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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:

      

     
    Heat Exchangers

 

 



Recuperator
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Devices that transfer energy between fluids at different temperatures by heat transfer modes such as conduction, convection, and radiation are called heat exchangers.

The simplest form of a heat exchanger is called a recuperator, in which a gas or liquid is separated from another gas or liquid by a wall through which energy passes by conduction.

The conservation of mass principle for a heat exchanger can be expressed as follows:

Under steady operation, the mass flow rate of each fluid stream flowing through a heat exchanger remains constant.

Hence,

      

There are several common assumptions that are made in the energy analysis of heat exchangers

     

Heat Transfer of Heat Exchangers
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  • If one takes the entire heat exchanger as a control volume, = 0. If only one of the fluids is selected as a control volume, heat will cross this boundary as it flows from one fluid to the other. Hence,

  • The only work interaction at the boundary of a control volume enclosing a heat exchanger is flow work and no shaft or electric work is involved.

  • The change in kinetic energy is insignificant.

  • The potential energy change is negligible.
     
    Pipe and Duct Flow


Pipe Are Used to Transfer Fluid from one Device to another

 

Pipes and ducts are used to transfer fluid from one device to another. Flow through a pipe or duct can be treated as a steady-flow process since the start-up and shut-down periods, which are transient process; are excluded.

When flow through pipes or ducts are analyzed, the following points should be considered:


  • If the pipes or ducts are insulated, heat transfer from the pipes or ducts to the environments is negligible. Under normal operating conditions, the pipes or ducts are not insulated, hence the heat gained or lost is large. Sometimes heat transfer is the main purpose of the flow, for example, in heat exchangers. In this case, heat transfer needs to be accounted for.

  • If the control volume involves pumps or fans or other work devices, the work interaction terms should be considered. Otherwise, the work term is zero.

  • The change in kinetic energy is insignificant, particularly when the pipe or duct has a constant diameter.

  • The change in potential energy is large when the fluid undergoes a considerable elevation change.