Ulvac CRYO-U Series, Instruction Manual

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CRYO-U ® Cryopump Instruction Manual
MA0E001E,2010.01 ULVAC CRYOGENICS INCORPORATED E-1
Appendix E
PRINCIPLE OF REFRIGERATOR OPERATION
Helium gas is used as heat-medium gas of a refrigeration cycle in refrigerator system,
because it does not liquefy even 10K.
First, after the helium gas is compressed into high pressure and being continuously
cooled by water-cooling or air cooling to room temperature within the compressor unit,
it will be introduced into the refrigerator when the valve A opens. Then the helium gas
is cooled by heat exchange between the regenerator and led to an expansion chamber
with a rise of a displacer. Next, because the valve B opens at the same time the valve A
closes, the high-pressure helium gas in the expansion chamber is exhaled toward the
low pressure part of the compressor unit with the differential pressure. During this
process, the pressure and temperature of the helium gas in the expansion chamber
decreases (called Simon expansion). The cooled low-pressure helium gas is discharged
completely from the expansion chamber by the descent of the displacer. As the helium
gas passes through the regenerator again, it will be warmed up to room temperature
and return to the compressor. At this moment, however, the regenerator is cooled down
conversely. In this way, a refrigeration cycle returns to the first state. By repeating this
refrigeration cycle, cryogenic temperature is obtained.
Figure E-1 Principle of Refrigeration
Low
pressure
Regenerator
Compressor unit
Displacer
Valve A
Valve B
High
pressure
Thermal load
(Low temperature)
Expansion chamber
Low
temp.
High
temp.