Ejectors and Vacuum Systems

Ejectors are steam jet vacuum pumps which use the energy of a steam jet to entrain, mix and compress low pressure gas (100 to 300 Torr) up to atmospherich pressure. When more vacuum is necessary, two, three, four or more stages with interstage condensers are required:





          One stage: up to 100 Torr
          Two stages: up to 25 Torr
          Three stages: up to 6 Torr
          Four stages: up to 0,75 Torr
          Five stages: up to 0,25 Torr













Its basic principle is the following: Inside the nozzle, steam expands up to to a pressure rather less than suction and get out supersonic speeds. First, the percent velocity rising is greater than percent density one, but afterward the inverse relation occurs. So, the profile must be convergent-divergent. The outgoing jet transfer its impulse to the secondary gas or vapor, which flow in through the suction opening, up to the mixing section, until speeds even up. Inside he diffuser, kinetic energy is transformed into compression work. The compression ratio depends of the initial mixture at the section 1. When it is 4 to 8 as for common ejectors, Mach number (ratio between fluid velocity and sound velocity) must be M > 1. The evaporators thermocompressor has a compression ratio 2 and the regimen is nearly sonic, M = 1. For jet fans or jet compressors, M < 1. The entropic diagrama shows the change traces occurring at an ejector steam-steam: 
Let's make a point of the fact that since those process involves a considerable number of looses, it also produces an entropic increase.  The required quantity of motive steam per kg of sucked vapor, geometrically is equal to ration of segment xo1/x1.