PRINCIPLES OF NAVAL ENGINEERING 



energy of the steam is transformed into work 

 as the steam impinges upon 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 con- 

 denser, where it gives up its latent heat of con- 

 densation to the circulating sea water. 



For the remainder of this cycle, energy is 

 required to get the water (condensate and feed 

 water) 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 tur- 

 bines. 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. 



ENERGY BALANCES 



From previous discussion, it should be 

 apparent that putting 1 Btu in at the boiler 

 furnace does not mean that 778 foot-pounds of 

 work will be available for propelling the ship 

 through the water. Some of the energy put in at 

 the boiler furnace is used by auxiliary machin- 

 ery such as pumps and forced draft blowers to 

 supply the boiler with feed water, fuel oil, and 

 combustion air. Distilling plants, turbogenera- 

 tors, steering gears, steam catapults, 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 which 

 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 

 piping. Friction losses occur in all machinery 

 and piping. Some heat must be wasted as the 

 combustion gases go up the stack. Some heat 

 must be lost at the condenser as the steam ex- 

 hausted from the turbines gives up heat to the 

 circulating sea water. We cannot expect all 

 of the heat supplied to be converted into work; 

 even in the most efficient possible cycle, we 

 know that some heat must always be rejected 

 to a receiver which is at a lower temperature 

 than the source. Thus, each Btu that is theoreti- 

 cally put in at the boiler furnace must be 

 divided up a good many ways before the energy 

 can be completely accounted for. But the energy 

 account will always balance. Energy in mustal- 

 ways equal energy out. 



Designers of engineering equipment use 

 energy balances to analyze energy exchanges 

 and to compute the energy requirements for 

 proposed equipment or plants. Operating engi- 

 neers use energy balances to evaluate plant 

 performance. The engineer officer of a naval 

 ship may find it necessary to make energy 

 balances in order to find out whether the plant 

 is operating at designed efficiency or whether 

 defects are causing unnecessary waste of steam, 

 fuel, and energy. 



An energy balance for an entire engineering 

 plant is usually made up in the form of a flow 

 diagram similar to (but more detailed than) the 

 one shown in figure 8-18. Anumber of numerical 

 values are entered on the flow diagram, the most 

 important of which are the quantities of the 

 working fluid flowing per hour at various points 

 and the thermodynamic states of the working 

 fluid at various points. The quantity of fluid 

 flowing per hour may be obtained by direct 

 measurement of flow through flow meters or 

 nozzles or by calculation; in some instances, 

 it is necessary to estimate steam consumption 

 of pumps and other units on the basis of avail- 

 able test data. Data on the state of the working 

 fluid is obtained from pressure and temperature 

 readings. Enthalpy calculations are made and 

 noted at various points on the diagram. The 

 complete energy balance includes tabular data 

 as well as the data shown on the flow diagram. 



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