Steam Boiler: Fire Tube Boiler

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Fire Tube Boiler Types

Fire tube boiler is one of boiler type in which hot air from combustion heats one or more pipes that contain water in the boiler. Fire tube boiler is one of two main types of existing boilers. Whereas the other type of boiler is water tube boiler. Fire tube boiler can be classified based on configuration such as horizontal fire tube boiler and vertical fire tube boiler. Boiler type is also often referred to as smoke tube boiler or boiler shell.

The types of fire tube boilers can be seen as following below:

• Cornish Boiler, boiler with single combustion flue container.

Figure 1: Cornish Boiler

• Lancashire Boiler, Boiler with double combustion flue container.

Figure 2: Lancashire Boiler

• Locomotive Boiler, boiler that has a lot of fire tubes.

Figure 3: Locomotive Boiler

Cause of Failure in Fire Tube Boiler

Fire tube boiler is the type boiler which is classified based on tube type. The combustion occurs and flow through inside tube and boiler water is distributed outside tubes. When fire tube boiler is running, there are possibilities of failure occur. Failure should be prevented to avoid damage in boiler itself and environment. Failure can be caused by water level in boiler exceed minimum low water level. This condition may be caused by operator who careless to maintain fire tube boiler keep in normal operating water level (NOWL). In modern boiler, fire tube boiler is designed automatic. If water level reaches minimum low level, fire tube boiler will sound alarm and automatically shut down.

Another cause of failure in fire tube boiler is the boiler age is very old. Material of boiler is fatigue so cause material strength can not resist the pressure. This condition will make pressure part such as tubes and header explodes. Furthermore other reasons for failure are may be safety valve does not work if the pressure exceeds maximum allowable working pressure. Safety valve does not open to vent steam so the pressure will be accumulated and lead to failure.

Too much scale in boiler water so heat is not transferred perfectly. This condition is similar with too much slag or smoky on inside tube or flue gas way. Heat transfer does not perform as expected condition. When metal temperature increases up to 600 F, the metal will be shrinkage and possibility of failure will be highly occur.

Advantage & Disadvantage Fire Tube Boiler

Each steam boiler type has its advantages and disadvantage as well as with the type of fire tube boiler. The following are advantages and disadvantages if the owner of power plant choose fire tube boiler as his boiler.

Advantages of fire tube boiler:
1. The water is supplied in shell and outside tubes while hot gas is supplied inside tubes so the water volume can not be shaken easily when the fire tube boiler is running.
2. Fire tube boiler is so easy to use, operate, clean and maintain
3. Fire tube boiler can be used in small scale industries.
4. Fire tube boiler is relatively cheaper than water tube boiler.

Disadvantages of fire tube boiler:
1. From the furnace combustion side, required time to fill water is longer than to increase temperature and pressure.
2. The efficiency of heat transfer (heat transfer efficiency) is bad enough because of the heat exchanger does not use thermal radiation.
3. In case of bombers fire tube boiler would be very dangerous if a large amount of hot water and steam have been accumulated inside (leakage occur).
4. The fire tube boiler can not produce steam at a pressure higher than 250 pounds per square inch.
5. Capacity of generated steam is limited.

Fire Tube Boiler Based on Configuration

Fire tube boiler is one of type boiler which is classified based on tube type. Fire tube it self can be classified based on configuration as Horizontal Fire Tube and Vertical Fire Tube. The following below is its classification:

1. Vertical Fire Tube Boiler

Figure 1: Vertical Fire Tube

Verictical fire tube consists of a bundle of tubes, pipe, shell and head. The heat from gas combustion flows inside tubes. Combustion chamber and nozzle may be located on the bottom side or middle of boiler. The fire combustion burn in the middle of the shell boiler with a fire hose. Volume water is maintained approximately 80% of theigh of shell.

The most suitable fuel for this boiler is gas because it is cheaper than the price of oil fuel and burn cleaner than diesel fuel. The range capacity of the boiler is between 10-1250 kg/hr and maximum allowable working pressure (MAWP) is not more than 1 MPa. It is ideal to use as a small boiler to produce steam, up to 800 kg / hr.

2. Horizontal Fire Tube

Figure 2: Horizontal Fire Tube

Horizontal fire tube boiler is type of fire tube which is configured as horizontal model (see figure 2). The firing flows inside tube and the water outside tubes. This type can generate evaporation 100-12000 kg/hr and pressures up to 1-2 MPa (150-300 psig). Regarding to the large diameter of the shell boilers, structural support is designed to enough strong to resist load of boiler. If the steam pressure increases, so steam temperature will be higher. The size of this boiler also has a steam capacity between 500-1200 kg/hr and pressures up to 1-2 MPa (150-200psig).

Fire Tube Boiler – Working Principle

Based on use classification, fire tube boiler is categorized as for process or heating boiler. It has range pressure not exceed 450 psi (small scale or medium scale). Design fire tube is regulated in ASME BPV Section I subpart PFT. Fire tube consists of tube and tube sheet.

Figure 1. Fire Tube Boiler

Fire tube design use tubes to flow hot gases and distribute it to the safe point of discharge through stack. Water at the outside of tubes is heated by hot tubes, so there is heat transfer between hot gases inside tubes and water outside tubes. Water is converted to steam for process purpose.

As long as combustion process there are some passes usually used. At the first pass, the hot gases in distributed in fire tube boiler through furnace. In Figure 1, the rear head of lower section had sealed the hot gas from first pass and direct it to the tubes at the second pass to front head. The hot gases from front head is redirected to the third pass then directed again to the fourth pass and the end the hot gas is discharged through stack.