Application of SFEE

SFEE governs the working of a large number of components used in many engineering practices. In this section a brief analysis of such components working under steady flow conditions are given and the respective governing equations are obtained.
 Turbines
Turbines are devices used in hydraulic, steam and gas turbine power plants. As the fluid passes  through the turbine, work is done on the blades of the turbine which are attached to a shaft. Due to the work given to the blades, the turbine shaft rotates producing work.

 
General Assumptions
1.      Changes in kinetic energy of the fluid are negligible
2.      Changes in potential energy of the fluid are negligible.

                                                                                           
 Compressors
Compressors (fans and blowers) are work consuming devices, where a low-pressure fluid is compressed by utilising mechanical work. Blades attached to the shaft of the turbine imparts kinetic energy to the fluid which is later converted into pressure energy.

General Assumptions
              1.    Changes in the kinetic energy of the fluid are negligible
              2.     Changes in the potential energy of the fluid are negligible
Governing Equation
              Applying the above equations SFEE becomes
                                              

 Pumps
Similar to compressors pumps are also work consuming devices. But pumps handle incompressible fluids, whereas compressors deal with compressible fluids.

General Assumptions
         1.   No heat energy is gained or lost by the fluids;
         2.   Changes in kinetic energy of the fluid are negligible.
Governing Equation
                                    
As the fluid passes through a pump, enthalpy of the fluid increases, (internal energy of the fluid remains constant) due to the increase in pv (flow energy). Increase in potential energy of fluid is the most important change found in almost all pump applications.
Nozzles
Nozzles are devices which increase the velocity of a fluid at the expense of pressure. A typical nozzle used for fluid flow at subsonic* speeds is shown in Figure 3.7.
General Assumptions
1.       In nozzles fluids flow at a speed which is high enough to neglect heat lost or gained                      as it crosses the entire length of the nozzle. Therefore, flow through nozzles can be                 regarded   as adiabatic. That is = 0. 
2.       There is no shaft or any other form of work transfer to the fluid or from the fluid;                   that is  = 0.
3.       Changes in the potential energy of the fluid are negligible.

Governing Equation
                      
Diffusers
Diffusers are (reverse of nozzles) devices which increase the pressure of a fluid  stream by reducing its kinetic energy.
General Assumptions
Similar to nozzles, the following assumptions hold good for diffusers.
1.           Heat lost or gained as it crosses the entire length of the nozzle. Therefore, flow through nozzles can be regarded as adiabatic. That is  
2.       There is no shaft or any other form of work transfer to the fluid or from the fluid;                   that is  = 0.
3.      Changes in the potential energy of the fluid are negligible


Governing Equation


 Heat Exchangers
Devices in which heat is transferred from a hot fluid stream to a cold fluid stream are known as heat exchangers.
General Assumptions
1.  Heat lost by the hot fluid is equal to the heat gained by the cold fluid.
2.  No work transfer across the control volume.
3.  Changes in kinetic and potential energies of both the streams are negligible.
Governing Equation
  Throttling
A throttling process occurs when a fluid flowing in a line suddenly encounters a restriction in the flow passage. It may be
      ·    a plate with a small hole as shown in Figure 3.10 (a)
      ·    a valve partially closed as shown in Figure 3.10 (b)
      ·    a capillary tube which is normally found in a refrigerator as shown in Figure 3.10 (c)
      ·    a porous plug as shown in Figure 3.10 (d)


General assumptions
     1.   No heat energy is gained or lost by the fluid; ie., = 0
     2.   There is typically some increase in velocity in a throttle, but both inlet and exit             kinetic energies are usually small enough to be neglected.
     3.   There is no means for doing work; ie.,  = 0.
     4.   Changes in potential energy of the fluid is negligible.
Governing Equation
                       h2  =  h1                                                                                                                                                                                     ...(3.16)

              Therefore, throttling is an isenthalpic process.