A heat exchanger for cooling fluid passing through the heat exchanger. The heat exchanger includes a hot fluid flowpath and a cold air flowpath. At least a portion of the cold air flowpath has a thermally conductive wall transferring thermal energy from hot fluid flowing through the hot fluid flowpath to cold air flowing through the cold air flowpath. The cold air flowpath includes a separator for separating ice particles from the cold air flowing through the cold air flowpath. The separator includes a passage having a bottom wall, an end wall, and a side wall including a porous wall through which a majority of cold air entering the separator passes. The end wall has an ice particle discharge opening adjacent the bottom wall permitting a minority of the cold air entering the separator to carry ice particles separated from the majority of cold air through the opening.