Steam Boilers
1. Fire Tube Boilers
In a fire tube boiler, hot gas passes through the pipes and boiler feed water is inside the shell to be converted into steam. Fire tube boilers are usually used for relatively small steam capacities with low to moderate steam pressure. As a guideline, fire tube boilers are competitive for steam rates up to 12,000 kg/hour and pressures up to 18 kg/cm. Fire tube boilers can use fuel oil, gas or solid fuels in their operations. For economic reasons, most fire tube boilers are constructed as “packaged” boilers (factory assembled) for all fuels.
2. Water Tube Boilers
In a water tube boiler, boiler feed water flows through the pipes into the drum. The circulating water is heated by the combustion gas to form steam in the steam area in the drum. This boiler is chosen if the steam requirement and steam pressure are very high as in the case of boilers for power generation. A very modern water tube boiler is designed with a steam capacity of between 4,500-12,000 kg/hour, with very high pressure. Many water tube boilers are constructed in packages if fuel oil and gas are used. For water tube boilers that use solid fuel, it is not common to design packages.
The characteristics of the water tube boiler are as follows:
Forced, induced and balanced drafts help to increase combustion efficiency
Less tolerant of the quality of water produced from water treatment plants
Allows for a higher level of heat efficiency
3. Boiler Package
It is called a packaged boiler because it is available as a complete package. When shipped to the plant, it only requires steam piping, water piping, fuel supply and electrical connections to operate. Boiler packages are usually of the shell and tube type with a fire tube design with high heat transfer both radiation and convection. The characteristics of packaged boilers are:
The small combustion chamber and the high heat released result in faster evaporation.
The large number of small diameter pipes makes it have good convective heat transfer.
Forced or induced draft systems produce good combustion efficiency. A number of passes/pass results in better overall heat transfer.
The level of thermal efficiency is higher compared to other boilers.
The boilers are grouped based on the number of passes, namely the number of times the combustion gas passes through the boiler. The combustion chamber is positioned as the first passage after which one, two, or three sets of fire pipes are then placed. The most common boilers in this class are three-pass units with two sets of fire-tubes and the exhaust gases exit the back of the boiler.
