Cooling fins are usually attached to the condenser

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Unformatted text preview: ns, or spirals. They can also be made as a flat layer with a thickness of about 0.3 cm. Flat heat pipes are very suitable for cooling high-power-output (say, 50 W or greater) PCBs. In this case, flat heat pipes are attached directly to the back surface of the PCB, and they absorb and transfer the heat to the edges. Cooling fins are usually attached to the condenser end of the heat pipe to improve its effectiveness and to eliminate a bottleneck in the path of heat flow from the components to the environment when the ultimate heat sink is the ambient air. cen58933_ch10.qxd 9/4/2002 12:38 PM Page 550 550 HEAT TRANSFER Evaporator end FIGURE 10–37 Variation of the heat removal capacity of a heat pipe with tilt angle from the horizontal when the liquid flows in the wick against gravity (from Steinberg, Ref. 18). TABLE 10–6 Typical heat removal capacity of various heat pipes Outside Diameter, cm (in.) 0.64(1) 4 0.95(3) 8 1.27(1) 2 Length, cm (in.) Heat Removal Rate, W 15.2(6) 30.5(12) 45.7(18) 15.2(6) 30.5(12) 45.7(18) 15.2(6) 30.5(12) 45.7(18) 300 175 150 500 375 350 700 575 550 ∆T = 3°C 180 W Heat pipe 0.6 cm L = 30 cm FIGURE 10–38 Schematic for Example 10–8. Power handling capability, % 100 Coarse wick Medium wick θ 80 Fine wick 60 Condenser end 40 20 0 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° Angle θ The decline in the performance of a 122–cm-long water heat pipe with the tilt angle from the horizontal is shown in Figure 10–37 for heat pipes with coarse, medium, and fine wicks. Note that for the horizontal case, the heat pipe with a coarse wick performs best, but the performance drops off sharply as the evaporator end is raised from the horizontal. The heat pipe with a fine wick does not perform as well in the horizontal position but maintains its level of performance greatly at tilted positions. It is clear from this figure that heat pipes that work against gravity must be equipped with fine wicks. The heat removal capacities of various heat pipes are given in Ta...
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