Oil sealed rotary vane pumps (aka rotary vane pumps) are the primary pumps on most vacuum systems used in the heat treatment industry. They are also referred to as a “backing” pump when used in combination with a booster pump, or with both a booster and secondary (“high vacuum”) pump, typically a diffusion style. A rotary vane pump can also be used alone when high vacuum is not required and slower pump-down is acceptable.

Two-stage designs are available, which utilize two rotors in series internal to the pump. Single-stage designs can provide a vacuum of 3 x 10^-2 Torr (4 x 10^-2 mbar), while two-stage designs can achieve 3 x 10^-3 Torr (4 x 10^-3 mbar). Due to the prevalence of rotary vane pumps, it is important for designers and users of industrial vacuum equipment to have a good understanding of how these pumps function. This series of articles will cover pump principles of operation, pump designs, pump oils, single-stage versus two-stage pump designs, contamination and gas ballast (manual and automatic), common accessories, applications, troubleshooting and pump maintenance.

Principles of Operation

Of the various vacuum pump technologies, rotary vane pumps are considered wet, positive displacement pumps. They are often called “wet” pumps because the gas being pumped is exposed to oil used as a lubricant to help provide the seal. For this reason, the oil is carefully selected and specially designed for the application. Positive displacement indicates that the pump works by mechanically trapping a volume of gas and moving it through the pump, creating a low pressure on the inlet side.

Selecting the correct vacuum gauge or gauges is critical to the success of a heat treatment process. It is important to know how they work and what options are available so that the correct choice can be made. There are several important considerations when using a vacuum gauge. They include the method of operation, the gas composition (inert or reactive, corrosive), the gas sensitivity (calibration factor), and the process being performed in your system.

Given the wide range of pressures encountered when running processes in vacuum furnaces (a staggering 9 orders of magnitude), no one gauge is adequate over the entire range of possible vacuum levels. As with vacuum pumps, multiple gauges are necessary to properly cover the entire operating range with the needed precision and accuracy. Given that it is critical to monitor the vacuum pressure at various points in the process and perhaps multiple locations throughout the vacuum system, the correct selection of each gauge ensures that we achieve optimal results.

Dry pumps are becoming increasingly popular as an alternative to oil sealed rotary vane pumps for many medium and high vacuum applications (e.g., in low-pressure vacuum carburizing where fine granular soot is carried from the process into the pump). Designers and users of vacuum furnaces must have a good understanding of how claw and screw pumps operate. This includes the principles of operation, pump design, sealing, operating characteristics, features, purging, and ancillary devices.

A dry screw pump (Fig. 1) is used alone when high vacuum is not required and a slower drawdown is allowable, or with a Roots blower (aka booster) when higher performance is required. For the highest system performance, both a Roots booster pump and a secondary high-vacuum (e.g., diffusion) pump are utilized with the primary pump, which provides the fastest pumping speed and highest ultimate vacuum. When used in series with either a booster pump or with both a booster and secondary high-vacuum pump, the primary pump is referred to as a “backing” pump.

The Edwards “RV” (simply meaning Rotary Vane) laboratory sized oil sealed rotary vane vacuum pumps have been in the market since 1993. They have a very unique design with no equal. This article will attempt to show the reasons for its design and introduction 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.