And the choice of the vacuum pump to vent, but by some of the following empirical data as a reference.
（
1)
When the diameter of the vent is 25 mm, with vacuum exhaust rubber tube length is 1000 mm, inner diameter of 20 mm, the exhaust velocity of 2.
5 l/S mechanical pump, liquid level of saturated vapor pressure can reach 1067 pa, the temperature of 1.
7 k。
（
2)
When the outlet diameter of 50 mm exhaust pipe length is 3000 mm, a diameter of 120 mm, the adoption of exhaust rate of 5 l/S pump, liquid level of saturated vapor pressure can reach 400 pa, the temperature of 1.
5 k。
when the pump capacity is 1700 l/MIN mechanical booster pump (
Distribution in the back pressure of pump of 300 l/MIN)
When the steam pressure to 13 pa temperature can reach 0.
98 k。
different experimental device has different characteristics and requirements, and specific needs as large diameter and length, and equipped with pump pumping speed, can according to the design of the liquid helium indicators,
Liquid helium evaporation)
Calculated according to the requirement of the design of vacuum.
when the evaporator pressure of liquid helium to pa 5000, liquid helium temperature drop to 2.
17 k。
At this point, the liquid helium superfluid phenomena, superfluid helium along the dewar wall up production & other;
Crawling & throughout;
。
Liquid helium in the absence of pressure difference of from bottom to top, from cold to hot & other;
Crawling & throughout;
And the thickness of the superfluid helium film is about 100 a.
According to different height, the thickness of the superfluid helium film is also different.
Near the surface, the Angle of superfluid helium membrane thickness, 1000 a month in the upper part of cunkou pulse liquid helium, in addition to the usual liquid surface evaporation, upwards, & other;
Crawling & throughout;
The superfluid helium membrane causes evaporation in an all-round way.
At this time, when using vacuum pump suction, vacuum pump can only speak crawling membrane evaporation of helium away comprehensively, and can't effectively reduce the evaporation pressure.
Superfluid helium film fully, therefore, will affect the efficiency of vacuum cooling evaporation, makes constant temperature continues to fall, in order to obtain a lower temperature, must adopt effective measures to limit the superfluid helium film & other
Crawling & throughout;
。
low temperature system for liquid nitrogen refrigeration, this subject to gain substantial breakthrough at an early date.
