Selection Of Marine Boilers During Ship Design

How To Select Marine Boilers During Ship Design

Today, marine boilers used on ships are primarily employed for auxiliary purposes, particularly on ships equipped with marine diesel engines or diesel-electric propulsion systems.

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

From the ship designer's perspective, they must be able to select the appropriate type of boiler for each ship based on the project requirements.

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

To evaluate the performance of the boiler, it is first necessary to correctly estimate the boiler steam output required 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, and 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:

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, and its high viscosity makes it difficult to flow.

However, to transfer the stored HFO to the settling tank and then to the HFO service tank, the viscosity must be maintained at a level that allows for easy flow. For this purpose, HFO tanks are equipped with heating coils to 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 and identify the area around each tank bulkhead. The heat transfer ambient temperature of each compartment baffle in the analysis is fixed according to the surrounding environment of that baffle (e.g., engine room, vacuum tank, ballast water tank, sludge tank, etc.).

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The steam 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

Since the heat transfer rate is known, the steam mass flow rate can 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 is used not only to compensate for heat losses in the fuel tank, but also to heat the fuel to the required temperature before it is used in the engine.

Therefore, the time (t) required in hours to heat each fuel tank 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.

This calculation involves two steps:

Calculate the amount of heat required to heat the contents of each tank (Q, in Watts), and add all the individual heat requirements to obtain 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)

Since 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, including:

• It is used as a heat exchange medium in purifiers for 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, high-pressure hot water storage units for gantry cranes, and 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).

Once the heat requirements for the three uses are obtained, they are combined to obtain the total heat rate and 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 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 it, or 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 to ensure that the actual steam capacity exceeds the steam flow (ms) obtained in the preliminary design calculations.

Rated Capacity (kW):

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

To calculate the actual steam flow 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 (depending 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, depending on the steam requirements under different navigation conditions. It must be ensured that the selected boiler meets the requirements under all these conditions and different load combinations. The designer must also select the type of boiler to be used on board 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, which provide a larger surface area for transferring heat to the water. In most cases, auxiliary boilers are arranged horizontally where space is not restricted, to prevent pressure fluctuations. This arrangement is more common in vertical boilers.

However, for economizer or exhaust gas boilers (boilers without a furnace. There are also tubular boilers where 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, while auxiliary boilers are used in port facilities.