Firetube vs watertube boilers
In a firetube boiler, hot combustion gases pass through tubes surrounded by water. In a watertube boiler, water and steam are inside the tubes and the hot gases are outside. Most of the differences that come up on the 4th Class Part A exam follow from that one change: how much water the boiler holds, how high a pressure it can take, and how it responds when steam demand changes.
Side by side
| Firetube | Watertube | |
|---|---|---|
| Inside the tubes | Hot combustion gases, with water around the tubes | Water and steam, with hot gases outside |
| Pressure on the tubes | External | Internal |
| Water held, for the same rating | More | Less |
| A short rise in steam demand | Stored heat in the water moderates the pressure drop | Less stored heat, so the boiler must respond quickly to pressure changes |
| Warming up from cold | Takes longer, because there's more water to heat | Reaches operating pressure and temperature sooner |
| Pressure range | General designs up to about 2.4 MPa | Above 2.4 MPa, and beyond the critical pressure of 22.09 MPa |
| Largest packaged units | Up to 25 000 kW | About 73 000 kW, with superheated steam above 15 000 kPa |
| Water circulation | Natural convection in steam boilers, forced circulation in hot water boilers | Natural circulation driven by density difference, and forced circulation in supercritical units |
| Tube ends | Expanded and beaded, so tubesheet contact keeps the end cool. Hotter regions may be expanded and welded | Flared to resist being pushed out by pressure. The ends are covered in water, so they need no cooling bead |
| Early design | Trevithick, early nineteenth century | Wilcox, 1856 |
Why water content matters
A firetube boiler holds more water than a watertube boiler of similar rating. That stored heat is why a firetube boiler loses less pressure when demand rises for a short time, and why it takes longer to warm up and recover.
Water content also sets the hazard. A boiler's explosive potential is proportional to the mass of saturated water inside it. Wilcox named his 1856 design, regarded as the first watertube boiler, the Safety Boiler because of its small internal water volume and small-diameter pressure parts.
Why watertube boilers go to higher pressure
With the same material and thickness, a smaller-diameter pressure part withstands a higher pressure than a larger one. Higher firetube pressures need thicker shells, which raise construction cost. General firetube designs stop at about 2.4 MPa and 25 000 kW, while watertube boilers run above 2.4 MPa and beyond the critical pressure. The largest modern units exceed 2 000 000 kg/h and 20 MPa.
Firetube boilers in more detail
- A modern firetube boiler keeps Trevithick's furnace, submerged in water, and firetubes surrounded by water. Its materials, assembly and engineering design have improved.
- Gas makes several passes through the tubes. A three-pass boiler runs gas rearward, forward and rearward again, which leaves the chimney at the opposite end from the burner. Each pass removes heat, and as the gas temperature approaches the water temperature, heat transfer slows. Extra passes also add gas-flow resistance and draft-fan demand, so practical designs are limited to four passes.
- Wetback construction carries less refractory to maintain than dryback construction, though dryback simplifies the pressure vessel.
- The furnace tube may be reinforced with rings or corrugations. Corrugations add strength with less thickness, add heating area, and take up unequal expansion between furnace and shell. Firetubes themselves are unreinforced.
- Flat tubesheets are supported mostly by the furnace tube and firetubes, with stays bracing any area left without tube support.
Watertube boilers in more detail
- Most watertube boilers have two drums. The upper steam drum separates steam from water, and the lower mud drum collects sediment for removal. Some older designs have three or more drums, some newer utility designs have one, and many once-through boilers have none.
- Water circulates naturally. Unfired downcomers carry water from the steam drum down to the mud drum, and heat in the fired risers makes steam, so the lighter mixture rises back to the steam drum. The bigger the density difference, the stronger the circulation. The difference disappears at the critical pressure of 22.09 MPa, so supercritical units use forced circulation and have no drums.
- Packaged watertube boilers come in three common layouts. The A-type has one larger steam drum over two small mud drums. The D-type and O-type each have one steam drum and one mud drum. The O-type is compact and suits limited space.
- Waterwalls line the furnace with water-cooled tubes, which absorb furnace heat and cut the refractory needed.
- Bent tubes came in with better water treatment. They let designers surround the furnace with tubes and fit more heating surface to smaller drums.
Where the two designs came from
The Haycock boiler dates from about 1720. Trevithick developed his firetube boilers in the early nineteenth century. Putting the furnace inside the boiler cut heat losses, and firetubes raised the heating surface compared with the water volume, so boilers made steam faster and used less fuel. Wilcox patented his first watertube design in 1856.
Study it in A11 Boilers
All of this is in Part A, A11 Boilers: CH01 Introduction to Boilers, CH02 Firetube Boilers and CH03 Watertube Boilers. Each chapter has a free quiz and a podcast you can play in the browser. By Pass 4's estimate, A11 is 15% of the Part A paper, one of the two highest shares. Listen to the chapters on the podcasts page, or try more questions in free 4A practice questions.
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