What are watertube boilers?
A watertube boiler, also known as a water tube boiler or water-tube boiler, is a type of boiler that uses water-filled tubes to generate steam. This is as opposed to a fire tube boiler which uses fire/exhaust-filled tubes to generate steam. The steam generated by the boiler can be used for a variety of purposes, including power generation via steam turbines, industrial process applications, and building heating purposes (district heating).
Note - to aid understanding and familiarisation, this article uses the various forms of water-tube, watertube, and water tube, throughout.
Heat Transfer to Watertube and Firetube Boiler Tubes
Watertube boilers are typically more efficient than other types of boilers and are also able to generate higher volumes of steam at higher operating pressures and temperatures; this makes them ideal for use in power plants and other industrial settings where higher capacity is demanded.
In this article, we will explore the workings of watertube boilers employed in coal-fired power stations. We will discuss the main systems associated with these large boilers, their main parts, and how we can use steam to generate electrical power. You can however learn more about different boiler types and power plant machinery in our Power Engineering Fundamentals Video Course.
Watertube Boiler
Good to know – ‘watertube’ is also spelled ‘water-tube’, or ‘water tube’, but the different variations all mean the same thing.
What are the main parts of a water-tube boiler?
The main parts of a water-tube boiler are:
- Steam drum
- Mud drum
- Downcomers
- Risers
- Headers
- Superheater (Primary, Secondary etc.)
- Reheater (Primary, Secondary etc.)
- Economiser
- Furnace (combustion space)
- Safety valve
- Primary air fan
- Secondary air fan
- Exhaust fan
- Attemperator
Watertube Boiler Piping and Flow Paths
How do watertube boilers work?
The below diagram shows the typical arrangement of equipment and systems associated with a coal fired power station; use the diagram as a reference as you progress through this article. It is necessary to understand all of a power station's systems in order to understand how a watertube boiler works.
Coal Fired Power Plant Systems and Equipment Diagram
Water Tube Boiler Fuel System
Before coal is fed to the boiler, it is stored in a coal yard, then conveyed to day silos. The day silos typically have enough capacity for 4-12 hours max boiler operation, which ensures that the boiler can remain in service even if there is an issue with the conveyor(s) in the coal yard. The day silos feed coal to the coal pulverisers (American English ‘pulverizers’).
Coal Pulveriser
The purpose of a pulveriser is to grind, dry, and classify the coal, ensuring that it contains a low amount of moisture and that it is the correct size, when it reaches the boiler. Pulverised coal is discharged from the pulverisers and conveyed pneumatically through piping to the boiler burners.
Pulverised coal is sprayed into the furnace through burner nozzles, where it is ignited and undergoes combustion. Air is blown into the furnace to ensure efficient combustion. Behind the side furnace tubes (side water walls) are windboxes; air accumulates in each windbox before entering the boiler. A windbox gives a small reserve of air and reduces the likelihood of air pulsation or inconsistent air supply to the boiler. There are two main air systems associated with watertube boilers, these are the primary air and secondary air systems.
Water-tube Boiler Windbox
Watertube Boiler Primary and Secondary Air Systems
Primary and secondary air systems have two separate functions.
- Primary air – controls the amount of fuel being burned.
- Secondary air – controls the efficiency of the combustion process.
The primary and secondary air fans used in both systems are typically of the centrifugal fan design, although the number and type of fans depends upon the boiler design. The primary air system feeds primary air to the coal pulverisers, while the secondary air system supplies air to the windboxes.
Primary Air System
The primary air system controls the amount of fuel being burned. By adjusting the amount of primary air supplied, the amount of coal fed to the furnace can be regulated. Cold primary air enters the coal pulveriser and mixes with hot primary air to create an optimal air-fuel mixture and temperature. The air-fuel mixture is then pneumatically conveyed through tubes to the burners, and into the furnace.
Secondary Air System
The secondary air system controls the efficiency of the combustion process; it supplies air to the windboxes, which distribute secondary air evenly within the furnace. The secondary air system helps maintain optimal combustion conditions by adjusting the amount of supplied air based on the carbon dioxide and oxygen levels in the exhaust gases. By monitoring the exhaust gas composition, the secondary air system can ensure combustion is as efficient as possible; this process is known as combustion efficiency control.
Air Preheater
Air preheaters use the exhaust gases from combustion to heat the primary and secondary air before it enters the boiler; this process improves boiler efficiency by preventing continuous cooling of the boiler due to the introduction of cold air.
Water-tube Boiler Air Preheater
Watertube Boiler: Working Principle
This section discusses the various components and systems used to convert water to steam within a watertube boiler. Common watertube boiler components such as the steam drum, downcomers, risers, mud drums, furnace walls, headers, and superheaters, will be discussed.
Flow Through a Water-tube Boiler
Economiser
Boiler feed water first enters the boiler via the economiser (American English ‘economizer’), which is a serpentine-type heat exchanger. The boiler water flows back and forth through the economizer tubes until it reaches the top of the heat exchanger, where it is discharged to the steam drum.
Good to know – ‘feed water’ is also spelled ‘feed-water’ or ‘feedwater’, but the different variations all mean the same thing.
Good to know – water that has been treated but has not yet entered the boiler, is classified as ‘feed water’. Water that is within the boiler, is classified as ‘boiler water’. Steam that has condensed back to water, is classified as ‘condensate’. Condensate becomes feed water after it has been treated.
Watertube Boiler Economiser and Steam Drum
Steam Drum, Downcomers and Mud Drums
The steam drum receives boiler water from the economiser. The steam drum is long, cylindrical in shape, and manufactured from flat metal plates. Due to the high pressure of the steam system (usually approx. 190 bar/2,755 psi), the water’s boiling temperature is correspondingly high (the boiling temperature of water is pressure dependent). The steam drum is responsible for separating water and steam. Steam is discharged from the steam drum to the steam turbines, whilst water is recirculated in the boiler until it becomes steam.
Watertube Boiler Steam Drum 3D Model
Relatively cold water from the steam drum -with a higher density due to its lower temperature- flows through large pipes called downcomers until reaching the mud drums. Mud drums are water distribution manifolds at the base of the boiler. If the boiler is a natural circulation boiler, the downcomers connect the steam drum at the top of the boiler to the mud drums at the bottom, without the use of a pump. If the boiler is a forced circulation boiler, a multistage centrifugal pump will be installed between the steam drum and mud drum. Downcomers get their name from the fact that the water ‘comes down’ from the steam drum.
Natural and Forced Circulation Watertube Boilers
Good to know – ‘manifolds’ and ‘headers’ are two words that are often used interchangeably. Both words mean ‘a central distribution point supplying smaller systems’.
There are typically six downcomers in a watertube boiler, this ensures adequate water flow to the mud drums. Mud drums are installed at the base of the furnace walls and serve as a collection point for sediment and other impurities circulating within the system. The mud drums need to be opened and cleaned at scheduled intervals to remove accumulated deposits.
Good to know – the name ‘mud drum’ derives from the ‘dirty’ material that accumulates within the drums.
Watertube Boiler Mud Drums
Furnace Walls and Risers
From the mud drums, water flows upward through tubes neighbouring the furnace; these tubes are called ‘risers’ because the water is ‘rising’ to the steam drum. The furnace is surrounded by risers on all four sides (front, rear, and both sidewalls), forming a rectangular-shaped box. Due to the rectangular shape of the furnace, and because the risers are full of water, each side of the boiler is often referred to as a ‘water wall’.
Watertube Boiler Furnace Wall Construction
Risers have a much smaller diameter than downcomers because their primary purpose is to absorb heat, thus they require a large contact surface area with the furnace. Riser tubes form the evaporator part of the boiler because evaporation takes place within the tubes.
Water Evaporating Within a Watertube Boiler Tube
When combustion occurs within the furnace, heat is transferred to the risers via radiation and convection. Radiant heat transfer requires line-of-sight between the heat source and the recipient, thus items such as the economiser are not heated via radiant heat (because there is no direct line of sight between the combusting fuel and economiser).
Conduction, Convection, and Radiation
Headers and Steam Formation
As water travels upwards through the furnace walls, it absorbs heat and begins to evaporate into steam. At the top of the furnace, wet steam (steam containing suspended water molecules) is discharged to headers, then to the steam drum. There are usually four headers, one per side of the furnace wall. Not all water evaporates to steam, thus water is also returned to the steam drum.
Wet steam and water enter the steam drum, where baffles, scrubbers, and cyclones, separate the suspended water molecules from the steam. Dry saturated steam is discharged from the steam drum i.e. the steam does not contain suspended water molecules.
Watertube Boiler Steam Drum Components
Good to know – steam is an invisible, odourless, gas. The steam that most people visualise when thinking of steam is actually ‘wet steam’. If steam contains suspended water molecules, it is referred to as ‘wet steam’; it is these suspended water molecules that are visible. If the steam contains no suspended water molecules, it is referred to as ‘dry steam’. The dryness of steam is measured by its dryness fraction (also known as ‘vapour quality’), with a value of 0 being completely liquid and 1 being completely dry.
Superheaters
The amount of energy a steam turbine can extract from steam is determined by the steam’s temperature and pressure. To further increase the steam's temperature -thus increasing the amount of power available to the steam turbine-, it is sent through a series of superheaters. Steam that is heated above its saturation temperature is referred to as ‘superheated steam’, this is what occurs in superheaters, and it is how they obtained their name.
Superheaters are heat exchangers manufactured from piping similar in design to risers; they are classified as primary or secondary depending upon their position within the boiler. Superheaters increase the temperature of the steam i.e., they add sensible heat. Superheaters do not add latent heat because the steam is already in a gaseous phase i.e., there is no phase change as the steam is heated.
The primary superheater adds heat to the steam as it flows through a relatively cool area of the boiler, this prevents the steam from cooling and condensing prior to it reaching the secondary superheaters. Dry saturated steam flows into the primary superheater and dry superheated steam flows out!
Watertube Boiler Primary and Secondary Superheaters
After the primary superheater, dry superheated steam passes to two secondary superheaters: the platen superheater and final secondary superheater. The secondary superheaters are located in hotter regions of the boiler, thus ensuring that the steam reaches its highest temperature before being discharged from the boiler to the high-pressure steam turbine(s).
At this stage, there is no further flow through the boiler unless the boiler is a reheat boiler.
Good to know – steam turbines are classified based upon the pressures at which they operate:
- High-pressure steam turbine – the first turbine that steam encounters after leaving the final secondary superheater; this turbine is referred to as the ‘HP Turbine’.
- Intermediate-pressure steam turbine – the turbine that steam encounters after leaving the secondary reheater superheater (if a reheat boiler), or, after leaving the HP turbine. This turbine is referred to as the ‘IP Turbine’.
- Low-pressure steam turbine – steam is delivered from the IP turbine to the low-pressure turbine via a crossover connection. This turbine is referred to as the ‘LP Turbine’.
Reheat Boilers
After the dry high-pressure superheated steam has passed through the high-pressure steam turbine, it is discharged and sent back to the boiler for reheating. Reheating of the steam occurs in heat exchangers referred to as ‘reheaters’, which are similar in design to superheaters. Like with superheaters, reheaters are also classified as primary or secondary depending upon their position within the boiler. After reheating, the steam is discharged to the intermediate pressure steam turbine, and finally the low-pressure steam turbine (which is connected via a crossover pipe).
Not all watertube boilers have a reheating cycle. The main purpose of reheating the steam is to increase its temperature and consequently increase the power available to the steam turbine; reheating also increases the overall thermal efficiency of the plant. Reheating the steam reduces the likelihood of suspended water molecules being present in the steam, thus reducing the risks of turbine blade erosion and corrosion.
Primary Reheater
A primary reheater is similar to a primary superheater (both use a serpentine heat exchanger design), but it is located in a neighbouring part of the boiler. Steam from the high-pressure turbine is discharged to the primary reheater. As steam travels through the primary reheater, it is reheated, and then discharged to the secondary reheater. The path of the steam is quite intricate, which ensures optimal heat transfer.
Watertube Boiler Reheaters
Secondary Reheater
After passing through the primary reheater superheater, steam enters the secondary reheater superheater, where it undergoes further heating before being discharged from the boiler to the intermediate pressure turbine.
Exhaust Gas Cleaning System
The exhaust gas cleaning system treats the combustion gases generated during the operation of the boiler.
Good to know – ‘combustion gases’, are also known as ‘flue gases’, or ‘gases of combustion’.
Exhaust gases travel through the boiler, passing over heat exchangers (superheaters etc.) and other components (air preheater etc.) as they do so. Once the exhaust gases have transferred the majority of their heat energy to the boiler, they are discharged to the exhaust gas cleaning system.
The exhaust gas cleaning system typically includes flue gas desulfurisers, and electrostatic precipitators or baghouses. Flue gas cleaning equipment is designed to remove pollutants and particulate matter from the flue gases before they are discharged to atmosphere.
Summary – How it All Works Together
- Coal enters the day silos from the coal yard.
- Coal is delivered to the pulverisers, where it is ground, dried, and classified.
- Pulverized coal is conveyed pneumatically to the boiler burners.
- Coal is sprayed into the furnace, where it is ignited and undergoes combustion.
- Secondary air is blown into the furnace to ensure efficient combustion.
- Relatively cold boiler water travels downwards from the steam drum to the mud drums via downcomers.
- Water flows from the mud drums upwards through riser tubes neighbouring the furnace. As the water flows upwards, it absorbs heat, and some water evaporates into steam.
- Wet steam exits through headers at the top of the furnace.
- Wet steam enters the steam drum, where the suspended water molecules are separated. Dry saturated steam is discharged from the steam drum.
- Dry steam passes through primary and secondary superheaters to reach its final temperature (it becomes superheated during this stage).
- Dry superheated steam is discharged from the boiler and can now be used by the steam turbine(s) for power generation.
- If a reheat system is installed, steam from the high-pressure turbine is returned to the boiler for reheating. If no reheat system is installed, steam is discharged from the high-pressure turbine to the next stages i.e. IP or LP turbines.