1. Definition of a Riser

A Riser, also known as a water wall riser tube, refers to the heated tubes located around the furnace walls.

• Its primary function is to receive water from the downcomer, absorb heat, generate a steam–water mixture, and deliver it back to the steam drum.

• Unlike downcomers, which carry single-phase water, risers carry a two-phase steam–water mixture.

2. Structure and Arrangement

• Location: Installed around the furnace as part of the water wall tubes.

• Connections: The lower end connects to the lower header, and the upper end connects to the steam drum.

• Material: Typically made of high-temperature, high-pressure alloy steel tubes, designed to withstand direct flame radiation and convective heat.

3. Working Principle

The circulation process involving risers can be summarized as follows:

1. Water from the steam drum flows down through the downcomers into the lower headers.

2. This water enters the riser tubes and absorbs radiant and convective heat from the furnace.

3. The water gradually vaporizes, forming a steam–water mixture that rises toward the steam drum.

4. In the steam drum, the mixture is separated: steam is sent to the superheater, while the remaining water re-enters the circulation loop.

The density difference between water in the downcomers and the steam–water mixture in the risers provides the driving force for natural circulation.

4. Key Characteristics

• High heat absorption: Risers are directly exposed to radiant and convective heat from the furnace.

• Two-phase flow: The tubes contain both water and steam, resulting in complex but efficient heat transfer.

• Circulation driver: The density difference between riser and downcomer flows maintains natural circulation in the boiler.

5. Engineering Importance

• Safety: Proper riser performance prevents overheating, dry-out, and tube rupture.

• Efficiency: Optimized riser arrangement enhances evaporation rates and heat transfer uniformity.

• Durability: Protective measures, such as wear-resistant shields in erosion-prone areas, help extend tube life.

Conclusion

The Riser in a boiler is the “driving force” of the water–steam circulation system. It not only enables water evaporation but also sustains the natural circulation process through its interaction with downcomers. Its design, material selection, and protective solutions are directly linked to the boiler’s safety, efficiency, and service life.