The fuel supply temperature must be within specified limits for optimum injector life and maximum power capability.
Fuel systems without fuel coolers rely on the day tank to dissipate the heat of fuel returning from the engine. Day tank temperatures are affected by the following conditions.
- Day tank wetted surface area (including tank bottom)
- Engine(s) fuel consumption rate
- Day tank replenishing level
- Storage tank fuel temperature
- Ambient temperature
- Spaces contiguous to the day tank (void tanks, cofferdams, vessel shell plating, etc.)
- Return fuel temperature
Tank temperature calculation are performed in five [5] steps. The first determines the fuel mass in the tank at each time interval. The second step is based on a fuel mix temperature resulting from the engine driven transfer pump flow
rate to the engine and the return flow rate to the day tank. The third step determines the day tank fuel height for each incremental time element. Typically, the calculations will be based upon a 30-60 minute iterative time function. The end point for the calculation is assumed to be when the day tank is refilled. The fourth step approximates the heat transfer from the tank to the surrounding environment due to the temperature difference between the fuel mix temperature and the ambient temperature. This convective heat transfer then determines the resultant tank temperature. The fifth step evaluates the impact of the final fuel supply temperature on the engineโs maximum power capability.
The included example calculations should only be used to provide general guidance. If the day tank size is marginal, use a fuel cooler.
To simplify the following calculations, it is assumed the day tank walls are surrounded by free moving air. If the tank walls are contiguous to the shell plating, heat transfer from the day tank will be enhanced. Conversely, if the day tank is bounded by void spaces and cofferdams, heat rejection from the day tank will be retarded. Typically, most day tanks are located with various combinations of the preceding boundary elements. The individual performing the evaluation
must be familiar with the installation as well as the fundamental engineering concepts of the formulas used in the calculations.