With the ongoing climate change and the research in the field of hybrid- and fully-electric aircraft engines, the interest in high-performing cooling systems for these engines rises. The main part of all cooling systems is the heat exchangers. For the design of those aero engines, the influences of the heat exchangers on the performance, aerodynamics, weight, installation space, and other disciplines must be included from the beginning. This paper’s main goal is to define a preliminary design process for a heat exchanger suitable for application in aero engines. A sizing or a rating approach shall be available to either calculate the total dimensions or the total heat transfer of the designed heat exchanger. Moreover, the design process shall be simplified for the user to apply and asses. A Corrugated Louver Fin and Rectangular Offset Strip Fin heat exchanger are chosen for this work. For the definition of the heat exchanger geometry, non-dimensional design parameters are derived. Correlations from experimental data are used to calculate the Colburn and Fanning friction factor. Specific properties relative to the volume of the heat exchanger are calculated right after the geometric definition. These design properties are used for an early assessment. The properties describe the heat transfer, pressure loss, and mass relative to the volume to asses for the specific application in aero engines. The dependencies of the design method are the geometry, the flow arrangement, the operating conditions, and the total dimensions, respectively, the total heat transfer. A sensitivity study is done to work out the influences of each design parameter on the design properties of the heat exchangers. Moreover, a recommended process for a specific design parameter choice is given. The scope of the design properties of each heat exchanger type is then compared. For the Corrugated Louver Fin heat exchanger, higher heat transfer and, at the same time, lower pressure loss and mass per volume ratios are possible. After an introduction, the design parameters are explained and derived in one chapter. In the following chapter, the whole design method is described. In chapter 4, the design parameters and design properties are investigated and the heat exchanger types are compared. In the end, a conclusion follows.