How do Ejectors Work?

Inside an Ejector - Nozzle and Diffuser

The operation of Ejectors is based upon Bernoulli’s Principle which states:

‘When the speed of a fluid increases its pressure decreases and vice versa’

Bernoulli principle
If you blow across the top of a piece of paper, what happens?
The air moving over the top of the piece of paper is moving quicker than the air underneath. Thus, the local pressure on the top surface of the paper is less than on the underside. The resulting pressure imbalance causes the paper to rise. This demonstrates the Bernoulli Principle.

 

Understanding the basics

ejector working principle stage one

Stage 1

Here is a length of pipe which includes a valve. The pipe is arranged to discharge to atmosphere. At the point of discharge there is a restriction (or a nozzle). The upstream side of the pipe is connected to a pressure source. The valve is closed so there is no flow. Upstream of the valve there is pressure energy. The arrangement is very similar to that of a garden hose connected to a domestic water tap

 

ejector working principle stage two

Stage 2

When the valve is opened the fluid can pass through the pipe and be discharged out of the end. Because there is a nozzle at the discharge of the pipe, we can make the following observations:

a) There is a pressure on the upstream side of the nozzle.
b) A jet of fluid, moving faster than the fluid within the pipe, emerges from the nozzle.

So, on the upstream side of the nozzle there is high pressure and low velocity and at the nozzle discharge there is low pressure and high velocity. The nozzle has converted the pressure energy available upstream of the nozzle into kinetic (or velocity) energy.

Now, if we were able to see the surrounding air in the region of the nozzle discharge, we would see that there would be eddy currents of air, circulating around the jet. In other words, the jet of fluid emerging from the nozzle has imparted some of it’s kinetic energy onto the surrounding air.

 

ejector working principle stage three

Stage 3

If we then placed a tube with open ends around the area of the nozzle discharge, we would see that the eddy currents had disappeared and that they had been replaced by a steady flow of air moving through the tube, in a direction from left to right, as shown in the diagram.

 

ejector working principle stage four

Stage 4

If we then blank off the upstream end of the tube and added a side inlet, we would see that the air would be sucked in through the side inlet and discharged from the end of the tube. We now have a simple device that is capable of pumping the surrounding gas. This is a very basic form of an Ejector.

 

Ejector Design

This diagram shows the basic components of an Ejector used in the Oil and Gas industry. This Ejector was designed for use with gas. It has similarities with the “basic” Ejector shown in the above diagrams.
These are:
1) There are three connections. One for the high pressure (HP) motive, one for the low pressure (LP) suction entrained and one for the medium pressure discharge.
2) The suction (in this case gas) comes in at the side.
3) There is a nozzle for converting the pressure energy of the high pressure motive into kinetic energy.

The Diffuser

The biggest difference between this and the first diagram is the venturi shape towards the discharge end of the Ejector. This part is called the Diffuser.
The Diffuser is designed to firstly mix the two incoming streams. Then, when mixing is complete, the diverging section slows the mixture down, thereby increasing it’s pressure. This is the reverse of the process occurring in the nozzle. This feature enables the Ejector to discharge at a pressure that is greater than that at the suction branch. Thus, the Ejector is capable of compressing or boosting the pressure of the fluid entrained.

Summary

Ejectors use a high pressure motive to compress a low pressure suction to discharge at an intermediate pressure.

Inside an Ejector - Nozzle and Diffuser