In this article, I will share the function, types and working principles of different types of desuperheaters used in steam boilers.
Desuperheaters are mainly used to reduce the temperature of superheated steam.
In most cases, the temperatures are reduced by bringing the superheated steam in contact with water or by using a heat exchanger.
The superheat from the steam is lost by evaporation of the water used to cool the gas.
And what can you understand from the word superheated gas? Superheated gas is a gas whose temperatures are beyond the expected ones.
So, there is a need to reduce the temperatures for optimal function of the steam at the points of applications.
Now that you have an idea of the function of the desuperheaters let me go ahead and share the types.
Types of desuperheaters
There are two main basic types of desuperheaters namely;
1. Indirect contact desuperheaters.
- Tube bundle type desuperheaters.
2. Direct contact desuperheaters.
- Water bath type desuperheater.
- Water spray desuperheater.
- Single-point radial injection spray desuperheaters.
- Multiple point radial injection spray desuperheaters.
- Axial injection spray desuperheaters.
- Multiple nozzle axial injection desuperheaters.
1. Indirect contact desuperheaters.
The name, suggests that this type of desuperheater uses a heat exchange to do the cooling.
There is no direct contact between the cooling medium and the superheated steam.
For you to understand deeply about this type of desuperheater, I will have to explain to you how this type of desuperheater works.
Tube bundle type desuperheaters.
The main components of the tube bundle type desuperheaters are the following;
- Heat exchanger.
- Shell.
- U-tube.
The U-tube of the desuperheater contains a heat exchanger on both sides.
It has an inlet and outlet at the top of the tube that allows the superheated steam to enter and leave the tube via the second point of the U-tube.
The U-tube is connected with a shell in between the bottom and also at the top there’s a float head.
The float head is similar to the shell that is connected at the bottom but it’s slightly slanted on one side.
The main function of the shell at the bottom is to store and provide passage of the cooling medium from one end of the tube to the other which is at a saturated pressure and temperature.
The float head is used to equalize the pressure between the two sides of the tubes.
You will see how that works in the working principle of this type of desuperheater.
Working principle of tube bundle type desuperheater.
The superheated steam enters one side of the tube through the inlet at the top.
The water in the heat exchanger is at a considerable level, not full to the brim, as this allows the water in the heat exchanger to evaporate thus cooling the superheated steam.
The superheated steam heats the water in the tubes as it flows down across along the U-tubes to the other side of the tube. The other side also contains a heat exchanger.
The superheated steam transfers its energy to the water which will cause evaporation thus cooling the superheated steam.
The evaporated cooling water will accumulate on one side of the float head and then it will flow to the slanted side of the shell and mix with the desuperheated steam.
After mixing up with the desuperheated steam, it will flow to the bottom of the U-tube where there is a float trap located and it will be eliminated from the tube.
The desuperheated steam will be saturated and will leave the U-tube via the next side of the tube.
Points to note: the cooling water is supplied by a pump if the pressure is insufficient.
The cooling water used in the tube and bundle type desuperheater should be at saturation pressure and temperature.
The U-tube has a float trap overflow and a drain on the side where the superheated steam enters the U-tube.
The safety valve is installed on this desuperheater for safety measures as the superheated steam tends to have a high pressure during entry.
There is no contact between the cooling water and the superheated steam when first entering the desuperheater.
Only the evaporated cooling water comes in contact with the desuperheated steam after flowing out from the float head.
2. Direct contact desuperheaters.
As the names suggest, there is direct contact between the superheated steam and the cooling medium.
The superheated steam is brought in contact with the cooling water to desuperheat it.
Water bath type desuperheater.
The main components of this type of desuperheater are these;
- The tank contains water.
- Pump.
- Mountings like a safety valve, water level controller, non-return valve, drain, pressure gauge and float trap.
This is a simple type of desuperheater in the industry because of how it’s designed.
Working principle of water bath type desuperheater
The superheated steam is injected into the tank containing the water bath thus resulting in the evaporation of the saturated steam.
Since the superheated steam has a higher temperature than the saturated steam, more steam will be evaporated from the tank and this will result in a decrease in the water level in the tank.
The steam leaving the vessel will be desuperheated since its temperature will drop significantly when it comes in touch with the water bath.
Water spray desuperheaters.
This type of desuperheater contains nozzles that are used to spray water on the superheated steam to result in desuperheating.
These are the types of water spray desuperheaters you should know;
- Single-point radial injection spray desuperheaters.
- Multiple point radial injection spray desuperheaters.
- Axial injection spray desuperheaters.
- Multiple nozzle axial injection desuperheaters.
1. Single-point radial injection spray desuperheaters.
This is a simple type of desuperheater that contains a single nozzle through which the cooling water is sprayed from.
This is how a single-point radial injection spray desuperheater works; the superheated steam enters through one side of the desuperheater and comes in contact with the water sprayed by the nozzle at the inlet.
The spayed cooling water cools desuperheats the steam and the steam leaves as desuperheated.
The effectiveness of this type of desuperheater is brought by controlling the amount of cooling water entering the desuperheater.
Varying the position of the valve that is located at the centre of the nozzle increases or decreases the amount of water sprayed in the desuperheater.
2. Multiple-point radial injection spray desuperheaters.
The main difference between the single and multiple-point desuperheaters is the number of orifices present to supply the cooling water into the desuperheaters.
The working principle of multiple point radial injection spray desuperheater is just similar to the single type, the only thing that changes are the number of orifices available.
In multiple, there are more than two orifices that spray the cooling water over the superheated steam.
The increased number of orifices increases the amount of cooling water supplied thus increasing the efficiency of desuperheating.
3. Axial injection spray desuperheaters.
In this type of desuperheater, the spray nozzle is moved in the axis of the pipeline which allows the superheated steam to pass through.
The nozzle placed along the axis of the pipe of the desuperheater encourages the mixing of cooling water and superheated steam which increases the rate of desuperheating.
The nozzle being at the axis of the pipeline facilitates even distribution of cooling water and also it creates a distraction that decreases the speed of the superheated steam thus increasing the time at which the water mixes with the cooling water due to the formation of turbulence.
Another design of axial injection spray desuperheater is the reverse flow type axial desuperheater which works on the same principle as the one I have talked about.
The only difference is that the nozzle is placed facing the inlet such that the superheated entering will have to go against the sprayed water jet.
The cooling water will be sent back due to the high pressure of the superheated steam and it will still be in contact with the cooling water as it moves along the pipeline.
This ensures that there is sufficient mixing of the cooling water and the superheated steam to increase the rate of desuperheating of the steam.
4. Multiple nozzle axial injection desuperheaters.
This type of desuperheater has multiple nozzles that are placed in the single pipe that is placed at the axis of the pipeline same as in an axial injection spray desuperheater.
This ensures that enough water is supplied thus increasing the efficiency of the desuperheater.
There are three types of multiple nozzle axial injection desuperheater namely;
Fixed area type; in this type of desuperheater, all the nozzles are opened when the desuperheater is working. The flow of cooling water is controlled by the water control valve.
The pipe supplying the cooling water via the nozzle is at a fixed position.
Variable spray type; this involves the mechanisms of detecting the temperature of the superheated steam to determine the number of nozzles that will be allowed to release water into the pipeline.
This is possible because of the actuator that moves up and down exposing the nozzles by either removing the disc arrangements on the nozzles or placing them at the nozzles.
When there is an increase in downstream temperature, the actuator moves the stem down thus exposing more nozzles to the superheated steam which in turn results in more water being sprayed.
If the temperature of the superheated steam drops the actuator moves the stem and disc arrangement up thus closing some of the nozzles and minimizing the number of nozzles spraying cooling water.
Spring-associated type; it’s a combination of the fixed and variable spray type desuperheater, but instead of having disc arrangements that are controlled by the actuator to either close or open the nozzles, there is a spring that moves in response to changes in the differential pressure between the cooling water and the superheated steam.
Final thoughts
This is a summary of desuperheaters that you should know about.
I have tried my best to give you value that will give you a comprehensive understanding of how these desuperheaters work.
The only thing you have to grasp here is how these desuperheaters work.
If you know their working principle the other things will be easier for you to understand.
There is still much I haven’t talked about in this article, like the factors that increase efficiency and the advantages and disadvantages of these desuperheaters.
Keep checking, I will be coming with these articles.
Thanks for reading, if you have any questions or want an extensive dive into these desuperheaters you should check the link below.
See you in my next article.
Reference: Spirax Sarco Desuperheater
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