The Doctor was called in because the patient was feeling poorly and was unable to go to work. On closer examination a milky, gassy, dark brown substance was found to be the culprit. Why, do you ask, is your vacuum furnace not producing acceptable parts? Perhaps part of the answer lies in the fact that your vacuum-pump oil is in need of gas ballasting. Let’s learn more.
Gas ballasting is essential for oil-sealed pumps such as rotary-vane (i.e., mechanical) pumps, but it is often poorly understood or neglected as part of the daily maintenance routine on a vacuum furnace. As a matter of good practice, it should be done for about 20-30 minutes each day and is normally performed before processing parts (not once the furnace is running a load!).
Today, some vacuum pumps are provided with an automatic gas ballast feature, but even these features should never be taken for granted. Put simply, gas ballast is a means of allowing a vacuum pump to handle gases containing condensable vapors or moisture without contaminating the pump oil.
In order to properly discuss the use of gas ballast, it is helpful to review the purpose of the pump oil.
Vacuum-Pump Oil – A Review
The oil in rotary-vane pumps serves several critical purposes. These pumps are designed with very tight physical clearances between the moving parts (the vanes or rotors) and the stationary parts (the pump housing or “stator”). A thin layer of oil is required in the gaps between these parts and acts as a seal, without which the pump could not generate a vacuum. In addition, the oil lubricates the sliding and rotating surfaces in what can be a very harsh environment of low pressure, high surface velocity and high temperature.
The oil also cools the pump by removing heat from the areas where it is generated and allowing it to be dissipated in the oil reservoir. For these reasons, the viscosity, vapor pressure and other qualities of the oil greatly impact pump performance. It is also critical for the oil to remain uncontaminated. In the world of vacuum, however, this is easier said than done.
If the oil does become contaminated with moisture or other impurities, it cannot seal, lubricate or cool the pump. The result is a failure to achieve the required vacuum level in a reasonable period of time. If the pump continues to operate with contaminated oil, it can overheat and potentially seize up, causing downtime and requiring an expensive repair or rebuild.
How does contamination of pump oil occur?
One of the chief causes of pump-oil contamination is moisture. When a vacuum chamber or furnace is opened and closed during the unloading and reloading sequence, moisture in the form of high-humidity air (Fig. 1) enters the chamber from the factory. In addition, moisture in the form of water may enter the furnace with the workload. This water vapor will attach to the interior chamber surfaces in the form of a monolayer, or single layer, of molecules (Fig. 2).
In the case of a vacuum furnace, the porous high-surface-area insulation can absorb a substantial amount of moisture in a very short time. The next time the chamber is put under vacuum, this water layer will form ice crystals on pump-down then evaporate into vapor as the furnace is heated and will be carried into the pump along with the pumped gas. Gas ballasting eliminates the water that winds up in the pump oil.
Water vapor is not the only contaminant that may be contained in the pumped gas. Solvent vapors from cleaning compounds or molecules from lubricants or greases found on the component parts placed into the vacuum furnace may also be released from the process, be carried in the pumped gas stream and enter the pump. Without gas ballast they will also mix with the pump oil and contaminate it, which destroys pumping efficiency.
The reason the moisture or solvent vapor mixes with the pump oil is related to the vapor pressure of the moisture or solvent. Vapor pressure is a property of all liquids that causes them to boil at any pressure below it and condense at any pressure above it. One can think of vapor pressure as the pressure required to maintain a liquid in liquid form. If the pressure in the environment drops below this threshold, boiling will occur.
The vapor pressure is also a function of temperature such that water, for example, will boil at room temperature at pressures below 24 mbar (18 Torr), which is the vapor pressure of water at room temperature.
Vapor pressure and its effect on boiling can be illustrated by a simple experiment using a syringe and a small amount of water (Fig. 3). First, pull back the plunger to draw a little water into the syringe. Then cap the open end of the syringe using a rubber cork so it is sealed from the environment. Next, quickly pull back on the plunger, reducing the pressure inside the syringe. The water will boil as soon as the pressure in the syringe drops below the vapor pressure of the water, even though it is at room temperature. Finally, move the plunger back in, and the water vapor will condense back into liquid water as soon as the pressure inside the syringe rises back above the vapor pressure of the water.
The same sequence followed in the syringe experiment occurs in a vacuum process. As a vacuum is drawn on a chamber, any residual water (or other volatile liquid) in the chamber boils into a vapor as soon as the chamber pressure drops below its vapor pressure. This water vapor, which is now held just barely above its vapor pressure, is drawn into the pump. After reaching the compression side of the pump, it is compressed above its vapor pressure, at which point it immediately condenses back into a liquid in the same way the water in the syringe condenses back into a liquid when the plunger is moved back in. This liquid in the pump then forms droplets that mix with the oil, causing the contamination. The gas ballast averts this by causing the outlet valve to open before the vapor condenses, and the vapor is discharged together with the ballast.
Now that you better understand how vacuum-pump oil is contaminated, next time we will discuss how gas ballasting is performed and address its advantages and limitations.
- Herring, Daniel H., Vacuum Heat Treatment, Volume I, BNP Media, 2012
- Herring. Daniel H., Vacuum Heat Treatment, Volume II, BNP Media, 2016
- The Vacuum Technology Book, Volumes 1, Pfeiffer Vacuum (www.pfeiffer-vacuum.com), 2008
- “Operating a Vacuum Furnace Under Humid Conditions,” Vacuum Furnace Reference Series, Solar Atmospheres, Inc., 2011.
- National Programme on Technology Enhanced Learning (www.nptel.ac.in)
- Mr. David Sobiegray, Edwards (www.edwardsvacuum.com), private correspondence.
- Dr. Sang Hyun Park, Busch LLC (www.buschusa.com), private correspondence.
- Mr. Mario Vitale, Oerlikon Leybold Vacuum USA, Inc. (www.oerlikon.com), private correspondence.