Peter Dyment, Technical Manager – Camfil Ltd, discusses the purpose and function of air filters and how they should be best utilised to ensure optimum data centre performance.
Data centre reliability is a top priority for data centre leaders and so must be acknowledged and ensured. Air filters must be best positioned to deal with the source of airborne contamination. High concentrations of airborne contaminants combined with higher humidity levels will usually show increased rates of equipment failure in data centres.
Air filters used to clean incoming supply air from outside and recirculated inside air must be capable of removing airborne particles and also corrosive gases when present. Tin or zinc whiskers can be an internal source of airborne contamination as well as white rust particles.
There is an increasing use of free cooling to reduce energy consumption in data centres. Use of outside air at lower temperature introduced directly into the data centre server hall can offer large reductions in Power Usage Effectiveness (PUE).
The current recommended particle filter class for data centre supply air systems is minimum BS EN ISO 16890:2016 ePM1 50% and for recirculated air systems is coarse 70%, the equivalent as recommended by ASHRAE in 2011 Gaseous and Particulate Contamination Guidelines for Data Centres. Air monitoring can be used to determine the concentrations of particles and corrosive gases present. Some air monitoring devices now include air cleaner/purifier control functions.
It is recommended that particle concentrations in data centre operational equipment areas align to meet the requirements of ISO Standard 14644-1 Class 8.
For corrosive gases, it is recommended that monitored levels of corrosion meet the requirements of ISA Standard 71.04-1985 to meet class G1 Mild. Where monitored levels of corrosion exceed this, then gas phase filtration needs to be used. Molecular gas filtration can be fitted to the main supply ventilation system or applied by use of standalone recirculation units. To target individual contaminant gases, filters tested to BS EN ISO 10121-2:2013 can be used. Broad spectrum filters can be used to target a combination of gases.
Air filters consume significant amounts of electrical energy through the fan motor, so selection of low energy air filters is recommended. These filter classes have the lowest operating pressure drop for their rated airflow. The Eurovent energy rating for air filters gives an A+ rating for the most energy efficient air filters. Use of Life Cycle costing can confirm the air filter selection with minimum energy use and lowest carbon effect.
For particle filters, best economic filter change can be made when the initial operating pressure drop of the air filter doubles at the given reference fan speed. Air filter pressure drop monitoring is recommended.
Ensure all contaminated air-conditioning filters are correctly removed from the live environment before cleaning; the filters should be placed into bags and sealed to prevent any particulate matter becoming airborne. The removing of contaminated filters inside the live data centre should be performed in a controlled and professional manner. Wearing of PPE, a mask, glasses, gloves and overalls is of course required.
COVID-19 air filters for air cleaning and standalone filter units
Increasingly, airborne-infectious particles are being seen as the primary route of transmission of COVID-19 infection. Waves of infection and new variants are constantly appearing.
Therefore, the use of air filtration as a mechanical intervention is sensible when people are present and occupying the data centre building indoor areas.
Particle air filters tested to International Technical Standards EN1822 and ISO16890 offer accurately measured performance of particle removal efficiency.
For standalone air cleaners and air purifiers, EN1822 Hepa filters class H14 are recommended. These are 99.995% efficient at MPPS most penetrating particle size which is typically about 0.12 to 0.16 micron (about the same size as the COVID-19 virus).
For general HVAC air filtration, ISO16890 at least ePM1 80% total efficiency filters are advised. PM1 particles are those sized at one micron and below and include the virus size and small airborne droplet aerosol sizes that stay airborne for long periods. ISO16890 allows calculation of multistage filter efficiency. So for example, two stages of ePM1 70% gives 90% PM1 efficiency and three stages give over 97% PM1 efficiency. Removal of infectious particles from air to this efficiency will give people a high-level of protection against infection when they breathe in the cleaner air.
Tightly sealed air filter mountings to EN1886 guidelines will ensure high filtration efficiency is achieved in HVAC systems using ISO16890 tested air filters.
The latest ASHRAE Core COVID-19 guidance advises using a combination of filters and air cleaners for air recirculated by HVAC systems. Air monitoring (PM1, PM2.5) and control devices are increasingly being used to confirm delivery of clean, healthy air.
Protecting people will also afford greater protection to the data centre plant and improve staff working efficiently.
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