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  1. 4 CHAPTER The First Law ofThermodynamics:Control Volumes

  2. A control volume differs from a closed system in that it involves mass transfer. Mass carries energy with it, and thus the mass and energy content of a system change when mass enters or leaves. 4-20 Chapter Summary

  3. The mass and energy balances for any system undergoing any process can be expressed as 4-21 Chapter Summary

  4. The mass and energy balances for any system undergoing any process can be expressed in the rate form as 4-22 Chapter Summary

  5. Mass flow through a cross section per unit time is called the mass flow rate and is denoted m. It is expressed aswhere = density, kg/m3 (= 1/v) = average fluid velocity normal to A, m/s A = cross-sectional area, m2 4-23 Chapter Summary .

  6. The fluid volume flowing through a cross section per unit time is called the volume flow rate V. It is given by 4-24 Chapter Summary .

  7. The mass and volume flow rates are related by 4-25 Chapter Summary

  8. A specific property is an intensive quantity obtained by dividing an extensive property ( or its flow rate) by the total amount of the process material. For example the specific Volume ( m3/kg) and specific kinetic energy is ( J/kg). We will use the symbol ^ to denote the a specific property: will denote specific volume specific internal energy The enthalpy of the system H =U+PV The specific enthalpy Specific Properties and Enthalpy

  9. Thermodynamic processes involving control volumes can be considered in two groups: steady-flow processes and unsteady-flow processes. During a steady-flow process, the fluid flows through the control volume steadily, experiencing no change with time at a fixed position. The mass and energy content of the control volume remain constant during a steady-flow process. 4-26 Chapter Summary

  10. Taking heat transfer to the system and work done by the system to be positive quantities, the conservation of mass and energy equations for steady-flow processes are expressed as where the subscript i stands for inlet and e for exit. These are the most general forms of the equations for steady-flow processes. 4-27 Chapter Summary for each exit for each inlet

  11. For single-stream (one-inlet--one-exit) systems such as nozzles, diffusers, turbines, compressors, and pumps, the steady flow equations simplify toIn the above relations, subscripts 1 and 2 denote the inlet and exit states, respectively. 4-28 Chapter Summary

  12. During a uniform-flow process, the state of the control volume may change with time, but it may do so uniformly. Also, the fluid properties at the inlets and the exits are assumed to remain constant during the entire process. The conservation of energy equation for a uniform-flow process reduces to 4-29 Chapter Summary

  13. When the kinetic and potential energy changes associated with the control volume and the fluid streams are negligible, the conservation of energy equation for a uniform-flow process simplifies to 4-30 Chapter Summary

  14. Steam Tables

  15. (fig. 4-6) 4-1 Velocity Profiles for Flow in a Pipe © The McGraw-Hill Companies, Inc.,1998

  16. Volume flow rate is the volume of fluid flowing through a cross section per unit of time 4-2 Volume Flow Rate

  17. 4-3 Mass Flow, Heat, and Work Affect Energy Content The energy content of a control volume can be changed by mass flow as well as heat and work interactions

  18. (Fig. 4-9) 4-4 Control Volume May Involve Boundary, Electrical, and Shaft Work

  19. (Fig. 4-10) 4-5 Schematic for Flow Work

  20. (Fig. 4-19) 4-6 During Steady Flow Process, Volume Flow Rates are not Necessarily Conserved

  21. . . . (Fig. 4-21) . . 4-7 A Water Heater Under Steady Operation

  22. (Fig. 4-25) 4-8 Steady-Flow Devices Operate Steadily for Long Periods

  23. (Fig. 4-27) 4-9 Nozzle and Diffuser Shapes Cause Large Changes in Fluid Velocities Nozzles and Diffusers are shaped so that they cause large changes in fluid velocities and thus kinetic energies

  24. 4-10 Schematic for Example 4-2

  25. 4-11 Schematic for Example 4-4

  26. (Fig.4-32) 4-12 Throttling Valve Devices Cause Large Pressure Drops in Fluid

  27. 4-13 Ideal Gas Temperature Does Not Change During a Throttling The temperature of an ideal gas does not change during a throttling(h =constant) process since h = h (T)

  28. (Fig. 4-35) 4-14 T-Elbow Serves as Mixing Chamber for Hot and Cold Water Steams The T-ebow of an ordinary shower serves as the mixing chamber for hot- and cold-water streams.

  29. 4-15 Heat Transfer Via Heat Exchanger Depends on System Selection The heat transfer associated with a heat exchanger may be zero or nonzero depending on how the system is selected

  30. . 4-16 Schematic for Example 4-9

  31. (Fig. 4-47) 4-17 Rigid Tank Charging From a Supply Line is an Unsteady-Flow Process Charging of a rigid tank from a supply line is an unsteady-flow process since it involves changes within the control volume

  32. The Temperature of Steam rises from 300 to 456°C as it enters a tank as a result of flow energy being converted to internal energy (Fig. 4-54) o o 4-18 Temperature of Steam Rises Entering Tank, Flow Energy Converts to Internal Energy

  33. In a pressure cooker, the enthalpy of the existing steam is Hg@P (enthalpy of the saturated vapor at the given pressure) 4-19 Enthalpy of a Saturated Vapor at a Given Pressure