Cryomech PT415, Инструкция по установке, эксплуатации и текущему обслуживанию

Страница: 30 / 93

Specifications
5-6
G. High Pressure Gasport
The high-pressure gasport fastens to the high-pressure helium flex line that supplies
compressed helium gas from the compressor package to the cold head.
H. Low Pressure Gasport
The low-pressure gasport fastens to the low-pressure helium flex line that returns helium
gas from the cold head to the compressor package.
I. Pressure Relief Valve
The pressure relief valve will begin to open when the pressure inside the dewar is
approximately
5 PSIG .
5.4 Operation of the Helium Reliquefier
The Helium Reliquefier and your cryostat must first be connected as shown in Figure 5-4.
The liquid return line is inserted into the fill port of your cryostat. The vapor line connects
the vent of helium vapor to the gas inlet of the reliquefier.
Please note that your open dewar/cryostat is now a component of a larger closed
system. The term “closed” means that no air should enter the system and no helium
should leave it. The helium that boils off and cools the components of the cryostat exits
the cryostat through the neck, carrying the heat from the cryostat to the reliquefier, where
the heat is removed from the system. When the heat is removed from the helium it re-
condenses in the Reliquefier Main Assembly. The natural convection establishes a flow
inside the system. This circulation loop, also called a thermo-siphon, forces the gas
cooling and condensing in the main assembly in one direction.
The liquid helium flows down the liquid return line into the cryostat. For maximum
performance, it is important that the liquid helium leaves the liquid return line inside your
cryostat at a level which is in the gaseous helium and < 8K. To install the Helium
Reliquefier, Cryomech recommends that you plan that the bottom end of the return line is
near the “full” liquid level in your cryostat. Please be sure the liquid return line is
matched with the fill port of your cryostat.
For optimum performance the helium vapor line should connect to the cryostat
instrumentation port so the helium vapor can be used to cool the leads and the shield
according to the dewars original design. There is a needle valve on the helium vapor line
to control the flow of the thermo-siphon loop. The thermo-siphon flow in some cryostats
could be very strong and decrease the liquefaction for your application. We recommend
that the needle valve is open to between 33-50%; and is set this way by Cryomech.
Minor adjustments might increase the liquefaction rate for some installations. If ice forms
on your vapor return line contact Cryomech for directions.
It is necessary to balance the rate of boil off of your cryostat with the condensing rate of
the Pulse Tube Cryocooler. Therefore, Cryomech, Inc. has included a pressure
controller to the system. There is a temperature sensor/monitor and heater assembly
attached to the 4K condensing surface of the Cold Head. Heat will be added to the 4K
condensing surface to ensure that the cryostat pressure does not go below 3 psig, the
desired vapor pressure/liquid temperature of the cryostat. Without the heating, the Pulse
Tube Cryocooler could cool to a temperature below the design temperature of the
cryostat and generate a vacuum in the cryostat.