With Craig Moller – Chief Engineer, Brad Stewart – Software and Controls Engineering Lead, and Steve Hartenberger – Electrical Retrofits Product Leader
Water vapor is everywhere. Even on a cold, dry winter day in northern Illinois, there is water vapor in the air. We measure the ambient water vapor by calculating the dew point: the temperature at which air must be cooled to reach 100% relative humidity at a given constant pressure.
So, when the temperature hits a chilly 10° F in the middle of winter, and the local weather expert reports a dew point of -7° F, it indicates that, assuming overnight temperatures follow their usual pattern and air pressure remains constant, we can expect water to precipitate out of the atmosphere. This can result in fog or frost forming on cold surfaces if the temperature drops below -7° F.
“-7° F would be a very bad dew point for backfill gas,” explained Craig Moller, Chief Engineer for Ipsen. “That’s too much water content. When we are monitoring process gases like argon, we’re typically looking for dew points between -60° F and -100° F.”
The high temperatures and low pressures inside a vacuum furnace can break water molecules into their component elements—hydrogen and oxygen. If oxygen is introduced into a heat-treating process, it can cause discoloration or oxidation of parts, as well as damage to hot zone.
Monitoring the Dew Point of Process Gases
Dew point sensors can play a big role in protecting furnaces and parts from water vapor entering the chamber through leaks in the process gas systems.
Dew point sensors are typically installed in-line with a process gas, like argon or nitrogen. The sensor tests the flow for the presence and concentration of water vapor just before the process gas enters the vacuum chamber. The sensor is connected to a hygrometer with a digital display that can provide analog, digital and dual relay alarm outputs. Dew point sensors can deliver accurate information as long as the gas is flowing, whether at a high pressure or a slow trickle. The sensors can be set up to deliver a reading for either manual inspection, or with active tracking through an integration with the PLC and furnace control software, so trending can be recorded and alerts can be generated.
“When it comes to our aerospace customers, many of their recipes specify that certain process and quenching gases must be very pure. Whenever they are ordering process gases like argon, the drier the gas, the higher the cost,” explained Brad Stewart, Software and Controls Engineering Lead. “Aerospace customers know that when they’re using argon for their thermal processing recipes, they have to meet specific dew point standards.”
Industrial gas providers can offer products ranging from 99% pure all the way up to 99.995% pure, but connecting large outdoor gas tanks to vacuum furnaces on a production floor can involve many feet of pipes. It’s not uncommon to encounter many joints, bends, fittings, and valves along the way. Each junction poses a risk of leaking, which even under positive pressure may allow for ambient air (and specifically water vapor) to enter the system and cause part oxidation.*
Furthermore, there are recipes that may involve introducing two different gases into the furnace, like argon and nitrogen, where each would have their own dew point specifications. “Most of our customers who use these processes know that they need (a dew point sensor) and will ask specifically to have them included in a new furnace or installed in a retrofit,” noted Steve Hartenberger, Product Leader for Electrical Retrofits.
*Bernoulli’s Principle shows that in a gas system with directional pressure generating from a large pipe that is reduced to a narrower pipe, gas pressure within the narrow pipe will be lower than in the larger pipe. A leak in a section of this narrow pipe can act like a vacuum, drawing ambient gases into the system.
Operators with inline dew point sensors are recommended to check and record the dew point readings prior to every cycle. Integrated systems can report and record dew point readings throughout the cycle. If a dew point reading falls outside specifications, the sensor can be instructed to set off an alarm.
“Dew point alarms can help operators identify one possible source of oxidation,” Moller explained. “The higher the velocity of gas moving through the pipe, the more chances to draw in air through a leak, and the more likely to set off the alarm.”
When a dew point system alarm is set off mid cycle, the operator will want to record the readings, the part of the process where the alarm went off, and allow the cycle to run through completion, followed by a close inspection of the parts and the hot zone.
“A dew point sensor alarm typically means it’s time to bubble test your fittings,” Moller suggested.
How to Identify the Source of a Dew Point Issue:
– Bubble test gas lines to check for leaks at all junctions (joints, bends, connection fittings, and valves)
– Test that the gas within the storage tank meets purity specifications
– After completing a bubble test on the junctions, pump down the furnace line going back to the gas accumulator tank and do a vacuum test using helium.
– If the dew point sensor is still out of range, perform another positive pressure bubble test from the furnace to the gas source.