the operating efficiency of indirect evaporative cooling processes and indirect evaporative cooling apparatus employing a dry side channel and a wet side channel separated by a heat exchange plate are improved by placement of holes in the heat exchange plate . further improvements are obtained when the flow direction in the wet side channel is cross - current to the flow direction in the dry side channel . placement of desiccant materials in the dry side channel also serve to improve the operating efficiencies of these processes and apparatus .