Figure 415 shows an ejector pump used in many engineering de

Figure 4.15 shows an ejector pump used in many engineering devices. Describe the working principle of this pump. Using the Bernoulli equation, determine the ratio Q_2/Q_1. Make any simplifying assumptions needed for your analysis.

Solution

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

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.

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.

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.

The above designed ejector pump has the following parts -
1) There are three connections. One for the high pressure (HP) motive Q1 , one for the low pressure (LP) suction entrained Q2 and one for the medium pressure discharge (Q1+Q2) .

2) The suction (in this case gas) comes in at the side B.

3) There is a nozzle for converting the pressure energy of the high pressure motive into kinetic energy (Portion C).