Principles of naval engineering - Historic Naval Ships Association

Chapter 8- INTRODUCTION TO THERMODYNAMICSthe exhaust products are returned to the atmosphere.In fact, another way to describe an opencycle is to say that it is one which includes theatmosphere at some point.The closed cycle is exemplified by the condensingsteam power plant used for ship propulsionon many naval ships. In such a cycle, theworking substance (water) is changed to steam inthe boilers. The steam performs work as it expandsthrough the turbines, and is then condensedto water again in the condenser. The water isreturned to the boilers as boiler feed, and isthus used over and over again.Thermodynamic cycles are also classifiedas heated-engine cycles or as unheated-enginecycles, depending upon the point in the cycle atwhich heat is added to the working substance. 1^In a heated-engine cycle, heat is added to theworking substance in the engine itself. An internalcombustion engine has a heated-enginecycle. In an unheated-engine cycle, the workingsubstance receives heat in some device whichis separate from the engine. The condensingsteam power plant has an unheated-engine cycle,since the working substance is heated separatelyin the boilers and then piped to the engines (steamturbines).There are five basic elements in any thermodynamiccycle: (1) the working substance, (2) theengine, (3) a heat source, or high-temperatureregion, (4) a heat receiver, or low-temperatureregion, and (5) a pump.The working substance is the medium bywhich energy is carried through the cycle. Theengine is the device which converts the thermalenergy of the working substance into useful mechanicalenergy in the form of work. The heatsource supplies heat to the working substance.The heat receiver absorbs heat from the workingsubstance. The pump moves the working substancefrom the low pressure side of the cycleto the high pressure side.The essential elements of a closed, unheatedenginecycle are shown in figure 8-8. This isthe basic plan of the typical condensing steampower plant.15 The terms heated ensine and unheated engine shouldnot be confused with the term heat engine. Any machinewhich is designed to convert thermal energy tomechanical energy in the form of work is known as aheat engine. Thus, both internal combustion enginesand steam turbines are heat engines; but the first hasa heated-engine cycle and the second has an unheatedenginecycle.PUMP• WORKING SUBSTANCEfmmmmmMHEATSOURCEI-UNHEATEDENGINEIHEATRECEIVER147.62Figure 8-8. — Essential elements of closed,unheated-engine cycle.In an open, heated-engine cycle such as thatof an internal combustion engine, the essentialelements are all present but are arranged in aIn this type of cycle,somewhat different order.atmospheric air and fuel are both drawn into thecylinder of the engine. Combustion takes placein the cylinder, either by compression or byspark, and the resulting internal energy of theworking substance is transformed into work bywhich the piston is moved. Since the space abovethe piston is a high pressure area when the pistonis near the top of its stroke and a low pressurearea when the piston is near the bottom, thepiston may be thought of as a pump in the sensethat it "pumps" the working fluid from the lowpressure to the highpressure side of the system.Thus, in terms of function, the piston-andcylinderarrangement may be thought of as includingthe heat source, the engine, and the pump.An open, heated-engine cycle might therefore berepresented as shown in figure 8-9.THE CONCEPT OF REVERSIBILITYWhen we put a pan of water on the stove andturn on the heat, we expect the water to boilrather than to freeze. After we have mixed hotand cold water, we do not expect the resultingmixture to resolve itself into two separatebatches of water at two different temperatures.When we open the valve on a cylinder of compressedair, we expect compressed air to rushout; we would be quite surprised if atmosphericair rushed into the cylinder and compressed179