Acid Rain Program.  The program was founded under the 1990 Clean Air Act and shows dramatic decreases in SO2 (4a) and Sulfate (4b) concentrations. (Link to Source)  Data centers that are immediately downwind of sources will need to monitor corrosive gas levels at the atmospheric level to understand if they are in areas of concern. Those considering air side economizers will want to place some coupons outside, specifically near the make-up air intake.

Figure 4a (Above): SO2 levels in Eastern USA in 1989-1991 and 2007-2009

Figure 4b (Above): Sulfate levels in Eastern USA in 1989-1991 and 2007-2009

For IT managers in locations with high levels of corrosive gases, what's the best plan of action? Fear not, there are solutions.  The next and final post in this series will provide some guidance.

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