Fire tube boilers, also known as smoke tube boilers, are the most common type of industrial boiler employed today. Compared to water tube boilers, they are much more suitable for smaller plant applications because they can better cope with sudden high demand steam fluctuations.
Combustion occurs within a furnace with the exhaust gasses (flue-gasses) passing through a series of tubes prior to exiting through a chimney. Because the exhaust gasses pass through tubes submerged in water, the boiler is referred to as a 'fire/smoke tube boiler'. Water tube boilers have tubes full of water that are surrounded by exhaust gas i.e. the opposite of a fire tube boiler configuration.
Fire Tube Boilers
Boiler Parts and Functions
Many of a fire tube boiler parts are housed within a long cylindrical shell that serves as a pressure vessel. The shell is full of water with space at the top for steam liberation.
Tubes pass from one end of the shell to the other; this may occur once, or multiple times. The shell and tubes may be installed in a vertical or horizontal orientation, although the vertical orientation is less common.
Several of the tubes may have a thicker diameter than the standard tubes, these tubes are referred to as 'stay tubes'. Stay tubes reduce the mechanical stresses placed upon the tube sheets when the boiler is pressurised; stay bars may also be used for this purpose.
The furnace is where combustion occurs; it is the place where the highest temperatures within the boiler are reached. Furnaces are usually corrugated to increase their mechanical strength, although non-corrugated furnaces are not uncommon.
Furnace, Tubes and Shell
Tube sheets are used to seal both ends of the shell and to provide a place for mounting of the tubes. Tube sheets will usually be connected to the shell via stay bars.
Fire Tube Boiler Tube Sheets
Fuel, air and heat, must be supplied to the furnace in order to achieve combustion. Fuel and air is fed into the furnace by the burner. A source of ignition is provided by an igniter (an electrode that creates an electrical arc, similar to a spark plug but bigger). Typical fuels include light fuel oil (LFO) and methane gas, although others are available. The 'front' of the boiler is located where the burner is mounted.
Burners Mounted Onto Boilers
There may be a single, or multiple, reversal chambers. Reversal chambers change the direction of the exhaust gasses as they exit one pass and are directed into another. Reversal chambers located away from the burner are 'rear reversal chambers' whilst those closest to the burner are 'front reversal chambers'.
The smokebox is the final part of the boiler that the exhaust gas passes through before exiting to the chimney.
Smokebox and Chimney
Fire Tube Boiler Designs
It is possible to vary the boiler design in several ways:
- Wet back - the rear of the furnace is surrounded by a water jacket.
- Dry back - the rear of the furnace is surrounded by sheet metal only.
- Steam dome - a dome mounted onto the top of the boiler shell to aid steam liberation.
- Number of furnaces - a boiler may have between one to three furnaces (general rule).
- Furnace design - corrugated or non- corrugated. The below image shows a corrugated furnace i.e. the furnace is not a straight cylinder.
Corrugated Furnace Indicated
- Number of passes - each time exhaust gas passes through the boiler, is referred to as a pass. Fire tube boiler typically have three or four passes, although one and two pass boilers are also in use.
Boiler 1st Pass
How do fire tube boilers work?
The below video is an extract from our Introduction to Steam, Boilers and Thermodynamics Online Video Course.
Air and fuel (usually light fuel oil (LFO) or methane gas) are fed into the furnace by a burner. The air/fuel mixture is ignited by an ignitor and combustion begins.
Heat from combustion is transferred immediately to the furnace wall and the surrounding water. Between 40-60% of all the heat generated in the boiler will be transferred to the water from the furnace, with the remainder being transferred to the water via the tubes. The furnace represents the 1st pass through the boiler.
Exhaust gasses from combustion then travel to the first reversal chamber. The gasses exit the reversal chamber and then enter the 2nd tube pass, second reversal chamber, 3rd tube pass, smokebox and finally the chimney.
Water is heated by the heat from combustion and the resultant exhaust gasses. As the water in the shell is heated, steam is liberated at the top of the boiler shell. Steam continues to accumulate until the desired pressure and temperature are reached.
3D Model Components
This 3D model shows all major components associated with a typical three pass fire tube boiler, these include:
- Shell – fired pressure vessel.
- Burner – used for combustion.
- Corrugated furnace – where combustion occurs.
- Tubes – used to direct exhaust gasses through the shell.
- Tube sheets – used to hold the tubes in position.
- Reversal chamber – gasses are reversed in this space and sent back in the opposite direction.
- Cover – retains insulation which increases the thermal efficiency of the boiler.
- Smokebox – located where the exhaust gasses exit the final tube pass, but before the chimney.
- Safety valves - used to relieve system pressure in the event of over-pressure.
- Gauges - local and remote gauges used for monitoring temperature, pressure and flow.