AUSTIN SCIENTIFIC M600-HV Manual - Página 3

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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.