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Exhaust Gas Boilers

What is the need of the exhaust gas boilers?

Why would someone invest in a industry if that industry does not give him profit in return? In the shipping industry the profit depends upon how efficiently the work is being done on board . Efficiency of any ship depends on the quality of its crew and the machines onboard. Among all the machines, the main propulsion plant of the ship is of utmost importance as far as the overall efficiency is concerned.

Exhaust gas boilers are one of the most important waste energy utilization system which are fitted on ships. They use the lost energy from the machinery systems to perform productive work, increasing the overall efficiency of the ship as a whole.

What is a Exhaust Gas Boiler?

The exhaust gas boiler may be considered as a huge heat exchanger that exchanges the heat energy between the exhaust gases from the main diesel engine to water and produces steam for the same purpose that auxiliary boilers produce steam.

The amount of steam generated by an exhaust gas boiler is usually more than enough for routine ship-board steam requirements, thus normally a ship with exhaust gas boiler does not need to fire the boiler while in passage.

Components of an Exhaust Gas Boiler

As seen the Fig 1 above, An Exhaust gas boiler comprises of the following components:

  1. A circulating pump which has to be kept continuously running at all times.The pump sucks the water from the bottom of the boiler and forces it through the exhaust gas heat exchanger tubes.
  2.  Outlet of the tubes is led back to the top of the steam drum of the boiler.The steam drum acts as a steam collector.
  3. A part of the saturated steam is also further heated in the exhaust gas boiler tubes thus generating superheated steam, which can then be used to drive a turbo generator .
  4. The feed pump has to be in normal operation to deliver make-up feed as required to maintain level in the steam drum.
  5. Safety valves are fitted to the boiler in order to prevent the pressure in the boiler exceeding the normal working conditions.

Types of Exhaust Gas Boilers:

There are various types of Exhaust gas boilers that can be found on board ships. Some of them are listed below:

  1. Aalborg Composite Boiler:

This boiler is a composite boiler, it has pressure shell in the form of three cylinders or drums. The lower drum contains the oil fired furnace, the centre drum which is made with two tube plates is essentially a water space whilst the upper drum is a steam and water space.

Exhaust Gas Boilers
Fig. Aalborg Composite boiler, Credits- Marine boiler by Flanagan

The tube plate of the lower drum and lower tube plate of the centre drum form the boundaries of the smoke box for the oil-fired section whilst the upper tube plates of the centre drum and the tube plate of the upper drum form the boundaries of the exhaust gas section. 

The three sections are connected by straight plain and stay water tubes, and the two large downcomers which connect the upper and lower drums are there  to promote efficient circulation.

  1. Osaka Water tube Composite Boiler:

It adopts round gas flow which allows higher boiler efficiency to be achieved . It’s simple structure makes boiler water circulation more effective . It has  wide steam space because of which high dryness steam can be generated . In case of trouble, one part of heating tubes is possible to exchange without cutting sacrificed heating tubes . Since it adopts insert type soot blower for exhaust gas side , high soot removing effect is achieved.

Fig. Osaka Water Tube Composite Boiler, Credits-Marine boiler by Flanagan
  1. Lamont Boiler:

The Lamont type boiler uses the waste heat from the main propulsion engine for the following purposes:

1.Air Preheating – (Cold air from blower to combustion chamber of Boiler)

2.Feed water heating -(Economizer-from Feed water pump to Boiler Drum)

3.Superheating– (Saturated Steam from steam drum is further heated to generate superheated steam which can be used for running Turbo generators and turbine driven feed water pumps)

4.Steam Generation–  ( Steam is generated and sent to steam drum of boiler using a radiant and convective method of evaporation)

Fig. Lamont boiler, Credits-Marine boiler by Flanagan

When can we use Exhaust gas boiler?

Exhaust gas boilers basically derive the heat from the waste steam from the main diesel engine and provide it to the water to produce  steam. This steam is used to meet the steam requirements on board .

An exhaust gas boiler can produce sufficiently enough steam to meet the steam demands on board ship. Therefore, there is no need to fire the auxiliary boiler during  maneuvering.

Function of the Exhaust gas economizer:

An exhaust gas economizer is present on board in order to improve the efficiency of the auxiliary boiler.

The exhaust gases from the main diesel engine are  still hot enough and can not be let out into the atmosphere, because that would be a waste. These exhaust gases are used to raise the temperature of the boiler feedwater. The feedwater thus arrives ito the boiler at an elevated temperature.Therefore,less energy is then required to produce steam.In broad terms it can be said that a 10 degree celsius increase in feedwater temperature will give an efficiency improvement of 2%.

Fig. Smoke tube type Economizer, Credits-Osakaboiler.co.jp
Fig. Water tube type Economizer, Credits- Osakaboiler.co.jp

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