Author: admin
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Refrigeration
Ordinary Vapor Compression Cycle The Carnot cycle is not practical for refrigeration for the same reasons as discussed for power production. Therefore, most refrigerators operate on the ordinary vapor-compression (OVC) cycle, shown in Fig. 5.8. Figure 5.8. OVC refrigeration cycle process schematic and T-S diagram. The ordinary vapor compression cycle is the most common refrigeration cycle.…
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Rankine Modifications
Two modifications of the Rankine cycle are in common use to improve the efficiency. A Rankine cycle with reheat increases the boiler pressure but keeps the maximum temperature approximately the same. The maximum temperatures of the boilers are limited by corrosion concerns. This modification uses a two-stage turbine with reheat in-between. An illustration of the…
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The Rankine Cycle
In a Rankine cycle, the vapor is superheated before entering the turbine. The superheat is adjusted to avoid the turbine blade erosion from low-quality steam. Similarly, the condenser completely reduces the steam to a liquid that is convenient for pumping. In Fig. 5.2, state 4′ is the outlet state for a reversible adiabatic turbine. We use the…
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The Carnot Steam Cycle
We saw in Example 4.4 on page 145 how a Carnot cycle could be set up using steam as a working fluid. The addition of heat at constant temperature and the macroscopic definition of entropy establish a correspondence between temperature and heat addition/removal. Steam is especially well suited for isothermal heat exchange because boiling and condensation are naturally isothermal and…
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Homework Problems
4.1. Extending Example 4.2 on page 141 from solids to gases is straightforward if you recall the development of Eqn. 1.13 on page 19. Consider N2 for example. Being diatomic, we should expect that Uig = 2(3NAkT/2) = 6RT/2 in the limit of classical vibrations. Vibrational energy means that heat can be absorbed in the vibration of a bond.…
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The Entropy Balance in Brief
In this section, we refer to a division of the universe into the same three subsystems described in Section 2.14 on page 74. 1. T is the system temperature at the location where Q is transferred. 2. Sin, Sout are state variables, and any pathway may be used to calculate the change from inlet to outlet. The pathway for calculation does not need to be the pathway for…
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Unsteady-State Open Systems
We end the chapter by providing examples of unsteady-state open systems. The first example shows that analysis of such systems can produce results quite consistent with expansion in a piston/cylinder. Example 4.19. Entropy change in a leaky tank Consider air (an ideal gas) leaking from a tank. How does the entropy of the gas in…
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The Irreversibility of Biological Life
A fascinating feature of living systems is that they organize small molecules into large structures. Towering pines grow with energy from the sun, CO2, water, and minerals extracted from the ground. Mammals grow into sophisticated thinking creatures by consuming small bits of food, consuming water, and breathing air. Small mindless flagella are known to swim…
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Optimum Work and Heat Transfer
Let us consider how to calculate the optimum work interactions for a general system. For an open system where kinetic energy and potential energy changes are negligible, where dSgen = 0 for an internally reversible process. If all the heat is transferred at a single temperature Tsys, elimination of dQ in the first balance provides If we wish to apply this…
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Strategies for Applying the Entropy Balance
When solving thermodynamic problems, usually the best approach is to begin by applying the mass and energy balances. The entropy balance provides another balance, but it is not always necessary for every problem. In this chapter, we have introduced some new terms which can specify additional constraints when used in the problem statement, e.g., “isentropic,” “reversible,” “internally…