COOLING BY EVAPORATION OF PERFLUOROCARBONS Why evaporate? 1. To enhance the heat transfer per unit area The heat transfer coefficient increases significantly from liquid convection

2. To reduce the temperature increase along the stave. The heat transferred to the liquid is used for phase change and not to increase its temperature. 3. To reduce the circulating coolant mass (the coolant is expensive!) The latent heat of evaporation is much higher than the sensible heat. 4. To reduce pipe dimensions (lower material budget) Smaller flow rates  smaller pipe diameter. Smaller flow rates  smaller pressure drops  thinner pipes may be used. 5. Possibility to have parallel channels at different temperatures by evaporating at different pressures (ex. by means back pressure regulators). 6. Lesser pipe insulation required Liquid can be kept at reasonable temperature all the way down to the expansion device (capillary), which is immediately upstream of the detector

Principle of Operation

Why an oil-free compressor? * Oil would undergo radio-induced polymerization Flow * Velocity in some parts of the system is not large enough to ensure safe oil return to the compressor. Oil pockets could be formed * An oil film on the inner wall of the cooling ducts would add an additional thermal resistance to heat transfer

Safety aspects of this system: * Consequences of a liquid spill are minimal because the amount of C3F8 in the system is small * In case of intervention in a section of the system, the C3F8 contained in those pipes can be previously drained out * Liquid reservoir can detect liquid loss when steady state operation is reached. * Liquid loss in an individual circuit can be detected by temperature rise and/or pressure variation

* Always a pressure relief valve in between two shut-off valves. * Pressure relief valves discharge to well ventilated locations * For detector protection, Back pressure regulators act also as safety valve. * A hard wired thermal interlock cuts the power to individual staves should their temperature exceed safe values.