The Foster Wheeler ESD III boiler is a compounded version of the ESD II type boiler. It is used when the final steam with superlative steaming conditions is required. It is asserted by recalling the previous knowledge that all ESD boilers are external superheater boilers with a single furnace arrangement and specific engineering apparatus for controlling the superheat temperature of the steam.
The ESD III units are mainly fitted in large tankers and give as much evaporative capacity as 100,000 kg/hr.
ESD III Boiler Design Modification
Like the ESD I design, the ESD III boiler-variation is provided with an attemperator: Between the primary and secondary superheater passes, attemperation of the steam is performed to regulate the final steam temperature needed at the inlet of the H.P. turbine. It is schematically illustrated below in Fig. 01.
Design and Construction
- It is provided with the attemperator between the two superheaters.
- The design of the furnace chamber is optimal: Being wider in area, it gives a conservative firing rate and is thus proportioned to provide ample spaces for the allocation of the burners to give sufficient flame clearances and flame length.
- In order to meet the required flame-length condition, the burners are provided on the roof of the furnace chamber.
- The furnace is surrounded by water-walls which are formed by close-pitched water-tubes to keep the refractory maintenance to a minimum.
- External downcomers are used to direct water-steam mixture from the steam drum to the water drum and lower front and rear water-wall headers. Likewise, water from the water-drum is supplied to the side water-wall headers through under-floor tubes. In this, the circulation of water in the boiler is maintained naturally under all possible steaming conditions.
- The superheater is of convection type. It is protected from the blow of hot gases coming from the furnace by a baffle of close-pitched generating tubes. The welded strips seal the gap between each tube.
- Likewise, the superheater side-wall refectory is also shielded from the hot gases by sealed close-pitched tubes.
- In the primary superheater, the steam flow is a counterflow to the gas flow.
- In the secondary superheater, the steam flow is parallel to the gas flow. It is done thus to minimize the tube-metal temperature.
- The economizer is of extended surface type. It is positioned above the primary superheater pass. The feed-water flow through the economizer is made counterflow to the gas flow.
- The economizer has two sections: one primary and the other secondary. The extended surface of the primary section is formed from the cast-iron grills and is called the low-temperature region of the economizer. Meanwhile, the extended surface of the secondary is formed from the steel-grills and is named the high-temperature section of the economizer.
- The economizer is given robust support in the boiler’s framework or through the ship’s structure.
- The design of the complete boiler structure is heavier and sturdier to carry heavy loads of the boiler itself and superheater convective-column.
- Compared with the economizer, which is single-cased, the boiler and the superheater are protected in the double-case. The intermediate spaces between the two are pressurized enough to prevent any loss of hot gas and, thereupon, any rise in the temperature of the casing.
- The refectory and the insulating linings are constructed out of monowalls known as membrane-walls. In the later version of the ESD III, the refractory-backed tangent tubes were replaced by gas-sealed all welded monowalls as shown in Fig. 02.
The overall working of the ESD III boiler is the same as that of the ESD I. The superheat temperature of the final steam is controlled by the assisted attemperator positioned between the primary and secondary passes of the superheater. As needed by the consumer of the boiler, the attemperation is provided either through drum-type desuperheater or by the external-spray-type setting.
Drum-type Desuperheater Control
If the attemperation of the steam is performed with the drum-type desuperheater, it follows with the proportion of steam after the primary superheater passes that is then diverted to the desuperheater placed inside the steam drum.
In the steam drum, the surplus heat from the diverted mass of steam is transferred to the water in the drum. Interestingly, now, this desuperheated steam is allowed to get mixed with the steam-flow that has bypassed the admission to the desuperheater before entry to the secondary superheater pass. The combined streams of steam now pass through the secondary superheat pass under all steaming conditions.
The metered amount of steam-flow to the desuperheater is controlled by the orifice plate in the by-pass line. Moreover, it can also be regulated by a control valve positioned in the line to the desuperheater. (fig. 03)
External Spray-type Desuperheater Control
In this modality of desuperheating, the steam flow after the primary superheater pass is allowed to pass through a desuperheater fitted and enclosed into a connecting steam pipe between the primary and secondary superheat pass.
While steam passes through the connecting pipe, high-quality feed-water is sprayed over the pipe in an attempt to remove the surplus heat from the steam, thereby desuperheating it to the required degree.
Special nozzles are provided in the desuperheater for water spray that is controlled by a sophisticated arrangement of sequential-valve settings, as shown in Fig. 04. The steam pipe is protected from erosion or thermal shock via a liner-pipe in the desuperheating area.
I am the author of Mechanical Mentor. Graduated in mechanical engineering from University of Engineering and Technology (UET), I currently hold a senior position in one of the largest manufacturers of home appliances in the country: Pak Elektron Limited (PEL).