Cryo 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.
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.