Reasons why ejector play an important role in chemical plants

ejector vacuum
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I will explain how to use the ejector for each drive source .

A convenient vacuum device that can be used even if you just have a vague understanding of it.

It is very active in chemical plants as an exhaust gas absorption device generated by reactions.

Combined with consideration for the environment, its position will remain stable in the future.

If you are a mechanical engineer, you want to understand one step ahead.

Principle of ejector

I will briefly explain the principle of the ejector.

When it comes to details, it will be the world of compressible fluids in fluid dynamics, but I will leave it to specialized documents and summarize only the outline.


The ejector has two inflows, the sucked gas and the driving fluid, and the mixture as the outflow.

Considering a major steam ejector, suction gas and steam are flowed into the ejector and the mixed gas is discharged.

Since the ejector is a device for the purpose of creating negative pressure , let’s first look at the balance related to pressure .

driving fluidP0
gas inletP1
gas outletP2

Naturally, P1 > P2 . Otherwise the gas will not flow.

P0 > P2 because the driving fluid is at a higher pressure than the gas outlet .

Also P0 > P1 since the driving fluid is higher than the gas inlet .

So the relationship is P0 > P1 > P2 .

The size of the difference changes depending on the operating conditions of the driving fluid, but the size relationship does not change.

Now, in order to obtain this pressure relationship, the speed becomes the control parameter.

Velocity (ejector)

The driving fluid has a nozzle at the tip.

We are trying to increase the speed by reducing the diameter of the nozzle .

On the other hand, there is a certain amount of pressure loss.

Mixing pressure (ejector)

Let the pressure at the nozzle outlet be P 3 .

The nozzle pressure loss is P0 – P3 .

P3 has a slightly lower pressure than P1 , and the driving fluid and gas are mixed.

Then it passes through the throat of the ejector and is discharged to the outlet.

Features of ejector

Introduce the characteristics of the ejector.

  • long life
  • Corrosion resistant
  • Space saving
  • Can be used in explosion-proof areas

The biggest feature of the ejector is that it does not use rotating or moving equipment .

The risk of breakdown is greatly reduced.

It can also be made from highly corrosion-resistant materials such as carbon. However, it breaks easily…

The facility size is small, and it is useful in Japan where it is narrow.

Since it does not use a motor, it can be installed inexpensively even in explosion-proof areas.

steam ejector

There are three types of driving fluids: steam, water, and air.

They are called steam ejector, water ejector and air ejector respectively.

I generally use a steam ejector.

The advantages of steam over other driving fluids are:

  • Steam with a wide pressure range can be used
  • condense

temperature range

Steam can be set to a fairly wide range of pressure if the boiler steam is decompressed .

This can be an advantage because the performance of the ejector can be changed simply by changing the pressure .

However, in reality, you will not adjust the operation with steam pressure.

This is because the suction flow rate and suction pressure, which are the performance of the ejector, are determined by the ejector.

If the pressure of the steam is changed when the suction flow rate is sufficient but the pressure is not enough, the pressure may be satisfied but the suction flow rate may decrease.

In such a case, attach an adjustment pipe in front of the ejector.


The drive fluid is run at a higher pressure, and the adjustment fluid is run while the ejector is trying to function with suction pressure lower than the operating conditions.

Operates at operating pressures in an attempt to handle more than the specified flow rate.

Since the steam has a high pressure, it becomes a high speed state at the nozzle exit and becomes sonic speed in the mixing section.

When the speed exceeds the speed of sound, a phenomenon occurs in which the flow speed increases as the cross-sectional area increases (under normal conditions below the speed of sound, the flow speed decreases as the cross-sectional area increases).

The ejector throat appears to be designed with the intention of making it supersonic.

Can be installed in several stages

Several steam ejectors can be installed.

This uses the condensability of steam.

This property is not found in water or air.

When the steam ejector is passed through one stage, gas with the volume of gas + steam is generated at the outlet.

When trying to connect several stages of steam ejectors in series in order to increase the suction capacity, the performance loss of each stage of the steam ejector is caused by the volume of steam.

Therefore, it is conceivable to condense the steam through a condenser.

Cooling water (ejector)

Since the steam itself is hot and the pressure inside the ejector is low, it will not condense unless it is cooled to a certain temperature.

If it still condenses, the steam volume becomes almost negligible.

high cost

The steam ejector has the greatest advantage of a large pressure adjustment range, but it also has disadvantages.

that’s the cost.

The cost of using steam is simply a bottleneck.

Carbon neutrality is now a problem.

I believe that many factories are switching to water ring vacuum pumps.

water ejector

You should think that the water ejector has lower capacity than the steam ejector.

  • Lower driving pressure than steam
  • Cannot be installed in multiple stages
  • low cost

The reason why it is impossible to install water ejectors in multiple stages is that the gas inlets of the second and subsequent stages are in the state of gas + water.

Water may accumulate somewhere in the ejector and the performance may not be demonstrated.

Like the condenser used in steam ejectors, water ejectors can also be installed in multiple stages if a mechanism for discharging water is provided along the way.

However, I don’t think there is a need to install multiple stages of water ejectors with low capacity per stage.

In terms of cost, it takes the power of the pump to lift the water, but it is not as expensive as steam.

Generally used in the form of a scrubber.

air ejector

Air ejectors are generally driven by the atmosphere.

Suction itself is not impossible if compressed air is used.

However, unlike steam, it does not condense and unlike water, it cannot be separated, so multiple stages cannot be installed.

The advantage of the air ejector is that it reduces the power consumption, so it is better to have the air sucked.

It is usually used before the water ring vacuum pump.

Normally, it is not possible to operate the air ejector alone.

If P0 -P2 > 0 is not achieved when P0 = 0, the drive air will not flow.


Vacuum is used in many industries, not just chemical plants.

For that reason, it is difficult to obtain information about one device such as an ejector.

You can study systematically with books such as:


I explained the features of the ejector separately for steam, water, and air.

Among the ejectors that have a long life, high corrosion resistance, space saving, and can be used in explosion-proof areas, steam is useful because of its wide range of applications.

There is a disadvantage in terms of cost, and water and air are used according to needs.

Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)

*We will read all the comments and reply seriously.