Exhaust gas boiler (EGB or EGE) or economiser described in this article is a smoke tube boiler with steam space, used for heat recovery from main engine exhaust gas. An illustration of the boiler is shown in figure below. The exhaust gas boiler is designed as a vertical boiler with a cylindrical shell surrounding the boiler tubes and water drum as well as the steam space. The tubes consist of a large number of smoke tubes and a small number of stay tubes. The stay tubes with an increased diameter act as support for the boiler. Both types are welded onto the lower and upper tube plates. The steam space is formed by the shell plate and internal cone. At the top it is closed by means of the end plate.
In the exhaust gas boiler tubes, heat from the main engine exhaust gas is transferred to the water side by convection. On the water side, the heat is transferred by evaporation of the saturated water adjacent to the tubes where steam bubbles are formed. As the steam bubbles have a much lower specific density than the water, they will rise rapidly to the steam space where water and steam are separated. The steam space of the boiler is designed to absorb the shrink and swell volumes. It is advisable, however, to avoid sudden and large load variations as this might create instability in the steam system, and cause level alarms.
As the pressure part is made of mild carbon steel with elevated temperature properties stress concentrations in corner welding are minimised. In emergency mode the exhaust gas boiler can therefore be operated with low water level and even without water with the full exhaust gas flow through the boiler tubes, provided the boiler is operated depressurised and the inside temperature does not exceed 400˚C.
For inside inspection, the boiler is arranged with both hand holes and manholes. Two manholes are arranged at the bottom end of the boiler shell for convenient access into the water drum. For visual check of the smoke tubes and steam space, a number of hand holes are arranged in a suitable distance at the bottom and top of the boiler shell.
The boiler foundation consists of four foundation consoles giving the necessary support and absorption of the thermal expansion.
Finally, the heating surface dimensions of the boiler are designed to maintain a sufficient exhaust gas velocity giving the best self-cleaning effect within the design limits. However, after long term operation soot deposits can be accumulated inside the boiler tubes. The heating surface of the boiler can be cleaned easily by means of water washing or by adding soot remover into the exhaust gas flow using compressed air.
Exhaust gas-piping system for marine engine:
The exhaust gas piping system conveys the gas from the outlet of the turbocharger(s) to the atmosphere. For designing the exhaust piping system, following important parameters must be observed:
- The exhaust gas flow rate
- Maximum back force from exhaust piping on turbochargers
- Exhaust gas temperature at turbocharger outlet
- Maximum pressure drop within the exhaust gas system
- Maximum noise level at gas outlet to atmosphere
- Sufficient axial and lateral elongation ability of expansion joints
- Utilisation of the heat energy of the exhaust gas.
The Exhaust gas from the cylinder unit is sent to exhaust gas receiver where the fluctuating pressure generated from different cylinders are equalised. From here, the gases which are at constant pressure are sent to turbocharger where waste heat is recovered to provide additional scavenge air to engine.
The most important thing to consider while designing the exhaust piping system is the back pressure on the turbocharger. The back pressure in the exhaust gas system at specified Maximum Continuous Rating (MCR) of engine depends on the gas velocity, and it is inversely proportional to the pipe diameter to the 4th power. It is general ship practice to avoid excessive pressure loss within the exhaust pipes, the exhaust gas velocity is maintained about 35m/sec to 50m/sec at specified MCR. The other factors which affect the gas pressure are the installation of EGB, Spark arrestor etc. in the path of exhaust gas travel.
At the specified MCR of the engine, the total back pressure in the exhaust gas system after the turbocharger (as indicated by the static pressure measured in the piping after the turbocharger) must not exceed 350 mm WC (0.035 bar). In order to have a back pressure margin for the final system, it is recommended at the design stage to initially use a value of about 300 mm WC (0.030 bar)