Resistance to gas flow through components that make up a vacuum system has a considerable effect on the pumping speed and pressure obtainable within the system. Any pipe or component that gas has to flow through is a hindrance to the flow of that gas, i.e. it offers resistance to the flow. It occurs in the roughing line, the foreline and the high vacuum piping and affects the amount of time taken to evacuate a vacuum chamber to its required base or process pressure.

For example, if a large vacuum chamber is connected to its vacuum pump using a long small bore pipe the gas flow down the pipe will be difficult and the gas will be removed from the chamber very slowly. There is a high resistance to gas flow and the conductance of that small bore pipe is low. Let’s first look at a simple vacuum system, using a single mechanical vacuum pump, the vacuum chamber if often mounted right above the inlet to the vacuum pump. By Howard Tring

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. By Howard Tring

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. By Howard Tring 

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.. By Howard Tring

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. By Howard Tring

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. By Howard Tring

This article completes the series of Five Main Reasons that vacuum is used in science and industry; To provide a working force, to remove active and reactive constituents, to remove trapped and dissolved gases, to decrease thermal transfer and finally to increase the mean free path to a useful dimension.

The article printed back in January this year talked about solid, liquid and gas states of matter. The following is a short excerpt from that article. “In a gas the atoms and molecules are generally much further apart than in solids and liquids. In air at atmospheric pressure and room temperature the actual space occupied by atoms and molecules is about 0.01 per cent or one ten thousandth of the volume. The equivalent for solid copper is about 74 percent or close to three quarters. (So much for being called a “solid”). In air the molecules are in constant random movement, typically in a straight line, and the interatomic forces have little effect due to the space between the molecules. The moving molecules will constantly collide with other molecules and then move away in a different direction. These collisions occur about 10,000,000,000 times per second at atmospheric pressure.”. By Howard Tring

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. By Howard Tring

This article continues the series of five reasons that vacuum is used in science and industry; to provide a working force, to remove active and reactive constituents, to remove trapped and dissolved gases, and to decrease thermal transfer.

If you commute to work by car and your drive is an hour or so, you may well take a coffee or another hot or cold beverage in your own personal container to drink on the way. Often this type of container is a vacuum insulated cup with a lid. A step up from that, if you work on a jobsite for example, would be a vessel that holds several cups of liquid for all day use. You may know these as “Thermos” or “Aladdin” flasks which are two of the trade names for vacuum insulated containers. Let’s discuss the thermodynamics of vacuum insulated vessels and then look at other uses for them. By Howard Tring

The Edwards “RV” (simply meaning Rotary Vane) laboratory sized oil sealed rotary vane vacuum pumps have been in the market for 21 years. They have a very unique design with no equal.

This article will attempt to show the reasons for its design and introduction in 1993 and then explain the features of the vacuum pump that make it one of the best small vacuum pumps available today. This is not an official Edwards account, although the engineering related content is based on Edwards information, it contains my personal knowledge, experience and understanding from working with these pumps for many years. By Howard Tring