First Law of Thermodynamics For A Control Volume
First Law of Thermodynamics For A Control Volume
First Law of Thermodynamics For A Control Volume
control volume
The velocity and the area correspond to a certain volume per unit time entering the
control volume, enabling us to relate that to the mass flow rate and the specific
volume at the state of the mass going in. Now we are able to express the rate of
flow work as
For the flow that leaves the control volume, work is being done by the control
volume,Pevem˙ e, and for the mass that enters, the surroundings do the rate of work,
Pi vim˙ i . The flow work per unit mass is then Pv, and the total energy associated with
the flow of mass is
THE STEADY-STATE PROCESS
The units for q and w are kJ/kg. From their definition, q and w can be thought of as the
heat transfer and work (other than flow work) per unit mass flowing into and out
of the control volume for this particular steady-state process
STEADY-STATE PROCESSES
Heat Exchanger
A steady-state heat exchanger is a simple fluid flow through a pipe
or system of pipes, where heat is transferred to or from the fluid.
The fluid may be heated or cooled, and may or may not boil,
changing from liquid to vapor, or condense, changing from vapor to
liquid.
One such example is the condenser in an R-134a refrigeration
system, where Superheated vapor enters the condenser and liquid
exits.
Assumptions:
constant pressure process.
no means for doing any work(shaft work, electrical work, etc.),
changes in kinetic and potential energies are commonly negligibly small
The heat transfer in most heat exchangers is then found as the
change in enthalpy of the fluid.
Nozzle
A nozzle is a steady-state device whose purpose is to create a high-velocity
fluid stream at the expense of the fluid’s pressure. thereby increasing its.
Assumptions:
no means to do any work—there are no moving parts.
There is little or no change in potential energy
and usually little or no heat transfer.
the kinetic energy of the fluid at the nozzle inlet is usually small and
would be neglected if its value is not know
Diffuser
A steady-state diffuser is a device constructed to decelerate a high-velocity fluid in a
manner that results in an increase in pressure of the fluid.
In essence, it is the exact opposite of a nozzle, and it may be thought of as a fluid
flowing in the opposite direction through a nozzle with the opposite effects.
The assumptions are similar to those for a nozzle,
large kinetic energy at the diffuser inlet and
a small, but usually not negligible, kinetic energy at the exit
These are the only terms besides the enthalpies remaining in the first law,
Eq. 6.13.
Turbine
A turbine is a rotary steady-state machine whose purpose is to produce shaft work
(power on a rate basis) at the expense of the pressure of the working fluid.
The first law for this process is either Eq. 6.10 or 6.13.
Assumptions:
• Usually, changes in potential energy are negligible, as is the inlet kinetic
energy.
• Often, the exit kinetic energy is neglected,
• Any heat rejection from the turbine is undesirable and is commonly small.
• therefore normally assume that a turbine process is adiabatic,
• Work output in this case reduces to the decrease in enthalpy from the inlet to
exit states
• In most problems where the change in
elevation is small, the potential energy terms
may be neglected.
Power Plant and Refrigerator