By Dan Herring

This is the fourth in a series of articles in our Vacuum Heat-Treatment Series. Here we conclude our discussion of vacuum pumping systems by reviewing the operation of diffusion pumps as well as offering troubleshooting tips for all types of vacuum pumping systems.

Vacuum pumps are the heart of a vacuum system. While mechanical pumps have the ability to work against atmospheric back pressure and booster pumps improve the speed and level to which we pump down, these pumps have the disadvantage of losing efficiency as the system pressure continues to lower. In order to reach extremely low vacuum levels, the use of diffusion pumps is required (Fig. 1).

Diffusion Pumps

The diffusion pump (Fig. 2) consists of a boilerplate system in which a high-grade silicon fluid is heated and then subsequently vaporized during boiling. This is typically accomplished by means of an external heating element and a stack assembly, or chimney (commonly referred to in the industry as a “Christmas tree assembly”), through which the vapors pass. These vapors exit the chimney through one or more levels of annular converging/diverging nozzles directed radially outward and downward at an angle of approximately 45 degrees and at speeds in excess of 190 km/hour (120 miles/hour). The hot vapors are accelerated by the action of the compression stacks within the diffusion pump, which serves as a venturi, creating supersonic velocities. As they travel outward and downward, they collide with molecules of the gases being drawn into the pump inlet by the pressure differential created during the boiling of the oils. This gives them an effective downward velocity toward the exit (foreline) from which they are removed efficiently by the mechanical pumping system.

Fig. 2. Diffusion-pump operation[2]

Diffusion pumps are, therefore, a type of vapor pump (without moving parts) and are used to help achieve lower system pressures than can be achieved by a mechanical-pump/blower combination alone. The diffusion pump is capable of pumping gas loads with full efficiency with/at inlet pressures not exceeding 8 x 10-2 torr and discharge (or foreline) pressures not exceeding 3 x 10-1 torr. The diffusion pump cannot operate independently. It requires a separate pump to reduce the chamber pressure to or below the maximum intake pressure of the pump before it will operate. Also, while operating, a separate or holding pump is required to maintain the discharge pressure below the maximum tolerable pressure.

While the diffusion pump is operating, a separate holding pump operates with the holding valve in the closed position. When the diffusion pump is in a standby mode, the holding valve is open and the holding pump maintains the discharge pressure at or below the maximum tolerable operational pressures.

The operation of the diffusion pump is as follows. The inlet of the pump is attached directly to the vessel via a right-angle main poppet valve assembly, and a mechanical pump is attached to outlet. The pressure of the entire system is reduced to about 2 x 10-1 torr. At this point, the diffusion-pump heaters are 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 the compression jet ring assemblies 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 on the water-cooled inner walls of the pump body).

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 it being knocked backward toward the inlet is less likely than the probability of it being carried (entrained) 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 compression stage, 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 pumping system.

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.

Today, several types of oil, based on silicones, hydrocarbons, esters, perfluorals and polyphenyl ethers, can be used as diffusion-pump fluids being vaporized in the range of 190-280ºC (375-535ºF). Each fluid has specific properties (Table 2). 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.

Fig. 3. Vacuum levels by type of pump

In order to produce an ultrahigh vacuum, one must be concerned with the characteristics of the pump (i.e. cleanliness and oil selection) and the vacuum vessel. Contamination from the chamber can be reduced by an intelligent choice of materials (ones that are less susceptible to outgassing), fabrication techniques and operating procedures. Trapping or processing procedures may reduce the gas load originating in the pump.

Although diffusion pumps have been replaced in some applications by more advanced designs, such as cryogenic or turbomolecular (ion) pumps, they are still widely used due to their reliability, simple design, operation without noise or vibration, and relatively low operation and maintenance costs. Industrial and laboratory vacuum levels (Fig. 3, Table 3) are achievable using these types of pumps.

Evacuation Effects

In general, the effects of evacuating a vessel can be summarized as follows: [4]

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)

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 1 x 10-4 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 1x 10-4 torr to 1 x 10-6 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 Problems – Troubleshooting Guide

Common problems with mechanical pumps also require routine maintenance and inspection, including:

• Oil contamination
• Sludge buildup
• 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 fins, metal chips, etc.)
• Excessive vibration (pipe connection or floor mounting)
• Exhaust filters (more than 12 months in age)
• Oil temperature not being regulated between 60-70ºC (140-160ºF)

Of the various mechanical-pump problems that can arise, contamination of the oil is the most common. Vapors present in the gas being pumped may condense and mix with the oil. Moisture (water vapor) is especially problematic, and if not removed will flash to vapor and tie up a large portion of the pump’s gas load capacity, thus creating a significant loss in pumping efficiency (resulting in either extremely long pumpdown times, failure to achieve a low vacuum level or both). In order to rid the oil of water and other liquid condensates, a gas ballast is used. A gas ballast may be used in conjunction with correctly regulating the operating temperature of the oil with a water-control valve assembly. A ballast valve on the pump can be opened (manually or automatically) to admit air, nitrogen or argon into the pump, disrupting its operating efficiency, resulting in 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. The gas-ballast valve is very effective in removing water vapor, but it is very ineffective in cleaning dirty oil or fixing oil that has cracked (fractionated) due to mixing with other downstream by-products.

In addition, the oil may break down chemically, forming a sludge that causes numerous (short- and long-term) problems with pump operation. This can cause severe wear on internal components, often to the point where rebuilding is not possible. Disassembly and cleaning of the pump is the only solution to this problem.

In replacing pump oil, be careful to use the type of oil recommended for the pump and be equally careful to apply precisely the right amount of oil. Either too much or too little oil in the pump reservoir will give rise to serious difficulties. Checking the amount of fluid in the pump reservoir during normal operation is strongly recommended. It is possible, due to improper operation, to have the pump oil backstream into the vessel in considerable quantities.

Common problems with diffusion pumps include:

• Power failures
• Excessively high foreline pressures
• Backstreaming
• Process by-products clogging oil returns in boilerplate
• Defective heaters and/or broken wiring on the boiler
• Water inlet temperature above 35ºC (95ºF)
• Water exit temperature above 55ºC (130ºF)
• Mixtures of hydrogen-based oils with silicone-family oils
• High leak rates on the system when being pumped on
• Water-cooled copper lines full of mineral (calcium) deposits negating proper heat transfer

Of the various diffusion-pump problems, exposure of the hot pump oil to the atmosphere or interruption of the coolant flow is most concerning. Accidentally introducing air when the diffusion pump is at too high a temperature almost inevitably leads to a pump malfunction or failure and often requires expensive and lengthy repairs. Severe cracking (breakdown) of the oil and oxidation will occur depending on the type of oil. This leads to excessive back pressure, and the products of the oil breakdown will deposit on the jet structure blocking the openings or deposit in the area of the oil heater, burning it out. Overheating due to inadequate coolant flow also decomposes the oil and can cause excessive backstreaming into the vacuum-furnace chamber. Depending on the actual amount of air in the hot pump, coupled with previously deposited materials in the base of the pump, the oil may expand excessively in vapor form with a significant pressure buildup.

Backstreaming

As the vacuum pressure continues to be lowered, some pump-fluid gas molecules attempt to reverse course and move up and back toward the vacuum chamber. This phenomenon is called backstreaming, and it increases in frequency as the inlet pressure exceeds 0.001 mbar (1 micron). The effects of backstreaming can be negated by use of a cold trap located in the line connecting the inlet of the diffusion pump and the vessel being pumped. Cold traps are cooled by water, refrigerants or liquid nitrogen and serve to limit backstreaming and stop the flow of contaminants coming from the chamber, which would otherwise carry into the pumping system and contaminate the pump fluids. Other methods – such as baffles and cold traps located above the top jet of the diffusion pump – have also been used. Each family of cold traps offers both favorable and unfavorable impacts on specific furnace operations, thus making selection of a trap methodology that best suits your specific operations of paramount importance.

Next time: Part five of this series begins a discussion of the types and characteristics of vacuum gauges and offers insights into which gauge should be used when working in a specific vacuum range.

Daniel H. Herring / Tel: (630) 834-3017) /E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it
Dan Herring is president of THE HERRING GROUP Inc., which specializes in consulting services (heat treatment and metallurgy) and technical services (industrial education/training and process/equipment assistance. He is also a research associate professor at the Illinois Institute of Technology/Thermal Processing Technology Center.

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