Gas Ballasting of Vacuum Pumps (Part 2)
Before the holidays we talked about how vacuum-pump oil becomes contaminated. It is now time to talk about how to overcome this problem using gas ballasting – how it is performed, its advantages and limitations. Let’s learn more.
What is gas ballast?
Gas ballast is the introduction of a non-condensable gas (e.g., air or nitrogen) into a rotary-vane pump, during the compression stage to intentionally impact the efficiency of the pump thereby heating the oil inside and helping to drive out water and other condensed liquids present in the pump oil. In addition to rotary-vane pumps, it is also used in scroll pumps and piston pumps, to name a few. Wolfgang Gaede developed the gas ballast principle in 1935, which was first applied to rotary-vane pumps.
The ballast gas is drawn into the pump chamber through a one-way valve (aka gas ballast valve) located on the pump. It is often said that the ballast gas is injected, but in actuality gas is being pulled into the pump by the rotating pump rotor, which produces reduced pressure inside the pump.
How does gas ballast work?
Gas ballast prevents condensation of vapors inside the pump by diluting the pumped gas with the air being drawn in through the ballast valve. Once the air is drawn in and mixes with the pumped gas, the water vapor being carried in the pumped gas makes up a smaller percentage of the gas, which now includes the ballast gas (air) as well. The water vapor is then too diluted to condense into a liquid. Another way to say this is that the ballast gas dilutes the vapor in the pumped gas so that the partial pressure of the water never reaches its saturated value during compression.
The ballast is introduced at the beginning of the compression cycle. After injection starts and the ballast gas is drawn into the pump, the pump rotor continues to rotate, which increases the pressure generated in the pump. This forces the one-way ballast valve closed but not until sufficient dilution has occurred. As the rotor continues to turn, the pump discharge valve is forced open and discharges the mixture of pumped gas, ballast gas and water vapor (Fig. 1).
Although the point in the pump cycle at which the ballast gas is injected is on the compression side of the pump, it is still below atmospheric pressure prior to the ballast valve opening, which forces the ballast valve to open and allow the ballast gas to be drawn into the pump. Although the pump is compressing the pumped gas at this point, it is not compressing it above atmospheric pressure. If it were, the ballast valve would not open. This can be confusing because common terminology describing gas ballast refers to the ballast gas being injected on the compression side of the pump, when it could be more intuitively described as being pulled into the second stage of the pump.
The ballast valve is a one-way valve that closes as soon as the pump pressure reaches atmospheric pressure, which is the exact point at which the pump outlet valve opens. This forces the pumped gas to immediately start exiting the pump after the ballast is introduced and before condensation of the pumped moisture or other vapor can occur inside the pump.
In addition to diluting the condensable water and other vapors, the gas ballast raises the temperature of the pumped gas by roughly 20°C (36°F), which also helps reduce condensation of vapors. Gas ballast is also used to decontaminate pump oil that has already been contaminated with condensed vapor, which can take several hours for severely contaminated pump oil. To prevent contamination during normal operation, pump manufacturers recommend running the pump with the inlet valve closed and the gas ballast on for 20-30 minutes after every cycle. This removes residual condensed vapors from the oil after each operating cycle, preventing them from accumulating and causing contamination. However, this recommendation is not often followed in practice given production demands on the equipment.
Although air is the most common ballast gas due to its low cost, it is not used when the moisture, oxygen or hydrogen contained in the air would react with the process gases. Nitrogen is preferred in these cases. Although more expensive, it is more inert than air.
The amount of ballast gas is selectable on many of today’s pumps, with a low flow and a high flow available (Fig. 2) or via a rotating knob to allow adjustment on a continuum as the knob is turned in or out. The negative effect of ballast on ultimate vacuum and oil loss is less in the low-flow mode than during high flow.
Although an essential tool for many applications, gas ballasting has several drawbacks that must be considered. The biggest is that it reduces the ultimate vacuum of the pump (Fig. 3) since the ballast gas negatively impacts the effectiveness of the pump by decreasing the pressure difference across the pump rotor. For this reason, the gas ballast valve should be closed in normal furnace operation. If it is not, the result can be failure to achieve the desired vacuum level for the process being run. Most pump manufacturers offer an automatic gas ballast feature, which avoids this problem but subjects the pump to ballasting even if no contamination exists.
Gas ballast also increases the rate of oil discharged from the pump. Although this is far more desirable than having it accumulate in the pump oil, measures must be taken to collect this oil to prevent its discharge from the plant. It is typically removed using either an oil-mist eliminator or coalescing filter (Fig. 4). When the discharged oil volume is high enough, it can be directed back into the pump for reuse via an oil return line.
Gas ballasting is an integral part of the successful operation of any vacuum furnace and should be considered a routine part of daily operation. Many heat treaters ballast their vacuum pumps for 20-30 minutes at the beginning of each day while planning their schedules and preparing loads to be run. Make it part of your routine. You will be glad you did.
- 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
- David Sobiegray, Edwards (www.edwardsvacuum.com), private correspondence
- Dr. Sang Hyun Park, Busch LLC (www.buschusa.com), private correspondence
- Mario Vitale, Oerlikon Leybold Vacuum USA, Inc. (www.oerlikon.com), private correspondence.