Fig. 1. Reliable and robust oil diffusion pump
In today’s economic climate, manufacturers are under increased pressure to reduce both their manufacturing cost and their environmental footprint. Recent developments in vacuum pumps have resulted in more environmentally friendly products that address these requirements. This article looks at the role vacuum pumps can have in these processes and how pump technologies have evolved to address the increasing environmental concerns for power and water consumption, noise levels, oil usage and disposal costs.
The Need for Vacuum
The benefits of performing the heat-treatment process under vacuum are well known: it produces bright parts with no oxidation and with no subsequent cleaning requirements; it is highly energy efficient because it eliminates heat loss via conduction or convection; it reduces the use of process gas; and it generates a fast, uniform heating, which reduces distortion in the final component. The overall benefit is consistent and reproducible product quality with faster processing and increased product throughput.Vacuum pumps vary in capacity and size depending on the application. For low vacuum (atmospheric pressure to 10 torr) and medium vacuum (from 10 torr to 10-3torr), a stand-alone mechanical vacuum pump with a capacity of 200-600 cubic feet per minute (CFM) or a Roots blower in series with a suitable mechanical vacuum pump with a system capacity of 500-20,000 CFM is typically used.
Some heat-treating applications involve large gas loads and require medium-to-high vacuum (10-3to 10-5torr). Traditionally, these applications call for secondary vacuum pumps such as oil diffusion pumps and vapor boosters with a capacity range of 3,000 l/sec for a 10-inch diffusion pump and up to 12,000 l/sec from a single vapor booster. Increasingly, turbo-molecular pumps, which offer pump capacities up to 4,000 l/sec, are also being used.
Oil Diffusion Pumps
In most of today’s applications, diffusion pumps are the most popular pumps to achieve high vacuum. They are relatively inexpensive, reliable and robust, offer good long-term performance, provide high-capacity pumping in the critical 10-3to 10-6torr range and have a high tolerance to gas surges (Fig. 1).The principle of a diffusion pump is to use a high-speed jet of fluid to transfer gas molecules through the pump and out of the exhaust. Boiling oil is used, and the high-energy oil vapor acts as the motive force to affect momentum transfer pumping of gas at the molecular level. Multiple jet stages provide multi-stage compression, enabling the pump to achieve vacuum pressures better than 10-7torr. Diffusion pumps depend on a low backing pressure, failure of which risks oil vapor flowing back to the process chamber and contaminating the product.
While there are no moving parts in a diffusion pump, these pumps do require regular maintenance intervals, such as oil replenishment and periodic heater replacement. Typically, multiple heaters are used with one or more requiring replacement yearly throughout the life of the product.
The pumping mechanism is very energy-intensive and results in high power usage of up to 24 kW in some cases. The pumps also have high cooling-water consumption, between 500-2,500 l/hour, and most of the energy is lost directly to the cooling water.
An energy cost results from the significant warm-up time that is required from the pump being turned on until the oil reaches operational temperature. It typically takes 30-60 minutes to achieve the operating temperature, which has to be sustained, even between cycles.
A new design of the boiler assembly has been engineered to reduce power consumption and increase performance. Pumps now have three power modes, enabling power savings to be made when the pump is on standby or lightly loaded. It can rapidly be switched to full power for processing. As a result, users are able to reduce their running costs.
Fig. 2. Maglev turbo-molecular pump
Turbo-Molecular Pumps
A turbo-molecular pump is a multi-stage axial-flow turbine that uses high-speed rotating blades to provide momentum-transfer pumping of the gas molecules. Work is done directly on the gas rather through another medium such as oil in the diffusion pump. Consequently, the turbo-molecular pump requires very little power or cooling water to run, and it has a very green environmental footprint as there is no waste oil generated.A compound molecular pump combines bladed turbo-molecular stages with molecular drag stages on the same rotor. This design allows high critical backing pressures of up to 3 torr, which is typically six times higher than the critical backing pressure of a diffusion pump. This enables the use of a smaller, lower-cost backing pump, which leads to energy savings.
Early turbo-molecular designs used conventional high-speed, oil-lubricated mechanical bearings. While still used today, they have limitations arising from bearing wear, noise, vibration and maintenance requirements.
The introduction of magnetic bearings and magnetic levitation of the turbo-molecular pump rotors has dramatically improved reliability (Fig. 2). The design reduces vibration levels and the pump suffers no mechanical friction losses, so the power demand is solely that needed for gas compression. At operating pressures in the 10-1to 10-4torr range, these powers are low, typically in the 1 kW range. Heat and noise generation is similarly low and results only from the drive motor, the onboard control electronics and gas compression heating. Therefore, cooling-water requirements are also low, typically in the range 100-180 l/hour.
In recent years, the costs of Maglev turbo-molecular pumps and their sophisticated control systems have significantly dropped, thus improving affordability. This enables metallurgy managers to realize the benefits of improved reliability and longer maintenance intervals. Table 1 gives a comparison of pumps with similar pumping speeds.
Oil-Sealed Pumps
For many metallurgical processes, users still use either an oil-sealed rotary pump or an oil-sealed piston pump because of their low capital cost. Through movement of the rotor or piston, depending on the type of pump, vacuum is created. Both pumps use oil to create the vacuum seal. Although simple to understand, these pumps do have limitations.In many processes, particulates, corrosives or condensable vapors will accumulate in the oil. This causes wear or damage to the pump mechanism, meaning it requires frequent maintenance or oil-change intervals. Oil disposal also presents an environmental and safety challenge as it is often considered hazardous waste.
Fig. 3. GXS screw pump to be previewed at the 2009 ASM Heat Treat Show
Dry Mechanical Pumps
Dry mechanical pumps, such as those that use a claw or screw mechanism, offer an oil-free alternative for heat-treatment vacuum-pumping applications. A dry pump has no lubricants within the pump’s swept volume. This reduces oil contamination, which can lead to discoloration of product, and makes the pumps ideal for use where vacuum cleanliness is important. The elimination of oil in the swept volume substantially reduces the maintenance requirements compared to oil-sealed mechanical pumps, which require regular oil-change intervals and have high oil disposal costs.The principle of operation of a typical dry vacuum pump is to have a pair of counter-rotating claw or screw-profile rotors operating with small clearances to pump gas from inlet to exhaust. In a screw pump, two “screws” rotate in opposite directions. Gas is drawn in through the inlet, trapped between the screws and moved axially downward to the exhaust, where it is discharged.
Each mechanism has its advantages in particular aspects of vacuum pumping. A claw-type rotor is very effective in light gas pumping and particulate handling capability, whereas a well-designed screw-type rotor can achieve vacuum levels below 10-2torr.
Edwards has traditionally provided claw-type dry pumps for most applications with great success. The company is now developing a screw pump that employs novel technology to advance the standard of dry pumping. In particular, the new GXS series of pumps directly address the need to reduce running costs and improve a user’s economic footprint (Fig. 3).
In terms of reducing power requirements, the profile of the new screw and the design of the gearbox have a direct influence over the amount of power the pump requires to run at each inlet pressure. As the amount of power required increases, so do the demands on the pump cooling. The innovative pump takes the lessons learned from the semiconductor and flat-panel manufacturing industry, where low-power products are required to reduce high manufacturing costs. For example, a 1,750 m3/hour booster system requires only 4.7 kW to run, whereas most comparative products require 50% more power.
The new design of screw-pump technology incorporates a number of innovative features that contribute to reducing mechanical and gas noise levels as well as vibration levels.
The pump motor is directly mounted to the shaft, which minimizes coupling loss. It is inverter-driven to reduce in-rush current on start-up. It also provides process-control capability and low-speed running to save power when the process is idle. Integrating onboard control in a water-cooled package means effective thermal management is achieved and subsequent improvement in reliability can be realized. The motor itself is a very high-efficiency design, which is matched to the inverter to achieve best-in-class efficiency.IH
For more information:Dave Sobiegray is product manager, Industrial Vacuum Systems, Grand Island, N.Y. and Dick Amos is applications manager, Vacuum Business, Burgess Hill, West Sussex, UK. Contact Edwards, Three Highwood Drive, Suite 3-101E, Highwood Office Park, Tewksbury, Massachusetts 01876; tel: 1-800-848-9800; e-mail: info@edwardsvacuum.com; web: www.edwardsvacuum.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: mechanical booster, diffusion pump, turbo-molecular pump, dry mechanical pump, screw pump, claw-type dry pump
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