Choosing between a firetube and a watertube boiler is one of the first decisions in any steam project. It shapes the capital cost, the fuels you can burn, the floor space you need and how the boiler behaves when your process demand changes.
Both designs are proven, and both have a place. This guide explains how each one works, where each one fits, and the questions to answer before you ask for a quote.
How a firetube boiler works
In a firetube boiler, fuel burns in a large furnace tube inside a cylindrical shell filled with water. The hot combustion gases then travel through banks of smaller tubes, also inside the shell, before they leave through the stack. Heat passes through the tube walls into the surrounding water, which boils to make steam.
Because the water surrounds the tubes, firetube boilers are also called shell boilers or smoke tube boilers. Most modern units are packaged boilers. They arrive as one complete unit, with the burner, level controls, feed pump and fittings already in place, ready to connect.
Wet back or dry back
Many firetube boilers send the gases through the shell more than once. At the rear of the boiler, the gases turn around in a reversal chamber. Where that chamber sits gives the two main designs.
- Dry back. The reversal chamber is lined with refractory and sits on the outside of the boiler’s rear plate. The design is simple and suits smaller installations.
- Wet back. The reversal chamber sits inside the boiler, surrounded by water. This adds heating surface and removes the refractory lining that a dry back design needs.
Maxitherm builds both. Model DBS is a three-pass dry back boiler designed for small installations, burning gas, light oil or a combination. Model WBS is a three-pass wet back design with a fully water-cooled combustion chamber. You reach its reversal chamber through a small hinged manway. Both are in Maxitherm’s range of firetube boilers for oil and gas.
How a watertube boiler works
A watertube boiler reverses the arrangement. Water and steam flow inside the tubes, and the hot combustion gases flow around them. The tubes connect a lower water drum to an upper steam drum. Water in the heated tubes warms and rises, while cooler water falls through unheated downcomer tubes, so the water circulates on its own.
Each tube has a small diameter, so its walls can hold much higher pressure than a large shell. In power stations, Spirax Sarco puts watertube boiler pressures at up to 160 bar, with superheated steam at up to 550°C. Smaller packaged watertube units overlap with the firetube range.
Maxitherm’s Model Mini D / D is a two-drum, D-type watertube boiler for liquid and gas fuels. It’s available from 1 MW to 50 MW and delivered factory assembled and tested. See all watertube boilers for oil and gas.
Firetube vs watertube at a glance
| Factor | Firetube | Watertube |
|---|---|---|
| Heat path | Hot gases inside the tubes, water outside | Water inside the tubes, hot gases outside |
| Pressure | Up to about 27 bar. Most run at 17 bar or less | Up to 160 bar in power stations |
| Output | Up to about 27,000 kg/h, the largest shell boiler that can travel by road in the UK | Small packaged units through to very large power station boilers |
| Water content | Large. Stored energy helps the boiler cope with sudden demand | Small. Responds quickly to changes in load and heat input |
| Construction | Simple, usually fully packaged | More complex. Larger units need more site work |
| Maintenance | Simple construction keeps routine inspection and cleaning straightforward | More complex construction. Ask your supplier what feedwater quality the boiler needs |
| Typical uses | Hotels, hospitals, food and beverage, laundries, small to mid-size process plants | Power generation, palm oil and sugar mills, large process plants, high-pressure steam |
Pressure and output figures are general industry limits from Spirax Sarco, not Maxitherm model ratings. Ask us for the rating of a specific model.
When a firetube boiler is the right choice
A firetube boiler is usually the better fit when:
- Your steam demand is small to medium and fits within one packaged unit, or two for backup.
- Your pressure is moderate. Most hotel, hospital, laundry and food processing loads sit well within firetube limits.
- Your demand swings quickly. The large volume of hot water acts as a buffer when a process suddenly draws more steam.
- You want a simple installation. A packaged firetube boiler arrives ready to connect, which keeps site work and commissioning short.
- Your maintenance team is small. The simple construction makes routine inspection and cleaning easier.
When a watertube boiler is the right choice
A watertube boiler is usually the better fit when:
- You need high-pressure or high-temperature steam, for example to drive a steam turbine.
- Your steam demand is large, beyond what one or two firetube boilers can deliver.
- You burn solid fuel at scale, such as palm fibre, shell, bark or coal, which needs a large furnace and a grate or stoker.
- You need fast response to load changes in a large system.
What about biomass and solid fuels?
Fuel often settles the question. Both designs can burn biomass, but they suit different scales and fuels. Here is how the solid fuel boilers in Maxitherm’s range line up.
From the firetube boilers for biomass and solid fuel range:
- Model EKW. A small-capacity wet back boiler for wood chip, sawdust, briquettes and low-ash coal, with pneumatic fuel feeding and fixed high-velocity grates.
- Model C / UFM. An underfired multi-tubular boiler that can be equipped for a wide range of fuels or fuel mixes, and suits waste heat recovery.
From the watertube boilers for biomass and solid fuel range:
- Model DLF. High-moisture agricultural waste such as palm fibre and rice husk, burned on a fixed, inclined water-cooled grate.
- Model JDW. High-moisture wood waste such as green sawdust, bark and wood chip, on an inclined water-cooled grate, or a dumping grate where ash is above 6%.
- Model FDB / FDW. Briquettes, brown coal, wood and agricultural waste, with a pneumatic fuel feeder and balanced draught.
- Model TDC. Coal across a wide range of grades, on a louvre traveling grate stoker made by Maxitherm.
For palm oil mills, which burn their own fibre and shell, a watertube boiler with a suitable grate is the usual choice. Smaller plants with a steady supply of wood chip or briquettes can often use a solid-fuel firetube boiler such as Model EKW instead.
Questions to answer before you specify
Have these answers ready when you talk to a supplier. They make quotes comparable and help you avoid an oversized or undersized boiler.
- Peak steam demand in kg/h or tonnes per hour, including planned expansion.
- Working pressure and temperature at the point of use, and whether you need saturated or superheated steam.
- Fuel. Its type, availability and cost. For solid fuel, its moisture and ash content.
- Load pattern. Steady, cyclical or with sudden peaks.
- Space and access in the boiler house, including room to pull tubes for maintenance.
- Feedwater. Its source, quality and existing treatment.
- Compliance. Check which inspections and permits your boiler needs before it goes into service, and choose a supplier who can provide the drawings and documentation for them.
Sources
- Spirax Sarco, Shell boilers. Firetube construction, wet back and dry back, packaged boilers, stored energy, and the 27 bar, 17 bar and 27,000 kg/h figures.
- Spirax Sarco, Water-tube boilers. Watertube construction and circulation, the 160 bar and 550°C power station figures, and load response.
- Maxitherm Boilers, product range. Descriptions of Models DBS, WBS, EKW, C / UFM, Mini D / D, DLF, JDW, FDB / FDW and TDC.