What is a water wall ？

A water wall is a heat-absorbing component inside a boiler furnace, composed of multiple rows of steel tubes, usually vertically arranged on the inner surface of the furnace wall. It is a boiler heating surface that primarily absorbs radiant heat emitted by the furnace flame and high-temperature flue gas. As the main form of evaporative heating surface in various modern boilers, it is also a basic component in the boiler water circulation circuit, thus serving the dual functions of cooling and protecting the furnace wall.

The original design purpose of the water wall was not to absorb heat, but to cool the furnace and prevent it from being damaged by high temperatures. Later, due to its excellent heat exchange performance, it gradually replaced the steam drum as the main heat-absorbing part of the boiler. In large-capacity boilers, the flame temperature inside the furnace is very high, and the intensity of thermal radiation is considerable. The water wall absorbs 40% to 50% or even more of the total heat in the boiler. Except for a few small-capacity boilers, all modern water-tube boilers use the water wall as the most important evaporative heating surface.

According to structural types, water walls are mainly divided into three categories: bare-tube, membrane, and studded (pin) types.

• The bare-tube water wall is composed of a whole row of seamless steel tubes, with the simplest structure.
• The membrane water wall is formed by welding many prefabricated finned water wall tubes together by spot welding to form a sealed combined heating surface. It not only improves the air tightness of the furnace and reduces air leakage, but also better protects the furnace wall, reducing its weight and simplifying its structure.
• The studded (pin) water wall is made by welding many pins (20–25 mm long, 6–12 mm in diameter) on the water wall tubes, then coated with chrome ore sand refractory plastic. This reduces heat absorption at that section, raises the temperature in the combustion zone, and is suitable for cases where fuel ignition is particularly difficult or for some liquid slag-tap boilers.

According to installation location, there are two types: wall-attached and division (double-sided heating) water walls. The former is mostly partially embedded in the inner furnace wall and absorbs heat from one side; the latter is mostly installed in the middle of the furnace of large boilers, dividing the furnace into two independent sections and absorbing heat from both sides.

1. Bare-Tube Water Wall

The bare-tube water wall is made of ordinary boiler steel tubes and is widely used in small and medium-capacity boilers. The compactness of the water wall layout is measured by the ratio of tube center-to-center spacing (s) to outer tube diameter (d), i.e., s/d.

For small-capacity boilers, s/d is large (1.5–2.0), resulting in high metal utilization but poor furnace wall protection; heavy furnace walls lined with refractory bricks are often adopted. As boiler capacity increases, s/d decreases (1.1–1.2), reducing metal utilization, and lightweight furnace walls can be used. When s/d < 1.1, tube-attached furnace walls are applicable.

The bare-tube water wall features simple manufacturing without large-scale mechanized equipment. However, it has poor boiler air tightness, complex and difficult-to-construct furnace walls, complicated supporting and hanging structures, and sufficient attention must be paid to thermal expansion during installation.

 

2. Membrane Water Wall

The membrane water wall is an airtight structure formed by welding tubes and fins.

Advantages:

• Low air leakage into the furnace;
• Enables the use of tube-attached furnace walls, reducing the load on the steel structure and lowering boiler cost;
• Facilitates the adoption of suspension structures and improves the thermal expansion system;
• Suitable for large-scale mechanized production with high assembly efficiency;
• Low heat storage capacity, shortening boiler startup and shutdown times.

Disadvantage: Relatively complex manufacturing process. It is mostly used in large-capacity, high-temperature, and high-pressure boilers.

Material and Tube Diameter

For medium-pressure, high-pressure, and ultra-high-pressure boilers, carbon steel tubes can be selected under normal water circulation conditions, as the tube walls can be well cooled. For subcritical and supercritical pressure boilers, due to possible heat transfer deterioration and high medium temperature (especially at supercritical parameters), internally ribbed tubes or Cr-Mo alloy steel tubes are often required. For membrane water walls, the maximum fin temperature and fin stress should also be verified.

The outer diameter of water wall tubes is generally 42–76 mm for natural circulation boilers, 42–51 mm for subcritical pressure controlled circulation boilers, and 22–42 mm for once-through boilers.

Advantages of Membrane Water Wall

1. Ensures furnace wall tightness and reduces air leakage into the combustion chamber.
2. Realizes full water cooling of the combustion chamber, helping prevent slagging under intensified combustion conditions.
3. Low heat storage capacity, enabling fast heating and cooling of the combustion chamber, shortening boiler startup and shutdown cooling times.
4. Suitable for tube-attached furnace walls, greatly reducing the load on the boiler frame and facilitating suspension.
5. Improves tube heat absorption compared with bare tubes.
6. Enhances the pre-assembly degree of boiler components, simplifying and reducing installation work.