Most ships run by diesel engines have fuel oil tanks that are used to store Heavy Fuel Oil (HFO). Since the viscosity of HFO is very high, stored HFO is almost as dense as tar, and its high viscosity makes it unable to flow.
But in order to transfer the stored HFO to the settling tanks and then the HFO service tank, the viscosity needs to be maintained at a level corresponding to which easy flow is possible. For this, HFO storage tanks are equipped with heating coils to maintain the fuel at a certain temperature.
The heating fluid in the heating coils is steam that is produced in the auxiliary boiler.
First, each HFO storage tank is located in the general arrangement drawing, and the surrounding space adjacent to each tank bulkhead is noted. Depending on the surrounding of each tank bulkhead (Engine Room, Void, Ballast Water Tank, Sludge Tank, etc.) the ambient temperature is fixed for heat transfer through each bulkhead of the tank in analysis.
The amount of steam flow rate required to maintain the temperature of the fuel in each such tank is calculated using the following steps:
Heat loss from tank bulkhead
Q1 = U A (T2 – T1)
Qb = heat loss from bulkhead (W)
U = overall heat transfer co-efficient (W/m2 0C)
A = Area of tank bulkhead under consideration (m2)
T2 = Temperature of the tank to be maintained (0C)
T1 = Temperature of the adjacent medium of the bulkhead considered (0C)
Heat loss from tank Qt = Sum of heat loss from all the six bulkheads of the tank
Q1 = sum of heat loss from all the tanks
As we know the heat transfer rate, the mass flow rate of steam can be calculated using the following formula :
ms = Q1 / ∆h
ms = mass flow rate of steam (kg/s)
Q1 = calculated heat transfer (kW)
∆h = enthalpy drop of the steam (kJ/kg)