In a one-phase heat transfer process, there is no phase change in the media. An example of such a process is a water-to-water application where the water flow on side 1 changes temperature from 30 to 50°C and the flow on side 2 changes temperature from 65 to 45°C, i.e. without a phase change. The most commonly occurring one-phase applications for BPHEs are water-to-water and oil-to-water. The purpose of the BPHE in oil applications is to cool engine oils, hydraulic oils or similar with water.