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The Gore filters used at Alliance have a 3- layer media construction. The first yellow layer captures most of the airborne dirt. Its efficiency and dust holding capacity are similar to existing lower efficiency MERV (Minimum Efficiency Reporting Value) or F-rated filters.
The middle white layer is a hydrophobic membrane which provides a HEPA rating of E12, i.e., removes 99.5% or greater of particles around 0.1 microns in diameter and even higher efficiency for larger particles. This e-PTFE (expanded polytetrafluoroethylene) membrane stops water ingress. This prevents captured particles from being forced through the filter during rain or fog events.
The membrane also prevents salt ingress thus preventing turbine corrosion. The third is a support layer added to provide strength and burst pressure resistance. This three-layer construction provides long life. The small particles that cause most of today's turbine fouling are less than 0.3 microns. As they do not weigh much, each HEPA filter would only need to stop less than 50 grams of these small particles per year.