SUPERHEATER | Its Important Concepts with 20 FAQs

CONTENT

WHAT IS A SUPERHEATER IN BOILER? | USE OF SUPERHEATER IN BOILER

A superheater is a coiled-shaped heat exchanger which is usually used in boilers to convert saturated steam to dry steam/ superheated steam. In power generation plants, superheated steam is used for the production of electricity.

A boiler is different from a superheater as a boiler uses both latent and sensible heat to raise the temperature of the fluid, but a superheater uses only sensible heat to increase the enthalpy of the liquid. The size of a superheater varies between 10 feet to 100 feet.

Image Attribution: “File:Locomotive fire tube boiler schematic (with superheater).png” by Emoscopes is licensed under CC BY 2.5

BENEFITS OF SUPERHEATERS IN BOILERS

  • Increases the efficiency of power plants
  • Erosion of turbine blades at a slow rate
  • Reduced levels of steam consumption
  • Condensation losses minimized in steam pipes
  • Helps in increasing temperature saturated steam thereby removing water droplets from the steam in the turbines.

USE OF SUPERHEATERS

BOILER SUPERHEATER COIL MATERIAL

The superheater coils used in the boilers should be a good conductor of heat and should be strong to withstand the temperature.  The superheater may exceed temperatures above 9000C and therefore the superheater is usually made of carbon but not plain carbon. Instead, superheaters tubes are made up of carbon-molybdenum steel or chromium-molybdenum steel. The important construction material of the superheater is Esshete 1250

BOILER SUPERHEATER DESIGN | BOILER SUPERHEATER DIAGRAM

Superheaters in a boiler are tubes that achieve the highest temperature and require adequate care because of their high operating temperatures. It is essential to design the superheater using materials that can withstand high temperatures. Further, fouling and corrosion should also be taken into consideration while designing the superheater.

It is essential to consider the following while designing a superheater:

1. uniform distribution of steam in all directions to avoid unbalanced flow.

2. steam velocities should be high enough to keep the metal temperature at minimal

3. to minimize steam pressure losses

4. Pressure losses should be less than 8% to reduce the load on pumping.

A superheater is usually U-shaped in design and depending on the need for increasing superheat, a double U design might be adopted. A loop of the superheater is often placed in the location of the furnace to achieve a high temperature.

SUPERHEATER EFFICIENCY

The superheater helps in increasing the efficiency of a steam generation plant by enhancing the temperature of the saturated steam in the boiler. Further, it also ensures that steam devoid of moisture enters the turbine thereby reducing the chances of erosion of the turbine blades and helps in increasing the overall thermal efficiency.

The superheater efficiency can be enhanced by using different stages of superheating and by employing reheaters.

PRIMARY SUPERHEATER AND SECONDARY SUPERHEATER

The heater that is placed just after the drum of steam is the primary superheater. The first superheater or the primary superheater is also known as the Low-Temperature Superheater or LTSH.

The heat that is released from the exhaust gases released after combustion is absorbed by the LTSH. After the steam is heated in the LTSH, it passes into the secondary superheater where it is heated again. This steam passes into the intermediate or final secondary superheater.

SUPERHEATER HEADER

The superheater header is a component that was added to the 1926 conversion with its main role to absorb the steam from the dry pipe. The absorbed heat is then sent through the superheater tubes for getting heated for the second round whereby the steam branches into the pipes down to the cylinders.

Superheater header with high temperature is difficult to handle and also for the assessment of risk. A superheater header in a power plant is usually subjected to high temperature and high-pressure loads. These power plants are operated in cycles to increase flexibility and reduced running costs.

A superheater subjected to baseload is often affected by the creep properties of the material of construction. A superheater that is subjected to two cyclic loads will be prone to fatigue which would increase significantly.

SUPERHEATER ATTEMPERATOR | WHAT IS THE USE OF DESUPERHEATER IN SUPERHEATER?

A superheater attemperator or a desuperheater is installed in a superheater to reduce the temperature of the superheated steam. The temperature of the superheated steam is reduced by bringing the steam in contact with a liquid such as water.

Desuperheaters or attemperators which are used for restoring the saturation state usually operate at discharge temperature close to saturation temperature. There are two types of superheaters that are installed which are:

1. Indirect type desuperheater: The fluid used for cooling the superheated steam does not come in direct contact with the steam. A fluid that is cooler than the steam is used as a cooling medium. An example of an indirect type superheater is a shell and tube, heat exchanger.

In this type, the steam passes through one side of the heat exchanger and the cooler fluid flows through the other side. The heat from the steam is lost as the cooler fluid carries away the heat from the steam. The superheated steam temperature could be controlled by either controlling the inlet steam pressure or the cooling water flow rate.

2. Direct type desuperheater: The fluid used for cooling the superheated fluid is in direct contact with the steam. The cooling fluid is often the same as the vapor that must be cooled.

In this type, the water used for cooling is added to the superheated steam which is mixed in the desuperheater. The water that is used for cooling evaporates from the superheated steam while carrying some amount of heat from superheated steam. In this manner, the temperature of the superheater steam is reduced.

TYPES OF SUPERHEATER IN BOILER | RADIANT SUPERHEATER | CONVECTION SUPERHEATER

Three basic types of superheaters are used in boiler and they are explained below:

Radiant Superheaters

In Radiant superheaters, the superheater is installed after the furnace in the radiation zone. When the temperature difference between the saturated and superheated is about 10000C, the superheated is usually located in the radiation part of the boiler. The spacing in the first superheated is approximated to be between 150 and 1000mm.

Since the superheater is the hottest surface where the heat transfer process takes place, it is often made of high-temperature-resistant alloys that are also corrosion-resistant. The design of a superheater must be often compromised in terms of technicality and cost.

Wall superheaters are a type of radiative superheater which is usually located at the wall of the furnace. The wall superheater is composed of a panel of tubes that hangs on the furnace membranes. The wall superheaters are the first superheater after the steam drum and are also referred to as the coolest superheater.

Convective Superheaters

When the difference in temperature between the saturated steam and the superheated steam does not extend 500C, for such conditions convective superheaters are used. A convective superheater is installed in the convective zone after the convective tubes.

Convective superheaters are installed in the region with low superheat gas temperature wherein the temperature ranges between 300 to 10000F. The superheater is protected by several rows of tubes such that the gas is cooled before it enters the superheater.

With a higher log mean temperature difference and lower heat transfer coefficient, a large surface area for heat transfer is required which increases the cost of the convective superheater in comparison to the radiative superheater.

Separately Fired Superheater

In this type of superheater, the superheater is located outside the boiler and the superheater has its corresponding combustion chamber. There are burners near the superheater tubes. This type of superheater is not often used because of its limited efficiency and the quality of steam is lower than other types of superheaters.

A separately fired superheater is a superheater that is placed outside the main boiler, which has its separate combustion system. This superheater design incorporates additional burners in superheater pipes.

DIFFERENCE BETWEEN RADIANT AND CONVECTIVE SUPERHEATERS

The main difference between a radiant and convective superheater is that a radiant superheater is exposed to the flames of the burner i.e., it is located in the combustion chamber. While a convective superheater is not located in the chamber which is exposed to the flames of the combustion process.

A radiant superheater is exposed to high temperatures up to 10000C while a convective superheater can handle only temperatures in the range of 50 -2000F.

ELECTRIC STEAM SUPERHEATER

In an electric steam superheater, the saturated steam is heated to produce superheated steam using electric power. An electric steam superheater does not have a combustion chamber and the heat is produced using electricity rather than from any source of fuel.

Electric tubular heaters are used for indirectly heating the saturated steam. The temperature for the superheated steam is a control variable. In some electric steam superheaters, even the flow rate is also taken to be a control variable.

GEOTHERMAL SUPERHEATER

A geothermal superheater uses heat from the hot steam that comes from beneath the rocks at the earth’s crust and is pumped to heat the saturated steam to superheated steam. The heat transfer takes place as water passes through the rocks and is transferred to the steam. Geothermal superheaters are preferred in cold regions where there is a higher temperature gradient between the surface temperature and the temperature of the rocks.

DIFFERENCE BETWEEN SUPERHEATER REHEATER AND AIR PREHEATER

A superheater reheater is similar in function to that of a superheater but the temperature of the exit fluid is comparatively lower, and the pressure is also observed to be 20-25% lower than observed in superheaters. They are usually made of fewer quality alloys because they are exposed to lower temperatures. They help in increasing the temperature of the superheated steam which would have experience heat loss

An air preheater is a basic heater that helps in heating the air before it enters the combustion chamber which thereby decreased the dependence on fuel and also increases the thermal efficiency of the system. The savings incurred by having a preheater is directly proportional to the increase in temperature due to a preheater.

Image Attribution: “Reheater” (CC BY 2.0) by Terry Wha

INTERVIEW QUESTIONS AND ANSWERS ON SUPERHEATERS

1. What is a primary superheater?

The heater that is placed just after the drum of steam is the primary superheater. The first superheater or the primary superheater is also known as the Low-Temperature Superheater or LTSH.

2. What is the working of Superheater in thermal power plant?

The superheater in a thermal power plant helps in reheating the steam that is produced by the boiler which indirectly increases the thermal energy of the steam. The steam is likely to condense in the engine. Superheaters help in increasing the thermal efficiency of the superheated steam and have been used in boilers in thermal power plants.

3. What is the role of a superheater in a steam power plant?

A superheater is a coiled-shaped heat exchanger which is usually used in boilers to convert saturated steam to dry steam/ superheated steam. In power generation plants, superheated steam is used for the production of electricity.

4. Why doesn’t the pressure of the steam increase in a superheater?

The pressure of the steam does not increase in a superheater because as the temperature of the steam increases, it can expand through the pipes away from the boiler. Here the steam behaves close to that of an ideal gas, wherein when the temperature increases, the gas is either suppose to expand or increase its pressure.

5. Which material is used for a superheater in a boiler?

Superheaters tubes are made up of carbon-molybdenum steel or chromium-molybdenum steel. The important construction material of the superheater is Esshete 1250.

6. What is the difference between radiant and convection superheater? How do they work?

The main difference between a radiant and convective superheater is that a radiant superheater is exposed to the flames of the burner i.e., it is located in the combustion chamber. While a convective superheater is not located in the chamber which is exposed to the flames of the combustion process.

A radiant superheater is exposed to high temperatures up to 10000C while a convective superheater can handle only temperatures in the range of 50 -2000F.

7. Why is an attemperation given in between a superheater to reduce the temperature?

A superheater attemperator or a desuperheater is installed in a superheater to reduce the temperature of the superheated steam. The temperature of the superheated steam is reduced by bringing the steam in contact with a liquid such as water.

8. Is an electric superheater radiant or convective?

An electric superheater is a radiant one as the heating of the saturated steam is carried out using radiant energy and it does not have a combustion chamber wherein convection zone is located.

9. Why there are many stages of superheater in the steam power plant?

In steam power plants, superheaters are often divided into multiple stages to ease the process of temperature control and for allowing heat recovery. The heating surfaces are often arranged into vertical or horizontal stages.

10. How to find a super heater coil jam while running?

If there is a coil jamming while running the superheater, the flow of the fluid will be restricted and there will be an unexpected elevation in temperature which can be monitored through temperature control sensors.

11. What is the maintenance procedure for the superheater of the boiler?

The maintenance procedure for the superheated are jotted down below:

1. To measure the TDS (Total dissolved solids) and conductivity rate which would help in determining the rate of blowdown.

2. To ensure that the heat transfer surfaced is regularly cleaned.

3. To avoid scaling of the surfaced and thereby reducing the corrosion rate.

4. To ensure that the air to fuel ratio is maintained. It should neither be too high nor too low.

12. Why superheaters are not included in chemical cleaning?

The reason for not including superheaters in chemical cleaning is because of the varying scale of the superheater tube. They are different in the microstructure, thickness of the tubes. Moreover, a high concentration of chemicals and a long time would be required for cleaning the superheaters using chemicals.

13. Why there is more difference between superheater outlet temperature and turbine inlet temp?

The reason for the difference in the superheater outlet temperature and turbine inlet temperature is due to the distance of the pipeline that carries the superheated steam. When the fluid gets heated, it expands and moves outside the boiler thereby behaving like an ideal gas. It loses some heat while moving through the pipes to the turbine.

14. What is the effect of outside scale deposit on superheater tube in coke oven based Whrb boiler and failure mechanism?

There are several reasons for the failure of the superheater tubes which include, corrosion, creep, fatigue, and erosion. Superheaters experience premature failure of their tubes due to the high temperature of the metal on the heat transfer surface and deposition of scales on these surfaces leads to high-temperature corrosion.

15. What type of superheating arrangement is best for steam load change?

It is desirable to over-size the convective superheaters to achieve the required steam temperature. A two-stage design with an attemperation interstage would be able to handle a wider steam load range between 30 to 100 %.

16. How to remove silica that is accumulated in the superheater zone in the thermal power station?

The silica can be removed from the superheater zone to increase boiler blowdown that is to decrease the acceptable limits of silica in water. Silica is usually soluble in water at high temperatures. Hence silica can be removed by allowing high-temperature water to dissolve the silica salts in the superheater.

17. How to identify superheater tube failure of a running boiler?

Failure of the superheater tube is usually due to the high temperature that the tubes are exposed to without proper cooling. A failure of the tube can be identified when there is an obstruction in the steam flow during the start-up of the boiler due to condensation.

18. Explain superheater column steam generation.

In a large heat exchanger such as a boiler, the heat from the combustion of fuel is transferred to the saturated water. The saturated water in the boiler is superheated using a superheater to ensure that steam enters the turbine to reduce erosion of the turbine blades and to enhance the efficiency of the steam generation process.

19. What is the standard superheater diameter?

The standard diameter for a superheater is 29 mm and the minimum thickness is 1.62 mm.

20. What is the secondary superheater material grade?

The secondary superheater is exposed to a higher temperature, therefore plain carbon cannot be used as the material of construction. Alloys with Esshete in them would be a suitable material grade for secondary superheaters.

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About Veena Parthan

I am Veena Parthan, working as a Solar Operation and Maintenance Engineer for the UK Solar sector. I have more than 5 years’ experience in the field of Energy and Utilities. I have completed my Bachelor’s in Chemical engineering and Masters in Thermal Engineering. I have a profound interest in renewable energy and their optimization. I have published an article in AIP conference proceedings which is based on Cummins Genset and its flow optimization.
During my free hours, I engage in freelance technical writing and would love to offer my expertise on LambdaGeeks platform. Apart from that, I spend my free hours reading, engaging in some sport activities and trying to evolve into a better person.
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