Novelty Steel supplies Firetube Boilers and Watertube Steam Boilers for Industrial Plants, Heating Projects and Production Facilities.

There are two major types of steam boilers: Firetube boilers and Watertube boilers.

The major difference between these two lies in the arrangement of fire (combustion gases) and water within the boiler tubes.

1. Firetube Boilers

In Firetube boilers, the combustion gases are contained within the tubes, while water surrounds the tubes. Firetube boilers have been a dominant boiler design for over a century, maintaining a majority share in high-pressure steam generating capacity within the normal size range. However, not all Firetube boilers share the same design characteristics. Historically, manufacturers commonly offered Scotch Marine Firetube boilers, featuring a horizontal multi-pass design with a large furnace. These boilers varied in the number of passes (2, 3, or 4-pass) and the furnace turnaround construction.

1.1 Scotch Marine Firetube Boilers

• Design: Cylindrical shell with a furnace for combustion and multiple tubes for extracting heat.
• Turnaround Chambers: Used to reverse combustion gases from the furnace to the tubes, generating multiple passes.
• Wetback and Dryback Turnaround: Wetback boilers have a water-cooled chamber for the furnace turnaround, while dryback boilers have a separate refractory chamber attached to the back of the vessel.
• Thermal Shock : Multi-pass Firetube boilers may experience “Thermal Shock,” caused by differential expansion and contraction rates of the furnace, tubes, and shell during heating. Slower warm-up cycles are performed to minimize temperature differences and prevent high stresses and fatigue failure.
• Hot water Applications: Multi-pass Firetube boilers are generally not used for hot water applications due to the thermal shock issue caused by sudden changes in water temperature.

1.2 Advantages of Firetube Boilers

1. High Heating Surface: Typically built with a lot of heating surface which leads to higher steady-state efficiency and resistance to fouling.
2. Large Steam Chest: The large diameter shell contains water and steam, providing a large steam chest and disengaging area for good steam quality without mechanical separation.
3. Fuel Flexibility: The furnace can be made large enough to support various fuels and emission requirements.
4. Sediment Accumulation: The large shell space allows for a fair amount of sediment accumulation without damaging the vessel, making it more forgiving in terms of water treatment and blow down maintenance.
5. Water Level Management: The large water content makes water level management easier, often requiring just an on-off control or a simple modulating level control.
6. High Energy Content: The boiler has a large energy content, allowing it to generate a higher steam flow rate for short periods with minimal steam pressure variations.
7. Reduced Heat Loss: Most of the boiler has water or steam on the inner side of the shell, reducing shell heat loss.

1.3 Disadvantages of Firetube Boilers

1. Slow Warm-up Cycle: Requires a slow warm-up cycle that can take several hours, subject to thermal shock. Improper warm-up can lead to failures over time, especially in larger boilers.
2. Backup Boiler Requirement: The longer warm-up cycle often necessitates keeping backup boilers hot for quicker response, adding cost and complexity to plant operation.
3. Pressure Limitations: The steam pressure is limited by the large diameter shell, usually around 300 PSI.
4. High Energy Requirement: The large steam and water content mean more energy is required to bring the unit online. If the boiler operates infrequently, start-up energy may be lost during off times, reflected in lower dynamic efficiency.

2. Watertube Boilers

In Watertube boilers, water is inside the tubes, and the combustion gases pass around the outside of the tubes.

Watertube boilers exhibit a wide range of design variations due to the ease of fabricating small-diameter tubing into different shapes. These variations cater to specific goals such as solid fuels, smaller footprint, truck mounting, lower costs, thermal shock resistance, and quick startup. The “Industrial Watertube” category comprises units with high capacities, high pressure capabilities, and the ability to provide superheat. While packaged Industrial Watertube boilers typically reach around 300,000 PPH, larger boilers may not ship as complete units.

2.1 Watertube Boiler Characteristics

• Steam and Water Configuration: Watertube boilers have steam and water inside the tubes, resulting in less water content than Firetube boilers.
• Multiple Drums and Tubes: Made up of multiple drums (steam drum and mud drums) and numerous tubes, providing a furnace and convection section.
• Bent-Tube Boilers: Feature multiple gas passes created by bending tubes without attachment between them, offering resistance to thermal shock.
• Flue Gas Sealing: Achieved through rigid casing, membrane walls, or a combination, with refractory and insulation protecting drums and tube connections.
• Special Types:
  • Bent Tube Boilers: Identified by shaped tubes creating multiple passes, available in smaller capacities and lower cost.
  • Coil Tube Boiler: Utilizes a single tube coil (or a few coils) for quick startup without thermal shock, with a separate chamber for steam separation. Has low water content, small size, and short start up time.

Watertube boilers, with their diverse designs and configurations, provide flexibility to meet specific application requirements. Whether Bent-Tube or Coil Tube, these boilers offer advantages in terms of efficiency, capacity, and start up time

2.2 Advantages of the Watertube Boilers

1. Higher Capacities and Pressures: Watertube boilers excel in higher capacities, steam pressures, and superheated steam.
2. Larger Furnace: The furnace in a Watertube boiler is generally larger which facilitates firing operations.
3. Handling Higher Pressures: Small-diameter tubes and drums allow Watertube boilers to handle higher pressures efficiently.
4. Energy Efficiency: The smaller water and steam capacity result in quicker warm-up times.
5. Quick Response to Load Changes: With less water and steam, Watertube boilers respond more quickly to load changes so that they can reflect variations in steam pressure promptly.
6. Reduced Energy Release in Vessel Failure: The smaller amount of water and steam means less energy to release in the event of a vessel failure.

2.3 Disadvantages of the Watertube Boilers

1. Scaling Sensitivity: Small-diameter tubes are prone to scaling, requiring good feed water quality and blow down practices to prevent scale build-up.
2. Smaller Heating Surface: Watertube boilers may have a smaller heating surface which leads to higher outlet flue temperatures and lower steady-state efficiency. Economizers are often employed to enhance efficiency.
3. Long Warm-up Cycle: Many Industrial Watertube boilers require a lengthy warm-up cycle to achieve even heating, which can take several hours.
4. Water Level Maintenance Challenges: Maintaining water levels can be challenging, especially with significant changes in firing rate and steam flow rates,
5. Impact of Load Swings: The small amount of water and steam results in larger steam pressure changes during load swings.
6. Steam Quality Challenges: The relatively small steam drum may require mechanical equipment to achieve good steam quality at lower pressures.

3.Bent Tube Boilers

Bent Tube Boilers are characterized by their smaller capacity and they do not offer high pressures or superheat. They feature tangent tubes forming a furnace space and additional gas passes due to the bent shape of the tubes. These boilers are popular for hot water heating applications and are known for their flexibility. They allow immediate operation at full input with a short warm-up time, usually minutes.

3.1 Advantages of the Bent Tube Boilers

1. Compact Design: Bent Tube Boilers can be very compact, often fitting through doorways in smaller sizes.
2. Flexible Tubes: The tubes are flexible, avoiding thermal shock. Most units can be started and reach full rate immediately.
3. Small Water and Steam Content: Limited water and steam in the vessel allow for quick response to load changes.
4. Quick Warm-up: The small amount of water in the vessel results in a short warm-up time .They require less energy to bring the boiler online.
5. Assembly Flexibility: Units with Ferrule connections can be assembled on the job site without the need for an certified welder. They are ideal for locations with difficult boiler room access.

3.2 Disadvantages of Bent Tube Boilers

1. Insulation and Condensation: Tubes surrounded by insulation can absorb condensation during cold start-ups. This may cause corrosion.
2. Limited Heating Surface: Bent Tube Boilers usually have a smaller heating surface and lower steady-state efficiency.
3. Steam Quality Concerns: The small drum size can raise steam quality issues, especially at lower pressures.
4. Water Level Control: Controlling water level is more challenging due to the small water content.
5. Load Swings Impact: Load swings can result in larger variations in steam pressure due to less internal energy to absorb the change

4. Comparison of Firetube and Watertube Boilers

• Design and Operation:

• Firetube Boilers:
  • Design: Hot combustion gases pass through small tubes within a pressurized cylindrical drum surrounded by water.
  • Operation: Heat energy is transferred to the water, heating it until steam is produced.
• Watertube Boilers:
  • Design: Water is circulated inside the boiler tubes, which are surrounded by hot gases.
  • Operation: Hot gases surround the tubes, heating the water inside.

• Pressure and Steam Output:

• Firetube Boilers:
  • Lower operating pressures.
  • Lower steam output.
• Watertube Boilers:
  • Can operate at much higher pressures.
  • Higher steam output; can produce saturated or superheated steam.

• Burner Configuration and Control:

• Firetube Boilers:
  • Typically powered by a single burner.
  • Limited control options.
• Watertube Boilers:
  • Often powered by multiple burners, allowing individual controlled shutdowns for maintenance.
  • Custom programmable logic controller (PLC) for boiler management and burner modulation.

• Responsiveness and Efficiency:

• Firetube Boilers:
  • Slower response to steam pressure fluctuations due to the large volume of water.
  • Lower efficiency of heat transfer.
• Watertube Boilers:
  • Exceptionally responsive, producing steam much faster.
  • High efficiency, operating at higher pressure.

• Fuel Compatibility and Environmental Considerations:

• Firetube Boilers:
  • Limited capability for high-ash fuels.
  • Environmental considerations may be a challenge.
• Watertube Boilers:
  • Designed to work with high-ash fuels, suitable for biomass applications and waste-to-energy plants.
  • Capable of meeting environmental regulations.

• Cost and Construction:

• Firetube Boilers:
  • Lower capital cost.
  • Typically factory-built.
• Watertube Boilers:
  • Higher capital cost.
  • Often constructed on-site, potentially adding to costs and timescale.

Firetube boilers have a larger water and steam content for a given capacity compared to Watertube boilers. The large reservoir of water and steam in Firetube boilers contributes to more stable steam pressure and water levels during fluctuating loads. Due to the large water and steam content, Firetube boilers require more energy to heat up to operating temperature and pressure.

» Please also visit our detailed article about Boiler Fabrication and our guide on How to select the right boiler?

» Novelty Steel supplies various types of Boilers and Pressure Vessels from its manufacturing facilities in Turkey.