Methods of Superheater Temperature Control Explained for Curious Minds
Introduction
A superheater is made up of several tubes. These tubes are usually connected in parallel between two headers. This design helps reduce the pressure drop inside the tubes. One header is linked to the steam drum to receive saturated steam, while the other is connected to the outgoing main steam pipe through the Main Steam Stop Valve (MSSV).
What is a Superheater?
Briefly explain what a superheater does in a power plant: heating steam beyond its boiling point to make it “superheated” and improve efficiency.
Example analogy: “It’s like making your tea extra hot so it stays warm longer.”
The superheater is subjected to high temperatures. So, special alloy steel is used here. Alloy steel is highly heat and corrosion-resistant. The tube is alloyed with chromium, molybdenum, nickel, titanium, and niobium. The superheater is placed at the flue gas path at various zones to gain heat from hot flue gas. Depending upon the mode of heat transfer, superheaters are classified as follows:
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Convective Superheater
This superheater is placed in the convection zone of the boiler. Heat transfer takes place here mainly by the convective method.
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Radiant Superheater
This type of superheater is located in the radiation zone of the boiler. Superheater tubes are exposed to the flame of the furnace. Heat transfer is done mainly using the radiation method in this case.
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Platen Superheater
This type of superheater is placed in such a location so that heat transfer can occur through radiation and convection. The superheater is located generally above the boiler furnace and starting of the convection zone.
Outlet Temperature at Various Loads for Different Types of Superheaters Radiant superheater is placed in the radiation zone. It absorbs radiation heat. So, during the low load period, when steam flow through the superheater tube is less, the outlet temperature remains high. As the load increases, more steam flows in the superheater tube, so the temperature of the steam falls.
Outlet temperature at various loads for different types of superheaters. In the case of a convective superheater, heat transfer to the steam takes place through the convection method. This type of superheater is placed at the flue gas path. During low load, the flue gas volume is less, as less fuel is fired in the boiler. So, the outlet temperature of the superheated steam is less. When the load increases, more fuel is fired in the boiler. So, the volume of flue gas increases. Hence, the outlet temperature of the steam also increases.
However, a combined superheater maintains a constant temperature at any load, so this type of superheater is preferred in most cases.
Types of Superheaters
Non-Drainable Superheater
In some boilers, the superheater tubes are arranged vertically. This setup makes removing condensate (water droplets formed inside the tubes) hard when the boiler is turned off. This type of superheater is called a non-drainable superheater. Proper precautions must be taken to avoid problems during boiler startup. For example, the vent of the superheater should be opened so that the condensate can evaporate safely.
Drainable Superheater
In horizontal setups, the superheater tubes are arranged with a slight slope. This sloping design helps any condensate drain out completely, making this type of superheater called a drainable superheater.
METHODS OF SUPERHEATER TEMPERATURE CONTROL
Maintaining the outlet superheated steam temperature within the limit is required. The temperature cannot fall below and rise more than a specific value. This range is very close when steam is used to drive a turbine. Maintaining the temperature within the limit is essential to minimize thermal stress. When steam demand decreases suddenly, flow in the superheater reduces, and the temperature of steam increases. For this, steam temperature decreases when flow increases in the superheater due to sudden load demand. Some control is required to maintain the outlet steam temperature. Various methods are adopted to control the outlet steam temperature.
Some of these methods are discussed here.
1. Gas Bypass Method:
In this method, a damper bypasses a portion of the flue gas, reducing the amount flowing through the superheater during low loads. By bypassing flue gas, the heat available to the superheater decreases, maintaining steam temperature under low-load conditions. When the load increases, the damper closes, allowing the entire flue gas to pass through the superheater. The damper operates in a high-temperature and erosive environment, leading to challenges like corrosion, erosion, and fatigue. Additionally, the draft loss varies depending on whether the damper is open or closed.
2. Excess Air Control Method:
The air supply to the furnace can also be controlled by varying steam temperatures. Increasing the air supply reduces heat absorption at the water walls, increasing the heat content of flue gas and thus raising the superheater steam temperature. Conversely, reducing air supply increases heat absorption at the water walls, lowering the heat content of the flue gas and decreasing the superheated steam temperature. This method is most effective with convective superheaters.
3. Tilting or Adjustable Burner Method:
Here, the burners are designed to be adjustable, allowing them to tilt upward or downward. When burners are tilted downward, the water walls absorb more heat, leaving less for the superheater, thereby reducing steam temperature. Tilting the burners upward has the opposite effect, increasing steam temperature. In boilers with multi-tier burners placed at different elevations, upper-tier burners are used during low loads to increase furnace exit temperatures. In contrast, lower-tier burners are added during high loads to maintain a constant exit temperature.
4. Separately-Fired Superheater Method:
This method involves two furnaces: the radiant superheater placed in a separate furnace and the convective superheater in both furnaces’ common flue gas path. Steam temperature is controlled by adjusting the firing rates of the two furnaces, allowing precise management of heat absorption.
5. Flue Gas Recirculation Method:
This method uses a fan to recirculate flue gas from the economizer outlet into the furnace. Recirculated gas acts similarly to the excess air method. Increasing recirculation decreases heat absorption at the water walls, raising the superheated steam temperature.
6. Coil Immersion in Boiler Drum:
This method involves passing part of the superheated steam through a coil immersed in the boiler drum. A bypass valve controls the flow through the coil. When the steam temperature is high, the bypass valve closes, forcing more steam through the coil, which cools. If the temperature drops, the bypass valve opens, reducing the amount of steam passing through the coil and increasing its temperature.
7. Attemperation or Desuperheating Method:
Attemperation is one of the most commonly used methods. In this approach, cold feedwater is sprayed directly into the steam to reduce its temperature. The superheater is divided into two sections: the primary and secondary superheaters, with the attemperator located between them. This setup ensures effective temperature control while eliminating the steam’s moisture risk as it passes through the secondary superheater after attemperation.
Attemperation can be implemented in two ways:
Spray Type Attemperator: Feedwater is sprayed directly into the steam via a nozzle. The amount of spray water is adjusted using a control valve in an auto mode, allowing precise control of steam temperature. To avoid thermal shock, a protective jacket is provided in the main steam header where feedwater spray occurs. Condensate from saturated steam can be used instead of direct feedwater spray for applications where steam purity is critical.
Surface Type Attemperator: Here, feedwater and superheated steam do not mix. Instead, feedwater flows through the shell of a heat exchanger while steam flows inside the tubes. Temperature control is achieved by varying feedwater flow through the exchanger using a control valve. This method ensures steam purity is maintained.
conclusion
Understanding the intricacies of superheater temperature control is essential for grasping the operation of modern power plants. This article aims to demystify these complex concepts for young and curious minds by explaining methods like gas bypass, excess air control, burner tilt adjustments, and various types of attemperation. The principles highlighted are critical to the efficient operation of boilers and play a vital role in ensuring safety, energy efficiency, and environmental compliance.
The methods discussed here underscore the precision and ingenuity required in thermal engineering. Whether redirecting flue gases or fine-tuning burner positions, each technique reflects a mastery of physics, thermodynamics, and engineering principles. By presenting this information clearly and engagingly, this blog post seeks to inspire budding engineers to delve deeper into the fascinating world of power generation.
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