Waste heat recovery in motor ships
A diesel engine loses approximately 30% of the energy to the exhaust gases. A
turbocharger recover some of it, but there is still heat losses that can be
recovered by means of an Exhaust Gas Boiler, EGB. Utilization of the energy in
the exhaust gas is restricted due to risk of low temperature corrosion on the
gas side of the tubes. The sulphuric acid dew point temperature is assumed to
be at 130-140°C. A temperature difference of 40°C must be considered
in order to obtain a certain margin when the gas temperature decreases due to
low load on the main engine. This means that an EGB shall not be designed to
reduce the gas temperature at the EGB outlet below 170-180°C at Continuous
Service Rating. Besides, at gas temperatures below 170-180°C, the
vaporized unburned oil particles in the gas condense to an adhesive mixture of
soot and oil resulting in troublesome deposits on the tubes and on the heating
surface of the EGB.
The reasonable optimal steam production of an EGB where no economizer is
incorporated is consequently reached at a steam pressure of 3-4bar (saturated
temperature 143-151°C). In practice it is found that the pinch temperature
shall be kept at minimum 25°C as a lower temperature difference will
substantially increase the heating surface and the price of the EGB.
An EGB is normally built to
regenerate the heat from comparatively large flue gas quantities at a not very
high temperature and with a limited pressure loss. To fulfill these demands,
the boiler ought to be provided with tubes with extended surface on the
gas-side. This can be achieved by means of pinned tubes, as in this example, or
finned tubes.
Utilization of the energy in
the exhaust gas is restricted due to risk of low temperature corrosion on the
gas side of the tubes. The sulphuric acid dew point temperature is assumed to
be at 130-140°C. A temperature difference of 40°C must be considered
in order to obtain a certain margin when the gas temperature decreases due to
low load on the main engine. This means that an EGB shall not be designed to
reduce the gas temperature at the EGB outlet below 170-180°C at Continuous
Service Rating. Besides, at gas temperatures below 170-180°C, the
vaporized unburned oil particles in the gas condense to an adhesive mixture of
soot and oil resulting in troublesome deposits on the tubes and on the heating
surface of the EGB. 
An EGB is normally built to
regenerate the heat from comparatively large flue gas quantities at a not very
high temperature and with a limited pressure loss. To fulfill these demands,
the boiler ought to be provided with tubes with extended surface on the
gas-side. This can be achieved by means of pinned tubes, as in this example, or
finned tubes.