This section describes the various types of Boilers: Fire tube boiler, Water tube boiler, Packaged boiler, Fluidized Bed Combustion Boiler, Atmospheric Fluidized Bed Combustion Boiler, Pressurized Fluidized Bed Combustion Boiler, Circulating Fluidized Bed Combustion Boiler, Stoker Fired Boiler, Pulverized Fuel Boiler, Waste Heat Boiler and Thermic Fluid Heater.
Fire Tube Boiler
In fire tube boiler, hot gases pass through the tubes and boiler feed water in the shell side is converted into steam. Fire tube boilers are generally used for relatively small steam capacities and low to medium steam pressures. As a guideline, fire tube boilers are competitive for steam rates up to 12,000 kg/hour and pressures up to 18 kg/cm2. Fire tube boilers are available for operation with oil, gas or solid fuels. For economic reasons, most fire tube boilers are nowadays of “packaged” construction (i.e. manufacturers shop erected) for all fuels.
Water Tube Boiler
In water tube boiler, boiler feed water flows through the tubes and enters the boiler drum. The circulated water is heated by the combustion gases and converted into steam at the vapour space in the drum. These boilers are selected when the steam demand as well as steam pressure requirements are high as in the case of process cum power boiler / power boilers.
Most modern water boiler tube designs are within the capacity range 4,500 – 120,000 kg/hour of steam, at very high pressures. Many water tube boilers nowadays are of “packaged” construction if oil and /or gas are to be used as fuel. Solid fuel fired water tube designs are available but packaged designs are less common.
The features of water tube boilers are:
Forced, induced and balanced draft provisions help to improve combustion efficiency.
Less tolerance for water quality calls for water treatment plant.
Higher thermal efficiency levels are possible
The packaged boiler is so called because it comes as a complete package. Once delivered to site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become operational. Package boilers are generally of shell type with fire tube design so as to achieve high heat transfer rates by both radiation and convection.
The features of package boilers are:
Small combustion space and high heat release rate resulting in faster evaporation.
Large number of small diameter tubes leading to good convective heat transfer.
Forced or induced draft systems resulting in good combustion efficiency.
Number of passes resulting in better overall heat transfer.
Higher thermal efficiency levels compared with other boilers.
These boilers are classified based on the number of passes – the number of times the hot combustion gases pass through the boiler. The combustion chamber is taken, as the first pass after which there may be one, two or three sets of fire-tubes. The most common boiler of this class is a three-pass unit with two sets of fire-tubes and with the exhaust gases exiting through the rear of the boiler.