LECTURE 12

Steady state steady flow process

For negligible change in kinetic and potential energies through the control volume,

If the control volume is well insulated (i.e. adiabatic), then, q = 0.

For steady flow devices, such as turbines, compressors and pumps, is power transmitted through a shaft.

The unit of ke is m²/s² which is equivalent to Joule/kg. The enthalpy is usually given in kJ/kg. So kinetic energy should be expressed in kJ/kg. This is accomplished by dividing it by 1000.

Kinetic energy term at low velocities is negligible, but should be accounted for at high velocities.

By similar argument, the elevation difference between inlet and exit of most industrial devices such as compressors and turbines is small and potential energy term is negligible (particularly for gases). The only time the potential energy term is significant is when a process involves pumping a fluid to high elevations.

Turbine

A turbine is a rotary steady state steady flow machine whose purpose is the production of shaft power at the expense of the pressure of the working fluid.

Two general classes of turbines are steam and gas turbines depending on the working substance used.

Usually, changes in potential energy are negligible, as is the inlet kinetic energy. Often the exit kinetic energy is neglected (if in a problem, the flow velocities are specified, the kinetic energy term should be included).

Normally, the process in the turbine is adiabatic and the work output reduces to decrease in enthalpy from the inlet to exit states.

Compressor / pump

The purpose of a compressor (gas) or pump (liquid) is the same, to increase the pressure of a fluid by putting in shaft work (power). There are two fundamentally different types of compressors:

1. The rotary type (either axial or centrifugal flow)

2. A piston/cylinder type compressor.

The first type is analyzed using control volume approach (steady state steady flow process). The working fluid enters the compressor at low pressure and exits at high pressure.

Usually, changes in potential energy are negligible as is the inlet kinetic energy. Often, exit kinetic energy is neglected as well (wherever, in a problem, velocities are specified, ke term should not be neglected).

The compression process is usually adiabatic.

Nozzle

A nozzle is a steady state steady flow device to create a high velocity fluid stream at the expense of its pressure. It is contoured in an appropriate manner to expand the fluid to a lower pressure.

Since the objective of the device is to increase the flow velocity, hence kinetic energy, the kinetic energy term cannot be ignored. Usually, the process through the nozzle is treated as adiabatic.

Since there are no moving parts, shaft work is zero. The potential energy term (for gases) is negligible and hence omitted.

Diffuser

A steady state steady flow device meant to decelerate high velocity fluid resulting in increased pressure of the fluid. It is the opposite of a nozzle as far as the purpose is concerned. The assumptions are similar to those for a nozzle.