approach) p is the pressure in the direction of the fluid motion (assumed laminar) P is the fluid density g is the gravity constant on the earth surface Q is the heath density T is the local temperature Cp is the air specific heath at constant pressure It is apparent that, even if the nature of the air motion in a well-designed cooling system allows several simplifications in the motion equations, solving the aforementioned equa- tions system with suitable boarder conditions is a formidable mathematical problem. The system designer has two possible options at his disposal. The first consists in using a trial and error design procedure supported by a specific software tool that allows, once the air flow is designed and the power dissipation of each part of the system is fixed, to solve the aforementioned equations, numerically SO deriving the regime temperature of all the system parts. Commercial tools exist to do that and the probability of finding a suitable technical solution in a short time is completely dependent on the experience and physical insight of the designer that is able to start from a tentative solution sufficiently near to the optimum. On the other hand, if some support in the first design is to be devised before analyzing the proposed structure with a software tool, either Equation 6.104 have to be drastically simplified or some heuristic rule has to be used. A heuristic model that is largely used is that of concentrated constant heat circuit, that in electronic engineers groups is easily adopted due to its similarity with concentrated electronic circuit analysis. In this model, every point in which a heath exchange is performed is modeled as a con- centrated constant heat circuit, SO talking in general of heat impedance and, in particular, of heat resistance, for example. Transmission Systems Architectures and Performances In this simple model, each part of the system that can be considered uniform from a thermal point of view (e.g., an interface card could be considered uniform in a first appro