2. DESCRIPTION
Compressed helium is used as a refrigerant when trying to achieve
cryogenic temperatures (as low as 4 Kelvin). The high-pressure helium is
delivered from the compressor to a cold head through the supply line (300-350
psi). The helium is expanded in the cold head to produce a temperature drop.
The cold head uses a motor and a piston like “displacer” to move the expanded
helium though regeneration materials to increase the thermodynamic efficiency of
the cycle. After expansion, the helium returns to the compressor through the
return line (50-100 psi). Theses magnets operate at cryogenic temperatures and
the chilled radiation shield helps maintain these temperatures.
The compressor itself consists of five main components: The compressor
capsule, the heat exchanger, the mist separator, absorber, and volume tank.
Compressing helium generates a large amount of heat. To remove this
heat from the compressor, oil is mixed into the helium as it is being compressed.
The compressor removes most of the oil from the helium stream before it leaves
the capsule. The oil is then pumped (by the differential pressure generated by the
compressor) out of the compressor, through the water-cooled heat exchanger, and
then back to the compressor.
The heat exchanger is a brazed, flat plate heat exchanger. It has two
circuits for hot fluids. One is for the oil (as mentioned above). The other circuit
is for the helium. Removing the heat of compression from the helium makes it
better able to chill the cold head. The two hot fluids are cooled by water. The
water enters and exits the compressor from the front panel.
The helium leaving the compressor still has a small amount of oil mixed
with it. If this oil makes it to the cold head, it will freeze and foul the cold head.
The function of the mist separator and absorber is to remove this oil from the
helium stream. The mist separator is filled with fiberglass. Oil agglomerates on
the glass fibers, drops to the bottom of the separator and is plumbed back to the
compressor, again by differential pressure. The absorber is filled with activated
charcoal. This charcoal adsorbs the remaining oil from the helium stream. The
absorber eventually fills with oil and must be changed. A typical time between
replacements is 20,000 hours.
The volume tank is an empty tank that provides extra volume on the low
side of the system to keep the low side pressure from going negative when the
system is running.
There are a number of subsystems that support these five main
components. There is an electrical box that contains the power and diagnostic
electronics and provides a user interface. The compressor electrical box also
supplies power to run the cold head motor though a connector on the front panel.
There are two thermostats attached to the plumbing lines that open should
temperatures get out of range. There is a solenoid valve that opens when the
compressor shuts down, providing a passage between the high and low side,
equalizing the two.
