ROTARY VANE PUMPS AND GAS BALLAST EXPLAINED

What is a Rotary Vane Pump?

Known to be the most common type of vacuum pump on the market, this style of pump has been around since the late 1800s. Common applications include automobile power-steering, espresso coffee machines, and fuel transfer.  Mechanically, a rotary vane pump has a circular rotor with vanes rotating inside a larger circular cavity. The two components are eccentric, meaning that the rotor with vanes does not share the same center as the larger cavity, they are offset which create a crescent shaped cavity. The circular rotor contains at least two vanes or more. The vanes must have contact with the sidewalls of the cavity. Depending on the pump design they can slide freely through the center of the rotor or they can have spring tension to help maintain full contact between the vanes and sidewalls of the cavity.

Fig.1: Rotary Vane Diagram

How do Rotary Vane Pumps Work?

As the circular rotor rotates gas molecules are inducted into the cavity through the intake port. The next stage is isolation, this is where the gas molecules are isolated in the cavity, done so by the vanes. Third, the gas molecules are compressed to allow for the final stage to take place. Once there is compression inside the cavity, the gas molecules can easily exit through the exhaust port and into the atmosphere or any desired location.

Fig.2: Pumping Flow Diagram

What is a Gas Ballast and How Do They Work?

When a pump is evacuating a chamber, all gas molecules in the chamber are being expelled through the pump. Gases included are water vapor, solvent vapor, and or any other contaminants. An increase in pressure as the vanes rotate through the compression stage causes some of these vapors to condense. As this “wet” vapor is exhausted from the pump through the oil reservoir, the condensate remains trapped in the pump oil. This can be seen in action through the pumps’ oil level sight glass, the oil will appear light colored and or bubbly. This problem eventually affects performance of the pump, as condensate will degrade the oil’s lubricating and sealing properties. A gas ballast is a passageway that leads into the compression side of the crescent cavity allowing for small air flow. The amount of air flow allowed is controlled through a cap located on the exterior of the pump.

Fig.3: Gas Ballast Diagram

Using a Gas Ballast

Some pumps have three positions for the gas ballast: closed, medium flow, and high flow. Others may only choose between fully open or closed. A check-valve located between the ballast valve and compression chamber opens to allow air to enter, but not leave back out through the gas ballast opening. Opening the gas ballast allows for air to enter the compression cavity and raise the pressure to atmospheric pressure. This prevents most evacuated vapors from condensing, allowing exhaust back out into the atmosphere. Because of this mechanism of introducing air into the compression chamber, operating a pump with the gas ballast open will affect the ultimate vacuum of the pump, and this information is commonly provided by the pump manufacturer. In applications requiring a deeper attainable vacuum than with the gas ballast open, it’s recommended to operate the pump with the ballast open to minimize wear on the pump while in primary evacuation, then to close the gas ballast as pressure decreases to attain the required vacuum level. Gas ballasts can be found on higher end vacuum pumps that may be higher in cost, more effective, and longer lasting. A gas ballast is highly recommended for applications that are known to have the presence of vapors harmful to the pump, such as water or other solvents.