Which way should the water separator be fitted to an air compressor?
the arrow (triangle) always points in the direction of air flow (from the compressor to the tool). In this case you would attach the filter (arrow side) to the air line usually after a valve (but not always) and on the opposite side a hose (or whatever accessories there may be). Use Teflon tape on all threads And don't use paste which may clog tool ports. And a good habit is to drain the tank of condensation regularly.