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Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

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Oil Sealed Rotary Vane Pumps

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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.

Proper Selection and Use of Vacuum Gauges

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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: Screw Type

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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.

Instrumentation and Process Control in Vacuum Furnaces

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The type and reliability of instrumentation and process controls used on vacuum furnaces in the heat treatment industry is critical to both the performance of the vacuum furnace itself as well as the results that are achieved when processing critical components. It is not an understatement to say that given the life expectancy of vacuum equipment, instrumentation and controls should be updated every few years to take advantage of the most advanced technology possible (e.g., remote communication and diagnostics, process monitoring and control by Internet-based devices and the like).

Demands on Instrumentation and Control Packages

Temperature control and as a result temperature uniformity can be difficult because of the heat transfer characteristics of the furnace as it moves, for example, from convection to radiant heating and convective/conductive heat transfer during quenching. For example, the ability to vary the furnace heating rate (e.g., 3°C/min – 25°C/min) demands precise and accurate measurement and control, including setpoint program control with guaranteed soak features.

Vacuum furnaces are often used for a variety of products and processes by the heat treater making recipe management an important function. Temperature overshoot of set points is typically not allowed. Setpoint program control is often applied to the temperature, vacuum level and gas pressure with extensive interaction between these programs and also with the logic control.

Process Applications Run in Vacuum Furnaces

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There is a constant need throughout the industry to produce the highest quality parts to the most stringent product specifications. Both long-established and new materials are being employed to meet the needs of lighter, stronger, smaller and more efficient designs, and the use of vacuum technology in manufacturing is of paramount importance in achieving these goals.

The strategy being adopted by manufacturing to meet these needs relies heavily on vacuum processes and equipment through:

  • Process development – New materials, new products, and new applications demand absolute cycle repeatability, flexibility, and control, and as such, designers are specifying vacuum processing over other heat treatment methods.
  • Process substitution – Older process technologies and the equipment associated with them are being replaced by vacuum equipment. The justification lies in reduced unit cost achieved by lowering the overall cost of manufacturing and/or through material and efficiency savings.
  • Process replacement – Product-performance demands are forcing designers to look toward vacuum processing and its ability to offer a superior product for the same or very similar cost.

Vacuum Systems for Composite Manufacturing

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More and more application uses are being found for composite materials and their use is expanding beyond just aerospace applications. As such, you may be wondering how they are manufactured. Composites are manufactured using multiple layers of material, each with different properties, combined into a single structure. By joining them in this manner, the resultant “composite” material is unusually strong and light. It has greater strength, flexure, and more favorable mechanical properties than any of the individual materials used to make the composite. Metal matrix and ceramic matrix composites enjoy widespread use in the wind power, automotive and aerospace industries, among others (Fig. 1), as they offer unique advantages over steel and aluminum. Composite manufacturing is a growing industry and has strong continued prospects for expansion.

The most popular composites, valued for their very high strength to weight ratio, are made of a woven base material such as fiberglass, aramid (Kevlar®) or carbon fiber impregnated with a resin that is hardened into a plastic using heat and/or pressure. The woven material is known as a “prepreg” since it has not yet been impregnated with resin. The resin can be epoxy, polyester, polyurethane, or other plastic in liquid form. The base fiber, such as carbon, has high tensile strength but is not stiff enough on its own to be used as a structural element. The resin can be molded and formed into various shapes but is not strong enough on its own to serve as a structural member. When the base fiber and the plastic are combined, the resultant composite material has both the strength of carbon fiber and the retained shape of the cured epoxy or other hard plastic. This feature, the ability to incorporate the most favorable mechanical properties of each of the constituent materials, is what makes composites such an attractive choice for a wide variety of lightweight structures.

The Rotary Vane (RV) Vacuum Pump

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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.