Fire Tube Boiler
A boiler is a closed vessel containing water, which by the application of heat, is converted into steam at a desired temperature and pressure. A boiler is often referred to as a ‘steam generator’.
As the boiler’s function is to generate steam from water, it must consist of the following:
There are two main types of boiler in use, these are the Fire Tube Boiler and the Water Tube Boiler.
Fire-Tube / Smoke Tube
In this type of boiler the combustion of fuel occurs within a confined space and the hot exhaust gases pass through the inside of steel tubes, which are surrounded by water.
In this type of boiler the water flows inside the steel tubes, which surround the combustion chamber. The tubing inside a water tube boiler may not only surround the combustion chamber, superheaters and primary air heaters also utilise the generated heat and are located outside of the main combustion chamber.
In this type of boiler combustion takes place in a furnace which is as far as possible surrounded by water. Tubes surrounded by water take the hot combustion gases away from the combustion chamber. As the burner and combustion chamber are contained within the water space, the result is a relatively large diameter vessel. If a boiler has a large diameter, it can only sustain small pressures due to the large hoop stresses generated on the shell. Steam pressures in this type of boiler are limited to approximately 20 bar(g) maximum with many fire-tube boilers have a safe working pressure below this. Higher pressures could be contained, but this would mean increased plate thickness', which would be difficult and expensive to manufacture, add greater weight, and result in poorer conduction of heat to the water (thus a drop in overall efficiency).
In this type of boiler the water and steam storage capacity is high, but the boiler occupies a large space and is heavy for the amount of steam generated. A large amount of heat is stored and a sudden stoppage of the steam demand results in heavy blowing of the boiler safety valves. Fire tube boilers operate best when operating slightly below their designed capacity. Boilers of this type are mostly used as auxiliary boilers on ships, or, for process steam required for production e.g. chocolate production and many other processes. Fire tube boilers typically operate with a working pressure of 7-18 bar(g) although vertically orientated high pressure boilers are often used for processes such as deoderising and can reach pressures exceeding 50 bar(g).
This type of boiler contains the water and steam within a system of drums and headers, which are interconnected by a large number of tubes. The hot combustion gases pass over the external surfaces of the water tubes on their path from the furnace to the funnel.
As the pressure vessels do not contain the furnaces, small diameters can be employed resulting in high pressures. Working pressures in excess of 50 bar(g) are common. The higher pressures allow for larger applications of the generated steam, typical consumers for high pressure steam are steam turbines.
The circulation of feed water and steam within this boiler is relatively quick, especially when the generating tubes are arranged vertically. Therefore steam can be produced rapidly from cold conditions (start-up), however, a minimum time of 3-4 hours is normally taken in order to protect the furnace refractory lining and the superheated sections. The minimum time required differs heavily depending upon the boiler, a large boiler could require several days before reaching maximum production capacity.
Steam and water storage capacity is relatively small compared with steam generation capacity. Due to the low storage capacity, sudden load changes can produce rapid changes in the water drum level.
Evaporation rates are 4-6 times greater than a fire-tube boiler of comparable size. The rate of heat transfer through the relatively thin walled tubes is high and thus only pure distilled water use is permitted to avoid scaling and overheating of the tubes. Many boiler failures are attributed to poor feed water quality, consequently the monitoring and conditioning of feed water is essential.
Example: Ship Exhaust Gas Boiler
The exhaust gases from a large diesel engine contain around 34% of the working energy liberated from the fuel. The temperature of this large volume of gas ranges from 280°c to 340°c . It would be pointless to waste fuel to maintain an oil-fired boiler to produce steam, when the vessel is at sea, if the heat from the engine exhaust gases could be utilised for the same purpose. This type of boiler is known as an exhaust-gas boiler although it belongs to the Fire Tube boiler family as the exhaust gases are fed through tubes surrounded by water.
For a variety of reasons only between 5-10% of the energy in the exhaust gases is recovered in the exhaust gas boiler and the working pressures are generally below 7 bar(g).
There are a number of exhaust gas boiler arrangements. In some the exhaust gas passes through the boiler permanently and this type can be operated with, or, without water. In others the boiler is bypassed when not in use.
Water tube or fire tube boiler?
Water tube boilers are typically installed for applications requiring a large amount of steam and higher operating pressures, this could include power stations and marine vessels etc. Fire tube boilers are fitted where steam volume and pressure demands are lower.
A ship driven by a steam turbine will have a water tube type boiler fitted as it requires large amounts of superheated steam to operate the turbines.
Power stations of moderate size are always installed with water tube boilers. Power stations operating with steam temperatures in excess of 500°C and steam pressures in excess of 55 bar(g) are not uncommon.
A diesel engine vessel could also be fitted with a water tube boiler, but only if it requires a large amount of steam for certain systems e.g. in the case of a large oil tanker where it would require large volumes of steam to heat the oil cargo in order to make discharge easier in colder climates. This type of ship would probably also be fitted with steam driven auxiliaries such as cargo pumps.
A diesel engine driven vessel requiring only small amounts of steam, such as a bulk carrier or container ship, would have electrically driven auxiliaries and would be fitted with a fire tube boiler probably of the ‘package type’.