USE OF PERFLUOROCARBONS IN HEP DETECTORS AT CERN

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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
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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
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Principle of Operation
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• 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

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• 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

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• 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.