Even as turbomolecular vacuum pumps have displaced most small laboratory sized oil diffusion pumps these days because of perceived ease of use and cleanliness, most high vacuum heat treating furnaces still rely on a large oil diffusion pumps to generate the pressures below about 10^-3 Torr needed for many metal conditioning processes.

The main reason for this is that turbomolecular vacuum pumps have a physical size limit due to the high rotational speed of the rotor. That size limit is around 320 mm or 13 inches inlet diameter and may vary a small amount from manufacturer to manufacturer. In many cases the pumping speed may not be high enough as it is directly related to the inlet size of the pump. Metal can disintegrate at very high speed, so the tip speed of the rotor blades has to be within the safe limit. Turbomolecular pump rotors have to move faster than the speed of the gas molecules they are pumping in order that the rotor blades can deflect the gas molecules downwards in the pump mechanism. The second reason that turbomolecular pumps are not used in many metal treating systems is they cannot tolerate any particulate matter entering them. They must only be used on clean vacuum systems.

When a vacuum system is designed it is often necessary to select a mechanical vacuum pump or pump set that will evacuate the chamber and associated piping in a certain amount of time. In laboratory or research situations that may not be as necessary as in a production environment where the time to complete a process has quite a lot to do with the cost of the manufactured part. In addition, the cost of the vacuum system has to be taken into consideration as well. Larger pumps may reduce the evacuation time, but also are more expensive. There has to be a balance between all the parameters.

There are two simple methods for calculating evacuation time; one for a rotary vacuum pump, vane or piston, on its own, and a second for larger volumes when a vacuum booster pump may be used. Both methods give good results for simple vacuum systems where the mechanical vacuum pumps are located close to the chamber and the chamber is relatively empty.

In the world of mechanical oil sealed rotary vacuum pumps there is a need for a variety of oils and fluids to suit the specific type of pump, its duty and the process it is used on. This discussion covers high vacuum pumps only, such as are used in the heat treating and vacuum furnace industry. These same vacuum pumps are used in many other industrial and scientific applications and have to work under many different types of conditions including one that many people expose their pumps too – neglect!.

Rotary vane vacuum pumps are available as direct drive (usually 1800 rpm) and vee belt drive (between 400 and 500 rpm) versions. Rotary piston vacuum pumps are generally vee belt driven and run at about 500 rpm. The work duty of a vacuum pump can vary between intermittent use and running continuously. They can also be used for cyclic duty, to evacuate a loadlock for example, where the pump evacuates a chamber from atmosphere to vacuum every few minutes. The vacuum process can also vary, from clean air pumping to hazardous gas, wet vapor pumping and dirty/dusty atmospheres..

This article is written for vacuum pumps such as the oil sealed rotary piston pumps used on many heat treating and vacuum furnace applications. The same information would also apply to the oil diffusion holding pump if it is used. This pump may be either a vee belt driven pump or a direct drive pump. The holding pump is used to keep the oil diffusion pump evacuated below the critical backing pressure when the main pump is in roughing mode.

All mechanical vacuum pumps need maintenance and the pump manufacturer usually lists the basic checks needed in the pump operation manual. This will vary with the application that the pump is used on but, at a minimum, will include the following: check oil level daily or weekly, depending on the application and use, change oil and check the shaft seal area for leaks every 6 months and inspect the exhaust valves and gas ballast valve seals every 12 months.

This article discusses inlet filters that are used on oil sealed mechanical medium vacuum pumps such as rotary vane and rotary piston pumps typically used on vacuum furnaces and, for smaller pumps used for many laboratory and light industrial applications. One of the downsides of any trap is that it will eventually require servicing. Many vacuum system operators prefer not to use traps for that reason. If the correct traps are used and maintenance is planned, the downtime and service costs can be kept in line.

There are four types of inlet filters used on vacuum pumps used in laboratories and in light industrial applications: foreline traps, catchpots, dust traps and vapor traps. The first, foreline traps, are used to prevent contamination coming out of the vacuum pump; and the other three are used to prevent contaminants from entering the vacuum pump. Foreline traps – This type of trap is to prevent oil vapor that moves out of the pump inlet under low pressure conditions when the gas is in molecular flow. That would be at a pressure lower than about 0.1 Torr or 100 microns. The ultimate vacuum of an oil sealed vacuum pump is reached when the hot oil in the pump starts to evaporate. Under these conditions some molecules of oil vapor will backstream from the pump inlet toward the vacuum system. Although back streaming of oil vapor occurs in larger pumps as well, it can be more critical in smaller vacuum systems where the piping is shorter. Instruments such as mass spectrometers, electron microscopes and ultra-high vacuum systems can be contaminated if oil vapor reaches them so most of these instruments use foreline traps. If these instruments become contaminated it can take several days to clean them out and return them to operation.

Oil diffusion pumps remain in popular use in the vacuum heat treating industry, possibly one of the few applications remaining for this type of high vacuum pump in the western world. The main reasons for their continued use are their longevity the lack of other options. When your process requires a pressure below that of a mechanical pump or mechanical pump and Roots pump combination a secondary vacuum pump has to be used. These are oil diffusion pumps, turbomolecular pumps and possibly cryogenic pumps.

Turbomolecular pumps are limited in physical size due to the high rotor speeds needed to create molecular flow into the pump mechanism; and both “turbos” and “cryos” are very susceptible to process contamination. Large cryos are often used in vacuum coating applications but, as far as I am aware, not in vacuum heat treating applications. I think that many oil diffusion pumps are still used for industrial and some scientific applications in the eastern parts of the world where the cost of a turbomolecular pump is still very high based on the local costs of doing business.

States of Matter – All matter consists of atoms, and some atoms combine with others in a chemical reaction to form molecules. For example, water consists of two atoms of hydrogen combined with one atom of oxygen (H₂O). Some gases such as argon (Ar), helium (He) and neon (Ne) are unlikely to combine with a similar or dissimilar atom, while others such as hydrogen, nitrogen, and oxygen and usually combine with an identical atom forming a molecule (H₂, O₂ and N₂) and are called diatomic.

Matter is divided into three states: solid, liquid and gaseous. Some matter can exist in each of the three states, for example, water that can be ice (solid), water (liquid) and steam (vapor) depending on the surrounding pressure and temperature (Fig. 1). Another example is carbon dioxide that can also exist in all three states.

The Solid state (Not to be confused with “solid state” electronic components)

In a solid, the atoms are bound tightly together in fixed positions relative to each other by interatomic forces. Therefore a solid has a fixed volume at a specific pressure and temperature. Changes in temperature will change the energy in the atoms and cause tiny vibrations about their position but the bonds are relatively strong and most solids can withstand reasonable temperature changes without breaking down apart from small expansion or contraction if the temperature changes. Even though a solid appears “solid” there are very small spaces between individual molecules. Metal solids, which are ones most prevalent in the vacuum furnace and heat treating industry, all have a melting point and if enough heat is applied the solid will eventually change to a liquid. This liquid, if more heat is added will eventually evaporate and change to a vapor.