How to Move Molecules in Vacuum Systems/Part Two: Diffusion Pumps and Troubleshooting Tips
Diffusion PumpsThe diffusion pump (Fig. 1) is a type of vapor pump (since it pumps vapors), and it is used to help achieve even lower system pressures. The diffusion pump is capable of pumping gas with full efficiency at inlet pressures not exceeding 2x10-2 and discharge (or foreline) pressures not exceeding 5x10-1 torr. The diffusion pump cannot operate independently. It requires a separate pump to reduce the chamber pressure to or below the diffusion pump’s maximum intake pressure before it will operate. Also, while operating, a separate or holding pump is required to maintain the discharge pressure below the maximum tolerable pressure.
The operation of the diffusion pump is as follows. The inlet of the pump is attached directly to the vessel, and a mechanical pump is attached to the outlet. The pressure of the entire system is reduced to about 5x10-2 torr. At this point the diffusion-pump heater is turned on, heating a fluid in the boiler portion of the pump. The rise in pressure forces the vapors up the chimney of the pump, where it is directed out spray nozzles into the surrounding area of lower pressure. The nozzles deflect the vapor as a jet downward and outward to the walls (where the vapor condenses).
Gas molecules from the vessel enter the pump throat and diffuse through the less-dense fringe at the edge of the vapor stream. When a gas molecule has penetrated into the high-density core of the stream, the probability of its being knocked backward toward the inlet is less likely than the probability of its being carried along the vapor stream toward the outlet. Thus the predominant direction of molecular travel is away from the inlet and toward the outlet. In a multistage pump, the gas molecules are directed toward the next nozzle, where the action is repeated. Several succeeding stages will compress the low-pressure gas at the inlet to a higher pressure at the outlet, where it is removed to atmosphere by the mechanical pump.
The movement of molecules from an area of low pressure to an area of higher pressure will only continue as long as the region of higher pressure (or forepressure) does not exceed a critical limit. Consequently, it is necessary for a diffusion pump to be “backed” by a mechanical pump. In practice, the backing pump has two or three times the minimum capacity required.
Oils based on silicones, hydrocarbons, esters, perfluorals and polyphenyl ethers can be used as diffusion-pump fluids being vaporized in the range of 190°C-280°C (375°F-535°F). Each fluid has specific properties (Table 1). Mercury is no longer used in vacuum pumping systems due in large part to its toxicity. The choice of the pump fluid depends on the required application (vacuum level) of the pumping system.
Although diffusion pumps have been replaced in some applications by more advanced designs – cryogenic or turbomolecular pumps – they are still widely used due to their reliability, simple design and operation without noise or vibration. They are also relatively inexpensive to operate and maintain.
Evacuation EffectsIn general, the effects of evacuating a vessel can be summarized as follows:
A. The effects of evacuating a vessel from 760 torr (atmospheric pressure) to 1 torr are:
1. Removing (high relative humidity) air
a. Water vapor condensation (due to cooling effect associated with a sudden drop in pressure).
b. “Fog” develops (a cloud “swirls” around with a turbulence that is characteristic of a gas flow at high pressure and high flow rate).
<br /><br />2. (Slow) Change in the composition of the gas remaining
a. Initially, air is the major component of the gas (certain other contaminants such as oils, grease and water exist on cold surfaces such as vessel walls).
b. Eventually, almost all of the air is pumped out. The grease and water will continue to evaporate, and their partial pressure will constitute a much larger portion of the total pressure. This is called outgassing.
B. The effects of evacuating a vessel from 1 torr to 1x10-4 torr are:
1. The ability of the gases remaining in the vessel to conduct heat begins to decrease rapidly.
2. A change in the electrical characteristics of the gas begins (voltage to start a discharge decreases).
C. The effect of evacuation from 1x10-4 torr to 1x10-6 torr is:
1. Decreasing molecular density
a. Molecules collide with the sides of the vessel as often as they will with each other.
b. There is an increase in sliding friction.
Pump ProblemsThe most common problems experienced with the various pumping systems can be summarized as:
- Contamination of the oil (mechanical pump)
- Gas leaking into the pump (mechanical pump)
- Solid particles (mechanical pump)
- Exposure of the hot pump fluid to the atmosphere (diffusion pump)
- Interruption of cooling fluid (diffusion pump)
- Power failure (diffusion pump)
- High forepressure (diffusion pump)
Troubleshooting Guide (Diagnosis of Problems)Of the various mechanical-pump problems that can arise, contamination of the oil in the pump is the most common. Vapors present in the gas being pumped may condense and mix with the oil. Moisture (water vapor) especially, if not removed, will flash to vapor, tie up a large portion of the pump’s capacity and create a significant loss in pumping efficiency, resulting in either extremely long pumpdown times, failure to achieve a low vacuum level or both. In addition, the oil may break down chemically, forming a sludge, which causes numerous short- and long-term problems with pump operation. In order to rid the oil of water and other liquid condensates, a gas ballast is used. A ballast valve on the pump can be opened – manually or automatically – to admit air (or another gas) into the pump, disrupting its operating efficiency. The result is a reduction in the compression necessary to exhaust the gases and, correspondingly, a decrease in the amount of vapor that condenses. The use of a gas ballast increases the amount of oil carried out in the exhaust.
Other common problems with mechanical pumps that also require routine maintenance and inspection include:
- Loose or slipping belts
- Improper oil level (too low or too high)
- Stuck discharge valve
- Clogged oil lines or valves
- Damaged discharge valve
- Ingestion of foreign contaminants (metal fines, metal chips, etc.)
- Excessive vibration (pipe connection or floor mounting)
Other common problems with diffusion pumps include power failures and excessively high foreline pressures. IH