How to prevent oil mist with an exhaust mist filter?

Oil-sealed rotary vane pumps emit oil mist from the exhaust ports when they are operated regardless of their manufacturer.

This “mist”, sometimes referred to erroneously as “smoke”, is actually a fine aerosol of the vacuum pump oil that forms as the oil is squeezed through the tiny clearances that are inside the pump mechanism and as an effect of the heightened temperature of operation.

This hydrocarbon oil mist may represent a flammable hazard.

When an oil-sealed rotary vane pump has high gas throughput, for example when roughing a large chamber down from atmospheric pressure, oil mist will be seen at the pump’s exhaust.

Examples of other occasions to generate oil mist at the exhaust include pumping a high gas flow introduced in the vacuum system or when running gas ballast. 

Over an extended time, this oil mist can cause the pump to lose a significant amount of its oil charge, possibly emptying the pump’s oil.

In a laboratory this can cause oil to be drawn into an exhaust ducting system or into the environment.

Even with a vacuum pump that is operating at its ultimate pressure there is generally a small amount of oil mist seen at the exhaust.

Most modern oil sealed rotary vane pumps use an ‘air bleed’ internally that does not affect the vacuum performance but prevents hydraulic “knocking” at very low pressures. This air bleed causes a small amount of oil mist to pass to the exhaust even with a pump that is apparently not pumping any gas.