THE ejectorsalso called jet devices or trumpetsare static devices intended to aspire, compress or mix gases, fumes, liquids and sometimes solids thanks to the expansion of a primary driving fluid. This can be gaseous, in subsonic or supersonic regime, or liquid, in incompressible regime, and provides the energy necessary for driving the secondary fluid.
These devices can also ensure the emptying of a cavity or a volume and, in certain applications, compete with mechanical vacuum pumps thanks to their absorption capacities which can reach 106 m3.h−1 d’air.
Compared to vacuum pumps, ejectors have the great advantage of being of a cost price significantly lower than that of a conventional pump, of having no moving parts, therefore of being virtually maintenance-free , and resist corrosion or attack from aggressive gases or solid particles. However, they do not always supplant mechanical vacuum pumps due to a narrower operating range and high energy consumption and, in certain applications, the combination of ejectors and liquid ring pumps is to be recommended.
In addition to their use in the production of vacuum, ejectors are devices used in a large number of very diverse applications, ranging from the chemical industry for the mixing of fluids or the desalination of water, up to the industrial aeronautics where the ejectors make it possible to increase the propulsion or to reduce the noise of thrusters, through the production of cold where the ejector, coupled to a heat source (which can be of renewable origin), ensures the compression of a refrigerant.
The applications of ejectors depend essentially on the nature of the fluids used. Given the large number of possible combinations between the different fluids, it was not possible to expose the fundamental physical laws that govern the operation of all these devices. However, in order to allow an engineer to get a rough idea of the sizing and characteristics of the device he needs, we deal with the three main combinations:
-
liquid jet liquid ejectors,
-
gas jet gas ejectors,
-
steam jet steam ejectors.
The first combination relates to incompressible fluids, while the other two relate to compressible fluids.
It should be noted that there are several manufacturers on the market, a non-exhaustive list of which is given in the appendix, who offer more or less detailed catalogs of standard devices or who are perfectly capable of studying and carrying out any particular project. In addition to the basic theoretical methods, they have the know-how and the experience essential to obtaining the required performance.
In this regard, the author would like to thank the various manufacturers cited for their documentation, which was largely used to write this article.
The reader will find at the end of the article a table of notations and symbols, as well as a table of the indices used.