Most of us are familiar with processing in the vacuum range up to around 1.33 x 10^-3 Pa (1 x 10^-5 torr) or slightly lower. There are also lessons to be learned from understanding the demands of ultra-high vacuum applications. What is an Ultra-High Vacuum? Practical high vacuum levels (Table 1) range down to approximately 1.33 x 10^-4 Pa (1 x 10^-6 torr) while ultra-high vacuum (UHV) levels are in the vacuum range characterized by pressures of about 10^-7 Pa (7.5 x 10^-10 torr) and greater. These vacuum levels demand the use of special materials of construction and processing techniques such as preheating (i.e. bake-out) of the entire system for several hours prior to processing to remove water and other trace gases, which adsorb on the surfaces of the chamber.

These vacuum levels demand the use of special materials of construction and processing techniques such as preheating (i.e. bake-out) of the entire system for several hours prior to processing to remove water and other trace gases, which adsorb on the surfaces of the chamber. At these low pressures, the mean free path of a gas molecule is approximately 40 km (24.8 miles), so gas molecules will collide with the chamber walls more frequently than they collide with each other. Thus, almost all gas interactions, therefore, take place on various surfaces in the chamber.

Acquisition of a vacuum furnace represents a major capital equipment investment and one that creates a long-term relationship with your supplier partner. Thus the choice of what to buy and who to purchase it from requires careful planning and considerable up-front research. You need to know when and how to apply vacuum technology if it will be the most cost-effective solution for what you need to do, what questions to ask and what information to provide.

The process begins by understanding your specific needs and asking all the right questions. Is it more prudent to upgrade an older piece of equipment, purchase new or purchase used? Is it better to have one large furnace or two smaller ones? Is a batch solution best or is a continuous approach better?

Ask yourself what the equipment must do, what productivity must be achieved (now and in the future) and what type of specifications or compliance requirements (e.g. Nadcap, CQI-9) must be met. The type of material(s) being run, the skill of your workforce, the (internal and external) support available and the type of controls and/or quality records required are additional considerations. All of these will help define how much training and support will be needed from your supplier partner.

Low-temperature vacuum heat treatment offers unique advantages to a variety of industries including Aerospace, Automotive, Electronics, Household Appliances, Machine Tools and Tool and Die as well as Commercial Heat Treaters who must serve all of these customers. Low-temperature heat treatments that involve a vacuum purge at the onset of the cycle have become increasingly popular throughout the industry. These operations are conducted in vacuum furnaces and furnaces that employ a vacuum purge prior to the beginning of the heat process with parts placed inside a special vacuum tight vessel or in a retort. Processing being run using these methods take advantage of a highly controlled environment designed to minimize surface interactions.

Low-temperature processing can be batch or continuous, either as stand-alone units or “modules” incorporated into a continuous vacuum furnace system. The following is a basic description of the operation of a typical batch vacuum furnace. Once a workload has been positioned into the unit, an outer door is closed and the vacuum process can commence. A mechanical vacuum pump, optionally equipped with a blower, produces a vacuum level as low as 1.3 x 10^-3 mbar (1 x 10^-3torr), a common vacuum level being under 1.3 x 10^-1 mbar (1 x 10^-1 torr). This is normally achieved in 10 – 20 minutes depending on the size of the pumping system and the nature of any contamination present on the workload. In some instances, a double pump-down sequence is initiated once an initial vacuum level lower than 6.7 x 10^-1 mbar (5 x 10^-1 torr) is reached. Once the desired final vacuum level is reached, the unit is backfilled in the range of 667 mbar (500 torr) to 3.4 x 10^-2 bar (0.5 psig) positive pressure with an inert gas such as nitrogen, argon or nitrogen/hydrogen (3% maximum) and heating begins. After reaching setpoint and soaking at temperature, a cooling cycle is initiated, typically with hot gases circulated through an internal or external heat exchanger to accelerate the process.

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.