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THERMODYNAMICS - THEORY

   
   

Many engineering devices operate essentially under the same conditions for long periods of time. Therefore, these devices can be treated as steady-flow devices.

Recall, the energy balance for a control volume is:

      

     
    Turbines, Compressors, and Pumps


How Does Turbine Work
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A Reciprocating Compressor
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In steam, gas or hydroelectric power plants, the device that drives the electric generator is the turbine. A turbine is a device in which work is developed as a result of a gas or liquid passing through a set of blades. The blades, which are attached to the shaft, force the shaft to rotate. The work is positive since it is done by the fluid.

Compressors are devices which raise the pressure of the gas that passes through them. In pumps, the working fluid is a liquid instead of a gas. Work is supplied to these devices through a rotating shaft from an external source.     

There are several common assumptions that are made in the energy analysis of turbines and compressors:


  • If there is no intentional cooling, the heat transfer for these devices is small relative to the shaft work.

  • The work term is important for all these devices. For turbines, it is the power output; for compressors or pumps, it is the power input.

  • The fluid velocities in turbines are high and the fluid experiences a significant change in kinetic energy. However, it is small relative to the change of enthalpy. The velocities involved in compressors and pumps are too low to cause any significant change in kinetic energy.

  • The potential energy changes of the fluid passing through turbines, compressors, and pumps are small and are neglected.
     
    Mixing Chambers


An Open Feedwater Heater is a
Mixing Chamber
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In engineering applications, the steady-flow mixing of two streams of the same fluid is another common process. The section where the mixing process takes place is referred to as a mixing chamber. An open feedwater heater is an example of mixing chamber. If 1 and 2 denote the inlets and 3 denotes the exit, a mass balance gives

      

There are several common assumptions that are made in the energy analysis of mixture chambers:


  • Mixing chambers are usually well insulated. Although the temperatures of the flow streams may be quite different from the temperature of the environment, the most important energy transfer is between the two fluids and not between the fluids and the environment.

  • No work is involved in the mixing process.

  • The increase in kinetic energy is insignificant.

  • The change in potential energy is negligible.

Hence, the energy balance for mixing chamber is: