Ejectors use Bernoulli's principle to pump one fluid using the pressure energy of another fluid. They work by accelerating a high pressure fluid through a nozzle, lowering its pressure and increasing its velocity. This creates eddies and suction that pull in a second fluid through a side inlet. The fluids then mix and their pressure increases in a diffuser section before being discharged at an intermediate pressure between the high and low pressure fluids. Ejectors are used in oil and gas applications to compress gases.

1. How do Ejectors Work?
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’.
The principle is demonstrated by air moving over the top of a 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.
First Diagram

2. This shows 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
Second Diagram
When the valve is opened the fluid can pass through the pipe and 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.

3. 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.
Third Diagram

4. 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would see that the eddy currents had disappeared and that they had been replaced by a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.
Fourth Diagram
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.

5. Summary
The ‘device’ we have created uses the available pressure energy in a fluid to do work on
(pump) a secondary fluid.
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 fluid, one for the gas sucked in
(or entrained) and one for the discharge.
2) The gas entrained comes in at the side.
3) There is a nozzle for converting the pressure energy of the high pressure fluid into
kinetic energy.

6. 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 fluid to compress low
pressure fluid to an intermediate pressure.