Optimisation of Marine Boilers using Model-based Multivariable ...

20 2. THE MARINE BOILER PLANT
heat absorbed in the WHR boiler for the same engine load depends on the outside
temperature (e.g. Arctic waters or the tropics). A typical WHR boiler for a 4000 kg
h
oil-fired boiler has a steam production in the range of 500 − 4000 kg
h of steam.
2.1.2 Operational situation
There is a collection of different operational situations for the marine boiler plant. This
gives a number of different states the system can be in. These states are sketched below:
�������� WHR
OF
off on start-up shutdown
off X11 X12 X13 X14
on X21 X22 X23 X24
start-up X31 X32 X33 X34
shutdown X41 X42 X43 X44
where OF means oil-fired boiler. The WHR boiler produces steam as long as the engine
is running, and start-up and shutdown of this boiler are completely dependent on the
engine operating condition. When to start or stop the oil-fired boiler is currently controlled
by monitoring the pressure. On/off control of the oil-fired boiler is necessary in
situations where the steam consumption is just above the capacity of the WHR boiler,
or if the WHR boiler is off when the steam consumption is lower than the minimum
capacity of the burner.
2.2 Water-steam Circuit
Here we look at the water-steam circuit for the marine boiler configuration: one oil-fired
boiler and one WHR boiler. The purpose is to introduce the most important terminology
used in the thesis and to make the reader more familiar with marine boiler plants. In
Figure 2.1 a principle diagram of the water-steam circuit is shown. Only the valves
important for the present work are presented, and none of the redundant systems have
been displayed.
The numbers in the figure have the following interpretation:
1 Oil-fired boiler.
2 WHR boiler.
3 Steam dump condenser.
4 Hot well. This works as the open feed water tank and further collects the condensate
returned from the consumer or dumped by the control system. The water in
the hot well is kept at a temperature > 80 ◦ C in order to increase boiler efficiency