Applications to real fluid processes in process plant equipments
In a typical process plant one encounters a variety of flow devices such pumps, compressors, turbines, nozzles, diffusers, etc. Such devices are not subject to heat transfer by design as are heat exchangers, condensers, evaporators, reactors, etc. However, the flow devices typically are subject to mechanical irreversibility owing to existence of dissipative forces such as fluid viscosity and mechanical friction, which results in reduction of their efficiency. In addition such devices may be subject to thermal irreversibility as their operation may not be truly adiabatic.
Therefore, it is necessary to compute the efficiency of such devices in relation to a perfectly reversible (isentropic) process between their inlet and outlet.
The performance of a flow device is expressed in terms of isentropic efficiency in which the actual performance of the device is compared with that of an isentropic device for the same inlet conditions and exit pressure. For example, the isentropic efficiency ηT of a turbine (which essentially converts fluid enthalpy to shaft work, fig. 5.7) is defined as:
Fig.5.7 Schematic of a Turbine
Where, H i = enthalpy of the fluid at the inlet of the turbine, He = enthalpy of the fluid at the exit of the actual turbine, = enthalpy of the fluid at the exit of the turbine, if it were isentropic.
Similarly, the isentropic efficiency ηc of a compressor (fig. 5.8) or a pump ηp (which convert applied shaft work to fluid enthalpy) is given by:
Where, Hi = enthalpy of the fluid at the inlet to the compressor (or pump), He = enthalpy of the fluid at the exit of the actual compressor (pump), and = enthalpy of the fluid at the exit of an isentropic compressor (or pump), and Ws represents the shaft work per mole (or mass) of fluid in the two situations.
Fig.5.8 Schematic of a Turbine
The isentropic efficiencyηN of a nozzle, which is used to achieve high fluid velocity at exit (by conversion of enthalpy to kinetic energy) is given by: