Every industrial vacuum furnace system uses a primary (aka mechanical) pump, which is also commonly referred to as a “backing” pump when either used in series with a booster pump, or used with both a booster and secondary (“high vacuum”) pump combination, such as a diffusion pump. These primary pumps are further divided into “wet” pumps (e.g., oil sealed rotary vane style or liquid ring pumps) or “dry” pumps (e.g., claw, hook or screw). Of the dry primary pumps, the most common types are the claw pump and the screw pump. 

Dry pumps are being increasing 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. By Dan Herring

Last time we focused on the principles of operation of oil sealed rotary vane pumps including basic pump design, and pump oil. We now continue that discussion focusing on operational features and the inner workings of these pumps. 

Single Stage vs Two Stage Pumps. One of the limiting factors of a rotary vane pump is the Duo Seal, which is the oil filled non-contact seal in the small 0.025 mm (0.001”) space between the rotor and the stator at the top of the pump. In a single stage rotary vane pump, the pressure difference across the seal can approach 100,000:1 (1000 mbar vs. .01 mbar). Above this, the duo seal will start to leak oil from the high-pressure side to the low-pressure side (Fig. 1). This creates backstreaming, that is, the movement of pumping oil back into the vacuum furnace chamber. By Dan Herring

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 pumpdown 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. By Dan Herring

We continue our discussion of vacuum gauges by focusing on the types of gauges in use throughout the heat treatment industry including their function and application.

Types of gauges. There are three common types of gauges used in heat treatment, namely mechanical, thermal conductivity, and ionization gauges. Each is unique and well suited for its intended purpose. Mechanical gauges. The use of mechanical displacement to indicate pressure is considered a type of “direct acting” gauge. Examples are Bourdon gauges, U-tube and capacitance diaphragm manometers. They measure the force per unit area utilizing Dalton’s law: “Total pressure of a mixture of gases equals the sum of the partial pressures of each gas”. Direct acting gauges are not subject to the issue of gas sensitivity and do not need to be calibrated for the gas being measured. Their sensitivity is such that theyare not used in the high vacuum range (below ~10-4 mbar). By Dan Herring

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. By Dan Herring 

When designing a vacuum system it is important to take into account the system conductance. What is Conductance and Why Does it Matter? Conductance is the characteristic of a vacuum component or system to readily allow the flow of gas and can be thought of as the inverse of resistance to flow. It must be closely considered when designing a vacuum system and selecting the pump and other components, otherwise your vacuum chamber will take too long to reach the pressure required.

Well-designed piping of vacuum equipment, as well as proper component selection, increases production efficiency by minimizing the vacuum pumping time. It also minimizes energy use, making your equipment less expensive to operate. Ignoring the principal of conductance and designing the system with only physical configuration and flow rates in mind, can cause delayed equipment startup, plant downtime and process inefficiency because if a problem is found after startup, it can take considerable time and money to correct. By Dan Herring

To successfully process component parts in a vacuum furnace, we need to create and control the “atmosphere” surrounding the work. In general, applications run in vacuum furnaces can be broken down into five main (5) categories: Processes that can be done in no other way than in vacuum; processes that can be done better in vacuum from a metallurgical standpoint; processes that can be done better in vacuum from an economic viewpoint; processes that can be done better in vacuum from a surface finish perspective and process that can be done better in vacuum from an environmental perspective.

A principal difference between vacuum heat treatment and all other forms of thermal processing is the absence of, or perhaps better stated, the precise control of surface reactions. In addition, vacuum processing can remove contaminants, and under certain circumstances degas or convert oxides found on the surface of a material. Typical vacuum applications include industrial, food and packaging, coatings, analytical and medical technology, solar, semiconductor technology and research and development. In the heat-treating industry typical processes involve: Brazing, Hardening, Annealing, Case Hardening (e.g. carburizing, nitriding), Sintering, Tempering and Special Processes (e.g. degassing, diffusion bonding). By Dan Herring