Editing Diffusion pump (section)

==Steam ejectors==
[[Image:DP_pumping_speed_plot.png|thumb|right|250 px|Plot of pumping speed as a function of pressure for a diffusion pump.]]
{{main|Vacuum ejector}}
[[Image:Langmuir mercury diffusion pump.jpg|thumb|Early Langmuir mercury diffusion pump ''(vertical column)'' and its backing pump ''(in background)'', about 1920.  The diffusion pump was widely used in manufacturing [[vacuum tube]]s, the key technology which dominated the radio and electronics industry for 50 years.]]  
 
The steam ejector is a popular form of pump for vacuum [[distillation]] and [[freeze-drying]]. A jet of steam entrains the vapour that must be removed from the vacuum chamber. Steam ejectors can have single or multiple stages, with and without [[Condenser (steam turbine)|condensers]] in between the stages. While both steam ejectors and diffusion pumps use jets of vapor to entrain gas, they work on fundamentally different principles - steam ejectors rely on viscous flow and mixing to pump gas, whereas diffusion pumps use molecular diffusion. This has several consequences. In diffusion pumps, the inlet pressure can be much lower than the static pressure of jet, whereas in steam ejectors the two pressures are about the same. Also, diffusion pumps are capable of much higher compression ratios, and cannot discharge directly to atmosphere.