This article contains everything you will need to know about water tube boilers and their use.
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A water tube boiler is a boiler that has a set of tubes through which water circulates and is heated by heated gasses from its furnace or combustion chamber that is located below the tubes. Fuel, burning in the combustion chamber or furnace, produces gasses that heats the metal tubes that contain water. The heated water rises to a steam drum from which saturated vapor is released.
The fuel for a water tube boiler is natural gas, propane, coal, diesel fuel, or heating oil, which is burned in the combustion chamber or furnace to create hot gas to heat up the tubes and water. Old, conventional style boilers completed the process in reverse where the water was heated by a furnace using coal or wood. As the temperature of the water rose, it heated tubes that were immersed in it. The high volume of water, which is a poor heat conductor, made the boiler process time consuming and is the reason for the development of the water tube design where water is in the tubes.
Old fashioned water boilers, that heated the water and then the tubes, had a very large footprint and took up a great deal of room. In order for a boiler to be operational for heating or industrial purposes, assigned personnel had to arrive very early in the morning to get the boiler operating before workers arrived.
The negative aspects of the old design led engineers to investigate the potential of using highly durable materials for tubes that would contain water and be heated to release steam. Their efforts led to the development of the water tube boiler, which has improved efficiency, generates more steam, and uses less fuel. Additionally, a major factor, water tube boilers are safer, easier to manage, and have a smaller footprint.
During the early uses of boilers, there were several tragic disasters due to the failure of pressure parts in the steam section of boilers. The materials used to construct early boilers were not strong enough to contain the power generated by a boiler. The first municipal boilers were smaller, which required several boilers to fulfill the needs of a community. In order to overcome this problem, larger boilers were built that produced higher steam pressure. The materials used to construct the larger boilers were not substantial enough to endure the higher pressure.
All boilers are designed to generate steam by heating water using a burner. With a conventional fire tube boiler, tubes are immersed in water in a tank filled with water. A furnace is used to heat the walls of the tubes, which heats the water that is converted to hot water and eventually steam. Since water is a very poor heat conductor, it takes a long time to be heated by the tubes and requires a great deal of energy.
For industrial purposes, conventional boilers require a great deal of space with most of them placed in a separate boiler room to accommodate their size. In many ways, this factor limits their use to large industrial and manufacturing operations.
Water tube boilers have become popular due to their efficient design, space saving sizes, and low energy conception. With the water tube design, the water is surrounded by heated gasses from the combustion chamber or firebox. As the hot gasses move through the boiler, it heats the water in the tubes to generate steam to provide power for electricity, heat, and other power requirements.
The four parts of a water tube boiler are the water drum or mud drum, an upper steam drum, water tubes, furnace, and super heater or economizer. The tubes on a water tube boiler connects the two drums as water circulates by convection between the drums. Cooled water passes down through a downcomer tube as heated water rises to the top of the boiler.
The steam drum collects steam and water and is where the steam and water are separated. It is a large, long cylinder made of flat metal plates and is located at the top of the boiler. Boiler water from the economizer is drawn into the steam drum, where the water and steam are separated by the scrubber or demister. The collected steam is discharged from the steam drum to provide power to various applications. The water from the separation process is recirculated back into the boiler through downcomers to be converted to steam.
Cooler water, with a higher density, flows from the steam drum through large pipes, known as downcomers, to the mud drums, water distribution manifolds, at the base of the boiler. The movement of the water can be by natural circulation or forced circulation using a multistage centrifugal pump. Most water tube boilers steam drums have at least six downcomers to ensure proper water flow.
Mud drums, at the base of the boiler, collect sediment and impurities. Due to the amount of material collected in the mud drum, they have to be frequently cleaned of the accumulated deposits.
The three types of steam drums are single steam drum, bi-steam drum, and multiple steam drums, which are the oldest types of steam drums. Bi-steam drums are used for power and steam generating. Single steam drum boilers are used in power plants and have a higher-pressure limit than the other types of steam drums.
The economizer or flue gas heat recovery unit is installed on the stack of a water tube boiler and is designed to recover heat that it transfers to the boiler feed water to preheat water entering the boiler. It is a power saving device that reduces the amount of fuel needed to heat the water in the boiler.
Types of economizers include condensing economizers and non-condensing economizers. Of the two types, condensing economizers are the most efficient but require flue gases to be cooled below the condensation temperature, which is a problem for boilers that burn fuel oil. By cooling flue gases below their condensation temperature, a condensing economizer makes it possible for water vapor in the flue gases to condense and release the latent heat that is transferred to the feed water.
Shell and tube economizers are the most common type and are made up of a shell that has a series of tubes. Flue gases flow through the shell as boiler water flows through the tubes. Heat from the flue gases transfers to the feed water.
Plate economizers have a series of plates arranged in a parallel or staggered pattern. Flue gases flow through the plates as feed water flows between the plates where heat from the flue gas transfers to the feed water.
The mud drum is located at the bottom of a water tube boiler and is connected to the steam drum by tubes. It is filled with water and collects the impurities in the water that settle to the bottom of the drum, which are removed through a process referred to as blow down, a method for removing small amounts of water from the boiler to reduce the concentration of impurities.
For the steam drum and mud drum, chemicals are added to the feed water to prevent fouling and corrosion. Feed water enters the boiler from the economizer into the steam drum and mud drum. As water moves through the downcomers, it picks up heat from the firebox and supplies water to the mud drum. Riser tubes move water or steam from the mud drum to the steam drum as a part of the constant circulation of water and steam.
Water tube boiler burners have a short flame and large diameter. The various types of water tube burners are single or multiple flame where a single flame burner can burn two kinds of liquid gas fuel while a multiple flame burner burns one. Water tube boiler burners have a central ventilation system and wind box for distribution of combustion air.
The factors that affect the performance of a water tube burner are the concentration of oxygen and nitrogen, combustion reaction temperature, and how long combustion species (NOx) is present, which are various forms of nitrogen oxides and appear in high amounts at temperatures above 2800°F (1540°C). As the above four factors increase, the concentration of NOx pollutant emissions increases, which makes it necessary to reduce one or more of the four factors. The design of water tube boiler burners is to reduce the effects of the four factors and the formation of NOx.
The key to the successful performance of a water tube boiler burner is a complete combustion process and oxidation of the fuel, which is dependent on the correct fuel to air ratio. Creating the correct fuel to air ratio and fuel rich zones at the front of the flame reduces the conversion of nitrogen to NOx and lowers their formation. The stoichiometric ratio for natural gas is 10:1, for oils 12:1, and for liquefied petroleum gas (LPG) 24:1.
Large water tube boilers used in power plants have multiple burners of 20 or more to provide the energy to heat the tubes. The furnace or radiant section of the boiler accommodates the flames from the burner and absorbs the heat from the burner or burners.
Burners are classified by their mechanism and fuel. The most basic types of burners are diesel, gas, and biomass fueled. Diesel burners use various types of diesel fuels that include fuel oil, kerosene, vegetable oil, and biodiesel fuel. The fuel for diesel burners has to be pressurized before combustion.
Gas burners use propane, butane, natural gas, hydrogen, and lighting gas. The fuel for gas burners burns easier than diesel liquid fuels and does not need to be pressurized before combustion.
Biomass or pellet burners are eco-friendly burners and are used for heating and the production of hot water. The term biomass refers to the natural elements used to create boiler combustion, flames, and heat. Biomass includes pressed sawdust, bioethanol, forest waste, fruit peels, and pits. The unique nature of biomass fuels requires that it be stored in a tank close to the boiler.
The flame from the burner burns inside the furnace or radiant section of the boiler and forms radiant heat. The hot gas in the furnace heats the water in the steam generating tubes. The walls of the furnace are made up of water filled tubes or risers that are heated by radiation or convection. The water in the risers comes from the mud drum at the bottom of the boiler. Since the risers are located on the four walls of the furnace, their formation is referred to as a water wall.
The risers have large contact areas due to the fact that they are designed to absorb heat. As they are heated by the radiant heat in the furnace, the risers become the evaporator part of the boiler as the water in the risers evaporates.
Tubes of a water tube boiler are an essential part of the steam creating process. Located in the walls of the furnace, they contain the water that is heated to create steam from the boiler. Water tube boiler tubes are seamless tubes made of carbon or alloy steel and can be medium or high-pressure types. The most frequently used steels for boiler tubes include low-alloy steels containing alloying elements such as chromium, nickel, molybdenum, and vanadium, 9% to12% chromium steels, austenitic stainless steels, austenitic heat-resisting steels, and nickel-base heat-resisting alloys.
Boilers are classified according to their maximum operating temperature and pressure. Tube material, an essential part of water tube boilers, requirements include resistance to oxidation and corrosion, fatigue strength, and resistance to creep and rupture. Of the various factors, tube metals are chosen based on their level of thermal conductivity, a factor that plays a significant role in the performance of a water tube boiler.
The riser tubes in the walls of the furnace contain the heat from the furnace. In some boilers, refractory cells are used that consist of tubes with welded studs and covered with refractory material in order to withstand the high heat. Most water tube boilers have tubes that are welded together by fins or strips. This type of tube is exposed to the heat of the furnace in order to generate steam.
The windbox is an essential part of the combustion process that takes place in and around the burner. It surrounds the burner and has fans that blow air into the box. The damper on the burner regulates, controls, and directs the air into the burner and dampens noise from the burner. Air flow is measured by a pitot tube, airfoil, or flow restricting method located in the duct system that leads to the windbox.
Windboxes can also be located behind the water wall to provide reserve air to reduce air pulsation and inconsistency. With coal water tube boilers, there are primary and secondary air systems where the primary air system controls the fuel being burned while the secondary system ensures the efficiency of the combustion process and feeds air to the windboxes.
Water tube boilers are classified by the type of fuel they burn, a factor that determines their operating cost. Over the years, various types of fuels have been removed from use due their environmental impact, a factor whose importance has rapidly risen in the 21st century. The search for eco-friendly fuels has led to the development of a wide range of uniquely designed fuels that take several different forms.
Fuels for water tube boilers fall into four classes, which are solid, liquid, gas, and agricultural mass. The cost of fuel is one of the most important costs related to the operation of a water tube boiler. The best fuels release heat energy to supply heat for the tubes and risers.
Coal is the most common type of boiler fuel and has been used for centuries to power boilers. It can be burned in lumps or crushed to make it more efficient for burning. Due to its environmental impact, coal is being phased out but is still used in large power plants. Coal is a brown or black combustible, sedimentary rock like material. The types of coal vary depending on the location where it is mined. Coal contains carbon, hydrogen, nitrogen, and oxygen with its impurities being ash, mercury, and sulfur.
Natural gas is mostly methane with varying amounts of ethane, propane, butane, and inert materials such as nitrogen, carbon dioxide, and helium. Of the various types of fuel, natural gas is the most common due to its low cost. It burns cleaner than oil and is readily available. The combustion of natural gas produces very small amounts of greenhouse gas emissions and is more efficient.
Liquid propane is similar to natural gas in that it is easy to use, economical, and is delivered for use in pressurized containers. As with natural gas, propane is cleaner than oil and costs less. It produces significantly fewer emissions than carbon energy sources and can be used in natural gas boilers. Propane is not recommended for industrial size boilers since it is not a sustainable source of fuel.
An oil fueled boiler functions like a gas boiler where the oil is ignited in the burner to produce the heat to warm the risers. Oil is more efficient than gas because all of the heat from the burning fuel is used with little waste. Unlike a gas boiler that receives its fuel from the main, oil fueled boilers have to have their fuel stored in tanks. Oil fueled boilers are not as environmentally friendly as other types of boilers but make better use of their fuel.
Steam atomization is used in oil fueled boilers to mix the oil with air for more efficient combustion. The steam injection process reduces the oil to a fine spray that mixes with the air to burn easily. As is typical for water tube boilers, the created flame is short and very intense.
Oil fueled boilers have flue gas recirculation (FGR), which is a process that takes flue gas from the combustion process and circulates it back to the burner. It is an effective technique for lowering NOx emissions and includes having an air fan on the burner that forces flue gases into the mixing port on the air inlet of the burner. The process of FGR has certain advantages in relation to better use of fuel and the elimination of toxic emissions but can impact the efficiency of the boiler.
There are several fuels that are included in the solid fuel category, which is a category where there have been experimentations with eco-friendly fuels. The range of solid fuels covers a wide spectrum of materials and includes coal and wood, which were the original fuels for boilers. In addition to these common fuels, solid fuels include bagasse or pulp residue, municipal waste, refuse fuel, tires, and agricultural waste as well as several other types of solid waste materials. Depending on the design requirements of a water tube boiler, the steam capacity burning solid fuels can range from 25,000 PPH up to 500,000 PPH.
Biomass fuel systems burn wood pellets or chips. It is a renewable energy source that generates steam by burning organic matter. Biomass burning releases carbon dioxide when burned but substantially less than that produced by fossil fuels. Boilers that use biomass as a heat source are bigger than fossil fuel boilers due to the need for a larger furnace to produce sufficient heat and burn the fuel.
A critical part of biomass boilers is the inclusion of a feed hopper that is capable of storing large volumes of biomass that is automatically fed into the furnace. The industrial use of biomass boilers has been rapidly growing in recent years especially in the food and beverage industry, dairy industry, and meat product production. The types of preferred biomass are wood pellets that are easy to store in large quantities.
ASTM has set technical standards for fuel oil grades and given them six classifications or grades. Grade No. 1 is for domestic and small industrial burners while Grades No. 2 to Grade No. 6 are used in atomizing burners, commercial and industrial boilers, and burners with increased viscosity and boiling range.
| Fuel Oil Grades | |||
|---|---|---|---|
| Grades | API Gravity | Density lb/gal | Heating Value Btu/gal |
| 1 | 38 to 45 | 6.950 to 6.675 | 137,000 to 132,900 |
| 2 | 30 to 38 | 7.296 to 6.960 | 141,800 to 137,000 |
| 4 | 20 to 28 | 7.787 to 7.396 | 148,000 to 143,100 |
| 5L | 17 to 22 | 7.940 to 7.686 | 150,000 to 149,400 |
| 5H | 14 to 18 | 8.080 to 7.890 | 152,000 to 149,400 |
| 6 | 8 to 15 | 8.448 to 8.053 | 155,900 to 151,300 |
Regardless of the type of fuel chosen, a proper air to fuel ratio ensures the best performance from a boiler and helps to avoid any damage. It defines the amount of air to burn with a particular fuel with the typical fuels burned in a boiler being oil grades #2, #4, and #6, diesel oil, gasoline, natural gas, propane, coal, and wood. A balanced air to fuel ratio prevents losing energy from infusing too much air and wasting energy from running too rich combustion and increases efficiency.
The many types of water tube boilers include single pass vertical water tube boilers and bent tube water tube boilers. In industrial and utility boiler markets, water tube boilers are preferred since they have design pressures up to 2000 psig, temperatures up to 1000°F (537.78°C), and capacities up to 500,000 pounds per hour (PPH).
The types of water tube boilers include A Type, D Type, and O Type, which are named for their shape. Other types of water tube boilers are simple vertical, Stirling, and Babcock and Wilcox who were early manufacturers of water tube boilers. Stirling water tube boilers were designed and built during the first industrial revolution and purchased by Babcock and Wilcox in 1906, who is still in operation and building modern technologically advanced boilers.
An A type water tube boiler has two water drums that have a shared header. Evaporated water is sent up by risers into the steam drum and steam header. A type water tube boilers have a water-cooled furnace with a vertical gas outlet that minimizes the footprint of the boiler to allow for high steam capacity in a small package. The arrangement of the two water drums with a steam drum has the shape of a triangle and is the reason for the name of A type water tube boilers.
With an A type water tube boiler, water fills the space between the drums with the furnace located in the center. A type water tube boilers are fired by coal or oil and have multiple fire doors for stoking. The design of A type water tube boilers was for battleships that required high power in a compact package.
D type water tube boilers have a large steam drum at the top that is connected to a mud drum at the bottom. Steam is generated in tubes or risers that moves up to the steam drum. The furnace has water filled tubes in its four walls, which are referred to as water walls. The design of the D type water tube boiler allows for exceptional water circulation to enable rapid steam production and generation.
The structure of D type water tube boilers can generate steam at 1,000,000 PPH at pressures of 2000 psig. D type water tube boilers are built for durability and endurance as well as longevity. They are ideal for applications that require high pressure superheated steam but need a restrictive footprint.
With an O type water tube boiler, the steam and mud drums are vertically aligned with one positioned above the other. The drums are connected by curved water pipes that form the letter O. O type water tube boilers are fired by liquid burners. The combustion chamber is located between the curved convection tubes and water wall tubes.
As with the D type water tube boiler, O type water tube boilers are ideally suited for conditions that have limited space. Access to an O type water tube boiler is provided at both ends of the boiler. They are a highly resilient style of boiler capable of long service. O type water tube boilers can reach an output of 250,000 PPH with pressures up to 900 psig.
Vertical water tube boilers are unlike other types of water tube boilers in that they are vertical and not horizontal. At one time, they were used to power vehicles such as early steam locomotives. Water is heated in the cylindrical shell of the boiler.
Fuel is added onto the grate in the furnace in the cylinder to create hot gasses to heat the vertical tubes. Ash residue from the fuel falls through the grate into an ash pit. The steam that builds up in the tubes is collected until it reaches a specific level, after which it is released to power machinery.
The firebox is tapered toward the top to allow for passage of steam to the top of the cylinder. The cross tubes are inclined to better expose their heat surface and increase water circulation. An uptake tube passes from the top of the firebox to the chimney of the boiler.
The Stirling boiler is a high-capacity bent tube boiler that can generate 50,000 KPH with pressure as high as 60 kgf per cum². It was introduced in 1888 by Alan Stirling and is used for central power stations. The Stirling boiler has three steam drums and two mud drums with the three steam drums at the top of the boiler and the two mud drums placed at the bottom. The steam drums and mud drums are connected by a system of bent tubes, which are not susceptible to expansion when they are heated.
In the tradition of the 19th century, the structure of the drum is enclosed with bricks with a fire door on the bottom side of the bricks. The steam drums are connected by a circulating tube that extends from the middle drum, which collects the steam. The furnace is located just below the mud drums such that it is able to heat the water in the mud drums. The steam from the steam drum is converted into superheated or dry steam by a superheater.
The water tube boiler designs described above are a sampling of the many varieties of water tube boilers with different designs and configurations being continually perfected through the use of technological advancements and innovations. The foundational principles of all types is the use of a fuel source, water, and tubes to generate steam.
The wide use of water tube boilers is due to their ability to handle high pressure and temperatures. Water tube boilers are exceptionally efficient at converting fuel into useful and productive energy. They are used in a wide range of industries due to their safe operation and reliability.
The main function of boilers is for power generation and was the reason that they were so crucial when they were first introduced during the first industrial revolution. The reliability of water tube boilers has made them a common part of power generating plants that produce steam to power turbines and generate electricity.
The chemical and petrochemical industries require consistent power, the ability to meet load demands, and compliance with emission regulations. The efficiency of water tube boilers makes it possible to meet all of these needs as well as provide reliable and consistent performance. They are used to produce steam for refining, distillation, and cracking processes.
In the pulp and paper industry, water tube boilers are necessary for drying paper, meeting energy requirements, and cooking wood chips in the initial steps of the manufacturing process in the digester. The provided steam produces uniform heating, temperature control, and efficiency.
Regardless of the many different ways to prepare food, the steam and hot water produced by water tube boilers is the best way to ensure smooth, soft, and easy to digest food products. The main benefit of water tube boilers for the food and beverage industry is their ability to produce high volumes of steam and hot water to meet the needs of mass production. Additionally, water tube boilers help maintain the environmental conditions of the facilities where food is processed.
As with the food and beverage industry, a great deal of demand is placed on water tube boilers for the pharmaceutical industry. They must be precise with extremely accurate temperature controls for the drying, sterilizing, and purifying of chemicals and instruments. A central issue for all forms of boilers is the production of clean steam, which is a necessity in the pharmaceutical industry. The processes of a water tube boiler and its filtration system are ideal to meet the need for clean and pure steam.
The main concern for the textile industry in regard to water tube boilers is the need to provide quick steam at 265o F (129.4o C) for dyeing and finishing processes. The use of boilers in the textile industry fluctuates in accordance with demands, which makes the use of fire tube boilers too costly and inefficient. Water tube boilers are essential in the initial steps of the textile production process since they provide the energy necessary for the pretreatment of fabrics and assist with specific heat settings for the printing process.
Water tube boilers have been used for many years as a method for heating buildings, factories, and commercial facilities. The popularity of water tube boilers for heating systems is their consistency and precision control. The radiant heat produced by a water tube boiler makes it possible to keep buildings and workplaces comfortable and productive.
The wide use and popularity of water tube boilers is due to their high efficiency with very little heat loss because of the direct contact of the water filled tubes with the combustion gases. They rapidly convert large amounts of fuel to useful energy.
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