Diffusion Pumps
Applications: The practical diffusion pump was invented by Langmuir in 1916, and this is the most common
high-vacuum pump when all vacuum applications are considered. It is far less dominant where avoidance of organic contamination is essential. Diffusion pumps are available in a wide range of sizes, with speeds of up to 50,000 L/s; for such high-speed pumping only the cryopump seriously competes.
A diffusion pump can give satisfactory service in a number of situations. One such case is in a large system in
which cleanliness is not critical. Contamination problems of diffusion-pumped systems have actually been somewhat overstated. Commercial processes using highly reactive metals are routinely performed using diffusion pumps.When funds are scarce, a diffusion pump, which incurs the lowest capital cost of any of the high-vacuum alternatives,is often selected. The continuing costs of operation, however, are higher than for the other pumps, a factor not often considered.
An excellent detailed discussion of diffusion pumps is available (Hablanian, 1995).
Operating Principles: A diffusion pump normally contains three or more oil jets operating in series. It can be
operated at a maximum inlet pressure of ~1 X 10-3 torr and maintains a stable pumping speed down to 10-10 torr or lower. As a transfer pump, the total amount of gas it can pump is limited only by its reliability, and accumulation of any hazardous gas is not a problem. However, there are a number of key requirements in maintaining its operation.
First, the outlet of the pump must be kept below some maximum pressure, which can, however, be as high as
the mid-10-1 torr range. If the pressure exceeds this limit,all oil jets in the pump collapse and the pumping stops. Consequently the forepump (often called the backing pump) must operate continuously. Other services that must be maintained without interruption include water or air cooling, electrical power to the heater, and refrigeration,if a trap is used, to prevent oil backstreaming. A major drawback of this type of pump is the number of such criteria. The pump oil undergoes continuous thermal degradation. However, the extent of such degradation is small, and an oil charge can last for many years. Oil decomposition products have considerably higher vapor pressure than their parent molecules. Therefore modern pumps are designed to continuously purify the working fluid, ejecting decomposition products toward the forepump. In addition, any forepump oil reaching the diffusion pump has a much higher vapor pressure than the working fluid, and it too must be ejected. The purification mechanism primarily involves the oil from the pump jet, which is cooled at the pump wall and returns, by gravity, to the boiler.
The cooling area extends only past the lowest pumping jet, below which returning oil is heated by conduction from the boiler, boiling off any volatile fraction, so that it flows toward the forepump. This process is greatly enhanced if the pump is fitted with an ejector jet, directed toward the foreline; the jet exhausts the volume directly over the boiler,where the decomposition fragments are vaporized. A second step to minimize the effect of oil decomposition is to design the heater and supply tubes to the jets so that the uppermost jet, i.e., that closest to the vacuum chamber,is supplied with the highest-boiling-point oil fraction. This oil, when condensed on the upper end of the pump wall, has the lowest possible vapor pressure. It is this film of oil that is a major source of backstreaming into the vacuum chamber.
The selection of the oil used is important (O’Hanlon,1989). If minimum backstreaming is essential, one can
select an oil that has a very low vapor pressure at room temperature. A polyphenyl ether, such as Santovac 5, or a silicone oil, such as DC705, would be appropriate. However, for the most oil-sensitive applications, it is wise to use a liquid nitrogen (LN2) temperature trap between pump and vacuum chamber. Any cold trap will reduce the system base pressure, primarily by pumping water vapor, but to remove oil to a partial pressure well below 10-11 torr it is essential that molecules make at least two collisions with surfaces at LN2 temperature. Such traps are thermally isolated from ambient temperature and only need cryogen refills every 8 hr or more. With such a trap, the vapor pressure of the pump oil is secondary, and a less expensive oil may be used.
If a pump is exposed to substantial flows of reactive gases or to oxygen, either because of a process gas flow
or because the chamber must be frequently pumped down after venting to air, the chemical stability of the oil
is important. Silicone oils are very resistant to oxidation, while perfluorinated oils are stable against both oxygen and many reactive gases. When a vacuum chamber includes devices such as mass
spectrometers, which depend upon maintaining uniform electrical potential on electrodes, silicone oils can be a problem,because on decomposition they may deposit insulating films on electrodes.
Operating Procedures: A vacuum chamber free from organic contamination pumped by a diffusion pump
requires stringent operating procedures. While the pump is warming, high backstreaming occurs until all jets are in full operation, so the chamber must be protected during this phase, either by a LN2 trap, before the pressure falls below the viscous flow regime, or by an isolation valve. The chamber must be roughed down to some predetermined pressure before opening to the diffusion pump. This cross-over pressure requires careful consideration. Procedures to minimize the backstreaming for the frequently used oil-sealed mechanical pump have already been discussed (see Oil-Sealed Pumps). If a trap is used, one can safely rough down the chamber to the ultimate pressure of the pump. Alternatively, backstreaming can be minimized by limiting the exhaust to the viscous flow regime. This procedure presents a potential problem. The vacuum chamber will be left at a pressure in the 10-1 torr range, but sustained operation of the diffusion pump must be avoided when its inlet pressure exceeds 10-3 torr. Clearly, the moment the isolation valve between diffusion pump and the roughed-down vacuum chamber is opened, the pump will suffer an overload of at least two decades pressure.
In this condition, the upper jet of the pump will be overwhelmed and backstreaming will rise. If the diffusion
pump is operated with a LN2 trap, this backstreaming will be intercepted. But, even with an untrapped diffusion pump, the overload condition rarely lasts more than 10 to 20 s, because the pumping speed of a diffusion pump is very high, even with one inoperative jet.
Consequently, the backstreaming from roughing and high-vacuum pumps remains acceptable for many applications. Where large numbers of different operators use a system, fully automatic sequencing and safety interlocks are recommended to reduce the possibility of operator error.
Diffusion pumps are best avoided if simplicity of operation is essential and freedom from organic contamination is paramount.
Applications: The practical diffusion pump was invented by Langmuir in 1916, and this is the most common
high-vacuum pump when all vacuum applications are considered. It is far less dominant where avoidance of organic contamination is essential. Diffusion pumps are available in a wide range of sizes, with speeds of up to 50,000 L/s; for such high-speed pumping only the cryopump seriously competes.
A diffusion pump can give satisfactory service in a number of situations. One such case is in a large system in
which cleanliness is not critical. Contamination problems of diffusion-pumped systems have actually been somewhat overstated. Commercial processes using highly reactive metals are routinely performed using diffusion pumps.When funds are scarce, a diffusion pump, which incurs the lowest capital cost of any of the high-vacuum alternatives,is often selected. The continuing costs of operation, however, are higher than for the other pumps, a factor not often considered.
An excellent detailed discussion of diffusion pumps is available (Hablanian, 1995).
Operating Principles: A diffusion pump normally contains three or more oil jets operating in series. It can be
operated at a maximum inlet pressure of ~1 X 10-3 torr and maintains a stable pumping speed down to 10-10 torr or lower. As a transfer pump, the total amount of gas it can pump is limited only by its reliability, and accumulation of any hazardous gas is not a problem. However, there are a number of key requirements in maintaining its operation.
First, the outlet of the pump must be kept below some maximum pressure, which can, however, be as high as
the mid-10-1 torr range. If the pressure exceeds this limit,all oil jets in the pump collapse and the pumping stops. Consequently the forepump (often called the backing pump) must operate continuously. Other services that must be maintained without interruption include water or air cooling, electrical power to the heater, and refrigeration,if a trap is used, to prevent oil backstreaming. A major drawback of this type of pump is the number of such criteria. The pump oil undergoes continuous thermal degradation. However, the extent of such degradation is small, and an oil charge can last for many years. Oil decomposition products have considerably higher vapor pressure than their parent molecules. Therefore modern pumps are designed to continuously purify the working fluid, ejecting decomposition products toward the forepump. In addition, any forepump oil reaching the diffusion pump has a much higher vapor pressure than the working fluid, and it too must be ejected. The purification mechanism primarily involves the oil from the pump jet, which is cooled at the pump wall and returns, by gravity, to the boiler.
The cooling area extends only past the lowest pumping jet, below which returning oil is heated by conduction from the boiler, boiling off any volatile fraction, so that it flows toward the forepump. This process is greatly enhanced if the pump is fitted with an ejector jet, directed toward the foreline; the jet exhausts the volume directly over the boiler,where the decomposition fragments are vaporized. A second step to minimize the effect of oil decomposition is to design the heater and supply tubes to the jets so that the uppermost jet, i.e., that closest to the vacuum chamber,is supplied with the highest-boiling-point oil fraction. This oil, when condensed on the upper end of the pump wall, has the lowest possible vapor pressure. It is this film of oil that is a major source of backstreaming into the vacuum chamber.
The selection of the oil used is important (O’Hanlon,1989). If minimum backstreaming is essential, one can
select an oil that has a very low vapor pressure at room temperature. A polyphenyl ether, such as Santovac 5, or a silicone oil, such as DC705, would be appropriate. However, for the most oil-sensitive applications, it is wise to use a liquid nitrogen (LN2) temperature trap between pump and vacuum chamber. Any cold trap will reduce the system base pressure, primarily by pumping water vapor, but to remove oil to a partial pressure well below 10-11 torr it is essential that molecules make at least two collisions with surfaces at LN2 temperature. Such traps are thermally isolated from ambient temperature and only need cryogen refills every 8 hr or more. With such a trap, the vapor pressure of the pump oil is secondary, and a less expensive oil may be used.
If a pump is exposed to substantial flows of reactive gases or to oxygen, either because of a process gas flow
or because the chamber must be frequently pumped down after venting to air, the chemical stability of the oil
is important. Silicone oils are very resistant to oxidation, while perfluorinated oils are stable against both oxygen and many reactive gases. When a vacuum chamber includes devices such as mass
spectrometers, which depend upon maintaining uniform electrical potential on electrodes, silicone oils can be a problem,because on decomposition they may deposit insulating films on electrodes.
Operating Procedures: A vacuum chamber free from organic contamination pumped by a diffusion pump
requires stringent operating procedures. While the pump is warming, high backstreaming occurs until all jets are in full operation, so the chamber must be protected during this phase, either by a LN2 trap, before the pressure falls below the viscous flow regime, or by an isolation valve. The chamber must be roughed down to some predetermined pressure before opening to the diffusion pump. This cross-over pressure requires careful consideration. Procedures to minimize the backstreaming for the frequently used oil-sealed mechanical pump have already been discussed (see Oil-Sealed Pumps). If a trap is used, one can safely rough down the chamber to the ultimate pressure of the pump. Alternatively, backstreaming can be minimized by limiting the exhaust to the viscous flow regime. This procedure presents a potential problem. The vacuum chamber will be left at a pressure in the 10-1 torr range, but sustained operation of the diffusion pump must be avoided when its inlet pressure exceeds 10-3 torr. Clearly, the moment the isolation valve between diffusion pump and the roughed-down vacuum chamber is opened, the pump will suffer an overload of at least two decades pressure.
In this condition, the upper jet of the pump will be overwhelmed and backstreaming will rise. If the diffusion
pump is operated with a LN2 trap, this backstreaming will be intercepted. But, even with an untrapped diffusion pump, the overload condition rarely lasts more than 10 to 20 s, because the pumping speed of a diffusion pump is very high, even with one inoperative jet.
Consequently, the backstreaming from roughing and high-vacuum pumps remains acceptable for many applications. Where large numbers of different operators use a system, fully automatic sequencing and safety interlocks are recommended to reduce the possibility of operator error.
Diffusion pumps are best avoided if simplicity of operation is essential and freedom from organic contamination is paramount.
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