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Selection Of Marine Boilers During Ship Design

Selection Of Marine Boilers During Ship Design: technology, equipment and fleet context for US, UK, Canada, Australia, Singapore and European maritime...

Marine Insight 360· Jul 1, 2025· 8 min read

How To Select Marine Boilers During Ship Design

Today, marine boilers used on ships are primarily employed for auxiliary purposes. That is true above all on ships equipped with marine diesel engines or diesel-electric propulsion systems.

For ships using steam turbines, the boiler is an integral part of the main propulsion system. Those vessels are typically high-speed naval vessels. However, this article will focus on auxiliary boilers. These are the boilers used to power the auxiliary systems of the ship.

From the ship designer's perspective, the appropriate type of boiler must be selected for each ship. That selection is based on the project requirements.

This process applies the basic principles, but in a slightly different way.

To evaluate the performance of the boiler, the required boiler steam output must first be estimated correctly. That output is the one needed for the ship being designed. There are three main requirements for this:

Requirement 1 - Steam consumption required to compensate for heat losses in the oil tanks.
Requirement 2 - Steam consumption required to increase the temperature of the fuel oil in the oil tanks.
Requirement 3 - Steam consumption for other services.

We will discuss these requirements one by one. After completing them, we will see how to use the collected data to estimate the boiler capacity.

Requirement 1 - Steam consumption for heat loss from fuel tanks:

Start with the fuel itself. Most diesel-powered vessels are equipped with fuel tanks for storing heavy fuel oil (HFO). Due to the high viscosity of HFO, the density of stored HFO is roughly equivalent to tar. Its high viscosity makes it difficult to flow.

The stored HFO must still be transferred to the settling tank and then to the HFO service tank. For that, the viscosity must be maintained at a level that allows for easy flow. HFO tanks are therefore equipped with heating coils. The coils maintain the fuel oil at a specified temperature.

The heating fluid in the heating coils is steam generated by the auxiliary boilers.

First, locate each HFO tank on the general layout diagram. Then identify the area around each tank bulkhead. Each compartment baffle in the analysis has a heat transfer ambient temperature. That temperature is fixed according to the surrounding environment of that baffle. The surrounding environment may be the engine room, a vacuum tank, a ballast water tank, a sludge tank, etc.

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Next comes the steam flow. The flow required to maintain the fuel temperature in each tank is calculated as follows:

Tank baffle heat loss.
Q1 = UA (T2 - T1).

Where:

Qb = Baffle heat loss (W).
U = Total heat transfer coefficient (W/m² °C).
A = Baffle area of the tank under study (m²).
T2 = Tank temperature to be maintained (°C).
T1 = Temperature of the medium near the baffle under study (°C).
Tank heat loss Qt = Sum of heat losses from all six tank baffles.
Q1 = Sum of heat losses from all tanks.

The heat transfer rate is now known. The steam mass flow rate can therefore be calculated as follows:

ms = Q1 / ∆h.

Where:

ms = Steam mass flow rate (kg/s).
Q1 = Calculated heat transfer rate (kW).
∆h = Steam enthalpy drop (kJ/kg).

Requirements 2: Steam consumption required to raise the temperature of the fuel in the tank:

Steam has a second job here. It does more than compensate for heat losses in the fuel tank. It also heats the fuel to the required temperature before it is used in the engine.

Time (t) is the number of hours required to heat each fuel tank. It is calculated as follows:

∆T/t

Storage tanks: 0.2°C/hour temperature rise. Service and settling tanks: 4°C/hour temperature rise. All other tanks: 1°C/hour temperature rise.

Practical considerations

This calculation involves two steps.

Calculate the amount of heat required to heat the contents of each tank (Q, in Watts). Then add all the individual heat requirements together. That gives the total heat transfer required to raise the tank fuel temperature (Q2).

Using the above heat requirements, calculate the required steam mass flow rate.

The heat required to heat the fuel tank can be expressed as follows:

Q2 = m Cp dT / t.
Where Q2 = Average heat transfer rate (kW).
m = Mass of fuel in the tank (kg).
Cp = Specific heat capacity of fuel (kJ/kg°C).
dT = Change in fuel temperature (°C).
t = Total time required for the heating process (hours).

Here too the heat transfer rate is known. The steam mass flow rate can be calculated using the following formula:

ms = Q2 / ∆h.
Where ms = Steam mass flow rate (kg/h).
Q2 = Calculated heat required for heating (kW).
∆h = Steam enthalpy drop (kg/J).

Requirement 3 - Steam consumption for other services:

Steam is also used for other heating needs of the vessel. These include the following:

It is used as a heat exchange medium in purifiers. Those purifiers handle heavy fuel oil, light diesel oil, and lubricating oil.
Steam is used as a heat exchange medium in booster units.
It is used to preheat the main engine cooling water.
It is used as a heat exchange medium in boilers. The same applies to high-pressure hot water storage units for gantry cranes and to sewage services.

The heat requirements of all these services are calculated separately and then combined. The final heat requirement is denoted as Q3 (for reference purposes only in this article).

The heat requirements for the three uses are then combined. The results give the total heat rate and the total steam mass flow required for the boiler.

Total heat rate required (Q) = Q1 + Q2 + Q3 (kW).
The total mass flow required is calculated using the following formula: mS = Q / ∆h (kg/h).
Where ∆h = steam enthalpy reduction (kJ/kg).

Boiler classification

There are two classification systems for selecting the right boiler.

“From” and “To” classification:

The vertical axis is the steam output as a percentage of the “From” rated power. The readings are taken at different pressures. For example:

At 15 bar pressure,
If the feed water temperature is 68°C,
The percentage of rated power (From/To) in the diagram is 90%.
So, if the rated steam capacity of the boiler is 2,000 kg/h, the actual boiler capacity will be 90% of that. That works out at 1,800 kg/h.

When selecting a boiler, the designer must determine the manufacturer's rated steam capacity. The manufacturer provides a table of rated capacities (from/to) for the proposed boiler. The above calculations are performed for various boiler pressures and feedwater temperatures. The aim is to ensure that the actual steam capacity exceeds the steam flow (ms). That steam flow comes from the preliminary design calculations.

Rated Capacity (kW):

Some boiler manufacturers prefer to use (from/to) capacity. Others prefer another system called rated capacity (kW). That is just another way of expressing the same data.

The actual steam flow can also be calculated from the boiler rated power (kW). The following relationship can be used.

Boiler Steam Output: In the above formula, the additional power is the energy added to the boiler by the feedwater. That energy depends on the feedwater temperature.

The designer must ensure that the actual steam output is greater than the steam flow (ms) calculated in the preliminary design.

The above tests must be done for different boiler operating pressures and feedwater temperatures. The range depends on the steam requirements under different navigation conditions. It must be ensured that the selected boiler meets the requirements under all these conditions. Different load combinations must be covered in the same way. The designer must also select the type of boiler to be used on board. That choice is based on the following criteria:

Boiler Function.

Space limitations. Most auxiliary boilers use tubular boilers. A water supply system is built into the boiler drum, along which smoke tubes run.

The hot gases from the burners are conveyed through flues. The flues provide a larger surface area for transferring heat to the water. In most cases, auxiliary boilers are arranged horizontally where space is not restricted. That layout prevents pressure fluctuations. This arrangement is more common in vertical boilers.

There are also economizer or exhaust gas boilers, which are boilers without a furnace. These are tubular boilers too. The engine exhaust gases pass through the flues to heat the water in the boiler drum. The vertical design is preferred because it creates less back pressure in the exhaust gas system. Exhaust gas boilers are used for propulsion. Auxiliary boilers are used in port facilities.

Next steps

For related machinery explainers and troubleshooting topics, continue with the marine machinery knowledge base.

Market context for high-compliance maritime regions

Readers in the United States, United Kingdom, Canada, Australia, Singapore and Europe share one comparison. Selection Of Marine Boilers During Ship Design should be set against technical procurement and maintenance planning. Vessel data, port operations and fleet compliance belong in the same review. The same maritime topic can have different practical meaning under USCG, MCA and Transport Canada. AMSA, MPA Singapore and European authority expectations differ again.

Use the market links below to compare how mature shipping markets evaluate maritime technology and equipment. Fleet tools and supplier decisions can be compared in the same way.