Boilers serve as the heart of heating and steam generation systems across various industries, playing a critical role in meeting energy demands. Among the wide range of boiler types, water-tube boilers have gained significant attention thanks to their unique capabilities, particularly their ability to operate effectively under high pressures and temperatures. With their smart and efficient design, these boilers can produce steam and hot water at high capacities, making them essential for industries such as thermal power plants, petrochemical facilities, and food production.
This article will explore the different components of water-tube boilers, how they operate, and their advantages and disadvantages. It also compares them with fire-tube boilers and discuss their various applications across different industries. As technology continues to advance and industry demands evolve, a thorough understanding of these boilers can greatly improve efficiency and optimize processes.
For an introduction to boilers and heating systems, you can start by reading the article: “A Comprehensive Overview of Industrial Boiler Types; Structures, Operations, and Applications”
What is a water-tube boiler?
Boilers are industrial devices commonly used to produce hot water or steam needed for various industries or heating systems. Based on their design and operation, boilers are divided into two categories: water-tube and fire-tube. Water-tube boilers have walls that allow water to flow through them and are typically used in higher capacity and higher pressure. These boilers are found in industries such as power plants, petrochemicals, and chemical plants, where they are used to generate hot water or steam.
Operation of water-tube boiler
A water-tube boiler works by using the heat generated from combustion to warm a fluid. These boilers consist of chambers with walls made up of closely arranged tubes. Water flows inside these tubes, while combustion flames are present within the chamber. Heat is transferred from the combustion products to the water through radiation, convection, and conduction mechanisms, resulting in the production of hot water or steam.
Differences Between Water-Tube and Fire-Tube Boilers
Water-Tube and Fire-Tube Boilers are named based on their design. As their names suggest, in water-tube boilers, water flows inside the tubes, whereas in fire-tube boilers, flames are generated inside a tubular chamber, and combustion products flow through the tubes.
Fire-tube steam boilers are generally used for lower capacities and pressures below 30 bar, while water-tube boilers are preferred for higher capacities or pressures exceeding this limit.
For more information on fire-tube boilers, you can read the article “Overview of Fire-Tube Boiler Types and Their Components“.
Structure of a Water-Tube Boiler
A water-tube boiler consists of several components, the most important of which are as follows:
Combustion chamber
The internal space of the boiler where combustion takes place is known as the combustion chamber. The walls of this chamber consist of riser tubes, arranged closely together. In the combustion chamber, fuel and air react chemically to produce thermal energy.
The walls or water tubes absorb the generated heat, which in turn heats the water flowing inside the tubes. If the boiler is a steam boiler, the water is then vaporized.
To prevent energy loss, the walls of the combustion chamber are covered with refractory insulation on the outside.
Burner
It is the central component of a water-tube boiler, responsible for supplying fuel and air, then combusting the fuel to generate the heat required by the boiler. An effective burner must perform this task with high efficiency, low emissions, and reliable stability. A water-tube boiler can have several burners, arranged in a specific configuration. Different fuels, such as natural gas, oil, liquefied petroleum gas (LPG), heavy oil, hydrogen, and others, can be used to generate the required heat, as long as the burner is compatible with the fuel being used.
A suitable and intelligent burner can enhance the efficiency and safety of the boiler, as well as make its operation easier. For this reason, raadman burners are among the best choices for use in steam or hot water boilers. With their modulation capability and smart controllers, these burners can play a key role in improving the boilers operation.
Steam drum
A horizontal cylindrical drum is positioned at the top of the boiler, with riser and downcomer tubes connected to it. Steam is produced in the riser tubes which surround the combustion chamber, and is collected in the upper drum. Inside the steam drum, components such as cyclone separators are used to remove liquid water droplets from the steam, ensuring that only dry steam exits the drum. Additionally, certain chemical compounds are injected into the steam drum, where the turbulence helps mix them effectively with the water.
Mud drum
This drum is positioned at the lowest part of the boiler, where it gathers the sediments and impurities in the water. In some cases, boilers are designed without a separate sludge drum, with the lower water headers performing the drum’s function as well.
Headers
These parts are seen as horizontal cylinders at both the top and bottom of the boiler, from which a large number of tubes branch out. They collect the working fluid from different tubes or distribute it across several tubes. Typically, there are two types of headers in a water-tube boiler:
- Upper header: It collects the hot water and steam from risers and transfers them into the steam drum.
- Lower header: It is located at the inlet of riser tubes and distributes the water between the risers.
Water tubes
The water in the boiler flows through the water tubes and receives heat from the combustion chamber.
The water tubes in a water-tube boiler are classified into two types:
- Riser: Hot water and steam flow upward from the bottom towards the upper drum through the riser tubes
- Downcomer: Cold water flows downward from the steam drum through the downcomer tubes.
Superheater
The steam leaving the steam drum is in a saturated state and loses heat to condense. To raise the steam temperature beyond its saturation point and convert it into superheated steam, a heat exchanger known as a superheater is employed. A water-tube boiler can have multiple superheaters. In the superheater, heat from the combustion gases is transferred to the steam, producing superheated steam that exits the unit.
Economizer
The exhaust gases or combustion products exiting the boiler still contain a large amount of heat. To make use of this heat and improve system efficiency, an economizer, a type of heat exchanger, can be utilized. The incoming cold water passes through the economizer before entering the main tubes. At the same time, the combustion products from the boiler also pass through the economizer. This process preheats the incoming water, allowing the thermal energy in the exhaust gases to be effectively used.
Reheater
In some steam power plants, the steam entering the turbine, after several steps of expansion, is returned to the boiler, where it absorbs some energy and then enters the subsequent stages of the turbine. To facilitate this process, another heat exchanger known as a reheater must be incorporated into the boiler.
Stack
In the final stage, the combustion products flow into the stack and are released into the atmosphere. However, to reduce harmful pollutants and minimize emissions, the exhaust gases can be filtered before being discharged into the environment.
Feedwater pump
The feedwater pump plays a crucial role in circulating water throughout the system and ensuring the necessary pressure for water or steam.
Other components of water-tube boilers
The auxiliary components of a boiler encompass control systems, precision instruments, and various valves, including the following:
- Water Level Indicator
- Relief Valve
- Blowdown valve
- Control system consists of controllers, sensors, actuators, indicators, and more
- FGR system
Water-tube boilers classification
Water-tube boilers are categorized from various aspects:
Type of Working Fluid: The working fluid can be water, oil, molten salt, and so on.
State of the Output Working Fluid: In hot water boilers, the output is in the form of warm liquid water, while in steam boilers, the output is in the form of saturated or superheated steam.
Water Circulation Mechanism: The mechanism for water circulation in a boiler can either be natural or forced. In water-tube boilers with natural circulation, the water flow is driven by the density difference between the fluid in the riser and downcomer tubes. In forced circulation water-tube boilers, a pump is installed between the steam drum and the mud drum, and this pump maintains the flow inside the riser and downcomer tubes.
Construction Method: In terms of construction, water-tube boilers are divided into two categories: conventional and packaged. Package boilers, used for lower capacities, are assembled in the factory and include all components of the boiler and its burner. Conventional water-tube boilers are assembled on-site and are used for higher capacities.
Design Type: Packaged water-tube boilers are usually categorized into three types—Type O, Type A, and Type D—based on the positioning of the drums and burner, as illustrated in the figure below.
Condensing water-tube boilers
These boilers are designed to efficiently capture the maximum thermal energy from combustion, lowering the flue gas temperature in the stack to below 50°C. Fossil fuel combustion products typically contain considerable water vapor. At this temperature range, part of the water vapor condenses, thus using the latent heat of the water vapor. Condensing boilers are typically limited to lower capacities and are primarily used for supplying hot water. Their design differs from that of standard water-tube boilers. The heat exchanger in these boilers is more compact and made from a different material.
Advantages of water-tube boilers
- Water-tube boilers can withstand higher pressures compared to fire-tube boilers.
- Water-tube boilers are capable of providing higher capacities than fire-tube boilers.
- A water-tube boiler, with the same capacity as a fire-tube boiler, contains less water, allowing it to respond more quickly to changes in thermal load.
- Water-tube boilers have a safer design and a longer operational lifespan.
Disadvantages of water-tube boilers
- Higher initial cost compared to fire-tube boilers.
- Occupy more space than fire-tube boilers.
- More challenging and expensive to maintain and repair.
- Limited ability to be manufactured as fully assembled packages due to high operating pressure.
Water-tube boilers applications
Water-tube boilers are used in different industrial and commercial sectors to supply hot water or steam. Some common applications of these boilers include:
- Thermal power plants
- Chemical and petrochemical sectors
- Food production industries
- Heating for buildings, especially district heating systems
- Textile manufacturing
- Pharmaceutical industries
Revolutionizing Industry with Water-Tube Boilers: Applications and Capabilities
Water-tube boilers are designed for higher pressures, temperatures, and capacities compared to fire-tube boilers. These boilers typically have a combustion chamber whose walls are made up of closely spaced tubes. Water flows through the tubes in these walls, and combustion takes place in the enclosed space within the chamber. As the fuel burns, the heat generated is absorbed by the water flowing through the tubes. The major benefit of water-tube boilers lies in their design capabilities for high pressures, temperatures, and capacities, making them widely used in large-scale industrial operations, especially thermal power plants.
