ximation, taking as a reference the worst card position, where the heat is gener- ated by the transmitter) and that is in contact with a heat sink is characterized by a thermal resistance OT, SO that the relationship between the temperature difference AT between the card and a static heat sink and the power Pd the card can dissipate can be written as AT = OTPd (6.105) Naturally, the thermal resistance depends not only on the system element, but also on the way it is connected to the heat sink. In our case, where there is a forced air flow that works as heat sink, there are technical graphics that, starting from measurements and analytical evaluations, give the thermal resistance existing between a subsystem and the air flow in the hypothesis of a complete immersion of the subsystem in the air flow, SO that the air is in contact with every part of the subsystem. One of these graphics is reported in Figure 6.48. At this point the procedure for the design is the following: Construct the thermal equivalent circuit remembering to insert all the heat resis- tances (also that related to a simple flow of the air along a space in the rack, since heat is exchanged with the metallic rack walls; Figure 6.49). For each subsystem evaluate the desired working temperature and the dissipated power. Construct the heat circuit model of the system also considering the incom- ing and outgoing air temperatures that are determined by the environmental conditions; the heat transfer model coming out are similar to the scheme of Figure 6.49, where a subrack with a few mounted cards is represented with a heat circuit. For each subsystem evaluate starting from Equation 6.90 and the heat circuit model the heat resistance that allows the power dissipation at the required temperature. Thermal resistance (C/W) FIGURE 6.48 Example of plot providing the thermal resistance versus Air speed (m/s) the air flow speed in conditions of laminar flow. Telecommunication Networks Output air MMM temperature Card 1 Air flow direction Card (n-1) Flow thermal 