On cold and cool days, the Oasis heat exchanger operates dry and acts as an air-to-air heat exchanger. Scavenger air indirectly cools the air through normal heat exchange, without the use of any water. Once the ambient temperature rises to a certain point, the heat exchanger will not be able to provide enough cooling while operating in dry mode. When this happens, water is pumped from the sumps (internal or external) to the air handler’s spray nozzles that wet the outside surface of the exchanger tubes, cooling them with a thin film of water. The scavenger air evaporates water on the exterior of the tubes, which causes heat to be extracted from the recirculating air flowing inside to the tubes. In this evaporation mode, the heat exchanger will be able to cool the recirculated air against ambient wet bulb, thus providing cooling even when ambient temperature is high. The system is suited for data center cooling, among other uses.