Key features of fire tube boiler systems include:
- Firetube boilers are often characterized by their number of passes, referring to the number of times the combustion (or flue) gases flow the length of the pressure vessel as they transfer heat to the water.
- Each pass enables the hot combustion gases to pass through the boiler tubes in the different direction. Every time, for conceiving next pass, the flue gases in the boiler make a shift of 180 degrees and go back via the shell.
- A typical three-pass boiler consists of three sets of boiler tubes whereas a four-pass boiler contains the four sets. The stack outlet in case of former one is normally positioned at the back end of the boiler while the latter one will have the stack outlet located at the front end.
- Various types of boiler tube arrangements are available for fire tube boiler systems depending upon the number of passes made by the hot flue gases emerging from the boiler furnace till the time they get cleared out of boiler system.
- In case of boilers made up of plain steel, the hot combustion gases going in to the reversal chamber are required to be cooled down to a temperature of 420°C prior to their entrance whereas in case boilers having alloy steel construction, this temperature should be at least 470°C. If the temperature of combustion gases happens to be more than recommended then problems like overheating and cracking of boiler tubes may take place.
- Since a fire tube boiler system consists of a considerable quantity of water within it maintained at saturation temperature point, it usually serves as significant energy storage ground to deal with very quick short term load applications. However, this fact can pose a major limitation also since re-buildup of energy reserve would be required which may consume large time.
- Nowadays, the largest shell boilers offering 1,500 boiler horsepower i.e. approximately 50,000 lbs/hr have been made available.
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