Operation
Figure 14 Steady State Cryopumping
During cooldown, the cold head may be noisy. When the unit is cooled down,
the noise level will decrease to normal. If this does not occur, refer to the
procedure in Cold Head Orifice Adjustment, which follows.
Cold Head Orifice Adjustment
PREVENT EQUIPMENT DAMAGE. The cold head’s variable orifice has been
factory set for optimal performance at customer’s stated electrical frequency.
This frequency, either 50 or 60 Hz, is identified by the label on the motor
housing. Check that the labeled frequency agrees with customer’s frequency.
Operating a cold head labeled “Factory set for 50 Hz operation” on 60 Hz
electrical service may damage the displacer. The orifice in a cold head labeled
“Factory set for 50/60 Hz operation” needs no adjustment.
The variable orifice has been set at the factory for maximum refrigeration capacity. The orifice
is located in the edge of the valve motor base, 135º from the return gas coupling and in the
same horizontal plane as the return gas coupling. See Figure 3.
In most systems tested, minimum temperature was achieved with the orifice 1 to 2 turns open
(out 1 to 2 turns from full in position) for 60 Hz. Because of manufacturing tolerances, not all
systems behave identically. An experienced operator can develop a technique to adjust the
displacer motion to achieve best system performance.
a) Make only small orifice stem changes (1/4 turn maximum).
b) Wait about one minute after making a change to observe the effect.
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