4.1 Thermodynamic Analysis of Control Volumes
4.1 Thermodynamic Analysis of Control Volumes 4.1 Thermodynamic Analysis of Control Volumes
CompressorShaftWork (-)TurbineShaftWork (+)‚ KE 0: The velocities involved with these devices, with the exception of turbines, areusually too low to cause any significant change in kinetic energy. In turbines, this change in kineticenergy is usually very small relative to the change in enthalpy, and is often disregarded.‚ PE 0: This energy term is typically small for turbines/compressors.Throttling Valves:Throttling valves are any kind of flow-restricting devices that cause a significant pressure dropin the fluid. Unlike turbines, throttling valves produce a pressure drop without any kind of work. Thepressure drop in the fluid is often accompanied by a large drop in temperature, and for that reasonthrottling devices are commonly used in refrigeration and air-conditioning applications. An adjustablethrottling valve is shown below.The relative magnitudes of the energy equation terms is discussed below ...‚ Q . 0: Flow through these devices is usually adiabatic.‚ Ẇ = 0: No work is done on or by the fluid.‚ KE 0: Even though the exit velocity is often considerably higher, the change in kineticenergy is insignificant.‚ PE 0: Potential energy changes are very small.The conservation of energy equation for a throttling valve readily reduces to:h eh i(4.3.1)... and for this reason, throttling valves are sometimes called isenthalpic devices.ENGS205--Introductory Thermodynamics page 39
Mixing Chambers:In engineering applications, mixing two streams of fluids is not a rare occurrence. The sectionwhere the mixing process takes place is commonly referred to as a mixing chamber.MixingChamberThermodynamic consideration of the mixing chamber reveals ...‚ Q . 0: Mixing chambers are usually well-insulated.‚ Ẇ = 0: There are no work interactions.‚ KE 0: Changes in kinetic energy are negligible.‚ PE 0: Changes in potential energy are negligible.Heat Exchangers:Heat exchangers are devices where two moving fluid streams exchange heat without mixing.Heat exchangers are used widely in industries, and they come in numerous designs. The simplest formof a heat exchanger is a double-tube (shown below) or tube-and-shell heat exchanger. It's composed oftwo concentric pipes of different diameters (i.e., a pipe within a pipe). One fluid flow in the inner pipe, andthe other in the annular space between the pipes. Heat is transferred through the wall separating the twofluids.Fluid BFluid AHere are the relevant energy terms ...‚ Q . 0: This must be absolutely true! Otherwise, the heat exchanger you're analyzing doesn'texchange heat!ENGS205--Introductory Thermodynamics page 39
- Page 2 and 3: Mass and Volume Flow Rates:The amou
- Page 4 and 5: The conservation of mass principle
- Page 8 and 9: ‚ Ẇ = 0: Heat exchangers typica
- Page 10: The total energy of a flowing fluid
CompressorShaftWork (-)TurbineShaftWork (+)‚ KE 0: The velocities involved with these devices, with the exception <strong>of</strong> turbines, areusually too low to cause any significant change in kinetic energy. In turbines, this change in kineticenergy is usually very small relative to the change in enthalpy, and is <strong>of</strong>ten disregarded.‚ PE 0: This energy term is typically small for turbines/compressors.Throttling Valves:Throttling valves are any kind <strong>of</strong> flow-restricting devices that cause a significant pressure dropin the fluid. Unlike turbines, throttling valves produce a pressure drop without any kind <strong>of</strong> work. Thepressure drop in the fluid is <strong>of</strong>ten accompanied by a large drop in temperature, and for that reasonthrottling devices are commonly used in refrigeration and air-conditioning applications. An adjustablethrottling valve is shown below.The relative magnitudes <strong>of</strong> the energy equation terms is discussed below ...‚ Q . 0: Flow through these devices is usually adiabatic.‚ Ẇ = 0: No work is done on or by the fluid.‚ KE 0: Even though the exit velocity is <strong>of</strong>ten considerably higher, the change in kineticenergy is insignificant.‚ PE 0: Potential energy changes are very small.The conservation <strong>of</strong> energy equation for a throttling valve readily reduces to:h eh i(4.3.1)... and for this reason, throttling valves are sometimes called isenthalpic devices.ENGS205--Introductory <strong>Thermodynamic</strong>s page 39
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