Boiler Details Study Notes
UNIT 1
BOILER DETAILS
Chapter 1: Watertube Boilers
1-1 Watertube Boiler Overview
Watertube boilers are categorized under heating, power, and tubular boiler types.
Learning Outcome
After completing this chapter, students should be able to describe various types of watertube boilers used in small industrial and heating systems.
Learning Objectives
Describe the construction of watertube and copper-tubular boilers.
Describe the water circulation in a longitudinal drum straight tube boiler.
Describe two-drum bent tube boilers and compare the advantages of a bent tube boiler to a straight tube boiler.
Describe the construction of “A” type, “D” type, and “O” type packaged watertube boilers and discuss their advantages.
Glossary of Terms
Baffle: A wall, barrier, or panel that changes the direction of flow of a liquid or gas. Used in boilers for directing flow inside drums or headers.
Drum: The boiler shell and heads form a drum to contain the fluid being heated; commonly used term in watertube boilers.
Economizer: Device used to preheat boiler feedwater in large high-pressure steam boilers.
Forced Draft Fan: Fan that blows air for combustion into the furnace.
Header: Larger pipes supplying or collecting from smaller pipes/tubes (also referred to as a manifold).
Heating Surface: Parts of the boiler that have water/steam on one side and are heated on the other. Determines the kW or horsepower rating.
Mud Drum: Lower drum in watertube boilers where mud/sludge collects (also called lower header).
Stack: Hollow duct for discharging combustion gases to the atmosphere; also known as chimney.
Superheater: In large high-pressure steam boilers, increases steam temperature above original boiling point for very dry steam.
Uptake: Duct that transfers spent combustion gases from the boiler to the stack (flue vent).
Water Line: Actual level of water in a boiler, where water and steam separate.
Watertubes: Tubes with water circulating inside and heat applied to the outside.
Introduction
Watertube boilers are standard in industries (power plants, gas plants, refineries) with high-pressure requirements.
They feature small water content, contributing to safety and rapid response to load changes.
Typically not favored for low-pressure steam heating systems due to high costs and supervision needs.
Packaged watertube steam heating boilers exist for low-pressure applications.
Objective 1: Construction of Watertube and Copper-Tubular Boilers
A popular watertube boiler includes lower and upper headers connected by numerous serpentine tubes (copper or steel) attached with threaded connections.
Figure 1 illustrates a low-pressure watertube boiler design.
Key components include:
Combustion chamber
Atmospheric gas burners
Insulated casing
Combustion gases rise between tubes, transferring heat via convection to water within the tubes; the design aids in efficient heat transfer due to turbulent gas flow.
Advantages include:
Flexible tube design to reduce expansion/contraction stress
Easy replacement of faulty tubes without welding.
Objective 2: Water Circulation in a Longitudinal Drum Straight Tube Boiler
Longitudinal drum boilers position the drum front-to-back compared to cross drum units.
Feedwater enters the lower drum, flowing down through a downcomer. Heat from the hot tubes decreases the water density, allowing it to rise and create natural circulation (illustrated in Figure 3).
Objective 3: Two-Drum Bent Tube Boilers
Figures 4 and 5 depict the two-drum bent tube boiler design.
Features include:
Upper and lower drums connected by bent tubes, forming waterwalls for the furnace enclosure.
Combustion gases navigate the boiler, allowing for enhanced heat capture.
Advantages over straight-tube designs include:
Better heat transfer through tube arrangement and gas direction changes.
Objective 4: Packaged Watertube Boilers
“A” Type Packaged Boiler: Utilizes two small lower drums and one larger upper drum for steam-water separation.
“D” Type Packaged Boiler: Offers flexible design; can accommodate superheaters and economizers.
“O” Type Packaged Boiler: Longer design due to transport limitations; typically symmetrical with lower exposed tube surfaces to radiant heat.
Advantages include:
Packaged design simplifies installation and provides ease of warranty coverage.
Chapter Questions
Why are low-capacity watertube boilers less used for low-pressure heating?
Answer: d) Require more supervision for water treatment.
What is the heating surface of a copper-tubular boiler?
Answer: b) A continuous copper tube.
The boiler type with an upper drum and two lower drums is:
Answer: c) “A.”
What is an advantage of packaged boilers?
Answer: a) Ease of installation.
The purpose of attaching “fins” to watertubes is to:
Answer: b) Increase the heating surface.
Chapter 2: Cast-Iron Sectional and Modular Boilers
Learning Outcome
Upon completion, students should be able to describe and explain the uses of cast-iron boilers.
Learning Objectives
Describe general construction of cast-iron sectional boilers.
List advantages of cast-iron sectional boilers over watertube and firetube boilers.
Describe arrangement of equipment in multiple cast-iron sectional boiler heating plants.
Describe construction/operation of cast-iron modular boilers.
Glossary of Terms
Combustion Chamber: Area where air and fuel mix for burning.
Combustion or Flue Gases: Hot gaseous products resulting from burning fuel.
Gas Passage: Areas in a boiler for flue gases to travel for heat transfer to water.
Water Legs: Water-filled sections around the furnace to increase heat transfer to the water.
Introduction
Cast iron sectional boilers are common in small to medium residential heating systems.
They can be disassembled for movement through tight spaces.
Larger buildings often use multiple boilers operating in parallel, allowing for improved fuel efficiency.
Objective 1: General Construction of Cast-Iron Sectional Boilers
Constructed of hollow cast-iron sections connected with push nipples.
Sections arranged either horizontally or vertically based on design requirements.
Objective 2: Advantages of Cast-Iron Sectional Boilers
Benefits include:
High resistance to corrosion.
Simplicity of assembly, requiring less skilled labor.
Ability to enlarge capacity through additional sections instead of full boiler replacement.
Objective 3: Arrangement of Equipment
Two options for large capacity systems:
Large single boiler.
Several smaller parallel units for efficiency and reliability.
Smaller units offer benefits including less space required, easier transportation, and reduced failure impacts.
Objective 4: Cast Iron Modular Boilers
Modular boilers combine horizontal sectional boilers into a single unit, often connected in parallel for continuous water flow.
Chapter Questions
Cast-iron sectional boilers have flue gas passages that:
Answer: b) increase heat transfer.
Machined piece of pipe found in boilers is:
Answer: c) push nipple.
An advantage of a cast iron boiler is:
Answer: d) choice of assembly location.
Small capacity heating boilers operate efficiently at:
Answer: a) continuous full load.
A modular cast-iron heating boiler consists of:
Answer: b) a number of individually operated sectional boilers.
Chapter 3: Firetube Boilers (Heating and Power)
Learning Outcome
Students should describe various types of firetube boilers used in heating/power systems.
Learning Objectives
Explain differences between power and heating boilers.
Describe early types of firetube boilers: HRT, locomotive, firebox.
Describe construction/application of wetback and dryback Scotch boilers.
Describe vertical firetube boilers and tubeless boilers used in heating plants.
Describe construction of packaged firetube boilers.
Glossary of Terms
Dry-Back Boiler: A boiler having a brick or insulated rear chamber for gas redirection.
Firetubes: Tubes through which flue gas travels, surrounded by water; mechanism in firetube boilers.
Internally Fired Boiler: A boiler with a combustion chamber inside the shell.
Packaged Boiler: A complete boiler supply unit from manufacturers, assembled and tested before shipment.
Introduction
ASME and CSA Boiler Codes guide design, installation, and safety standards for boilers. Power and heating boilers are determined by pressure and temperature limits.
Objective 1: Power vs. Heating Boilers
Power boilers adhere to ASME Section 1 for pressures exceeding 103 kPa for steam, 1100 kPa or 121°C for hot water boilers.
Heating boilers (Section 4) remain below these thresholds.
Objective 2: Historical Firetube Boiler Designs
Horizontal Return Tubular (HRT) Boiler: Early model with coal burning; labor-intensive and inefficient due to high heat loss.
Locomotive Boiler: Internal furnace with efficient heat surroundings; design allows for mobility.
Firebox Boiler: Similar to locomotive boiler with additional support and minimized brickwork.
Objective 3: Wetback and Dryback Scotch Boilers
Scotch boilers are predecessors of modern units, designed for effective heat transfer in packed formats.
Objective 4: Vertical Firetube and Tubeless Boilers
Vertical Firetube Boiler: Efficient for small high-pressure steam; tube arrangement causes effective heat transfer and efficient steam space management.
Vertical Tubeless Boiler: Commonly used in dry cleaning/laundry; compact design enhances user-friendliness and ease of operation.
Objective 5: Packaged Firetube Boilers
Modern boilers are manufactured as complete units with assembly and components, enhancing efficiency and control.
Chapter Questions
Maximum low-pressure steam boiler pressure: a) 103 kPa.
Main part of high-pressure firetube boiler: c) shell.
Pressure limitations for steam units are: c) 103 kPa for steam, 1100 kPa for hot water.
Steam space location in vertical firetube boiler: d) top of the shell.
Forerunner of modern packaged firetube boilers: d) externally fired HRT boiler.
Chief advantage of packaged heating boiler: b) mass production ability.
Vertical firetube boiler steam space: d) upper part.
Common term for the inner pipe in vertical tubeless: a) combustion chamber.
Firing direction in vertical tubeless: c) downward.
Unique feature of vertical tubeless: d) water column.
Chapter 4: Electric Boilers
Learning Outcome
Students should be able to describe electric boilers, their use, and design.
Learning Objectives
Compare electric boilers to fuel-fired models.
Describe construction/operation of electrode-type electric boilers.
Describe construction/operation of immersion-type electric boilers.
Glossary of Terms
Electrode: Conductor for passing electric current, in various forms.
Furnace Explosion: Ignition of gas accumulation in a furnace.
Steam Trap: Device for removing condensate from steam lines without steam loss.
Introduction
Electric boilers convert electrical energy to heat without combustion; used in various industrial settings.
Objective 1: Electric vs. Fuel-Fired Boilers
Advantages of Electric Boilers:
Compact design; no combustion space, thus eliminating ductwork/chimney requirements.
Quick installation without needing fuel storage. 98% of electrical energy is converted to heat, with no emitted pollution or potential for explosions.
Disadvantages:
Higher costs for electricity usage compared to fuel-fired units and pressure limitations around 2100 kPa.
Objective 2: Electrode-Type Electric Boilers
Electrodes located in a central generating chamber; where electrolyte (like NaCl) allows for conduction, producing steam when current flows.
Objective 3: Immersion-Type Electric Boilers
Current flows through submerged elements instead of water, arranged for maintenance access.
Chapter Questions
Immersion-type electric boiler has: c) elements.
Electrode-type boiler load maintained by: b) amount of electrode covered by water.
Output of immersion boiler regulated by: b) water level relative to elements.
Main disadvantage of electric boiler: b) high cost of electricity.
Electric boilers have efficiencies close to: d) 100%.
Salinity feeder is important in: b) electrode-type electric boilers.