1. An equipment enclosure free air cooling assembly, comprising:an air outlet arranged to deliver air that has passed completely through said free air cooling assembly to inside an interior equipment chamber of an equipment enclosure;
an air inlet arranged to receive said air from outside said equipment enclosure;
an air flow pathway through which said air entering said air inlet must pass to reach said air outlet;
a primary air filter removably disposed in said air flow pathway, said primary air filter being configured to filter dust from said air;
an adjustable screen assembly having a screen filter element disposed upstream of said primary air filter in said air flow pathway;
an adjustment mechanism operable to selectively utilize different regions of said screen filter element for filtering said air passing through said air flow pathway at different times;
said screen filter element being a secondary air filter configured to perform pre-screening to remove large particulates from said air before it reaches said primary air filter while passing said dust to said primary air filter, said large particulates being larger in size than said dust filtered by said primary air filter, thereby extending a useful service life of said primary air filter; and
said free air cooling assembly being operable to receive said air from outside said equipment enclosure into said air inlet, pass said air through said adjustable screen assembly and said primary air filter via said air flow pathway, and deliver said air that has passed through said adjustable screen assembly and said primary air filter through said air outlet to inside said interior equipment chamber.