Evaporative CO_2 cooling is under design for several new silicon particle detectors at CERN. In general the cooling lines inside the detectors are long (up to 25 m) and have small diameters (1 to 3 mm). Heat loads typical vary from 50 to 350 watt per cooling line. The individual cooling lines are manifolded and operate in parallel with each other. The long and small diameter cooling lines cause relative high pressure drops. These pressure drops cause temperature gradients which must be minimized to meet the strict temperature requirements of silicon detectors. The parallel operations of the several cooling lines which are operating with different heat loads require a careful design of the flow distribution to ensure a good heat transfer in all the channels. This paper describes the ongoing developments at CERN concerning the understanding of a proper design for long evaporative channels. A simulation program called CoBra is built to investigate the theoretical aspects. Test results on long evaporators under development for the Atlas-IBL and CMS-pixel experiments at CERN are presented and compared with the simulation model.
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