A pumping system is the heart of a vacuum furnace system. A variety of pumping system combinations are used in vacuum furnaces for evacuation of atmospheric pressure from the vacuum chamber to meet the level required for specific processes.
The use of a suitable pumping system allows a vacuum furnace to deliver optimum performance. This article discusses the measures required to maintain your vacuum furnace system and keep it running in optimal condition.
Vacuum Furnace Pumping Systems
Keeping the pumping system in good condition, as outlined in the operator's manual, is essential for the performance of the vacuum furnace. For instance, special accommodations may be required depending on the processes being ran.
A vacuum furnace pumping system typically consists of three subsystems; namely, the roughing pump, the vacuum booster pump and the diffusion pump. These pumps are usually placed under the categories of mechanical and diffusion vapor pumps.
Roughing pumps, also known as mechanical pumps and blowers, are used in a vacuum furnace’s initial pumpdown phase from atmospheric pressure up to a prescribed pressure level. A diffusion pump is employed to generate a system pressure less than the pressure normally generated by a mechanical pump and booster package alone.
It is not possible to operate the diffusion pump independently as it needs a holding pump to be operated concurrently during idle modes, thus lowering the inner pressure of the diffusion pump. When the diffusion pump is in operation, the holding pump valve is isolated and the mechanical pump and blower serve as the backing pump for the diffusion pump.
Customers who do not require a low system pressure can employ a two-stage system rather than the three-stage system, which features a diffusion pump. Using the two-stage system, the pressure level of the vacuum furnace can be increased from atmospheric pressure to 4.5 x 10-2 Torr (as compared to 8.0 x 10-6 Torr for a diffusion-pumped furnace).
Factors, such as gas volume, pressure, desired evacuation rates and the specific vacuum furnace and process specifications, need to be considered in the selection of the appropriate system for your equipment.
Common Problems with Vacuum Furnace Pumping Systems
Knowing the most common problems associated with mechanical and diffusion pumps, as well as corresponding corrective procedures, is crucial for the efficient operation of a pumping system.
Common Problems with Mechanical Pumps
The most common problem experienced with the use of mechanical pumps is oil contamination. Oil often mixes with the vapors present in the gas being pumped, which can then decrease the efficiency of the pumping system. Oil contamination can be prevented by removing water vapor through a gas ballast valve. Particulate-laden oil, wrongly set oil temperatures, clogged oil lines, unacceptable oil levels, loose belts and sludge buildup are other common problems that occur with mechanical pumps.
Common Problems with Diffusion Pumps
Backstreaming is one of the most common problems associated with diffusion pumps. It can occur when the furnace is allowed to sit cold pumping for 24 hours or when it overheats due to inadequate coolant flow. The continuous drop in the vacuum pressure prompts the fluid gas molecules to counter flow in the direction of the vacuum vessel.
Backstreaming can be offset by employing a cold trap between the vacuum chamber and the diffusion pump throat. The use of a cold trap lowers the ability of the oil to backstream into the furnace. High leak rates on the system while pumping, defective heaters, clogged oil return ports in the boilerplate, excessive high foreline pressures and power failures are other common problems experienced when working with diffusion pumps.
Presence of Water Vapor
Since water vapor can negatively affect the vacuum heat-treating process, preventative measures need to be taken against water or air absorption. Leaks at all joints, pumps, valves, seals and welds should be identified and corrected to ensure successful operation of the pumping system and the vacuum furnace.
The amount of time the furnace chamber is exposed to the atmosphere needs to be limited during the process of loading and unloading. Backfilling the furnace with argon or nitrogen is one way to reduce the impact of moisture and avoid the trapping of water vapor inside the hot zone.
Efficient maintenance is crucial for ensuring the successful performance of your vacuum furnace system. Understanding the actual condition of the system and taking the necessary measures for proper care allows the system to run smoothly with minimal downtime.
This information has been sourced, reviewed and adapted from materials provided by Ipsen.
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