The heart of every vacuum furnace is the pumping system, and it is just as critical as the heart of the human body. To get the best performance out of your vacuum furnace, we put together the following information on the basics of pumping systems, common problems, and best practices.
Pumping System Basics
Vacuum furnace systems utilize various types of pumping system combinations to evacuate atmospheric pressure from the vacuum chamber to required ranges for specific processes. Since the heart of the furnace is the vacuum system, it is essential to maintain the pumping system as specified in the operator’s manual, taking into consideration any special accommodations that the type of process being conducted may require.
There are typically three subsystems included in each vacuum furnace pumping system: the roughing pump, the vacuum booster pump and the diffusion pump. Mechanical pumps and blowers, often referred to as roughing pumps, are utilized during the initial pump-down phase of the vacuum furnace from atmospheric pressure to a predetermined pressure level.
A diffusion pump is utilized to achieve a lower system pressure than what is typically achieved by a mechanical pump and booster package alone. The diffusion pump cannot operate independently; it requires a holding pump to operate simultaneously during idle modes to reduce the diffusion pump’s inner pressure. During the operation of the diffusion pump, the valve to the holding pump is isolated and the mechanical pump and blower act as the backing pump for the diffusion pump.
Customers not requiring such a low system pressure may use a two-stage system instead of the three-stage system. The two-stage system will take the vacuum furnace pressure level from atmospheric pressure to a pressure level of 4.5 x 10-2 Torr as opposed to 8.0 x 10-6 Torr for a furnace with a diffusion pump.
When determining which system is right for your equipment, you must consider pressure, gas volume and desired evacuation rates, as well as your specific vacuum furnace equipment and process requirements.
Maintaining Your Vacuum Pumping System: Common Problems
To maintain an efficient pumping system, you need to recognize the most common problems that may occur with mechanical and diffusion pumps and know how to correct them.
Common Problems with Mechanical Pumps
Oil contamination is the most common problem with mechanical pumps. Vapors within the gas being pumped often mix with oil and can reduce pumping efficiency. To prevent contamination, open a gas ballast valve to the pump to help remove water vapor. Other common problems with mechanical pumps include sludge buildup; loose belts; improper oil level; clogged oil lines; incorrectly-set oil temperature; and particulate-laden oil, which causes valve damage.
Common problems with Diffusion Pumps
Backstreaming is one of the more-reported problems with diffusion pumps. This issue can happen when you allow the furnace to sit cold pumping for 24 hours or to overheat because of inadequate coolant flow. When the vacuum pressure continues to decline, the fluid gas molecules attempt to counter flow toward the vacuum vessel. Using a cold trap reduces the oil’s ability to backstream into the furnace. Other common problems with diffusion pumps include power failures, excessively high foreline pressure, clogged oil return ports in the boilerplate, defective heaters, and high leak rates on the system during pumping.
Enemy Number One – Water Vapor
Water vapor is the number one enemy in the vacuum heat treating process. Water vapor can have a negative effect on a process, so it is important to take preventative measures against air or water absorption. Leak detection of all joints, welds, seals, valves and pumps is critical to successful pumping and furnace operation. Limit the amount of time the furnace chamber is exposed to atmospheric conditions during the loading and unloading process. Utilizing nitrogen or argon to backfill your equipment helps minimize the effects of moisture and prevents water vapor from becoming trapped within the hot zone.
Daily Checklist for Effective Pump Operation:
- Check the oil levels on all pumps. Diffusion pump oil levels that are too high or too low can cause backstreaming.
- Check the oil consistency. Cloudy or discolored oil may indicate the presence of moisture or other contaminants.
- Make sure the operating temperature of the roughing pump is at 140 °F (60 °C).
- Check the water temperature to the diffusion pump. Inlet water supply should be at or under 85 °F (29 °C). Water outlet should be at or under 125 °F (52 °C). An overheated diffusion pump can be the result of poor water flow and leaks in the pumping system.
- Make sure the crossover setting on the diffusion pump is 60-80 microns.
- For older furnaces, make sure the vacuum level is −28 to −29 inches of mercury when the booster is activated. For newer furnaces with a VFD, the booster is activated at atmospheric pressure.
Just as the health of your body reflects how you have treated it, the performance of your vacuum furnace system reflects the care you have invested in it. Understanding what goes into the system and how to take proper care of it is vital to maintaining a smoothly running system with minimal downtime or unplanned servicing. Treat it right and it will treat you right!