The
most common of them is pumping the cryogenic pump, by one mechanical
pump. So that, when the main valve of pump (valve, between pump and
vacuum chamber) is closed, heating the cryogenic pumps, and then, one
carrier gas (usually nitrogen) is inserted into a cryogenic pump and on
the other hand is pumped by rotary pump and thus, being outside all
gases and vapors through the pump
ryo Pump Operation
A cryo pump works by freezing water vapor, oil, and others gases out
of the atmosphere within the users vacuum chamber or system. The water
and gases are frozen onto the various arrays located inside the cryo
pump. The cryo pump operates on the principle of closed-cycle
refrigeration. This process is very similar to how a standard household
refrigerator works, but using a slightly different arrangement. The
cryo system consists of a helium compressor, a cryo pump, and
interconnecting helium lines. Inside the cryo pump there is a motor
driven multi-stage piston assembly (called a displacer). The compressed
helium from the compressor flows into the piston assembly, and as the
piston moves the helium gas expands and cools. As the piston returns it
pushes the helium out of the cryopump and back to the compressor. This
helium is re-compressed and the cycle repeats. During each cycle a
little more heat is removed from the arrays and they continue to cool
down until the second stage reaches about 9 to 10 Kelvin. This is the
lowest operating temperature of most cryopumps.
Cryopump Layout
The inside components of a cryo pump usually consist of the following:
a) a thermal shield
b) a condensing array
c) a low temperature array
d) a cold head cylinder
The thermal shield is attached to the first stage of the cold head
cylinder and helps to minimize the temperature loss between the inner
array and the cryo vacuum vessel wall. The 80K condensing array
attaches to the top of the thermal shield thus maintaining a temperature
of roughly 60 to 120K. This condensing array freezes out most of the
water and oil that enters the cryo pump. Lower freezing point gases
pass through this array and freeze out on the 15K lower temperature
array which is attached to the second stage area of the cold head
cylinder. The 15K array usually consists of metal fins that have a
cryo-adsorbant (such as activated charcoal) attached to the fins in a
shielded area. Most of the gases freeze out on the 15K array fins, but
since helium and hydrogen won’t freeze at this temperature the cold
charcoal adsorbs and traps the helium and hydrogen.
Precautions and Misconceptions Regarding Cryopumps
Cryo pump users should always remember that a cryo pump stores all
the gases that exist within the users vacuum chamber during operation.
If the cryo pump is not isolated from the vacuum chamber during
shutdown and warm-up, all of the stored gases within the cryo pump will
re-enter the users vacuum chamber. If the cryo pump is valved off or
isolated from the vacuum chamber, as the cryo pump warms, the stored
gases will build up pressure within the cryo pump until the pressure
reaches the level (approx 14 to 16 psi) that opens the pressure relief
valve on the cryopump. The gases stored are then released to the
atmosphere.
Caution: Pumping
oxygen, hydrogen, or other combustible gases with a cryo pump can create
a real danger of explosion during cryo pump warm-up. Follow all the
safety methods provided by the cryo pump manufacturer when pumping such
gases. Pumping of toxic or corrosive gases also require proper safety
methods in order to prevent the release of these gases to the
atmosphere. Altering the over-pressure valve on the cryo pump can have
severe consequences, and should not be done.
Misconception#1 –
Many users often believe that a high background of helium gas in their
chamber is caused by the helium within the high pressure circuit of the
cryo pump leaking into their chamber. Although this is possible, it is
almost never the case. A high helium background is usually caused by
too high a base temperature within the cryo pump, or a condition where
the 15K array is too saturated or frozen over to where it can no longer
trap the helium within the pump. The solution to the high base
temperature problem is to check the base temperature of the cryo pump
with the pump blanked-off and under no load. The base temperature
should be less than 15K. If it is running at a higher temperature, then
the displacer in the pump may need an overhaul that can be performed
here at TFS Technologies, Inc.. It is also possible that the cryo pump
was not roughed out to a low enough pressure during its initial
cool-down or that there is an external leak. This would cause the
arrays in the pump to freeze over and overload the ability of the pump
to reach a lower temperature. This is often indicated by a condensation
or frost layer forming on the outside of the cryo pump body. To solve
the saturated or frozen 15K array problem, first solve the temperature
problem if there is one, or else replace the 15K array. The 15K array
can get contaminated by pump oil during the rough down or the operating
cycle. It is always recommended to have an oil trap in the rough line
or use a drypump with cryo pumped systems.
Misconception#2 –
“My cryo pump sounds like it is tearing itself apart! It must be
broken!” A cryo pump will sometimes make a severe grinding noise or
ratcheting sound. This can mean that something inside the displacer has
broken or come loose, but usually it is simpler than that. Dirty
helium in the compressor circuit usually caused by water vapor that
entered the lines during hookup and disconnect will cause this
ratcheting noise. The water in the helium freezes out on the piston
cylinder walls in the displacer and the piston grinds against this ice
making the ratcheting or grinding noise. If not resolved soon the
piston will eventually jam and seize up the pump. Even worse, the
jamming can cause the motor shaft to break in half meaning an expensive
repair. To solve the ratcheting problem, decontaminate the helium
circuit in the pump, compressor, and helium lines following the
instructions in your pump manufacturers manual. This problem can also
occur if the compressor adsorber has not been changed at its regular
service interval. It can also occur if the compressor starts blowing
oil due to age. You can help prevent this problem by changing the
compressor adsorber every 10000 hrs of use (TFS Technologies can supply
your replacement adsorbers), and by being careful not to leak helium
from the connectors during hookup and removal of the helium lines. If
none of these solutions work, then ship your cryo pump to TFS and we
will evaluate the problem, inform you of the results, and repair the
pump for you
Refrence
Book: Coating fundamental and nanostructure analysis, jahanbakhsh mashaiekhy, iup,2015
http://www.tfstechnologies.com/how-does-a-cryo-pump-work/