Subsection 3.3.3 Energy Relationships in the Shipboard Propulsion Cycle
The movement of a ship through the water is the result of a number of energy transformations. Although most of these transformations have been mentioned, they have not been discussed in the proper sequence.
The first energy transformation occurs when fuel oil is burned in the boiler furnace. By the process of combustion, the chemical energy stored in the fuel oil is transformed into thermal energy. Thermal energy flows from the burning fuel to the water in the boiler, and steam is generated. The thermal energy is now stored as internal energy in the steam, as we can tell from the increased pressure and temperature of the steam. When the steam is admitted to the turbines, the thermal energy of the steam is converted into mechanical energy, which turns the shaft and drives the ship. Two main energy transformations are involved in converting thermal energy to work in a turbine. First, the thermal energy of the steam is transformed into mechanical kinetic energy (energy of motion) as the steam flows through one or more nozzles. Second, the mechanical kinetic energy of the steam is transformed into work (mechanical energy in transition) as the steam strikes the projecting blades of the turbine and thus causes the turbine to turn. The turning of the turbine rotor causes the propeller shaft to turn also, although at a slower speed, since the turbine is connected to the propeller shaft through reduction gears. The steam exhausts from the turbine to the condenser where it gives up its latent heat of condensation to the circulating seawater.
For the remainder of this cycle, energy is required to get the water (condensate and feedwater) back to the boiler where it will again be heated and changed into steam. The energy used for this purpose is generally the thermal energy of the auxiliary steam. In the case of turbine driven feed pumps, the conversion of thermal energy to mechanical energy occurs in the same way as it does in the case of the propulsion turbines. In the case of motor-driven pumps, the energy conversion is from thermal energy to electrical energy (in a turbogenerator) and then from electrical energy to mechanical energy (work) in the pumps.
As you can see, putting in 1 BTU at the boiler furnace does not mean that 778 foot-pounds of mechanical energy will be available for propelling the ship through the water. Some of the energy put in at the boiler furnace is used by auxiliary machinery such as pumps and forced-draft blowers to supply the boiler with feedwater, fuel oil, and air for combustion. Distilling plants, turbo-generators, steering gears, heating systems, galley and laundry equipment, and many other units throughout the ship use energy derived directly or indirectly from the energy put in at the boiler furnace.
In addition, there are many “energy losses” throughout the engineering plant. As we have seen, energy cannot actually be lost. But when it is transformed into a form of energy that we cannot use, we say there has been an energy loss. Since no insulation is perfect, some thermal energy is always lost as steam travels through the piping. Friction losses occur in all machinery and piping. There is also an energy loss at the condenser as the steam exhausted from the turbines gives up heat to the circulating seawater and turns into condensate.
Although some energy losses can be partly avoided by designing the machinery and equipment to minimize friction losses, by insulating hot surfaces, and by lubricating moving parts, some energy losses are still unavoidable. Consider, for example, the unavoidable energy loss that occurs at the condenser. To allow the flow of heat, the condenser must be at a lower temperature than the boiler since heat flow can occur only from a higher temperature area to a lower temperature area. Therefore, the energy loss at the condenser is not only unavoidable, but is also actually required for the conversion of heat to work.
As you can see, each BTU that is put in at the boiler furnace has to be divided in many ways before all the energy can be accounted for, but the energy account will always balance. Energy in will always equal Energy out.
