Steam is also used in ships to cater to other heating requirements, some of which are listed below:
Heat requirements by all such services are calculated individually and added. The obtained heat requirement is termed Q3 (for the purpose of this article only).
Once the heat requirements for the three purposes (mentioned above) are obtained, they are added to obtain the Total Heat Rate and Total Steam Mass Flow Rate Required for the boiler:
Total Heat Rate Required (Q) = Q1 + Q2 + Q3 (kW)
The total Mass Flow Rate Required is calculated from the relation: mS = Q / ∆h (kg/hr)
Where ∆h = enthalpy drop of the steam (kJ/kg)
Now, there are two rating systems to obtain a suitable boiler:
From and At Rating:
In the above graph, the vertical axis corresponds to the steam output as a percentage of the form and at the rating, at different pressures. That is, for example:
At 15 bar,
If the feed water temperature is 68 degrees Celsius,
Then the percentage from and at rating from the graph is 90%
So if a boiler has a rated steam output of 2000 kg/hr, the actual steam output of the boiler will be 90% of the rated output, which is 1800 kg/hr.
Now, when a designer chooses a boiler, he/she needs to specify the rated steam output to the boiler manufacturer. The boiler manufacturer, along with the boiler, provides the boiler from and at the rating graph for the proposed boiler, and the above calculation is carried out for various boiler pressures and feed water temperatures, to check that the actual steam output is more than the steam flow rate (mS) obtained in the initial design calculations that we have previously discussed.
While some boiler manufacturers prefer from and at ratings, some others prefer another system called the Kilowatt rating system, which is however just a different way of expressing the same data.
In order to obtain the Actual Steam Flowrate from the kW rating of a boiler, the following relation is used:
In the above expression, the energy to be added refers to the amount of energy added to the boiler by the feed water (which in turn depends on the feedwater temperature).
The designer should make sure that the steam output obtained above is more than the steam flow rate (mS) obtained in the initial design calculations that we have previously discussed.
The above checks are to be carried out at various working pressures of the boiler, and different ranges of feed water temperature, depending on the steam requirement at various sailing conditions. It is to be ensured that the chosen boiler meets the requirements in all such conditions, at different load combinations.
The type of boiler to be used in the ship is also to be chosen by the designer based on the following criteria:
For most auxiliary boilers, shell and tube boilers are used, where the boiler drum holds the water reserve, and fire tubes run along the length of the drum.
The hot gases produced by the burner are carried in the fire tubes that provide more surface area for heat transfer to the water. In most cases, the auxiliary boilers are horizontally oriented in case there are no space constraints, as they prevent pressure fluctuations which is more in the case of vertically oriented boilers.
However, exhaust gas economisers or exhaust gas boilers (These are boilers that do not have a furnace. They have also fired tube boilers, where exhaust gases from the engine are passed through the fire tubes to heat the water in the boiler drum.) vertical configurations are preferable, as it provides less back pressure on the exhaust gas system. Exhaust gas boilers are used when the vessel is on the voyage, and when in port, the auxiliary boiler is used.