Ejectors are devices that rely upon the change in flow properties when flowing through variable cross-sectional geometries to bring about the compression and increase the gas/vapour pressure in the device. The change in flow properties occurs due to involvement of complex aerodynamic phenomenon. The dominant among them are mixing of supersonic flow with subsonic flow and compression due to Shock waves. The mixing process is responsible for a considerable wastage of energy which leads to reduction in Ejector efficiency.

Multi Jet Ejectors employ multiple nozzles to inject the motive flow through multiple supersonic jets into the mixing volume. This helps to Distribute the energy from motive flow more evenly into the sucked flow which reduces the energy wastage.

But the efficiency enhancement cannot be generalized for all ejector applications. The incremental advantage offered by a multi jet ejector differs from application to application and in some applications no added advantage is observed.

Multi Jet nozzles should be preferred in the following cases:

Low molecular weight of Suction Medium.
Low density of Suction Medium
When the Motive gas flow is less in comparison to the Suction flow i.e in Gas Jet ventilators.
In case of very high Gas/Vapour flow.

Multi Nozzle assembly for a Triple Jet Ejector

Designing a Multi Jet Ejectors is a complicated process as just distributing the motive flow into multiple jets doesn’t yield favourable results. The geometry of the multi jet apparatus plays a crucial role in delivering an added advantage. Jet spacing and Jet angles being the important parameters.

A Multi Jet Ejector with improper geometry will infact deliver inferior performance compared to a Single Jet/Nozzle.

From a constructional and cost point of view Multi Jet ejectors have a more complex construction and have a higher construction cost compared to a Single Jet ejector.

Hence the pros and cons of a Multi-Jet ejector system must be carefully evaluated to obtain an Optimum Solution.