The steam generated in the boiler must be conveyed through pipework to the point where its heat energy is required. Initially there will be one or more main pipes, or &#;steam mains&#;, which carry steam from the boiler in the general direction of the steam using plant. Smaller branch pipes can then carry the steam to the individual pieces of equipment.
When the boiler main isolating valve (commonly called the &#;crown&#; valve) is opened, steam immediately passes from the boiler into and along the steam mains to the points at lower pressure.
The pipework is initially cooler than the steam, so heat is transferred from the steam to the pipe.
The air surrounding the pipes is also cooler than the steam, so the pipework will begin to transfer heat to the air.
Steam on contact with the cooler pipes will begin to condense immediately. On start-up of the system, the condensing rate will be at its maximum, as this is the time where there is maximum temperature difference between the steam and the pipework. This condensing rate is commonly called the &#;starting load&#;. Once the pipework has warmed up, the temperature difference between the steam and pipework is minimal, but some condensation will occur as the pipework still continues to transfer heat to the surrounding air. This condensing rate is commonly called the &#;running load&#;.
The resulting condensation (condensate) falls to the bottom of the pipe and is carried along by the steam flow and assisted by gravity, due to the gradient in the steam main that should be arranged to fall in the direction of steam flow. The condensate will then have to be drained from various strategic points in the steam main.
When the valve on the steam pipe serving an item of steam using plant is opened, steam flowing from the distribution system enters the plant and again comes into contact with cooler surfaces. The steam then transfers its energy in warming up the equipment and product (starting load), and, when up to temperature, continues to transfer heat to the process (running load).
There is now a continuous supply of steam from the boiler to satisfy the connected load and to maintain this supply more steam must be generated. In order to do this, more water (and fuel to heat this water) is supplied to the boiler to make up for that water which has previously been evaporated into steam.
The condensate formed in both the steam distribution pipework and in the process equipment is a convenient supply of useable hot boiler feedwater. Although it is important to remove this condensate from the steam space, it is a valuable commodity and should not be allowed to run to waste. Returning all condensate to the boiler feedtank closes the basic steam loop, and should be practised wherever practical. The return of condensate to the boiler is discussed further in Block 13, &#;Condensate Removal&#;, and Block 14,&#;Condensate Management&#;.
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The more we learn about the steam-generating industry, the more we can appreciate its diversity and rich history. Most people have never even been to a power plant, let alone know anything about the history of the power industry. Their knowledge of both extends only to the stacks they see in the distance.
If you ask someone who is credited with starting or inventing the automobile and the automobile industry, he will likely answer, &#;Ford.&#; But how many people know who started the steam-generating industry? Obviously the automobile industry has played an important role in shaping our country, but so has the power industry.
A boiler is a box formed by tubes that uses fire inside that box to heat water into steam. Surrounding those tubes and completely encasing the tube walls and the firebox area are the bril (brick, refractory, insulation, and lagging) materials. The number and size of the tubes, the type of fuel, and the overall physical dimensions of the boiler will all vary depending on what the boiler is designed to produce (water, steam, or heat) and the industry it is intended to serve (e.g., utility, industrial, medical).
Many components make up or act as a support system for the boiler to meet its designed steam or heat requirements. There are the tubes that carry the water and/or steam throughout the system; soot blowers that keep the unit free of fly ash or dust by blowing steam water or air into the boiler; burners that burn the fuel (oil, gas, coal, refuse); economizers that recover heat from the exit gas and pre-heat the water used for making steam; and many more such systems, including brick, refractory, insulation, and lagging, which help the steam-generating boiler be energy and thermally efficient.
The steam-generating boiler&#;s roots go back to the late s and early s with the development of the kettle-type boiler, which simply boiled water into steam. The water was placed above a fire box and then boiled into steam. It wasn&#;t until around , with the development of the convection boiler, that the steam-generating industry began.
It may be debated who developed the first steam-generating boiler; however, most will agree that George Babcock and Steven Wilcox were two of the founding fathers of the steam-generating boiler. They were the first to patent their boiler design, which used tubes inside a firebrick-walled structure to generate steam, in , and they formed Babcock & Wilcox Company in New York City in . Their first boilers were quite small, used lump coal, fired by hand, and operated at a very low rate of heat input. The solid firebrick walls that formed the enclosure for the unit were necessary because they helped the combustion process by reradiating heat back into the furnace area.
The Stirling Boiler Company, owned by O.C. Barber and named for the street (Stirling Avenue) the facility was on in Barberton, Ohio, also began making boilers in . Their eighth Stirling boiler design was called the H-type boiler (&#;h&#; being the eighth letter in the alphabet) and had a brick setting design. The Stirling boiler was much larger than the Babcock & Wilcox boiler and used three drums to help circulate the water and steam flow throughout the boiler.
In , the Stirling Boiler Company merged with the Babcock & Wilcox Company. They renamed their boiler the H-type Stirling, and it became one of best-selling boilers of its time, probably because of its ability to produce up to 50,000 pounds of steam per hour.
However, they were not the only boiler manufacturers during the late s. The Grieve Grate Company and the American Stoker Company were also making boilers of similar all-brick-wall design. They both used a traveling or screw-type grate at the bottom of the boiler to transport the fuel (lump coal) across the inside of the boiler. As the fuel traveled across the inside of the boiler, it was burned and the ash or un-burned fuel would drop into a hopper. These two companies later formed the Combustion Engineering
Company in . The new Combustion Engineering Company offered their version of the Grieve and American Stoker boilers and called it the Type E stoker boiler.
With the advent of these new types of boilers and boiler companies, utility companies formed across the country to generate and distribute electricity to the industrial and residential markets. Many cities and towns had their own utility or electric company. Larger cities had numerous utility companies scattered around the city due to the limited amount of steam pressure each boiler and electric generator could produce (on average, approximately 50,000 pounds of steam per hour per boiler). These early utility companies might have as many as 10 to 16 boilers at each facility. Industrial companies that needed a lot of electricity or steam to run their facilities (e.g., Eastman Kodak, which made film and cameras in Rochester, New York, and The Box Board Company&#;later called the Packaging Corporation of America&#;which made the boxes for cereal companies in Rittman, Ohio) had their own steam-generating boilers.
These brick-wall-constructed boilers, sometimes referred to as brick-faced boilers, were the first in the evolution of boiler design, but they were limited in size and capacity. As the size of the boiler increased, so too did the furnace heat input, the boiler rating (pressure), and steam temperature. Thus, continually increasing the size of the boiler furnace raised the temperature the brick was subjected to. These three factors (heat input, pressure, and steam temperature) had a direct effect on the development of boiler furnace designs. The severe furnace conditions began to exceed the temperature limits of the brick walls, and the structural loads became excessive as the boilers kept getting bigger and taller. The young boiler industry needed to eliminate the all-brick-wall design and find an alternative construction that would keep the boiler thermally and energy efficient, generate more steam per hour, and cost less to build.
This led to the &#;tube and tile&#; boiler design around the early s. A tube and tile boiler used large, widely spaced tube walls (6 in. diameter tubes on 9 in. centers) to help cool the surface temperature of the brick. This was a new and radically different design. Unlike the original boiler design, which used 22-in.-thick firebrick walls that required no insulation, the tube and tile boiler used thin tile (2½ in. thick) or firebrick (4½ in. thick) to keep the fire inside the fire box and added insulation over the brick or tile to keep the boiler thermally efficient. With this new development, the boiler industry began to grow just as the boilers began to grow in size and capacity.
By this time there were many more companies manufacturing these tube and tile boilers: Riley Stoker, Foster Wheeler, Erie City, Zurn, Nebraska, Peabody, Keeler, Union Iron Works, and The Trane Company (to name just a few), with the two largest by sales being the Babcock & Wilcox Company and Combustion Engineering. Each had their own unique loose tube wall constructed boiler designs with multiple boiler types depending on the required capacity. To save in engineering costs, each boiler company developed a line of boilers much like the automobile industry did with the Model-T Ford.
For example, Babcock & Wilcox developed their version of the tube and tile boiler starting with their type FF, which was a two-drum boiler capable of producing up to 54,000 pounds per hour of steam. For higher capacities they offered the FH, FJ, FL, and the FP, with the largest design and highest steam capacity (100,000 pounds per hour). The same goes for Combustion Engineering Company, which developed their V2M8 and V2M9 (vertical two drum) super-heater boilers.
The next two most important industry changes occurred in the late s and early s with the introduction of the flat studded tube and the loose tube wall constructed boilers. These two designs allowed the boilers to get the most heat out of burning pulverized coal. The flat studded tube increased heating surface between tubes by adding flat studs all along the tube wall surface. The loose tangent tube design used more tubes close or tangent spaced (touching each other) to increase the heating surface of the tubes. The flat studded tube wall required refractory, insulation, and outer casing to keep the fire inside the fire box, whereas the loose tube tangential wall design used a smear coat of refractory between the tubes and a steel inner casing over the refractory.
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These two designs led to the development of larger, higher-capacity boilers, with the radiant boiler design the largest of all. The radiant boiler used one drum and an increased tube wall and super-heater surface area in the back pass, sometimes referred to as the convection pass (cp) or heat recovery area (hra), to increase steam capacity. For example:
The steam capacities of these radiant boilers ranged from 400,000 to 1,000,000 pounds of steam per hour. Consequently, the small city- and town-owned power plants became obsolete, as the utility companies could now produce enough electricity for larger residential areas and industrial companies.
The biggest change in boiler design came with the development of the membrane tube wall in the late s and early s. Seamless tubes were welded together in a tube shop, using a steel membrane bar between the tubes, and made into a large tube panel. This eliminated the need for refractory for keeping the fire inside the fire box, reduced construction cost, shortened erection schedules, and increased the size of the boilers. The radiant boiler designs could now reach up to 4,000,000 pounds of steam per hour. Later the industry developed the largest of the boiler designs, the universal pressure and supercritical boilers. These steam-generating behemoths could now reach over 1,300 megawatts of electricity or 9,000,000 pounds of steam per hour.
During the past 100 years, the steam-generating industry has modified or developed boilers specifically suited for and in response to industry needs. For example, around the late s many medical, industrial, college, and government facilities wanted the ability to generate their own steam and electricity. In response to this need, the package or shop-assembled boilers were developed. A package boiler is a pre-engineered steam-generating boiler that ranges in size and steam capacity (typically from 10,000 to 600,000 lb/hr) built in a shop and shipped by rail or barge. Many companies manufactured these small shop-assembled boilers.
Another example is boilers for the pulp and paper industry, which have been around a very long time and began with the kraft recovery process developed in Danzing, Germany, in . In , the kraft recovery process was introduced in North America. The pulp and paper industry needed a boiler that could generate large quantities of steam and electricity to help run their driers, help them be energy self-sufficient, and, most importantly, help them make smelt. Using the designs described above, the boiler manufacturers developed the &#;recovery&#; boiler.
The recovery boiler&#;s furnace area is designed to melt the sodium salts in black liquor (the byproduct left over from the pulp-making process). Black liquor droplets fall onto the char bed or furnace floor of the boiler, and the molten inorganic chemicals, or smelt, remains on the furnace floor and flows by gravity through the smelt spout openings into a dissolving tank. The smelt will then be recovered by the paper mill for use in pulp processing. Two such designs were Combustion Engineering&#;s chemical recovery boiler, called the V2R (vertical 2 drum recovery boiler), and Babcock & Wilcox&#;s process recovery boiler, called simply a PR boiler.
The steam-generating industry also had to develop new boilers in response to non-commercial or industry demands. By the late s and early s, the growing disposal costs for landfills, the passage of the Public Utility Regulatory Policies Act of , and an increased demand for electric power in the United States led to the development of alternative fuel&#;burning boilers. Many different types of boilers began to be designed to burn alternative fuels such as refuse (trash), wood, and biomass (vine clippings, leaves, grasses, bamboo, and sugar cane or bagasse). A boiler using fluidized bed technology was also designed as an alternative method of burning solid fuels such as coal. Each alternative fuel&#;burning boiler has the basic components of its predecessors. The boiler manufacturers only modified the fuel input equipment or modified the basic boiler parts to accommodate the transfer of additional air, ash, or the fuel itself.
Refuse, wood, and biomass boilers are similar to the utility radiant boilers and industrial boilers that burn coal. They fall into the category of &#;waste-to-energy&#; boilers. They differ only in the type of refuse, wood, or biomass they burn, and the fuel they burn may vary depending on the time of the year (e.g., autumn may bring more leaves). Due to the many variables of the fuel, the lower furnace environment is constantly changing. There are two basic methods of burning refuse: mass burning, which uses the refuse as received, and prepared refuse or refuse-derived fuel (RDF), for which the refuse is separated and sorted, with the remaining non-recycled material going to the boiler. The burning of either mass refuse or RDF can cause serious corrosion on the tube wall surface. Choosing the right refractory material for the lower furnace walls is critical for efficient boiler operation and tube protection.
Fluidized bed boilers have most of the basic components of all boilers (steam drum, tubes, economizers, super-heaters, etc.). However, its basic design is different from most other boiler designs. A fluidized bed boiler, depending on the boiler manufacturer, may have cyclones (not to be confused with a cyclone burner), fuel chutes, over-bed burners, collection hoppers, combustion chambers, and stripper coolers. Though the technology of gasification has been around since the s, its use as an alternative fuel&#;burning method of generating electricity and power began in the late s. The fluidized bed boiler uses a process by which solid fuels are suspended in an upward-flowing gas or air stream at the bottom of the unit. The burning fuel exists in a fluid-like state that has a high heat transfer but with lower reduced emissions. Like the refuse boiler, the lower furnace walls must be protected from the environment created by the burning of the fuel.
Unfortunately, power plants are often depicted as dirty, with air pollution spilling into the air. The truth is that power plants are in some of the prettiest areas of the country, along rivers and lakes; spend millions of dollars annually to protect the environment and their neighbors; and keep their facility clean and tidy. If allowed, everyone should visit a local power plant and see how beautiful the country around the plant is and how clean the plant is&#;and at the same time take note of what type of boiler it has.
References
The information contained in this article was obtained primarily from public sources, without direct input from any of the boiler manufacturers.
Combustion Fossil Power, Combustion Engineering, Inc., 4th Edition ().
Steam, its generation and use, Babcock & Wilcox Company,
40th Edition ().
Babcock & Wilcox a corporate history, Carlisle Printing Company,
N.W. Eft ()
Refractories in the Generation of Steam Power, McGraw-Hill Book
Company, F. H. Norton ().
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