4AE3 U11 C2 Review
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What is a key characteristic of a firetube boiler?

  • The boiler is only used for high-pressure applications.
  • The products of combustion surround the water and steam.
  • The boiler is only used for low-temperature applications.
  • The water and steam surround the products of combustion. (correct)

    • What was a major issue with early firetube boilers?

  • They were too efficient and wasted energy.
  • They were too expensive to construct.
  • Much heat was passed to brickwork rather than boiler water. (correct)
  • They were too large and heavy.

    • What was an improvement made to early firetube boilers?

  • Using only riveted construction.
  • Decreasing the number of flue gas passes.
  • Increasing the size of the boiler.
  • Directing combustion gases through firetubes submerged in boiler water. (correct)

    • What is an advantage of firetube boilers over watertube boilers of comparable rating?

    <p>They contain more water, storing more heat energy.</p> Signup and view all the answers

    Why can firetube boilers tolerate lower quality feedwater?

    <p>Because they have no small water passages to block.</p> Signup and view all the answers

    What has led to the modern firetube boiler being safer than its early counterparts?

    <p>The development of design codes and advancements in welding methods.</p> Signup and view all the answers

    What is a major disadvantage of firetube boilers?

    <p>They are limited in operating pressure to about 2.4 MPa.</p> Signup and view all the answers

    What is the primary reason locomotive boilers were taken out of service and placed in stationary service?

    <p>They were no longer needed for steam traction engines.</p> Signup and view all the answers

    What is the purpose of staybolts in locomotive boilers?

    <p>To support the waterlegs.</p> Signup and view all the answers

    What is a characteristic of HRT boilers?

    <p>The combustion gases make two passes on their way to the chimney.</p> Signup and view all the answers

    What is the main reason HRT boilers are no longer constructed?

    <p>They are inefficient due to extensive field-erected brickwork.</p> Signup and view all the answers

    What is the steaming capacity range of HRT boilers?

    <p>500 to 7000 kg/h</p> Signup and view all the answers

    What is the main advantage of firetube boilers over watertube boilers?

    <p>They are less expensive to purchase.</p> Signup and view all the answers

    What is the type of boiler shown in Figure 1?

    <p>Locomotive boiler.</p> Signup and view all the answers

    What is the primary application of firebox boilers?

    <p>Low-pressure heating service.</p> Signup and view all the answers

    Why are locomotive boilers expensive to construct?

    <p>They require a lot of staying, which increases construction costs.</p> Signup and view all the answers

    What is the primary advantage of moving the furnace to within the shell of a boiler?

    <p>Increasing the heat transfer to the boiler water</p> Signup and view all the answers

    What is the name of the boiler design that is suitable for both high-pressure and low-pressure service?

    <p>Scotch boiler</p> Signup and view all the answers

    What term does ASME use to refer to all types of boilers, regardless of their design?

    <p>Firetube boilers</p> Signup and view all the answers

    What is the purpose of the reversing chamber in a multi-pass boiler?

    <p>To redirect the flow of flue gas</p> Signup and view all the answers

    What is the disadvantage of adding extra passes to a boiler?

    <p>Increasing the cost of the boiler</p> Signup and view all the answers

    What is the reason for the development of multi-pass boilers?

    <p>To lengthen the flue gas path without increasing the boiler length</p> Signup and view all the answers

    What is the term for the pathway through which the products of combustion flow in one direction?

    <p>Pass</p> Signup and view all the answers

    Why is the Scotch boiler often called a Scotch Marine boiler?

    <p>Because it was originally designed for use on ships</p> Signup and view all the answers

    What is the purpose of the corrugated furnace in a Scotch boiler?

    <p>To contain the water below the furnace</p> Signup and view all the answers

    What is the term for the type of Scotch boiler that has a water-cooled surface at the rear of the boiler?

    <p>Wet-back boiler</p> Signup and view all the answers

    What is a major drawback of modern firetube boilers with multiple passes?

    <p>They require more powerful draft fans.</p> Signup and view all the answers

    What is the primary heat transfer mechanism in the furnace tube of a two-pass firetube boiler?

    <p>Radiation</p> Signup and view all the answers

    What is the main advantage of a single-pass firetube boiler?

    <p>It has more radiant heat transfer surface.</p> Signup and view all the answers

    What is the purpose of the reversing chamber in a two-pass firetube boiler?

    <p>To change the direction of the flue gas flow.</p> Signup and view all the answers

    What is the main reason why multiple-pass boilers have more tubes than single-pass boilers?

    <p>To increase the heat transfer surface area.</p> Signup and view all the answers

    What is the main difference between a dry-back and a wet-back three-pass firetube boiler?

    <p>The pressure vessel design.</p> Signup and view all the answers

    Why do multiple-pass boilers require fewer tubes in each pass?

    <p>To reduce the total cross-sectional area of each pass.</p> Signup and view all the answers

    What is the main advantage of a vertical tubeless boiler?

    <p>It is a simple, packaged design.</p> Signup and view all the answers

    What is the purpose of the flame retainer rings in a vertical tubeless boiler?

    <p>To swirl the hot combustion gases.</p> Signup and view all the answers

    What is the main difference between a firetube boiler and a vertical tubeless boiler?

    <p>The presence of firetubes.</p> Signup and view all the answers

    What is the advantage of using corrugated furnaces?

    <p>Increasing heat transfer surface area</p> Signup and view all the answers

    What is the primary purpose of the flame retainer rings in a firetube boiler?

    <p>To slow the progress of the combustion gases and maximize contact time between the water and the hot gases</p> Signup and view all the answers

    What is the purpose of staybolts in firetube boilers?

    <p>To support the flat surfaces of the boiler</p> Signup and view all the answers

    What is the function of the fins welded to the outside of the boiler shell?

    <p>To increase the convective heat transfer to the water inside the shell</p> Signup and view all the answers

    What is the difference between through stays and staybolts?

    <p>Staybolts are used where through stays would block manhole access</p> Signup and view all the answers

    What is the maximum allowable working pressure that special designs of firetube boilers can be manufactured to?

    <p>3450 kPa</p> Signup and view all the answers

    What is the primary mechanism of water circulation in firetube boilers in steam service?

    <p>Natural convection</p> Signup and view all the answers

    What is the purpose of the small hole drilled into the end of through stays and staybolts?

    <p>To serve as a telltale for corrosion or cracking</p> Signup and view all the answers

    Where is the lowest permissible water level typically taken in locomotive and firebox boilers?

    <p>25 mm above the crown sheet</p> Signup and view all the answers

    What is the advantage of using reinforced furnace tubes?

    <p>They can be made thinner for the same working pressure</p> Signup and view all the answers

    What is the purpose of corrugations and reinforcing rings on furnace tubes?

    <p>To strengthen the furnace</p> Signup and view all the answers

    How are tubes in a firetube boiler typically joined to the tubesheets?

    <p>By expanding, welding, or both</p> Signup and view all the answers

    What is the typical rating range for vertical tubeless boilers?

    <p>40 to 600 kW</p> Signup and view all the answers

    What is the purpose of the doors in a firetube boiler?

    <p>To contain the combustion gases, direct the products of combustion, and provide access for inspection and maintenance</p> Signup and view all the answers

    What is the effect of corrugations on furnace turbulence?

    <p>They increase furnace turbulence</p> Signup and view all the answers

    What is the function of the reversing chamber in a firetube boiler?

    <p>To direct flue gas around the outside of the boiler shell</p> Signup and view all the answers

    What is the purpose of the inner door and baffle in a firetube boiler with multiple passes?

    <p>To redirect the flue gas</p> Signup and view all the answers

    What is the consequence of uneven water circulation in firetube boilers?

    <p>Sludge accumulation directly below the furnace tube</p> Signup and view all the answers

    What is the purpose of the tubesheets in a firetube boiler?

    <p>To accept firetubes and the furnace tube</p> Signup and view all the answers

    Why are tubes in a firetube boiler installed differently in each pass?

    <p>Due to differences in temperature</p> Signup and view all the answers

    What was the primary reason for the development of submerged furnace tubes in early firetube boilers?

    <p>To reduce heat loss to the brickwork</p> Signup and view all the answers

    What is the primary benefit of the large water volume in firetube boilers?

    <p>Quick response to load changes without pressure loss</p> Signup and view all the answers

    Why did design codes become necessary for firetube boilers?

    <p>To ensure consistent design calculations and construction methods</p> Signup and view all the answers

    What is the primary advantage of firetube boilers over watertube boilers of comparable rating?

    <p>More water volume and tolerance for lower quality feedwater</p> Signup and view all the answers

    What led to the improvement in safety of modern firetube boilers compared to their early counterparts?

    <p>Improved control systems and welding methods</p> Signup and view all the answers

    What was a major issue with early firetube boilers?

    <p>High heat loss to the brickwork</p> Signup and view all the answers

    What is the primary reason for the limited operating pressure of firetube boilers?

    <p>Thicker shells would increase construction costs</p> Signup and view all the answers

    What is the main advantage of locomotive boilers in steam locomotives and steam traction engines?

    <p>They are portable and can be easily moved</p> Signup and view all the answers

    What is the primary advantage of a single-pass firetube boiler over multi-pass designs?

    <p>Increased radiant heat transfer surface</p> Signup and view all the answers

    Why are horizontal return tubular (HRT) boilers no longer constructed?

    <p>All of the above</p> Signup and view all the answers

    Why do multiple-pass firetube boilers require more powerful draft fans?

    <p>Because the boiler becomes more restrictive to flue gas flow</p> Signup and view all the answers

    What is the purpose of the waterlegs in a locomotive boiler?

    <p>To support the crown sheet and wrapper sheet</p> Signup and view all the answers

    What is the primary difference between a dry-back and a wet-back three-pass firetube boiler?

    <p>The pressure vessel design</p> Signup and view all the answers

    What is the main difference between a locomotive boiler and a firebox boiler?

    <p>The design of the furnace</p> Signup and view all the answers

    Why do four-pass firetube boilers have fewer tubes in each pass?

    <p>To reduce the total cross-sectional area of each pass</p> Signup and view all the answers

    What is the advantage of using an all-welded construction in modern locomotive boilers?

    <p>It is stronger and more durable</p> Signup and view all the answers

    Why are firetube boilers slow to heat up and bring to operating pressure and temperature?

    <p>The large water volume takes longer to heat</p> Signup and view all the answers

    What is the advantage of a vertical tubeless boiler over other firetube designs?

    <p>It occupies very little floor space</p> Signup and view all the answers

    What is the purpose of the flame retainer rings in a firetube boiler?

    <p>To facilitate hot combustion gases swirling</p> Signup and view all the answers

    What is the purpose of the radial stays in a locomotive boiler?

    <p>To support the upper sections of the end plates</p> Signup and view all the answers

    Why are HRT boilers often retrofitted for gas or oil firing?

    <p>To extend their lifespan</p> Signup and view all the answers

    Why are single-pass firetube boilers more economical to maintain?

    <p>Due to the absence of internal baffles and refractory</p> Signup and view all the answers

    What is the primary mechanism of heat transfer in the furnace tube of a two-pass firetube boiler?

    <p>Radiation</p> Signup and view all the answers

    What is the primary application of firetube boilers?

    <p>Low-pressure heating service</p> Signup and view all the answers

    What is the consequence of reducing the number of tubes in each pass of a four-pass firetube boiler?

    <p>Decreased total cross-sectional area of each pass</p> Signup and view all the answers

    What is the primary application of vertical tubeless boilers?

    <p>Dry-cleaning, laundry, and small industrial applications</p> Signup and view all the answers

    What is the primary reason for moving the furnace within the shell of a boiler?

    <p>To eliminate heat losses through the brick setting</p> Signup and view all the answers

    What is the main advantage of a multi-pass boiler over a single-pass boiler?

    <p>Increased efficiency due to longer combustion gas paths</p> Signup and view all the answers

    What is the primary purpose of the reversing chamber in a multi-pass boiler?

    <p>To redirect the flow of flue gas</p> Signup and view all the answers

    What is the term used to describe the pathway through which the products of combustion flow in one direction?

    <p>Pass</p> Signup and view all the answers

    What is the main disadvantage of adding extra passes to a boiler?

    <p>Increased complexity and cost due to additional reversing chambers</p> Signup and view all the answers

    What is the primary advantage of a Scotch boiler over a firebox boiler?

    <p>Reduced heat losses due to the elimination of brick settings</p> Signup and view all the answers

    What is the main reason why modern firetube boilers are more efficient than their early counterparts?

    <p>Internal firing and multi-pass designs</p> Signup and view all the answers

    What is the primary reason why multi-pass boilers require more complex door designs?

    <p>To accommodate the additional reversing chambers</p> Signup and view all the answers

    What is the primary advantage of a wet-back boiler over a dry-back boiler?

    <p>Increased heating surface due to water-cooled surfaces</p> Signup and view all the answers

    What is the primary reason why firetube boilers are more suitable for low-pressure and high-pressure service?

    <p>Due to their internal firing and multi-pass designs</p> Signup and view all the answers

    What is the primary purpose of the fins welded to the outside of the boiler shell?

    <p>To increase the convection heat transfer to the water inside the shell</p> Signup and view all the answers

    What is the consequence of poor water circulation in firetube boilers?

    <p>Sludge accumulation and overheating of the furnace tube</p> Signup and view all the answers

    What is the typical lowest permissible water level for locomotive and firebox boilers?

    <p>25 mm above the crown sheet</p> Signup and view all the answers

    What is the purpose of the tubesheets in a firetube boiler?

    <p>To attach the firetubes and furnace tube to the boiler</p> Signup and view all the answers

    What is the primary mechanism of water circulation in firetube boilers in hot water service?

    <p>Forced circulation</p> Signup and view all the answers

    What is the maximum allowable working pressure that special designs of firetube boilers can be manufactured to?

    <p>3450 kPa</p> Signup and view all the answers

    What is the purpose of the reversing chamber in a multi-pass firetube boiler?

    <p>To direct flue gas around the outside of the boiler shell</p> Signup and view all the answers

    What is the typical rating range for vertical tubeless boilers?

    <p>40-600 kW</p> Signup and view all the answers

    What is the primary advantage of using corrugated furnaces?

    <p>Improved structural strength and resistance to deformation</p> Signup and view all the answers

    What is the term for the type of boiler that is suitable for both high-pressure and low-pressure service?

    <p>Firetube boiler</p> Signup and view all the answers

    What is the primary benefit of using corrugated furnaces in boilers?

    <p>Improved heat transfer through increased surface area</p> Signup and view all the answers

    Why are tubes in a firetube boiler installed differently in each pass?

    <p>To accommodate varying temperatures across passes</p> Signup and view all the answers

    What is the purpose of the small hole drilled into the end of through stays and staybolts?

    <p>To act as a telltale for damage</p> Signup and view all the answers

    What is the primary function of the tubesheets in a firetube boiler?

    <p>To support the furnace tube</p> Signup and view all the answers

    What is the advantage of using ring-reinforced furnaces?

    <p>Thinner furnaces for the same working pressure</p> Signup and view all the answers

    Why are firetubes not reinforced like furnace tubes?

    <p>Because they are not subject to external pressure</p> Signup and view all the answers

    What is the purpose of the doors in a firetube boiler?

    <p>To direct the products of combustion through the flue gas passes</p> Signup and view all the answers

    What is the primary benefit of using diagonal stays in firetube boilers?

    <p>They allow for manhole access to the waterside</p> Signup and view all the answers

    Why are staybolts often attached by welding?

    <p>To provide a stronger joint between the tubesheet and the tube</p> Signup and view all the answers

    What is the primary purpose of the inner door and baffle in a firetube boiler with multiple passes?

    <p>To redirect the flue gas</p> Signup and view all the answers

    Study Notes

    Firetube Boilers

    • Advantages:
      • Less expensive to purchase than watertube boilers
      • Cost less to maintain
      • Simpler to maintain and repair
      • Have simple, rugged construction
      • Can tolerate lower quality feedwater
    • Disadvantages:
      • Limited to operating pressure of about 2.4 MPa
      • Large water volume makes them slow to heat up and bring to operating pressure and temperature
      • Take longer to recover from load increases

    Locomotive Boilers

    • Also known as steam locomotive boilers or steam traction engines
    • Can produce up to 45,000 kg/h of superheated steam at 2,400 kPa
    • Most were much smaller
    • Were either internally fired or externally fired
    • Most were riveted designs, but some newer ones are all-welded construction
    • Are extensively stayed to support flat surfaces
    • Have poor water circulation, are difficult to maintain, and provide poor cleaning and inspection access

    Horizontal Return Tubular (HRT) Boilers

    • Developed around the same time as locomotive boilers
    • Not designed for portability
    • Range in steaming capacity from about 500 to 7,000 kg/h, at pressures up to around 1,700 kPa
    • Were either top or bottom supported
    • Have extensive field-erected brickwork, which requires maintenance and repair

    Firebox Boilers

    • Are externally fired, two- or three-pass, horizontal firetube boilers
    • Most are designed for low-pressure heating service
    • Early firetube boilers were very inefficient and unsafe
    • Design codes were developed to standardize design calculations and construction methods
    • Welding methods progressed, and eventually replaced riveted construction, leading to stronger boilers

    Scotch Boilers

    • Also known as "Scotch" boilers, "horizontal return tubular" boilers, or "return tubular" boilers
    • Designed to maximize heat transfer and increase efficiency
    • Suitable for high-pressure and low-pressure service
    • Are the predominant style of firetube boiler manufactured today
    • Often called "Scotch Marine" boilers

    Gas Passes

    • One of the advances in boiler technology was to add tubes within the shell of the boiler
    • Multi-pass boilers were developed to lengthen the flue gas path without the need for a longer boiler
    • Each pass provides more time for heat transfer to occur
    • Designs range from one-pass to four-pass, with each additional pass providing more heat transfer

    Vertical Tubeless Boilers

    • A special two-pass firetube design
    • Does not have firetubes, but has a central furnace tube
    • Has many advantages, including simplicity, compactness, and automatic controls
    • Widely used in dry-cleaning, laundry, and small industrial applications

    Firetubes

    • Cylindrical steel components subject to external pressure
    • Designed like furnace tubes, but not reinforced
    • Can be made of welded or seamless tubing
    • Tubes can be joined to tubesheets by expanding, welding, or both

    Stays

    • Support the flat surfaces of the boiler
    • Many different styles of stays, including diagonal stays, through stays, and staybolts
    • Diagonal stays are used where through stays would block manhole access to the waterside
    • Through stays are used where such interference is not a problem
    • Staybolts are short through stays, usually attached by welding### Firetube Boilers
    • Firetube boilers heat water from the inside of the furnace tube and the outside of the shell.
    • They can fire natural gas, propane, or fuel oil.
    • Available in ratings from 40 to 600 kW (4 to 60 BoHP).
    • Can produce nearly 100% dry and saturated steam at up to 1035 kPa.
    • Special designs can be manufactured with maximum allowable working pressures up to 3450 kPa.
    • Individually, the largest of these designs can produce up to 940 kg/h of steam.

    Water Circulation

    • All boilers must be designed to provide adequate water circulation.
    • Firetube boilers in steam service rely on circulation caused by natural convection.
    • Firetube boilers in hot water service use forced circulation.
    • The greatest heat transfer is from radiant furnace heat.
    • More steam is produced adjacent to the furnace tube, causing a rise in fluid above the furnace tube and a general settling of fluid adjacent to the shell.

    Lowest Permissible Water Level

    • All boiler heat transfer surfaces require cooling.
    • Boiler tubes are cooled with water.
    • Superheater tubes are cooled with steam.
    • The lowest permissible water level is usually taken as being 25 mm above the crown sheet for locomotive and firebox boilers.
    • For HRT, Scotch marine, and packaged firetube boilers, the lowest permissible water level is usually taken as 25 mm above the top surface of the top row of tubes.

    Firetube Boiler Shell

    • A complete firetube boiler shell is constructed of courses, heads, furnace tubes, firetubes, stays, and access openings.
    • Tubesheets are flat heads with formed edges called flanges.
    • Furnace tubes are cylindrical components made of steel, subject to external pressure.

    Firetube Boiler Development

    • Early firetube boilers were inefficient and unsafe.
    • Improvements included submerged furnace tubes and multiple flue gas passes.
    • Design codes were developed to standardize design calculations and construction methods.
    • Welding methods progressed, and eventually replaced riveted construction, leading to stronger boilers.

    Firetube Boiler Advantages

    • Simple, rugged construction.
    • Contain more water than watertube boilers of a comparable rating, making them quick to respond to load changes.
    • Can tolerate lower quality feedwater without disrupting water circulation.
    • Less expensive to purchase and maintain than watertube boilers.
    • Simpler to maintain and repair than watertube boilers.

    Firetube Boiler Disadvantages

    • Limited in operating pressure to about 2.4 MPa.

    • The large water volume makes them slow to heat up and bring to operating pressure and temperature.

    • Take longer to recover from load increases due to the greater volume of water to heat.### Firetube Boilers

    • Modern firetube boilers come in one-, two-, three-, and four-pass designs, which vary in their flue gas paths and heat transfer mechanisms.

    • The relative location of the burner and chimney determines whether the boiler has an even or odd number of passes.

    • The two-pass design is a dry-back, and the three- and four-pass designs are wet-back.

    One-Pass Boilers

    • One-pass boilers have no central furnace tube, and each firetube is a small-volume furnace, creating more radiant heat transfer surface.
    • They have no internal baffles and no refractory, making them more economical to maintain.
    • Draft fans can be smaller due to short flue gas travel paths and little furnace pressure drop.

    Two-Pass Boilers

    • Two-pass firetube boilers have a reversing chamber at the back of the boiler, with primarily radiant heat transfer in the furnace tube and convective heat transfer in the firetubes.
    • They are economical to purchase, simple to maintain, and commonly installed.

    Three-Pass Boilers

    • Three-pass boilers provide greater economy than two-pass designs, with the flue gas traveling a greater distance and having more opportunity to transfer heat to the water.
    • They are available as dryback or wetback designs.

    Four-Pass Boilers

    • Four-pass boilers are considered the most economical design, with the flue gas transferring heat and decreasing in specific volume, resulting in a decrease in flue gas velocity.
    • Additional passes must have fewer tubes to maintain proper conditions for combustion.

    Vertical Tubeless Boilers

    • Vertical tubeless boilers are a special two-pass firetube design with a central furnace tube, but no firetubes.
    • They are simple, packaged designs, ready for hook-up with automatic controls, and occupy very little floor space.
    • They are widely used in dry-cleaning, laundry, and small industrial applications.

    Firebox Boilers

    • Modern firebox boilers do not have brick settings, and have minimal refractory, with water-cooled furnaces and usually 3-pass designs.
    • They are heavily stayed, difficult to clean, and hard to inspect.

    Scotch Boilers

    • Scotch boilers were designed to maximize heat transfer and increase efficiency by utilizing the heat that would be otherwise lost through brickwork.
    • They are suitable for high-pressure and low-pressure service, and are the predominant style of firetube boiler manufactured today.
    • They are also called "Scotch" boilers, "horizontal return tubular" boilers, and "return tubular" boilers.

    Gas Passes

    • The term "pass" refers to a passageway through which products of combustion flow, in essentially one direction.
    • Multi-pass boilers were developed to lengthen the flue gas path without the need for a longer boiler.
    • Each additional pass increases heat transfer, but also increases door complexity and refractory requirements.

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    Explore the evolution of firetube boilers, from their early inefficiencies to significant design improvements. Learn how the direction of combustion gases and furnace tubes contributed to increased efficiency. Test your knowledge on this fundamental concept in boiler manufacturing!

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