pollution

When the vacuum chamber is evacuated, the vacuum chamber must be cleaned in order to achieve the desired pressure as soon as possible. Typical contaminants in vacuum systems include

■ Grease and lubricant on the surface, screws and seals of the residue produced by the production process of the vacuum system

■ Application related contaminants process reaction products, dust and particles

■ Environmentally relevant pollutants condense vapors, especially moisture adsorbed on the walls of the vessel.

Therefore, when assembling vacuum equipment, it is necessary to ensure that the parts are as clean as possible. All components installed in the vacuum chamber must be clean and grease free. All seals installed must also be oil-free. If the use of vacuum grease cannot be avoided, it must be used very carefully, in short, as a lubricant to assist in installation, rather than as a sealing agent. If high vacuum or ultra-high vacuum systems are involved, clean, non-shed, dust-free gloves must be worn during assembly.

Condensation and evaporation

All substances have three states: liquid, solid, or gas. Their aggregation state is determined by pressure and temperature. Liquids are converted to gas by evaporation and solids by sublimation. Separation of a liquid or solid from the gas phase is called condensation. Since normal ambient air contains about 10 g of water vapor per cubic meter, condensed water vapor is present on all surfaces. The adsorption on the surface is particularly pronounced due to the strong polarity of water molecules. Natural fibers, especially paper, can escape a lot of moisture during drying under vacuum conditions. Condensers are used to separate water vapor. Even some metals (cadmium, zinc, magnesium) can evaporate in large quantities at high temperatures of several hundred degrees Celsius. Therefore, the use of these metals in the manufacture of equipment should be avoided.

Desorption, diffusion, penetration, and leakage

"In addition to water, other substances, such as the vacuum pump working fluid, can also be adsorbed on the surface." Material can also diffuse out of the metal walls, which can be detected in the residual gas. In particularly demanding cases, the stainless steel container can be baked under vacuum conditions, thereby encouraging the release of most volatile components from the metal walls. Desorbed gas molecules, especially water, adhere to the inner surface of the vacuum chamber by adsorption and absorption and gradually desorb again under vacuum. In the vacuum system, the amount of gas discharged from the metal and glass surfaces due to desorption decreases with decreasing coverage and with time. A good approximation is generally obtained by assuming that after a given time point t > t0, the decrease will vary with time in a linear law. t0 is usually assumed to be one hour.

Diffusion with desorption

In operations below 10-6 hPa, the impact of surface desorption from plastics, especially seals, is greater. Plastic releases mainly the gas dissolved inside the plastic, which must first diffuse to the surface. As a result, the gas desorbed from the plastic is more dominant than from the metal surface as the pump downtime is prolonged. Although the surface area of the seal is relatively small; The decrease of the desorption rate with time is much slower than that of the metal surface. As an approximation, the square root of the analytical downdescent process with time can be assumed.

Infiltration and leakage

Seals, and even metal walls, can be permeated by small gas molecules, such as helium, through diffusion. Since the process is not a function of time, it leads to a continuous increase in the ultimate pressure. The amount of permeating gas is proportional to the pressure gradient across the wall thickness and the material dependent permeating constant.