This article describes the function and principle of gas ballasting at a professional level.
Condensable gases
When a vacuum pump is used, the removed gas contains a combination of gases that can condense into liquid form and gases that cannot condense. This is because there is water vapor in the atmosphere that contributes to the mixture. When gases are compressed beyond their saturation vapor pressure, condensable gases will condense and mix with the pump oil.
Accumulation of condensed substances reduces pump’s ultimate vacuum and pumping speed over time. Higher humidity in pumped gas contaminates pump oil, affecting sealing, lubrication and cooling. Consequently, frequent oil replacements become necessary.
Principle of gas ballasting
Figure 1 depicts the gas ballasting process.
It includes the gas intake phase (“ab”) with an intake pressure of Pv.
The pump with an open gas ballast valve undergoes a slight compression to point b’. From there, some dry air is introduced to the pump chamber until it reaches the exhaust pressure PT2 (“b’c'”). And then, the mixed gas is discharged (“c’f”).
Figure 1: gas ballast operation
For a pump without gas ballast, gas is compressed to the saturation pressure PST at the pump’s temperature (“bc”). Gradual liquefaction occurs thereafter (“cd”). Further compression leads to a sudden pressure increase to the exhaust pressure PT2 (“de”). The liquid is expelled into the oil tank, mixing with the pump oil (“ef”).
During the gas mixing process, the total pressure of the gas matches the exhaust pressure, and the partial pressure of the vapor achieves only PST, when the compressible gas reaches point “c”. The exhaust valve is therefore opened to release non-condensed vapor and gas. This is the working principle of a gas ballast.
Limitations of gas ballast
- Gas ballasting removes vapor but impacts the pumping speed and ultimate vacuum. During gas mixture, the pump’s ultimate pressure decreases significantly. A recommended solution for it: Once vapor is almost eliminated, close the gas ballast valve and keep the pump running. The maximum ultimate vacuum can be gradually reached after a period of time.
- Gas ballasting aims to prevent vapor condensation during compression. However, some vapors, which can dissolve in the oil, may not be efficiently removed by the gas ballasting process. As a result, their presence in the pump may persist, potentially affecting the overall performance and efficiency of the system.
Additional benefit of gas ballast
A mechanical vacuum pump with a gas ballast valve has an additional benefit of self-purifying the pump oil.
When the pump oil becomes slightly contaminated with condensate, sealing the pump inlet and opening the gas ballast valve allows for a certain period of vacuum operation. This process enables the pump oil to restore its original performance by effectively removing the contaminants. As the pump operates, the gas ballast helps to purge the contaminants from the oil, restoring its purity and ensuring optimal pump performance.
In summary, the gas ballast in a vacuum pump prevents vapor condensation and oil contamination by introducing dry air during compression. It maintains pressure, improves pump efficiency, and self-purifies pump oil for optimal performance.
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