Many heat-treating applications put difficult demands on vacuum pumps in general and oil-sealed pumps, in particular. Byproducts from the heat-treating process can contaminate the vacuum pump oil and create higher vapor pressures that cause deteriorated vacuum levels in the heat-treating chamber, or buildup and blockages in the pump mechanism.
Whether the vacuum system in question is an oil-sealed or dry pump technology, the driving force is what works best for the heat-treatment process, the application at hand, and the needs of the customer. Here’s what to know about choosing the right vacuum pump for demanding heat-treating applications.
Examine the Level of Process Contamination
The level of process contamination varies considerably from application to application within the world of heat treatment. Where brazing may be considered relatively clean, carburizing is considered very dirty.
Though it may seem counterintuitive, a dry pump is not the ideal solution for this situation; in reality, the proven solution is an Oil-Sealed Rotary Piston (OSRP) Pump, such as the Edward Stokes 412J. Its capacity to handle large amounts of soot by-product stems from the fact that it has a large oil reservoir. The large amount of oil allows the pump to operate for reasonable periods of time under high contamination loads. On the other hand, it produces an oil waste stream that must be dealt with which requires more time and energy.
The Edward Stokes Microvac oil-sealed rotary piston pump.
On the opposite end of the spectrum are applications such as tempering, annealing and nitriding where cleanliness of the overall system is becoming increasingly important. Dry pumps provide a very suitable and reliable solution, especially dry screw pumps with their clean and dry swept volume and non-hydrocarbon lubrication.
Depending on the type of filler material used, brazing applications, although not considered dirty, may produce contaminants. Temporary binders, pastes and other fillers will sublimate to the vacuum system and contaminate the pump but the contamination is managed through the vacuum system using appropriate strategies.
Developments in Metal Additive Manufacturing
While there is a long history of providing suitable vacuum solutions to these heat-treat applications, this is not a static field. We’ve seen a number of new processes come from development of metal additive manufacturing systems for metal components manufacturing.
In these early days of metal additive manufacturing, processes were rapidly being developed for a wide range of applications. Each of these process solutions puts different demands on the vacuum systems, and in some cases, vacuum has been seen to enable metal additive manufacturing process development.
Not only do the additive manufacturing processes themselves rely on vacuum to enable the new process, but downstream heat treatment is also benefiting from the range of vacuum solutions available. These treatment processes are presenting new challenges. It’s important to assess chemistries for binders, and temperature and pressure parameters as part of the possible control set for vacuum pumps as these solutions develop.
Vacuum Technologies Evolve
Heat-treating applications that require, or benefit from vacuum pumps, have used oil-sealed piston pumps for years. These durable, reliable systems are certainly a standard recognized in the industry, but new technology development has provided some additional options for vacuum solutions to address the challenges of demanding applications.
Historically, some standardization has occurred due to available pumping speeds. For example, Stokes oil-sealed vacuum pumps have a limited range, which includes only a couple of base pumps. It’s likely this limitation is evolving, as more pumping speed ranges have become available. Additionally, variable speed drives are now maturing and accepted as an alternative to fixed speed systems for control and energy saving opportunities. These can provide a tuned speed alternative to the historically limited pump speed selection available.
New technology developments have provided some additional options for vacuum solutions to demanding applications. The use of dry vacuum pumps, for example, eliminate the oil in the vacuum-generating mechanism. This can introduce other undesirable attributes such as high capital cost, however, and over time the reduced maintenance expense can mean overall lower cost of ownership.
The Edwards GXS dry screw vacuum pump.
Understand the Vacuum Environment
It’s important to understand the vacuum environment. Often times, that means looking at in-house vacuum applications with the same or similar process conditions and evaluating the merits of the given type of vacuum pump for the given application.
The focus of this practice is reliability and frequency of maintenance. Of course, it is important to detail the make, model and configuration of the existing vacuum pump and then consider the frequency of oil changes, or other types of maintenance, while also weighing the pros and cons. There are always trade-offs: an Oil-Sealed Rotary Piston (OSRP) Pump is very reliable but requires frequent oil changes. Sometimes the dry pump works well but needs a full rebuild after two years of operation. A simple cost of ownership model will point decision makers in the right direction.
Knowledge of the same or similar applications in a given facility and one’s personal experience juxtaposed to the extent of maintenance activity/cost of ownership provides an avenue to propose alternative solutions and/or technologies.
In some instances, the frequency of oil changes is daily or weekly, so the solution may involve a change in pump technology but may also require additional equipment such as an inlet filter. Regardless of the type of pump, it may be possible to reduce cost of ownership with improved setup, looking to see if the extent of contamination can be controlled. In other instances, the vacuum pump operator is dealing with challenging vapors, which cannot be easily trapped. These may plate out in a vacuum pump resulting in gummy or sticky buildup.
It is incumbent on the user to be aware of a variety of these additional factors that affect vacuum and therefore, heat-treating performance. These can include binder chemistries in brazing applications where, for example, the high temperatures and low pressures can turn acrylic binders into a char that ends up in the pump. This can be detrimental for both oil-sealed and dry vacuum pumps.
In carburizing applications, depending on the gas source (butane or acetylene for example) deposition of degraded polymer chains can have a similar impact on the vacuum pumps in terms of maintenance requirements and/or performance.
Regardless of the situation, a reputable vacuum solutions provider will ask many questions in order to understand the vacuum pump challenges so the proposed solution will solve the problem without creating other problems. This is all part of a value add proposition users should expect from their suppliers.
Pump Sizing Software Aids in Selection
Size matters when it comes to investment dollars. When looking to make a large investment in vacuum, there needs to be some degree of certainty that the dollars invested will equate to desired furnace throughput.
Getting it wrong puts production targets in jeopardy and reputations on the line. It’s why years of vacuum expertise in conjunction with software tools, can help ensure success. One such tool is Edwards’ PumpCalcTM, a software suite used for sizing vacuum pumps and systems. This advanced tool provides invaluable insight into the expected performance of a proposed vacuum system and allows for comparison of various sizes and configurations to determine what will work best. PumpCalcTM is expanded into TransCalcTM, a tool within the software suite that simulates the modeling of large, distributed and dynamic vacuum systems as might be the case on very large metallurgical furnaces utilizing large boosters and vacuum pump combinations.
The Right Information Results in The Best Solution
Gathering the right information to select the appropriate vacuum pump for a given application is essential. In practice, however, the process of evaluating an application for vacuum pump selection is not always straightforward – the best place to start is to examine what is already known and to proceed from there.
The practice of gathering the facts, applying applications knowledge and incorporating all resources and tools to solve the problem at hand is a best practice. It’s especially important when designing a customized system to best fit the needs of the specific heat-treatment process.
About the Author
David Sobiegray received a Bachelor’s Degree in Marketing from Niagara University in 1980. He has worked in the vacuum industry for 35 years and has held various positions in industrial sales, technical, and global product management. He currently holds the position of Senior Product Manager, Industrial Vacuum Equipment and Systems, Edwards.
Edwards is a leading developer and manufacturer of sophisticated vacuum pumps, vacuum system products, abatement solutions and related value-added services. Our products are used for analytical instruments, a wide range of R&D applications and are integral in the manufacturing processes for semiconductors, flat panel displays, LEDs and solar cells. Edwards’ products are used within a diverse range of applications including power, glass and other coating applications, steel and other metallurgy, pharmaceutical and chemical processes. For more information, visit www.edwardsvacuum.com, or email: firstname.lastname@example.org.
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