Vacuum/Surface Treatment: Selecting the Right Vacuum Pump for the Application
November 15, 2006
To select a vacuum pump, first thoroughly understand what you need your vacuum system to do. But just as important, you must know the impact the selected vacuum system will have on the overall cost to produce your product. The pumping-system configuration can be just as important as the pump technology you select and even small changes in configuration can make significant improvements to vacuum-system reliability, reducing overall user intervention.
In this discussion, we will concentrate on vacuum systems capable of low or "rough" vacuum from atmospheric pressure to 10 mmHg and medium vacuum from a 10 mmHg to 10-3 mmHg (torr). For low and medium vacuum, you will typically utilize 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. The configurations vary with roots blowers in series or in parallel backed by one or more mechanical vacuum pumps. Some heat-treating applications call for secondary vacuum pumps such as diffusion pumps and vapor boosters with a capacity range of 7,000 CFM for a 10-inch diffusion pump up to 25,000 CFM from a single 30B5M vapor booster (Fig. 2). That is a lot of pumping speed from a pump with no moving parts! We will focus on choosing a mechanical vacuum pump, looking at oil-sealed rotary pumps (primarily piston pumps) and dry-running claw or screw-type vacuum pumps. A brief discussion on secondary pumps (i.e., diffusion pumps and vapor boosters) will round out the review, giving coverage to the specific types of vacuum pumps widely used in the heat-treating industry.
Applications that evolve large gas loads and require medium-to-high vacuum (10-1 mmHg to 10-5 mmHg) may call for secondary vacuum pumps known as diffusion pumps or vapor boosters (Fig. 2). A well-designed diffusion pump for heat-treat applications will offer high throughput (mass flow) and have a high tolerance to gas surges. Vapor boosters operate in a similar way to vapor diffusion pumps, yet are distinctly different in that they generate boiler pressures approximately 10 times higher than what is typical for a vapor diffusion pump. The high boiler pressure feeds motive fluid to powerful ejector nozzles specifically designed to enhance the mass-flow capability of the pump. They are well suited to applications in the 0.1mmHg range to 0.0005mmHg range where mechanical pumps are often at their limit. Classical diffusion pumps become unstable as pressure tends towards 0.1mmHg. Diffusion pumps and vapor boosters are incapable of exhausting to atmosphere and require a mechanical rough pump to be operated continuously in series with it, so mechanical-pump selection remains important. Turbo-molecular pumps (Fig. 3) are another type of medium-to-high-vacuum secondary vacuum pump that, when backed by a dry pump, may have uses in the heat-treat industry when extremely clean, hydrocarbon-free vacuum atmospheres are required.
Define Your ApplicationAn integral part of the selection process is detailing what you need your vacuum system to do. Clearly define your application in order to determine a vacuum-system solution to meet your specific needs. An important question to ask up front: Do you need to protect the process from the pump as it pertains to hydrocarbon contamination, or do you need to protect the pump from the process as it pertains to reactive gas or dust, particulate, and corrosives? In some cases, the furnace operator will need to deal with both problems at once. Also, ask yourself if the pump system will be required to simply pump down a chamber to a certain vacuum level and then be taken off-line or remain at the attainable blank-off pressure. You may need the vacuum system to handle a specific mass flow while holding at a specific pressure. Or you may need rapid pump-down to crossover pressure of a sec-ondary vacuum pump and then use the system in series to back a secondary vacuum pump such as a vapor booster or diffusion pump. Often the vacuum system will execute a pump-down and then hold at vacuum under specific mass-flow conditions, or you may require rapid cycling from atmosphere to vacuum.
Size Your SystemBefore moving on to the subjective task of weighing the attributes of various pumps against a particular application, let us consider system-sizing criteria. To size a vacuum pump for a specific pump-down, you must know chamber volume, chamber surface area, chamber materials of construction, vacuum-system leakage, target vacuum level and desired time to achieve the required vacuum level. Also required is a thorough description of the vacuum piping connecting the vacuum pump and chamber. This includes length and inside diameter of pipe, number of 90° bends and if there are inline components such as valves, filters, knock-out pots, dead legs, etc.
Sizing a vacuum pump for a specific pump-down requirement assumes a clean, dry and empty chamber with a leakage rate of zero. The fluid being pumped is assumed to be air at 68°F. Although the chamber is assumed to be dry, chamber out-gassing is associated with microscopic layers of water vapor that adhere to the chamber walls. Therefore, there is a direct correlation between surface area and the overall amount of chamber outgassing. The more surface area exposed to vacuum, the more out-gassing, so considering the surface area of chamber fixtures is important. Usually the engineer executing pump-down calculations uses published out-gassing rates for specific materials of construction. The rates used are widely accepted in the vacuum-engineering community. Zero chamber leakage is usually a reasonable assumption given the quality of modern vacuum furnaces in relation to the size of the vacuum-pumping systems employed. With that said, if atmospheric leakage is known, it is easily incorporated into vacuum pump-down calculations.
Furnace applications often require that the vacuum system be able to evacuate the chamber of atmospheric air in a reasonable period of time, while at the same time the material being processed contributes significantly to the overall gas load. Sizing a vacuum pump for a new application such as this can be challenging. Using past experience on similar furnace applications will help you make a reasonable determination of the vacuum-pumping speed required. It is also a challenge to determine the vacuum-pumping speeds required to hold a chamber at a specific vacuum level for a given chamber load. Again, past experience will help you to make a reasonable determination of the vacuum-system capacity required. Generally speaking, furnace capacities and associated vacuum systems must be reasonably sized to meet overall size, footprint, utilities and budget.
Sizing a vacuum system to back a secondary pump, such as a diffusion pump or vapor booster, is fairly straightforward. A secondary pump will have a published maximum mass flow and a published critical backing pressure, sometimes referred to as critical fore-pressure. The mechanical system required as the backing pump for a secondary pump must be capable of handling the published mass flow of a single secondary pump or the combined mass flow of multiple secondary pumps while maintaining a backing pressure well below the published critical fore-pressure of the secondary pump(s). The manufacturer of secondary pumps often specifies the required backing speed to further simplify sizing.
Consider Specific Selection CriteriaNow we will consider how to select the right vacuum pump for your application as it relates to specific selection criteria and the challenges you face in your specific application and installation. Below is a summary list of some selection criteria.
- Environmental health and safety (e.g., emissions and waste generation, noise, general equipment safety)
- Operating cost
- Maintenance cost
- Capital cost
- Affect on process/product
Your list may be much longer, but these are some of the primary considerations. Examining each of the criteria as it relates to your particular application and installation will allow you to draw conclusions on what criteria matter most in your particular situation. Although this discussion is not intended to be a discussion about wet vs. dry pumping technology, the case for wet vs. dry is pertinent at this point in the selection process.
The mechanical vacuum pump, historically the workhorse of the industry, is the oil sealed rotary piston (OSRP) pump such as the Stokes 412 (Fig. 4). In the last decade, however, dry-pump technology is becoming widely accepted in the heat-treat industry. In some applications, dry pumps reduce cost of ownership, improve product quality and increase uptime, but the argument of wet vs. dry is still a subjective one. Generally speaking, oil-sealed pumps offer industry-proven reliability and performance. The trade-off is coping with oil-contamination issues, which in the end can significantly degrade reliability and performance, impacting product quality and contributing to total cost of ownership. Some applications may require frequent oil changes and experience rising disposal costs, which further adds to the overall cost of ownership.
Improvements in dry-pump technology allows the user greater reliability than ever before, minimizing contamination and oil-handling issues associated with oil sealed pumps. There is no better time than during the pump-selection process to detail the issues and concerns with the present technology, to investigate alternatives and to collaborate with industry experts. In a few cases where hydrocarbon oil contamination cannot be tolerated, dry vacuum pumps (Fig. 1) are clearly the best choice. Most applications are not so clear-cut, requiring more investigation to better understand the impact of vacuum-pump selection.
Following is a list of heat-treating processes where vacuum is employed. We will break down each application and present a case for dry and wet pump technology.
- Hardening: Conventional Nitriding
- Hardening: Ion Nitriding
- Hardening: Low-Pressure Carburizing (LPC)
- Hardening: Quenching
- Vacuum Brazing
Hardening: Conventional NitridingProcess gases are typically ammonia and nitrogen with residual water vapor present from the chamber load. Roots pumps with OSRP pumps or dry pumps are used to remove ammonia and flammable hydrogen by-products. Not a particularly dusty application although adequate filtration is recommended to protect the vacuum pumps. Moisture, which may condense in pump oil, is present from the large surface area of the chamber load. Moisture combines with ammonia to cause corrosion of pump components, degradation of elastomers and shaft seals resulting in leakage and may attack pump lubricant/seal fluid. Dry pumps offer greater reliability and a more cost effective solution as they do no contain lubricants in the swept volume or retain moisture as long as condensation of vapors in the pump is avoided.
Hardening: Ion Nitriding / Plasma NitridingProcess gases used are typically nitrogen and hydrogen. The absence of ammonia avoids many of the problems associated with conventional nitriding. Both dry pumps and oil sealed pumps are equally robust in this application. Dry pumps offer stable performance to give a predictable and repeatable roughing cycle. Pumping cycles for oil sealed pumps may take significantly longer if the pump oil becomes contaminated with water and other condensable vapors. Other selection criteria pertinent to your installation may help you to decide which pump technology is best.
Hardening: Low-Pressure Carburizing (LPC)Process gases used are typically pure hydrocarbons like propane, ethylene or acetylene. With the exception of acetylene, these gases form soot and sticky tar deposits in the furnace and vacuum system, which result in damage causing lock-up when the pump is shut down and a restart is attempted. A flow of pump oil having detergent and dispersant properties acts as a solvent to aid in the solubility of the tars. Oil sealed pumps require an oil purifier circuit to continuously remove insoluble particles. Regular and frequent oil changes are necessary. Dry pumps require a solvent flush through the mechanism to minimize tar buildup. As the solvent does not need to provide lubrication, a wider range of lower-cost solvents can be used and less volume of solvent is required. Both dry and wet technologies can be robust on this application if configured properly.
Hardening: QuenchingProcess gases used are hydrogen (flammable), argon, helium or nitrogen. Gases are removed from the chamber by vacuum pumps. Hydrogen is a low-viscosity gas. Therefore, dry pumps do not perform as well on hydrogen gas as compared to air. The loss of performance is due to leak-back of the low-viscosity gases through the pump clearances. Oil sealed pumps do not have this problem due to oil sealing of the pump mechanical clearances. Hydrogen poses a flammability risk that must be carefully managed. Argon at high concentrations will reach a high gas temperature during compression due to its high gamma value and low thermal conductivity causing a vacuum pump to run hot compared to pumping air. For dry pumps, this can be a problem unless certain precautions are taken. Oil sealed pumps will run hotter but do not typically have problems. Dry pumps offer stable performance to give a predictable and repeatable roughing cycle. Oil sealed pumps' pumping cycles may take significantly longer if the pump oil becomes contaminated with water and other condensable vapors.
Vacuum BrazingRoots pumps with OSRP pumps or dry pumps are used with vapor boosters or diffusion pumps to provide a desired vacuum environment in the range of 7.5x10-4 torr. Vacuum pumps must handle small amounts of particulate along with solvents and residues from braze paste, binder and "stop-off" used. Environmentally friendly acrylic-based binders have replaced traditional solvent-based binders, resulting in problems with the vacuum pumps. These materials can polymerize and build hard deposits in oil sealed pumps. The problem is minimized by applying a gas ballast purge at the vacuum pump, but absorption and subsequent polymerization is inevitable. Oil mist from the oil sealed backing pump may contribute to hydrocarbon contamination in the chamber even when used in series with a diffusion pump. The theory is the mechanical-pump oil slowly diffuses into the diffusion pump over a long period of time. The mechanical-pump oil has a higher vapor pressure than diffusion-pump fluid and contributes largely to backstreaming in the chamber. Dry pumps eliminate the potential for this problem. Polymerization in the dry pump can be reduced or avoided by proper purge strategies.
Keeping a vacuum system running in a safe and efficient manner is a top priority and proper selection of a vacuum pump will make this task easier. It is hoped that the material presented will help you better understand the selection process to determine which solution is best for the given application and situation. Borrowing a few words from my distinguished colleagues - good design, good procedures, good maintenance and good housekeeping equals good vacuum!