In order to utilize the waste heat or exhaust gas that is coming from the main engine, exhaust gas economizers are built and are located after the turbocharger. The exhaust gas from the main engine passes through the water tubes and generates steam whilst heating the tube.
With the use of an economizer, a great amount of fuel can be saved from firing the auxiliary or composite boiler during the voyage to generating steam.
However, during the port stay or at anchor, we can’t rely on the exhaust gas economizer since there is no exhaust coming from the main engine because it is not running. In this situation, the auxiliary or composite boiler will automatically run to maintain the steam pressure that is needed on the ship.
Another situation for the exhaust gas economizer is during maneuvering, the auxiliary or composite boiler will run together with the exhaust gas economizer to aid and supply the lacking steam pressure that is generated from the exhaust gas economizer.
Generating steam onboard is very essential in order to heat heavy fuel oils to their desired temperature and achieve the right viscosity. Also, steam is used to pre-heat water for the accommodation or used as a heater during cold weather. Without steam to heat the fuel, there will be problems with the engine’s combustion and atomization thus making it unable to give the engine efficiency.
What is the difference between a boiler and an economizer?
The boiler and economizer have the same purpose, which is to produce steam onboard. However, these two have different ways of generating steam.
The boiler uses a burner to generate steam. Burners of the boiler mix up fuel and air to make sufficient combustion and make a high degree of combustion efficiency. Mostly, there are a series of settings to fire a burner in a sustainable manner because if there is too much air on the mixture, it would cool the furnace and heat would be neglected.
Meanwhile, if there is less air in the ratio, the combustion will be incomplete and the unburnt fuels will be moved over and more smoke will be produced.
The burner is fitted with a fan which is made to produce high steady air pressure that is required to get good combustion and an efficient ignition for the modern combustion chamber. A servomotor is equipped to control the amount of air that is needed for the mixture of fuel-air ratio.
Economizer uses the exhaust gas of the main engine to be able to produce steam. The exhaust gasses heat the water tubes inside the economizer, the water boils continuously and generates steam after.
However, exhaust gas economizer water tubes should be cleaned thoroughly and routinely because the soot that is brought up by the exhaust gas will eventually stick on the pipes and may become thicker over time. The soot on the pipes will lessen the efficiency of the steam production because it will block most of the heat transfer needed from the exhaust gas to the pipes.
Safety equipment is also installed on both economizers and boilers to avoid damage during running. These are the following:
- Safety valves – for protection against excessive pressure
- Pressure regulating valves – for maintaining a constant pressure
- Pressure reducing valves – for reducing pressure
- Temperature control valves – for controlling temperatures
- Steam traps – for separating condensate from steam
- Feedwater control valves – for controlling levels
How does the exhaust boiler economizer generate steam pressure?
Inside the economizer is a series of pipes that is arranged accordingly. These pipes contain water that is later on heated by the exhaust gas coming from the man engine while running. The exhaust gas flows over the tube stacks and heats the water, thus producing steam.
What is the purpose of an exhaust gas economizer?
Exhaust gas economizers are built on board to lessen the consumption of fuel for producing steam. It utilizes the waste heat or the exhaust gas of the main engine by running it passes through a series of arranged water pipes and heating it.
\As the exhaust gas passes through the pipes, it creates a heat transfer thus heating the water until it boils and generates steam. In this manner, the charterers and the owners can save enough fuel during the ship’s voyage.
What are boiler uptake and exhaust gas economizer fire?
There are different kinds of exhaust gas economizer fires that may occur on board. They may happen on every ship that’s why every crew must be attentive enough and clean the economizer to prevent these fires onboard.
Normal Soot Fire
Soot from the exhaust gas can build up on the water tube pipes inside the economizer. During at slow-speed, the exhaust temperature of the main engine may vary from 150oC to 250oC and this temperature is enough to ignite “wet soot” whose ignition temperature is around 150oC.
Meanwhile, if the soot is “dry”, it will not ignite unless the temperature of the exhaust gas reaches 300oC with the help of excess oxygen from the deposits of combustible materials that liberates sufficient vapor which can ignite a spark or a flame.
Hydrogen fire in an economizer may occur when there is a chemical reaction between carbon monoxide and hydrogen which happens when the water separates or dissociates at a temperature of more or less 1000oC.
2H2O= 2H2 + O2 (separation of water leading to the formation of hydrogen)
H2O + C =H2 + CO (reaction of water with carbon deposit leading to the formation of carbon monoxide)
At this stage, the chain reaction of oxidation of iron metal starts at a high temperature of 1100oC which means at such a high temperature the tube will start burning itself, leading to a complete meltdown of tube stacks.
Actions to stop and prevent economizer fire
There are ways to prevent fire on the economizer. Some of these are regularly done on routine maintenance while others are orders and instructions directly from the office.
Steps for Prevention of Fire
- A regular soot blow should be done on the boiler tubes
- Make sure to ensure good fuel combustion in the main engine to lessen soot production
- If possible, minimize the slow steaming of the main engine
- The feedwater circulating pump should not be turned off at any time while the main engine is running to avoid overheating the pipes which may lead to a fire.
- Ensure fuel is treated and is of good quality while supplying to the engine
- Start the circulating pump 2 hours before starting the main engine
- Let the circulating pump run for at least two hours after the main engine is stopped
- Do water washing in ports at regular interval
In case of preliminary fire on the exhaust gas economizer, stop immediately the main engine and remember always that the feed water circulating pump must run continuously. Avoid using soot blowers for fire-fighting as they will accelerate the fire inside.
Also, make sure that the exhaust valves and the turbochargers of the main engine are closed and covered to cut any possible air supply that may feed the fire. If water or steam washing is fitted, this may be used to extinguish the fire, along with external boundary cooling of the economizer.