The Edwards “RV” (simply meaning Rotary Vane) laboratory sized oil sealed rotary vane vacuum pumps have been in the market for 25 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.
In this article we will only discuss one and two-stage “medium vacuum” oil sealed rotary vane vacuum pumps that can produce a catalog ultimate vacuum of about 1 x 10-2 Torr (0.01 Torr or 10 microns) for a one stage model and about 1 x 10-3 Torr (0.001 Torr or 1 micron) for a two-stage model. We will review the Vee Belt Drive design and the Direct Drive design.
Smaller vacuum pumps such as those used in the heating, ventilating and air conditioning industry (HVAC) are not included as they are often only for intermittent use and do not have the design features built into the laboratory sized continuous running vacuum pumps used in industry and science. Larger rotary vane vacuum pumps, ones that require ball or roller bearings to support the weight of the rotor are not included either. Although they have many similar features to the laboratory sized vacuum pumps, they also have a variety of options to suit different applications.
In this article, we’re going to take a step away from vacuum pumps and systems and write about general applications that use vacuum in the process. There may be some applications you have heard about and some, hopefully, that may be new to you. Whenever a vacuum (a pressure lower than the surrounding atmospheric pressure) is used in a process it will generally fall into one of the Five Main Reasons for using Vacuum. In some cases, a process may use vacuum for two of the five reasons. This month I will discuss the first of these reasons, in no specific order.
First, a short explanation of the two vacuum measuring units used in this article. We know that standard atmospheric pressure is 14.7 lbs. in-2 and that your real life atmospheric pressure varies up and down a few percentage points from the standard depending on a) weather conditions in your area and, b) your altitude above sea level. Remember that “low” pressure is the same as “high” vacuum, and conversely “low” vacuum is the same as “high” pressure – but still below atmospheric pressure in the vacuum industry. I have written a small number and read many technical articles about vacuum and it is very difficult to ensure that these terms are consistent throughout. In the example below we also have to understand that the vacuum measuring units read in opposite directions and we have to change one of them. Does that sound confusing? Yes, it is.
By now, most maintenance departments should have a plan in place for schedules repairs and maintenance during the summer months, especially if the plant has a shutdown for vacation. The part of the vacuum furnace system that I know best is the vacuum system, so I will concentrate on this.
The summer maintenance schedule should cover all the vacuum pumps on a vacuum furnace. For some it will just be mechanical pumps, the rotary piston pumps and the Roots booster (or blower); for others it will include the oil diffusion pump or pumps and the small rotary vane holding pump, if used.
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..