The oil booster pump operates in a similar manner to a diffusion pump, by employing oil of low pressure. The high speed jet is generated through a jet assembly and the oil is gaseous when entering the nozzles. Within the nozzles the flow changes from laminar to molecular.
Often, several jets are used in series to enhance the pumping action. The outside of the diffusion pump is water cooled, as the vapor jet hits the outer cooled shell of the diffusion pump, the working fluid condenses and is recovered and directed back to the boiler. the pumped gases continue flowing to the base of the pump at increased pressure. The oil booster pump has an additional ejector stage which increases pumping speed between 10-4 and 10-5 mbar, which is a requirement for many metallurgical processes.
Oil contamination from the pump is an issue, and baffles at the high vacuum port are recommended to minimize contamination of the product. However, these will result in a noticeable reduction in pumping speed and this needs to be factored in when determining pumpdown times.
You will also notice a small oil catch pot at the forevacuum port, which is designed to reduce carry over into the dry pump. This alone is not an issue, but particles are also carried into the pump. These can mix with the silicone oil of the oil booster pump and, if left unchecked, can act as a grinding paste, increasing the small rotor tolerances of the pump. This is in turn will result in poorer ultimate pressure over time. It is recommended that a fine inlet particle filter is used to avoid the process producing significant particles. Cleaning or replacing this filter should form part of the routine maintenance schedule.