How to Move Molecules in a Vacuum Systems

7/28/2019 How to Move Molecules in a Vacuum Systems

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How to Move Molecules in Vacuum Systems -1: Mech. & Booster

Pumps

by Daniel H. Herring

In order to create a vacuum within a closed container, or vessel, we need to

remove the molecules of air and other gases that reside inside by means of

a pump. The vacuum vessel and pumps (mechanical, booster, diffusion,

holding) together with the associated piping manifolds valves, gages and

traps comprise a typical vacuum system.

Mechanical Pumps

To reach the various vacuum levels, different vacuum pumping systems are

required. The foundation of any of these systems is the positive

displacement mechanical, or roughing, pump. The roughing pump so

called because it is used to produce a rough vacuum is used in the initial

pumpdown from atmospheric pressure to around 2 x 10-2 torr, depending on the type of pump.

The internal components of the mechanical pump (Fig.2) help us understand its operation. Basically, it is

an eccentric cylinder driven about an axis by an electric motor. During operation, the rotor turns with the

shaft, which causes the piston to sweep the volume between it and the stator. The piston does not turn in

this case, but the vane-like extension on the piston (called the slide, or slide valve) moves up and down in

an oscillating seal (called the slide pin or slide-valve pin).

At the start of a rotation, the ported slide valve is open. As the rotation occurs, the slide valve closes,

trapping a given volume of gas. This volume is compressed as the revolution continues. Near the end of

the revolution, the pressure is above atmospheric, and the gas discharges through a spring-loaded

poppet valve. On the completion of the revolution, the slide valve opens, and another increment of gas isadmitted.

A vacuum pump will remove a number of molecules with each rotation. How many molecules will depend

largely on the actual pump displacement, rotational speed and vacuum-system pressure. Each time

molecules are removed, the remaining molecules spread out in the vacuum chamber to occupy the

available volume. This repeats (molecules are removed by the pump) the pressure reduces and there are

less and less molecules to expand into the pump inlet with each rotation.

A vacuum pump will remove a number of molecules with each rotation.

How many molecules will depend largely on the actual pump displacement, rotational speed and vacuum-

system pressure. Each time molecules are removed, the remaining molecules spread out in the vacuum

chamber to occupy the available volume. This repeats (molecules are removed by the pump) the pressure

reduces and there are less and less molecules to expand into the pump inlet with each rotation.

Fig. 2. Mechanical pump

operation[4]
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Mechanical pumps can be single or dual stage. A single-stage design will achieve a pressure of about 1

x10-2 torr, while a dual-stage pump is capable of reaching pressures around 1 x 10-3 torr. A two-stage, or

compound, pump has two pumping chambers connected in series. The exhaust of the first stage is

coupled to the inlet of the second stage.

Lower pressure, less molecules and more speed in the same volume results in less pumping efficiency.

Why mechanical pumps start off with high efficiency and fall of at these pressure ranges can be explained

as follows. Consider one cubic foot of volume at atmospheric pressure (760 torr). If we were to put this

volume of gas in a container that was twice as large, the pressure would be exactly half, or 380 torr. If we

double the volume, we halve the pressure. Thus, doubling the volume again to 4 cubic feet results in a

pressure of 190 torr. So, to evacuate a chamber to 1 x 10-3 torr theoretically requires that we remove a

volume of 760,000 cubic feet. In everyday operation, a mechanical roughing pump will have great

difficulty achieving this ultimate pressure (lowest attainable pressure) since its efficiency begins to fall off

at 1 x10-1 to 8 x 10-2 torr.

An alternative to wet mechanical pumps those that use mechanical pump oil are the so-called dry

mechanical pumps. These pumps are used in applications where pumping efficiency and process

contamination concerns are important issues. They have positive environmental impact due to reduced oil

consumption and minimal disposal issues, and they operate with less noise and vibration.

Dry pumps operate on the compressor principle. As the two rotors rotate, gas is drawn in through an inlet

slot aligned with the cavity in one of the rotors. Further rotation closes the inlet while the lobes, or claws,

compress the trapped volume of gas until the cavity in the second rotor exposes the outlet or exhaust

slot. A small volume of gas remains trapped and is carried over into the next pumping cycle. These

designs produce high compression ratios and operate at high efficiency.

Booster Pumps

Enter the booster pump, or blower, a different type of mechanical pump

that is placed in series with the roughing pump. It is designed to cut in at

around 700 torr and provide higher speeds in the pressure range of 100

torr to 1 x10-3 torr. In this intermediate pressure range, the roughing

pump is losing efficiency while the diffusion (vapor) pump is just starting

to gain efficiency.

The operation of the booster (Fig.3) is as follows. Two impellers are

mounted on parallel shafts and rotate in opposite directions. They are

geared together so that the correct relative position of each impeller to

the other can be maintained. The impellers do not touch each other, and

no sealing fluid is used. Any back leakage is small compared to the total speed of the pump in its useful

range.

During operation, gas from the inlet side is trapped between the impeller and housing. No compression

takes place as this gas is moved from the inlet to the discharge port. When the leading lobe of the

impeller passes the discharge port, gas from the discharge area (at higher pressure) enters, but it is swept

away by the trailing lobe.

Fig. 3. Booster pump

operation [1]
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Mechanical booster pumps have a useful compression ratio of 10:1. Therefore, they must be backed by a

mechanical roughing pump in order to reach their maximum efficiency. The mechanical booster pump is

highly efficient in reducing the time required to evacuate a large or gassy system to the operating

pressure at which the diffusion pump is efficient.

How to Move Molecules in Vacuum Systems -2: Diff. Pumps &Troubleshooting

by Daniel H. Herring

Diffusion Pumps

Vacuum pumps have been called the heart of a vacuum system. Lets look at how we can reach low

vacuum levels using diffusion pumps. And we need to know how all pumps should be maintained to keep

the vacuum system running trouble free.

The 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 pumps 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 ofthe 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.

Fig. 1. Diffusion pump

operation[2]
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Oils based on silicones, hydrocarbons, esters, perfluorals and polyphenyl ethers can be used as diffusion-

pump fluids being vaporized in the range of 190C-280C (375F-535F). 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 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 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 Problems

The most common problems experienced with the various pumping systems can be summarized as:

o Contamination of the oil (mechanical pump)o Gas leaking into the pump (mechanical pump)o Solid particles (mechanical pump)o Exposure of the hot pump fluid to the atmosphere (diffusion pump)o Interruption of cooling fluid (diffusion pump)o Power failure (diffusion pump)o High forepressure (diffusion pump)

Fig. 2. Typical Fluid

Properties
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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 pumps 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:

o Loose or slipping beltso Improper oil level (too low or too high)o Stuck discharge valveo Clogged oil lines or valveso Damaged discharge valveo Ingestion of foreign contaminants (metal fines, metal chips, etc.)o Excessive vibration (pipe connection or floor mounting)

Of the various diffusion-pump problems, exposure of the hot pump oil to the atmosphere or interruption

of the coolant flow is of the most concern. Accidentally introducing air when the diffusion pump is at toohigh a temperature almost inevitably leads to a pump malfunction or failure, and this often requires

expensive and lengthy repairs (most often at the manufacturer). Severe cracking (breakdown) of the oil

and oxidation will occur depending on the type of oil. These lead to excessive backpressure, and the

products of the oil breakdown will deposit on the jet structure blocking the openings or, 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.

Other common problems with diffusion pumps include power failures and excessively high foreline

pressures.

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How to Move Molecules in a Vacuum Systems