Fire Tube vs Water Tube Boiler: Which One Fits Your Plant?

A fire tube boiler — also called a shell boiler — is a large horizontal or vertical vessel filled with water. The burner fires into a furnace tube inside the shell. The hot flue gas then turns and passes through banks of smaller smoke tubes before it leaves for the chimney. All the tubes sit under water, and the steam collects in the space above the water line.

In a three-pass design the furnace is the first pass and two banks of smoke tubes are the second and third. Wet back means the chamber where the gas turns at the rear is surrounded by water instead of being lined with refractory, so that part of the boiler also takes up heat and needs less maintenance. The WNS fire tube steam boiler is built this way, from 0.5 to 20 t/h at 0.7–1.25 MPa.

In a water tube boiler it is the other way round: water and steam flow inside the tubes, and the flame and flue gas pass around them. Because each tube has a small diameter, it can hold a high pressure with a thin wall. The boiler holds far less water than a shell boiler of the same output, so it heats up and responds quickly. Large power-station and process boilers are almost all water tube designs.

At the small end, a vertical water tube boiler is a compact way to raise steam fast. The LSS vertical water tube steam boiler has the burner on top, three gas passes and covers 0.5–2 t/h for gas or diesel.

Boilers for coal and biomass often combine both principles. A chain-grate boiler such as the DZL biomass steam boiler has a drum with smoke tubes (the fire tube part) and water-wall tubes around the grate (the water tube part). The water walls take the radiant heat of the burning bed, and the smoke tubes recover heat from the flue gas.

• Steady process steam, 1–20 t/h, gas or oil, up to 1.25 MPa: a three-pass fire tube boiler (WNS).
• Small loads, little floor space or frequent starts: a vertical water tube boiler (LSS) or a steam generator.
• Biomass pellets, wood chips or coal: a chain-grate DZL boiler.
• Pressures above 2.5 MPa or superheated steam for turbines: a water tube boiler built under a higher licence class — Wilford’s class B licence covers boilers up to 2.5 MPa.

If you are not sure, send the output, working pressure, fuel and how the steam demand changes over a day. Our engineers will propose the type and size, with the parameter table for each option.