In the first part of this article, we focus on heat exchangers used in vacuum gas quenching furnace systems (Fig. 1) and examining their design and function. In the second part, we review the similarities and differences between internal and external types of heat exchangers as well as the advantages and limitations of each design.

In vacuum processing, the load being heated in the furnace is rapidly cooled at the end of the heat cycle to impart desired physical properties in a process referred to as quenching. Although the metallurgical reasons for quenching vary depending on the process used, in all cases the goal is to quickly cool the load. Gas quenching involves introducing an inert gas (i.e. nitrogen, argon, or helium) into the furnace and rapidly circulating it through the heating chamber under pressure to remove the stored thermal energy from the load. While being circulated, the gas is forced through a heat exchanger to remove its heat.

There are several interesting and important factors that play a role in the design of a heat exchanger used for quenching and cooling of workloads in a vacuum furnace. In order to understand the function of the heat exchanger in the quenching process, we first want to review the fundamentals of the heat exchanger operation.

A heat exchanger is a device that transfers heat from one fluid (liquid or gas) to another fluid (liquid or gas) without the two fluids coming in direct contact. The type of heat exchanger typically used in a vacuum furnace is the finned tube type. Heat is first transferred from the hot gas to the fins and then to the tubes by convection, then through the tube wall by conduction and finally from the tube interior to the cold fluid inside the tube, again by convection. The efficiency of a heat exchanger is highly dependent on the mode of heat transfer, with convection being the dominant form of heat transfer in fluids. However, the conductivity of the materials must also be considered.
The basic equation (Equation 1) governing convection heat transfer is Newton’s law of cooling.

The heart of any vacuum furnace is said to be its hot zone and if properly constructed and well maintained will help ensure that the furnace performs in an optimal manner. One of the most important aspects of the hot zone is its insulation and the choice of materials used in its construction. In this article, we will focus on the two main Hot Zone configurations namely all-metal hot zones and carbon-based hot zones.

Thermal Insulating Systems: We begin, however, with a brief discussion of the three modes of heat transfer, namely convection, radiation, and conduction. To perform properly, the hot zone insulating system must contend with all three. In simplest terms, conduction and radiation may be thought of transferring energy to the surface of the part, while conduction is responsible for the heat to penetrate into the center of the parts. Conduction or conductive heat transfer occurs by direct molecular collision. On a microscopic scale, the kinetic energy of molecules varies in direct proportion to their temperature. So, as the temperature rises the molecules increase in motion and gain kinetic energy. They then collide with cooler surface molecules, transferring their kinetic energy and hence their heat, to the part surface.

A frequently asked question is, “How can I keep my vacuum furnace performing like it was when it was brand new?” This goes hand in hand with the question “How should I operate and maintain my vacuum furnace to maximize my investment and produce repeatable high-quality results?” In this article will provide seven (7) important tips for doing just this.

We plan to cover the following subjects:

Tip #1:     Maintaining Your Vacuum Pumps
Tip #2:     Selecting the Right Vacuum Level for the Job
Tip #3:     Avoiding Eutectic Melting
Tip #4:     Maintaining Your “O” Rings Seals
Tip #5:     Avoiding Diffusion Bonding
Tip #6:     Controlling Partial Pressure Additions
Tip #7:     Properly Supporting Your Work (Grids, Baskets & Fixtures)

Heating element design and selection is critical to the proper functioning of any vacuum furnace and is dependent on a number of factors: maximum operating temperature; power load; partial pressure and quench gases and life expectancy. The vast majority of vacuum furnaces are electrically heated. As such, heating elements are constructed of either high-temperature metallic alloys such as stainless steel, nickel-chromium, molybdenum, tungsten, tantalum, or of non-metallic materials such as graphite and silicon carbide.

Stainless steel and nickel-chromium alloys are commonly used for lower temperature applications such as aluminum brazing and at higher partial pressures, while graphite, molybdenum, and tungsten are more common for higher temperature processes such as hardening, sintering and nickel or copper brazing. Since the heating elements create the heat and transfer it to the load, the importance of choosing the proper alloy for its construction is critical in maximizing the heating element’s longevity, reliability, efficiency, and ultimately the process results. The different heating element types, as well as their advantages and limitations, are discussed here.

The most important materials used in man’s history have lent their names to the periods concerned, namely the stone, copper, bronze, and iron ages. Although it would be an exaggeration to call modern times the ‘graphite age’, graphite has become an indispensable and reliable material for the manufacture of modern high-performance products made from metal and ceramics. Graphite-based composites, in particular, are gaining constantly in importance. In the high-temperature applications field, these composites include carbon fiber-reinforced carbon (C/C for carbon composites) and carbon fiber-reinforced silicon carbide (C/SiC), as well as rigid and flexible felts based on pitch or viscose fiber. Carbon fiber-reinforced carbons (C/C for short) are used in modern vacuum or protective gas furnaces in the form of heating elements or charging systems. They are characterized by thermal shock resistance, the absence of distortion, low mass and strength increase with rising temperature. These features enable users to operate their plants more effectively, minimize reject rates and therefore reduce the cost of production. CIC is thus a key element in many process optimization steps and helps companies to improve their competitiveness.

Towards the end of the 19th century, carbon fibers based on pyrolyzed bamboo were used to manufacture incandescent filaments. In the 1950s, fibers with aligned crystalline structures were first produced in Great Britain. The starting materials now used for the production of carbon fibers are viscose (rayon), PAN (polyacrylonitrile) and pitch. Among these raw materials, the one with the poorest electrical and thermal conductivity is viscose. It is therefore often used as a thermal insulation material in the form of felts. Felts can be used up to a temperature of 2700°C. Pitch materials cost less than PAN, but the subsequent treatment required makes the pitch more expensive. by Alexander Racek, SGL CARBON GmbH