Foster Wheeler ESD-I Boiler: Merits, Demerits, Applications

It is a D-type boiler designed by Foster Wheeler some 35 years ago. It falls in the category of the D-type because it is a single furnace boiler and the nest of the boiler tubes is arranged in a manner that it takes up a D-shape geometry as in the case of D and DSD versions of the Foster Wheeler boiler class.

Boiler Specification Facts

The ESD-type boiler is designed to have an evaporative capacity ranging from 13500 kg/hr. to 118000 kg/hr. at 52 bar steam pressure and superheat temperature as high as 516°C.

What does ESD stand for?

In the catalog of Foster Wheeler boilers, the ESD stands for External Superheater D-type boiler. It is called so because the superheater is situated external to the bank of generating tubes yet before the economizer unlike in D and DSD types in which the superheater is fitted inside the boiler tube mesh: between the banks of generating and screen tubes.

Design Modifications to the D-type Boiler for ESD I Boiler

It is a more sophisticated form of D-Type boiler. ESD stands for external superheater D-type boiler. As is implied by its name, in this type, the superheater is located after the bank of main generating tubes in the direction of the gas flow yet below the economizer. It is shown below.

design modifications to d type boiler for esd i boiler
Fig. 01

Between the first and second superheater passes an air attemperator is provided to regulate the temperature of the final superheated steam. This brand of foster-wheeler genius is advantageous in terms of controlling the steam temperature at the superheater level through the air-assisted attemperation of the steam.

The air-attemperator arrangement is useful in terms of its added benefit in the reduction of metal-tube temperature which is a characteristic of all water-tube boilers that operate at the superheat temperature of the steam.

Moreover, such an arrangement also prevents the slagging as well as corrosion of the supports provided near the super-heating zone of the boiler which is located just a few rows away from the furnace.

So where exactly is the air-attemperator situated? It is placed in the combustion air duct. Steam is provided for attemperation to the air-attemperator after it completes the first superheater pass.

In the air duct, the air coming from the air-heater is allowed to pass through the air-duct where an air-attemperator is provided. This air acts as a coolant though it is hot.

esd i boiler
Fig. 02

There are multiple means by which the quantity as well as the direction of this hot air is managed: a simple interlocked air-bypass route splits the incoming air into divided streams or the shut-off dampers would stop the flow of air within the ductwork either partially or fully depending upon the final superheat temperature of the steam between the two superheater passes. The control of superheat temperature via air-attemperation is either thermostatically managed or through manual handling.


  1. Since the generating tubes are of relatively larger diameter and consist of few rows, their maintenance is not a difficult task as such.
  2. Because the superheaters lie outside the main tube-bank, the whole assembly can be easily cleaned, examined, and water-washed.
  3. The external superheaters in the low gas zone also benefit in terms of commonplace tube-plate problems as well as minimal slagging and flame impingement.
  4. The superheat temperature of the steam is well controlled through the attemperation mediated by air.
  5. The option is available for low-temperature steaming.
  6. The small yet optimal size of the combustion chamber.


  1. The air-attemperation method for controlling the superheat temperature of the steam offers some limitations when put into practice such as more space, additional steam piping, weight, fan-power, and elevated capital.
  2. Another draw-back associated with the ESD-I design is increased heat input to the superheater at higher loads and subsequently more attemperation for removing any excess heat provided in the first superheat pass.


A number of vessels including large passenger liners are fitted with the ESD I boiler design with evaporative capacity as high as 118000 kg/hr.