Two significant areas currently challenging the global heat-treatment industry are the major requirements to increase energy efficiency and to reduce the environmental impact of operations. 

Added to those challenges are the ever-increasing needs to improve the productivity and reliability of vacuum furnaces, to reduce both installation and operating costs and, in particular, to enable more technological advances to take place in order to satisfy market demands for improved functionality and performance.

   In view of these strong influences, equipment manufacturers have been making significant technological strides in new vacuum pumping mechanisms to provide both end-users and system builders with advanced vacuum pumps capable of fulfilling these requirements. These pump developments not only assure the user of excellent vacuum performance and reliability but also offer significant savings in energy use and a dramatic improvement in ease of integration with furnace control systems.

    This leap forward is particularly evident in the new intelligence now installed in industrial dry vacuum pumps such as the recent GXS series. These have onboard controllers that can undertake basic process-control functions themselves (e.g., pressure control) and can also integrate easily and seamlessly into the vacuum furnace’s control system using a range of digital or hard-wired methods. This results in total installation flexibility for all types of vacuum metallurgical applications.

    These advanced dry pumping mechanisms provide very low energy consumption and low noise operation together with enhanced vacuum pumping performance and a very compact installation size. The significant design features also mean that servicing intervals can be greatly extended – up to five years in between services in many cases. Furthermore, the onboard intelligent control system in these pumps can protect the pumping mechanism from adverse process conditions by using smart start-up, warm-up and shutdown sequences and by providing automated self-cleaning where required.

 

Applications and Parameters

Vacuum in metallurgy is primarily essential to avoid hot-metal reactions with atmospheric gases and moisture. Vacuum is also needed to degas liquid metals to ensure homogeneity, to remove light-metal impurities with higher vapor pressures, to evacuate molds prior to filling to avoid trapped gas pockets, and to ensure good control of reactive gas environments for the various metal surface treatments.

    Metallurgical processes needing vacuum can be broadly divided into the following key areas:

•   Preparation, purification and alloying– vacuum degassing, vacuum induction melting and vacuum arc refining processes

•   Shape forming – precision investment casting, metal injection molding and powdered-metal sintering processes

•   Reactive heat treatments – carburizing, nitriding and other surface-coating or modification techniques

•   Nonreactive heat treatments – annealing, tempering and quenching operations

•   Clean assembly processes – electron-beam welding and vacuum brazing furnaces

 

    The common feature of all of these processes is the requirement for good vacuum pumping performance. This is achieved by using a combination of primary vacuum pumps, which are pumps that can exhaust direct to atmosphere, with secondary vacuum pumps (typically mechanical boosters) that do not usually exhaust to atmosphere themselves. Secondary pumps produce deeper vacuum levels needed for clean assembly, purification and casting processes, but they must be backed by primary vacuum pumps.

    A typical vacuum metallurgy system would use combinations of mechanical boosters with primary vacuum pumps to do the initial evacuation of atmospheric gases and to rapidly take the furnace pressure down to typically well below 0.1 mbar.

    For processes requiring deeper vacuum, other secondary vacuum pumps (such as oil diffusion pumps) take over and go on to remove residual atmospheric gas and (most importantly) the outgassing load from the chamber internal surfaces to create the deep vacuum levels of 10-3 or 10-4 mbar or lower typically required for purifying and casting operations.

    In selecting the appropriate vacuum-pump types, the furnace designer will have some key requirements that must be met.

•   Pumping capacity: In terms of volumetric pumping speed, this must be adequate to pump down the chamber to the required vacuum in the required time. The correct low pressure is then held against the expected process gas loads (and ideally has enough reserve performance to also allow for some unexpected gas loads from minor leaks, etc.).

•   Utility consumption: The consumption of power, cooling water and purge gas (where needed) should be minimized.

•   Reliability: The pump system must have proven reliability and minimal maintenance requirements.

•   Survivability: The pumping mechanism should be able to handle the abrasive dusts and other materials that might come out of the metallurgical process and head into the pump set.

•   User convenience: Compact size, low noise and being easy to install and operate are all very desirable qualities.

 

Operating-Cost Savings

In terms of the primary vacuum pumping options available, the traditional form of oil-sealed primary vacuum-pump mechanism (so-called “wet” vacuum pumps of either rotary vane or rotary piston type) has steadily given way over the years to “dry” vacuum pumping technologies in many metallurgical processes. In terms of both operating costs and routine maintenance requirements, the older oil-sealed rotary pumps are usually more costly and more labor-intensive to run than modern dry vacuum pumps and do not offer the same operational consistency as their “dry” counterparts. Figure 1 shows a typical cost-of-ownership comparison plot between a rotary piston “wet” vacuum pump and the equivalent size of the GXS dry screw pump based on power, water and purge-gas consumption plus typical service costs in a metallurgical application.

    The recent GXS series of screw vacuum pumps (Fig. 2) is ideally suited to the metallurgy market, and the design incorporates a wide range of innovative features to provide best-in-class pumping performance together with very low energy consumption and low noise generation – all in a very compact installation size.

    The energy consumption of these dry screw pumps at ultimate pressure is typically only 60% of the equivalent oil-sealed wet-pump power consumption. This compact pump uses a new, patented screw rotor that delivers excellent pumping speed and deep ultimate vacuum capability with a remarkably quiet and low-vibration operation. It is also equipped with an advanced pump temperature-management system that maximizes pump performance and life of the seals, bearings and motor.

    Long service intervals are further assured by the use of a non-oxidizing, non-hydrocarbon and recyclable gearbox lubricant, which enables service intervals of up to five years to be achieved, providing virtually maintenance-free operation during this period. The GXS is fitted with a high-efficiency, water-cooled motor with integrated inverter drive system and advanced shaft seal technology to provide greatly reduced power consumption.

    With its onboard intelligence and compact size, the pump is simple to install and operate because it can be easily wheeled or forklifted into position, coupled up to the vacuum furnace, connected to its utility services using the supplied connectors, and immediately run at the push of a front panel button. This has coined the expression “plug & pump,” reflecting the speed at which installation and commissioning can be accomplished.

    The GXS is also easily integrated into simple or complex control systems, as it is supplied with both serial and Ethernet-based communications, plus a web-serving capability through its Ethernet port – meaning the pump’s real-time operating parameters can be viewed by any standard web browser connected to the same network.

 

Remote Control and Intelligence

Optional remote-control modules can add both parallel hard-wired contact inputs/outputs or Profibus network connectivity. The standard pump’s controller also provides onboard PID loop pressure-control capability. This can be used to allow the pump itself to control the vacuum chamber pressure against a steady gas load on simple applications without the need for additional external control valves or loop-control hardware. This intelligence is a key feature in reducing the cost of integrating the pump into the furnace control system and in making increased sophistication of pump control and data monitoring available. The features provided as standard on the GXS can potentially save up to €5,500 ($7,115) in typical control and installation costs compared to the use of a non-intelligent pump requiring add-on systemization to provide the same facilities.

    GXS pumps are offered with a range of gas purging options to provide protection from various levels of process risk. For the harshest of metallurgical processes, this can extend to a built-in high-flow purging and solvent-flushing accessory, which will enable the pump to routinely clean its own internals with solvent flushing and air drying without any need for manual disassembly.

    A typical GXS installation is shown in Figure 3. Pump sizes are available from 160 m3/hour up to 750 m3/hour, and combinations with integrated mechanical boosters are also available up to 4,200 m3/hour.

 

Investment-Casting Installation Example

A typical precision investment-caster operator has used a variety of oil-sealed rotary piston pumps, plus associated mechanical booster pumps, to provide primary vacuum pumping in several small investment-casting furnaces. The melt shop for these furnaces is a busy environment with elevated levels of ambient noise – partly due to the rotary piston pumps – and furnaces very close together. The proximity of the furnaces and vacuum-pump groups, together with the busy production schedule, mean that conducting routine maintenance (such as regular oil changes for the rotary piston pumps) can be problematic. The opportunity arose to replace a 412J large rotary piston pump on one casting furnace with a GXS dry screw vacuum pump and booster combination unit.

    The dry vacuum combination unit selected was a GXS450/2600, which provides both higher total pumping speed and a deeper ultimate vacuum than the rotary piston pump it replaced. The very compact size of the GXS450/2600 combination and its castor-wheel mounting enabled it to be maneuvered into place very simply. It was easily connected into the customer’s existing control system using an optional miniature hardwiring interface module, which provides basic control inputs and outputs using dry contacts. This pump and booster combination provides quiet operation (its noise specification at ultimate vacuum is <64 dBA), and it is virtually unnoticeable in this melt-shop environment. The freedom from routine oil changes and from any other major routine maintenance items meant it reduced the demand on the plant maintenance department. The provision of a hand-held display terminal that can be plugged into the GXS at will provided an accessible method of monitoring the pump’s performance and operation whenever desired. 

    In terms of performance, this installation significantly improved the investment caster load-lock pump-down time, going from 68 seconds with the original rotary piston primary pump to 52 seconds with the new GXS dry pump and booster combination. The actual installation is shown in Figure 5.

 

Other Installations

The new generation of GXS intelligent dry vacuum pumps have high performance and excellent energy efficiency, plus extended maintenance intervals of up to five years. They are also very easily integrated into the vacuum-furnace controls using a range of digital or hardwired methods. This gives lower cost of ownership and increased operational flexibility in many metallurgical processes. The high pumping speed, small size and ease of installation can provide compact systems with very high primary pumping speeds, such as the 1,500 m3/hour used on a steel degasser shown in Figure 5. Other recent GXS installations in metallurgy have included various furnaces for sintering, quenching, carburizing and nitrocarburizing. IH

 

For more information: Contact Dr. Simon Bruce, Market Sector Manager, Edwards Ltd., Dolphin Road, Shoreham-by-Sea, Sussex, UK  GB-BN43 6PB  e-mail: simon.bruce@edwardsvacuum.com