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online_ch_power_refrig_cycles

online_ch_power_refrig_cycles - Online Chapter 1 POWER AND...

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Online C hapter 1 POWER AND REFRIGERATION CYCLES | 1 T wo important areas of application for thermodynamics are power generation and refrigeration. Both power generation and refrigeration are usually accomplished by systems that operate on a thermodynamic cycle. Thermo- dynamic cycles can be divided into two general categories: power cycles and refrigeration cycles. The devices or systems used to produce a net power out- put are often called engines , and the thermodynamic cycles they operate on are called power cycles . The devices or sys- tems used to produce refrigeration are called refrigerators, air conditioners, or heat pumps , and the cycles they operate on are called refrigeration cycles. Thermodynamic cycles can also be categorized as gas cycles or vapor cycles , depending on the phase of the work- ing fluid—the substance that circulates through the cyclic device. In gas cycles, the working fluid remains in the gaseous phase throughout the entire cycle, whereas in vapor cycles the working fluid exists in the vapor phase during one part of the cycle and in the liquid phase during another part. Thermodynamic cycles can be categorized yet another way: closed and open cycles. In closed cycles, the working fluid is returned to the initial state at the end of the cycle and is recirculated. In open cycles, the working fluid is renewed at the end of each cycle instead of being recirculated. In auto- mobile engines, for example, the combustion gases are exhausted and replaced by fresh air–fuel mixture at the end of each cycle. The engine operates on a mechanical cycle, but the working fluid in this type of device does not go through a complete thermodynamic cycle. Heat engines are categorized as internal combustion or external combustion engines , depending on how the heat is supplied to the working fluid. In external combustion engines (such as steam power plants), energy is supplied to the work- ing fluid from an external source such as a furnace, a geo- thermal well, a nuclear reactor, or even the sun. In internal combustion engines (such as automobile engines), this is done by burning the fuel within the system boundary. In this chapter, various gas power cycles are analyzed under some simplifying assumptions. Steam is the most common working fluid used in vapor power cycles because of its many desirable characteristics, such as low cost, availability, and high enthalpy of vaporiza- tion. Other working fluids used include sodium, potassium, and mercury for high-temperature applications and some organic fluids such as benzene and the freons for low-tem- perature applications. Steam power plants are commonly referred to as coal plants , nuclear plants , or natural gas plants , depending on the type of fuel used to supply heat to the steam. But the steam goes through the same basic cycle in all of them.
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