A typical diff pump has a chimney that sets in a pool of heated oil. The chimney has a number of downward (back toward the pool)facing annular jets. The chimney and oil are contained in a tubular housing with inlet at the top and outlet just above the oil reservoir. It has cooling coils around it.
The oil is heated to boiling by an external heater. The oil vapors go up the chimney and out of the jets at a supersonic speed. When the oil hits the wall of the pump, it condenses and goes back to the boiler.
Gas molecules that wander into the pump, when they get to the oil vapor stream are whacked by the oil molecules and given a kick downward toward the outlet. This is unlike the mechanism in mechanical pumps where a piston or piston-like action compresses the gas and expells it. Diff pumps only work in transition or molecular flow (under ~50 mTorr) where these molecule to molecule interactions work. At higher pressures the sonic stream doesn't develop and all you have is an oil vapor generator.
Steve
Created on Friday, March 30, 2001 10:35 AM EDT by Steve Hansen