They are popularly known as Shell boilers since in these boilers all the surfaces contributing towards heat transfer are enclosed inside a shell made up of steel. They can be also called as Smoke tube boilers. As the name suggests, in fire tube boiler systems, the hot exhaust gases emanating out of combustion reaction are passed through the boiler tubes. These tubes are in turn surrounded by the water which needs to be heated. Heat is then transferred to the circulating water via the hot gases produced by the burner. “In firetube boilers, the combustion gases pass inside boiler tubes, and heat is transferred to water on the shell side.”1
In this way the water gets converted into steam for utilization into various process applications. The most widely used boilers for industrial applications are Scotch marine boilers.
Scotch Marine Boiler
“Scotch marine boilers are typically cylindrical shells with horizontal tubes configured such that the exhaust gases pass through these tubes, transferring energy to boiler water on the shell side.”2
This type of fire tube boiler is considered to be an industry workhorse owing to its several advantages. Most common of them include:
- They are highly proficient and strong.
- Their use proves to be very economical because they involve very low initial price.
- These types of boilers usually hold quite huge quantities of water due to which they are capable of retorting to load changes occurring with reasonably small variations in pressure.
However, this category of boilers suffers from few limitations too which are listed below:
- Since, these types of boilers consist of large amount of water; they consume somewhat extra time to set off steaming operation and put up with variations occurring in steam pressure.
- In these types of boilers steam generation takes place in the shell portion which offers a big surface area. Due to this fact, the quantity of pressure generation tends to be less.
- Typically, the use of Scotch marine boilers is not recommended in areas requiring high pressures (normally beyond 300 psig).
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.”3
- 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.
Fire tube boilers come in variety of configurations. The two most common configurations include:
- Dry back boiler configuration
- Wet back boiler configuration
The above mentioned two-pass boiler configuration types are shown in the figure4
In the first configuration i.e. dry back boiler configuration, the reversal of hot combustion gases is done via a refractory lined chamber located at the external covering of the boiler.
The another one is wet back or water back boiler configuration which is considered to be extra efficient than the former one since in this kind of arrangement the main chamber for reversal of hot gases is totally enclosed inside the boiler. No refractory lining is required in this configuration since the turn around zone is water cooled in this case. Hence, it makes available larger area for transfer of heat. Also, it helps in heating boiler water at the side of the chamber wall where the heat generated from the furnace is usually found to be of maximum intensity.
The fire tube boiler systems mainly find their use in following areas:
- For operation of steam locomotives
- In small factories and industrial processes
- For home heating systems
Following are the major advantages offered by fire tube boiler plants:
- “The entire plant may be purchased as a complete package, only needing securing to basic foundations, and connecting to water, electricity, fuel and steam systems before commissioning.”5
In this way, the installation cost of fire tube boiler systems gets reduced.
- Also, due to their availability as packaged systems, they can be easily relocated from one place to another.
- The construction and maintenance routine of a typical fire tube boiler happens to be very simple.
- Fire tube boilers usually consist of a single furnace tube and a burner. Hence, the control systems meant for heating purpose inside these boilers is kept very simple.
- Due to their comparatively low operating pressures, the accessories needed to support fire tube boiler systems can be obtained at very economical rates.
- These types of boiler systems are found to be very fuel efficient. Besides, their operation is very easy.
- They provide very cost-effective heating solutions.
- Their cleaning procedure is very simple.
- They are available in compact size and form. They generally exist in size ranging from 600,000 btu/hr to 50,000,000 btu/hr
- They are designed in such a way that replacement of shell boiler tubes is very simple
- They are widely put into use for space heating purposes and variety of industrial applications.
Major disadvantages associated with fire tube boilers are mentioned below:
- As a general rule, the maximum output generated out of fire tube boiler is around 27 000 kg / h. In case of high steam requirements, one needs to combine multiple boilers together.
- Besides, the fire tube boiler systems are not capable of working with applications which involve high operating pressures i.e. beyond 250 psig. This limitation is experienced due to the large diameter cylindrical construction of fire tube boilers. For getting higher pressures, water tube boilers are generally employed.
- Also, these boiler systems are not considered suitable for processes where high capacity steam is required.