Vacuum furnace users are striving to increase productivity and reduce operating costs. Higher performance and reduced maintenance is now the target. Through use of dry-compressing vacuum pumps, this target can be achieved. This article will introduce the modern design of a dry-compression vacuum pump with cantilevered screw rotors and feature advantages for use in demanding vacuum heat-treatment applications including sintering, brazing and carburizing.

Fig. 1. Screw rotors with pumping chamber


While there are obvious environmental benefits in using dry-compressing, screw-type vacuum pumps – no contaminated waste oils and emissions, spent filters and related disposal – the main drivers are performance, ease in maintenance and cost-effectiveness when pumping in harsh heat treatments that include or produce condensable vapors, particles, binders, fluxing agents and even cracked residues such as soot and tar. In contrast, when entering an oil-sealed vacuum pump, those items create the need for frequent and costly oil changes, filters and replacements of internal shaft seals. The productive time of the furnace is reduced by the necessary time for maintenance and service. Therefore, the risks of unplanned downtime are increased.

Suitable dry-compression, screw-type vacuum pumps can reduce maintenance to a simple cleaning without the need for costly spare parts. Use of a modern monitoring system can detect real-time status of critical pump parameters, thereby allowing the user to conveniently schedule maintenance and service.

Fig. 2. Cantilevered vs. conventional rotor

Why use Dry-Compression, Screw-Type Vacuum Pumps in Heat-Treat Applications?

Vacuum pumps that require simple care and maintenance are an essential component in a modern vacuum furnace and play an important role in reaching the goals of increased productivity and operation cost reduction. Unexpected pump failures can stop production. The maintenance and service needs of vacuum pumps reduce uptime and productivity of a furnace. Vacuum pump performance losses over time can lead to an unstable product quality.

Oil-sealed vacuum pumps combined with high-speed boosters are today’s standard equipment for most industrial vacuum furnaces. These vacuum pumping systems do have relatively low investment costs and are still the norm for a broad field of heat treatments. Applications such as annealing, hardening and tempering cause only a benign load with negligible impact on the vacuum system. Therefore, oil-sealed vacuum pumps offer efficient, cost-effective and very reliable performance.

Conversely, if oil-sealed vacuum pumps are used in more demanding heat-treatment applications such as sintering, brazing or carburizing, where the gas load entering the vacuum system contains vapors or particles, the impact is more pronounced. Oil-sealed pumps can still work here, but depending on the exact application details, they can require extensive maintenance. Very frequent oil, oil-filter and exhaust-filter exchanges are typical. Performance of oil-sealed vacuum pumps degrades over time since it is influenced by the vapors that condense inside the pumps’ oil. Further, with vapor and particle-laden gas loads, these pumps often require an annual or even semi-annual rebuild where exchange of all wear parts such as bearings, shaft seals and internal valves is not untypical.

Additionally, reduced maintenance staffing at user sites can also increase the risk of unexpected vacuum pump failure caused by lack of routine maintenance, resulting in the loss of an expensive batch of products.

Fig. 3. SCREWLINE SP 630

Concept of Dry-Compression Vacuum Pump with Cantilevered Screw Rotors

A screw is capable of pumping a gas-load while moving particles in the pump chamber without slowing the particle velocity. This eliminates the chance for particle buildup and clogging and is a great advantage if significant quantities of particles are present. Moreover, the pump chamber is formed by only three components (two screw rotors and rotor housing) so that disassembly and cleaning is very simple (Fig. 1).

The preferred pumping mechanism is two screw rotors that are housed in a cantilever arrangement (Fig. 2a). Thus, bearings and shaft seals on the vacuum side of the pump – a notorious weak point of vacuum pumps – are avoided (Fig. 2b). The bearings for the rotors are located in a segregated gear housing and are lubricated directly by the gear oil. This arrangement allows for rapid on-site disassembly of the rotor housing, thereby permitting a cleaning of all internal surfaces within the pump in contact with the medium.

Other desired features when selecting a dry-compression, screw-type vacuum pump in view of the aforementioned application requirements are:
  • A decreasing pitch of the screw profile from the intake to the delivery side can create an internal compression, resulting in very low power consumption.
  • A shaft-seal design on the delivery side of the screw rotors that, due to a low pressure differential with respect to the gearbox, allows for the use of a combined piston ring and labyrinth seal. By means of purge gas, the seal may, if required, be effectively protected against any harmful process media. This also prevents any escape of process gas to the surroundings of the pump. A combined piston ring and labyrinth seal are virtually wear-free when compared to traditional lip seals commonly used in vacuum pumps.
  • A modern monitoring system that allows for continuous real-time monitoring of the vacuum pumps’ critical parameters – vibration level, gear oil temperature and level, exhaust pressure and pump run time. Such monitoring can detect the formation of deposits on the screw rotors, and bearing wear can be detected at an early stage to avoid unscheduled downtimes.
  • A flushing-kit facility that allows the cleaning of the screw rotors in-situ while the vacuum pump is running. Even when using only a periodic regimen of water, cleaning is efficient due to the water-jet effect coming from the high screw-rotor speeds. Depending on the type of contamination layers present, other solvents have been successfully qualified.
  • Optional accessories such as adapters for directly fitting a booster stage onto the vacuum pump (Fig. 3). Others include exhaust silencers for proper noise abatement and drainage capability at the exhaust side.


Fig. 4. Inside a dry-compression pump

Application Examples

Vacuum Sintering of Cutting Tools
During the de-waxing step of vacuum sintering or metal injection molding, binders and lubricants that are mixed with the powder metal evaporate. Since polymers such as polyethylene (PE), polyethylene glycol (PEG) or polypropylene (PP) are used, these consist of very long hydrocarbon chains, which cannot simply evaporate. Caused by the heat, they crack toward smaller chain lengths, which then can evaporate. These newly formed chemical compounds, which are more or less undefined, then enter the vacuum pump. In an oil-sealed pump this leads to problems, such as heavy buildup and clogged and sticking parts. Inside a dry-compression pump, the vapors tend to build layers inside the compression room (Fig. 4).

These layers do not damage the pump, but frequent cleaning is required. The necessity of a cleaning will be detected by the vibration sensor in the pumps’ monitoring system. Whenever the layers start to build up, the clearance in between the two screws is decreased, thus causing vibration to increase. The user will get a vibration warning and can schedule a pump cleaning at the next convenient period.

Fig. 5. In-situ water flushing

The user simply cleans the pump with periodic in-situ water flushing (Fig. 5), which can be completed without any pump disassembly and reduces the cleaning time to less than 30 minutes. Other users have further automated this cleaning step by programming it into their process control. Every time a vibration warning comes up, the cleaning is automatically done at the end of the batch. An operator will only be involved if the vibrations cannot be removed by this automatic flushing.

In contrast with polymers, paraffin-based binders are short-chained hydrocarbons that can evaporate without cracking. Entering the vacuum pump, which is colder than the furnace, the vapors condense into the liquid phase. The oil inside an oil-sealed vacuum pump has a certain tolerance for the paraffin, but when the oil becomes saturated or when the pump cools down after being switched off, the paraffin wax starts to solidify and blocks oil channels and exhaust filters inside the pump. This causes a reduced lubricating effect for the pump. High wear of internal components, pump housing and bearings can follow. If the pump is not well maintained, degradation of pump performance is imminent or even pump seizure, which may result in the loss of a production batch.

Fig. 6. Dry-compression, screw-type pump housing covered with solidified paraffin wax

When using a dry-compression, screw-type vacuum pump, paraffin vapors will also condense, but in contrast to an oil-sealed pump, the dry screw-type pump can handle the liquid wax. Since the dry screw-type pump is warm enough to hold the paraffin above the solidification temperature, the liquid wax is pumped out through the exhaust by use of the gas-ballast valve (an on-board controlled gas leak into the compression stage), where it can be continuously drained out of the warm exhaust silencer. For long interval, trouble-free operation of a dry screw-type pump in such an application, it is also important that the pump temperature is not too high. Excess heat for a long period of time can cause a cracking of the paraffin binder, which would lead to an unnecessary demand for pump cleaning. The desirable exhaust temperature range is 80-100°C (176-212°F), which is well suited for this application.

Experience gained using a dry-compression, screw-type vacuum pump in both applications revealed that the pump did not degrade over time nor did it require any additional service. As a result, the users’ product quality remained stable. Even restarting the cooled-down pump filled with solidified wax (Fig. 6) was not problematic.

Fig. 7. Dry-compression, screw rotors with no buildup of fluxing agents

Vacuum Brazing
During the brazing process, the fluxing agents used partly evaporate and enter the vacuum pump. In an oil-sealed pump these agents condense inside the oil. The result is a degradation of end-pressure achieved by the pump and a reduced oil lifetime due to chemical attack. Also, the pumps tend to corrode if maintenance is not well executed.

Using a dry-compression, screw-type vacuum pump with open gas-ballast valve combats condensation from forming inside the pump, so these pumps are not influenced at all by the fluxing agents. The dry screw-type pump does not require any additional process-related maintenance. Even after a one-year operation the pump did not show any corrosion or buildup of layers (Fig. 7).

Fig. 8. Dry-compression, screw rotors covered with layers

Vacuum Carburizing
In vacuum carburizing, hydrocarbon gases are introduced into the furnace. Touching the hot steel surface, the hydrocarbons decompose into radical carbon and hydrogen. The carbon partly diffuses into the steel and hardens the steel’s surface. Depending on the gases used – commonly acetylene or propane – and the process conditions, the decomposed hydrocarbons tend to buildup new compounds. This reaction is relatively uncontrolled, so formation of soot and tar is typical. The vacuum pumps must therefore be able to handle not only those hydrocarbons that have not decomposed but also soot and tar.

Oil-sealed pumps have some tolerance for these compounds when choosing suitable oils, external oil-filtration accessories and regular oil exchanges. Nevertheless, the needed pump accessories increase the investment for such systems, and the operation costs are high because of the frequent maintenance demands. Even still, oil-sealed pumps normally require a regular rebuild overhaul every six to 12 months in demanding applications.

In a dry-compression, screw-type vacuum pump, the buildup of layers can be detected (Fig. 8). A regular screw rotor and housing cleaning are therefore necessary and can be manually executed by the user or via an automated flushing process.

Alternatively, the buildup of layers can be avoided if the screw rotors are continuously wetted with suitable oil (Fig. 9). This will avoid the buildup of sticky layers attaching to the inner pump surfaces. The oil will continuously flush out the vapors entering the pump. Even if this hybrid method of lubricating a dry-compression, screw-type vacuum system seems counterintuitive, a continuous operation of the vacuum system is possible by use of this measure. Additionally, the screw rotors and housing did not show any wear and therefore need no additional process-related service. The monitoring system will enable the user to detect any wear or buildup of layers over time so that the furnace can be switched off in a convenient and controlled way.

Fig. 9. Hybrid dry-compression, screw-type vacuum system with continuous oil lubrication

Conclusion

Modern dry-compression, screw-type vacuum pumps are clearly a step ahead in demanding heat-treatment applications. They offer the furnace user increased uptime and cost of ownership reductions – not only in the aforementioned harsh application examples but also in benign applications where low maintenance and trouble-free operation is easily achieved. Modern on-board monitoring systems now enable the user to gain an internal view of critical vacuum pump parameters in real-time. This allows the user enhanced control of the vacuum process, whereby maintenance may be scheduled in accord with planned factory maintenance.

Dry-compression vacuum pumps have evolved over the last decade. Even with a higher initial capital cost, the benefits of dry-compression, screw-type technology are setting new standards and will start to pay off quickly.

For more information: In the U.S. - Mario Vitale, regional market manager - process industry, Oerlikon Leybold Vacuum USA Inc., 5700 Mellon Rd., Export, PA 15632; tel: 724-325-6565; fax: 724-325-3577; e-mail: mario.vitale@oerlikon.com; web: www.oerlikon.com. International contact: Uwe Zoellig, senior global market support manager, Oerlikon Leybold Vacuum GmbH, Bonner Str. 498, 50968 Cologne, Germany; tel.: +49 (0) 221 347 1375; fax: +49 (0) 221 347 31375; e-mail: Uwe.Zoellig@oerlikon.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: dry-compression vacuum pump, screw-type vacuum pump, oil-sealed pump