VacuumWorld.com offers Welch Scroll Pumps. 

Welch SCSpro 15 has a built-in self-cleaning air flush that removes that vapors that make it past an in-line cold trap and condense inside the pump.  The air flush doesn’t impact the ultimate vacuum or pumping speed of the SCSpro 15.

 
In applications where a vacuum pump is connected to a sealed container, chamber, or a system with an in-line cold trap, gases or vapors can condense into a liquid state inside of the pump. These vapors must be flushed out to prevent contamination and ensure peak efficiency. The self-cleaning air flush is there to protect your system against damage and disruptions from liquefied vapors. You don’t have to worry about your pump’s vacuum performance, it remains unaffected by the air flush function.