With Jesse Lawrence, Southwest Field Service Engineer and Larry Dahm, Southeast Field Service Engineer
After a long, cold winter, spring is a welcome visitor, and few rituals are more rewarding than a proper spring cleaning. It’s also a great time to review best practices for cleaning your vacuum furnace. Field Service Engineers Jesse Lawrence and Larry Dahm have seen a wide range of furnaces in varying conditions, and they both agree—a little daily cleaning goes a long way in keeping a furnace in production for a long time.
“Every time you take out a load of parts from a furnace, the first thing you should do is make a visual inspection of the elements,” Lawrence suggested. “Make sure that if there are any parts that fell from the basket or any obvious debris, clean it up right away.”
Dahm added “When parts drop onto the elements below the hearth, those elements can crack or break. Be sure to make those repairs right away.” It’s also a good idea when something like this is spotted to review loading and unloading protocols, and to make sure the fixtures are being loaded as designed to avoid dropping parts.
Next, “Look for deterioration of the insulation. Then inspect the lip seal for debris that may be trapped,” Lawrence recommended. Loose debris can cause punctures in the insulation, or cause leaks in the door seal either when the chamber is pumped down or when it’s under pressure during a quench cycle.
Removing Debris from the Hot Zone
While some maintenance crews and operators use common shop vacuums to tidy up debris found in the furnace, Lawrence has a better suggestion.
“I prefer using a source of dry, compressed air to keep the vacuum furnace clean. In our shop at Cherry Valley, we have a compressed air line with a ball valve and a J-shaped nozzle that will push the debris towards the front of the chamber and out,” Lawrence explained. This design helps keep debris from being pushed back into the chamber where it could damage the heat exchanger and the back wall.
Shop vacuums can also have trouble getting in between elements and insulation. “Small pieces of molybdenum, stainless steel wire, debris hiding up around the hangers; there are some things that dry compressed air will simply be able to do better than a shop vacuum,” Lawrence said.
Beyond Visual Inspections
The definition of a “clean” furnace can vary widely depending on the processes it runs. “If you look at a brazing furnace, for example, the coating of residual brazing material on the hot zone might make some operators think that they couldn’t run a process, but brazing experts will recognize that it’s just the nature of brazing,” Lawrence explained.
“On the other hand, if you’re operating an all-metal hot zone, and you’re running more exotic materials in a high-pressure quench process, that furnace may need to be spotless. Anything loose in there that could blow around while cooling could end up contaminating parts,” Lawrence noted. “In that case, a cleaner furnace means cleaner parts.”
Regarding furnace maintenance, Dahm emphasizes the importance of basic best practices for operators. “The best thing you can do for a furnace is a burnout – at minimum we recommend running one a week.”
One important way to maintain a clean hot zone is by keeping water out of the furnace. It’s important to close and pump down the furnace during any down time, preventing atmospheric humidity or condensation from getting into the system.
Between humidity and outgassing, there is a chance that a visually clean furnace isn’t as clean as it appears. “Fluids like water and oil that find their way into the insulation can cause erosion of the insulation or contamination of the processed parts,” Lawrence explained. “Regularly checking your oil levels in your pumps is a good way to see if there is any sign of leaking or outgassing.”
Checking the oil can reveal slow leaks, excessive oil entering the chamber, or water vapor contamination if the oil appears milky.
“We also recommend changing the oil every three months. The instructions and recommendations for when and how to replace pump systems oil are all in the maintenance manual,” Dahm suggested. “It’s best when you stick to the general guidelines at a minimum.”
“Even if you start by using the general guidelines in the manual, it’s important to fine tune your furnace maintenance schedule to meet your specific usage and circumstances,” Lawrence added. “For example, in high-humidity environments, you may need to do a burnout more than once a week.”
Spring Cleaning Can Help Prevent Hidden Problems
Dahm pointed out “If during your visual inspection you spot a big black spot on an element, or any soot or carbon discoloration on the graphite elements, that’s a good time to check to see if any of the connections are loose.” Connections can be loosened from the wind currents generated as a furnace gets pumped down into vacuum, or as quenching gas is pumped into the vessel. “Tightening the elements may take 15 minutes, but it can make a real difference in the life of your furnace.”
Also, using a multimeter to check resistance to ground can help operators avoid arcing in the chamber.
“Checking each element for resistance to ground, it should be consistent from one week to the next. Typically, we see 100-ohm resistance, within a few ohms, at every good test,” Dahm noted.
“If you’re getting readings down in the 70-80 ohm range, that’s a good indication that a burnout is in order.” Lower than that, and you may have an element shorting out in need of repair or replacement.
Typically, between the nut and the element, there is a Grafoil washer. By inspecting each element and tightening each nut by hand – no wrench is needed – you can make sure to identify if a nut or washer has either been damaged or has fallen out. It’s just another opportunity to make sure each element is working properly.
“Whenever you have an element that isn’t functioning properly, the rest of the furnace will have to work harder to make up for the energy and heat loss from the malfunctioning element.”
Uniformity can become a real challenge when one element isn’t operating properly. “As the rest of the furnace needs to run at higher temperatures over a longer period of time to ensure that the chamber temperature meets the recipe requirements, there could end up being as much as a 20 °F difference from one area to another,” Dahm explained.
Checking thermocouple readings while the furnace is processing and tracking any changes to the time it takes for the furnace to get up to temperature are also good ways to identify potential issues.
While the cycle is running, it’s a good idea to listen for any strange noises coming from the pumps, and test to see if the water-cooling system is operating properly. “Does the outside of the furnace feel warm? You may want to check the water flow, and see if there are any signs of rust,” Dahm recommended.
Why do maintenance every day? Every week? Every month? Every quarter?
Spring cleaning is a great time to remind everyone from owners to operators and maintenance staff that a small amount of prevention is much less costly than frequent repairs.
“Spotting even minor issues before they become much larger can mean the difference between an occasional hour of downtime and spending days, not to mention money, waiting for a replacement part or a new hot zone,” Lawrence said.
“Most importantly, don’t take shortcuts when you are inspecting and testing your furnace,” Lawrence concluded. “Take the time to check everything. Check the resistance, look for debris, inspect the elements, check the oil. Don’t rush any repairs or hurry through your checklist.”
Skipping burnouts, ignoring signs of deterioration, and leaving debris in the hot zone can cost operators in lost uniformity, lost functionality, increased processing time, and a shorter life for the hot zone. Fixing even the small issues, like tightening a nut, can significantly reduce the costs that come with replacing elements and hot zones over time.
“Reducing your inspection time always leads to an increase in downtime,” Lawrence concluded. “Take your time and do it right.”