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.
