Ejector is a device whose task is to transport liquids through the pipeline. It is similar to a pipe fitting and it has the same function as a pump. Primary (working) liquid is its source of driving energy. For this reason, it is referred to as “jet pump”. According to type and state of driving and driven liquid, the ejectors are divided to water (water is both primary and secondary medium), gas, steam ones etc. Working and sucked substance may differ. In that case the ejector releases mixture of two liquids.
The ejector works on the principle that the working substance enters the suction nozzle, that can be either classical tapering or Laval’s one, downstream which there can be supercritical flow. Due to expansion in the nozzle the pressure is dropped and the velocity of the primary liquid is increased. Thanks to that there is a negative pressure formed in the suction chamber and it begins to suck-in the secondary (driven) liquid. At the outlet of the suction chamber of the nozzle the admixture of substances has the same pressure but different velocity profile, that is to be conditioned in the mixing chamber. In case of subcritical flow, the mixing chamber has a shape of a cylinder and in case of supercritical flow it is tapered to slow down the flow. At the inlet of the divergent section (expanding channel) there must be subcritical flow according to Huguenot theorem, so that compression (i.e. reduced velocity) as well as increase in liquid pressure was caused. Then admixture of working and suctioned substance with the steady pressure and velocity profile leaves the ejector.
Ejectors can be produced in wafer as well as so-called welded-in designs. The weld insertion in the pipe has advantages such as material and mass savings on the flanges as well as minimization of leakage at the flange joints. Wafer type design has advantage of easy replacement of ejector in case of eventual modification of technological parameters as well as better access for servicing. The body of the ejector can be cast, machined from a forge, or it is welded from bent centring. Suction nozzle can be manufactured by machining.
Mostly used materials are carbon or stainless steels. Upon agreement, the injector can be made of special refractory steels and materials based on nickel titanium alloys, or tantalum. It can be made of plastic materials, too.
Selection of the design as well as the material of the body and suction nozzle of the ejector depends on operational and max. parameters and the type of primary and secondary liquid. It is suitable to use the ejector at locations with sufficient working substance for device powering. Pivotal dimensions are calculated from specified operating parameters of working and suctioned liquid.
The biggest advantage of the ejector is the lack of rotating parts as well as its construction simplicity. The ejector doesn’t need electricity, only sufficient quantity of working substance.