Water Tube Boilers: Design, Operating Principle and Advantages

Large facilities such as power plants, refineries, and paper, sugar and chemical plants need steam continuously, at high flow rates and often at high pressure. Once steam capacity and pressure exceed a certain level, typically from around 30 t/h and 20 bar, fire tube boilers give way to the water tube boiler.

This article covers the design of water tube boilers, membrane wall technology, the differences from fire tube boilers and the boiler fittings needed for safe operation.

What Is a Water Tube Boiler?

As the name suggests, a water tube boiler is a boiler in which water circulates inside the tubes while the combustion gases pass outside them. In a fire tube boiler it is the other way round: hot gases flow through the tubes and the water surrounds them in a large shell.

In a water tube boiler, water in the tubes is heated and partly evaporates; the steam-water mixture collects in the upper steam drum, where the steam separates from the water. The separated water returns to the heated tubes through downcomers. Where superheated steam is required, the saturated steam leaving the drum passes through a superheater.

Membrane Wall Construction

In modern water tube boilers, the furnace is enclosed by gas-tight membrane walls formed by welding flat bars between adjacent tubes. This construction:

• makes the furnace gas-tight,
• turns the entire furnace wall into a heating surface that absorbs radiant heat,
• supports convective heat transfer in the gas passes,
• reduces the need for heavy refractory brickwork.

Fast Start-Up and High-Pressure Capability

Water tube boilers contain a relatively small volume of water in their tubes and drums for their capacity. As a result:

• Fast steam raising: with a small water volume to heat, the boiler can be brought on line more quickly.
• High pressure: small-diameter tubes are better suited to high pressure in terms of wall thickness; water tube boilers can operate above 100 bar.
• Lower consequence of failure: because they hold less water and steam, the energy released in a tube failure is more limited than in large-shell boilers.

Fire Tube vs Water Tube Boilers

Advantages of Water Tube Boilers

• High efficiency: with proper design and heat recovery equipment such as economisers, high thermal efficiencies are achieved, reducing fuel consumption.
• Fuel flexibility: can be designed for solid, liquid and gaseous fuels.
• Long life: long economic life with correct water treatment and proper operating conditions.
• Controlled operation: level controls, safety valves, feedwater inlet and surface blowdown connections on the steam drum allow safe, controlled operation.

Boiler Fittings and the Importance of Water Quality

Safe and efficient operation of a water tube boiler depends as much on its valves, fittings and controls as on the pressure parts. The main items are:

• Safety valves and main steam stop valves
• Level gauges, level controls and low-water safety devices
• Feedwater check valves and feedwater control valves
• Surface (TDS) and bottom blowdown systems
• Sample coolers and measuring instruments

When the total dissolved solids (TDS) concentration in boiler water rises, foaming and water carryover with the steam occur, while sludge settling at the bottom creates a risk of tube overheating. Regular surface and bottom blowdown is therefore essential in water tube boilers.

Frequently Asked Questions

Conclusion

Water tube boilers are a fast-starting, efficient and safe steam generation solution for plants that need high capacity and pressure. Consult the ORFA technical team on blowdown systems, valves and fittings for your boiler, and explore our boiler surface and bottom blowdown systems product group.