This screencast is going to be about energy
balances and energy balances are similar to
material balances since both mass and energy
are conserved. In fact if you take a look
at the first law of thermodynamics it states
that energy can be neither created nor destroyed.
Probably the best way to state this is that
accumulation equals input minus output. We
have seen a similar balance when we have talked
about material balances however there is no
consumption or generation terms. Why is that?
Because again energy can be neither destroyed
nor created. So let's look at three types of
energy in the total energy of the system and
those three types are kinetic, potential and
internal. If we start with kinetic energy.
That is the energy of motion. We call this
E sub K and this is 1/2 times the mass or
mass flow rate times the velocity squared
and sometimes you will see instead of v, you'll see a u.
Potential energy is the energy caused by
a change in elevation. We write this as mgz
where m again is the mass or mass flow rate,
g is the gravitational acceleration and z
is the height of the object or fluid above
the reference height. The third type of energy
that we look at is what is known as internal
energy. This is often described as all the
other energy and what it is is it is due to
molecular interactions. These are things like
rotational, translational motion, electro-
magnet interactions. We call this internal
energy U. In addition we have heat so that
is Q and what that is is energy that transferred
due to a temperature difference. Now this is
between the system and the surroundings. For
example you have a reactor and the air around
it is 25 degrees C. A reaction takes place
and the products are at 500 degrees C. What
will happen is that heat will flow from the
system to the surroundings since there is
a temperature difference. Q is considered
positive when it is transferred to the system
from the surroundings. So in this case is the
Q negative or positive? Let's take a look.
The system is at 500 degrees. The surroundings
is at 25 degrees. It is transferred from the
system to the surroundings so our Q will be
negative. We also have work which is a form
of energy and what this is is a response to
any other driving force other than temperature.
This can encompass a number of things such
as pistons, pumps, turbines. There is a difference
of opinion on how to describe the sign of
work. So in Felder and Rousseau it is defined
that work is positive when it is done by the
system on the surroundings however in other
textbooks including thermodynamic ones it
is the opposite. That work is negative when
the system does work on the surrounding. You
might wonder how the two can reconcile and you
will see in a future screencast when we talk
about the equation for the energy balance
you will see how it does end up being reconciled.
These are the important elements of starting
energy balances. If we look at the types of
energy, kinetic, potential, and internal as well
as the heat and work we are going to use these
definitions to determine the amount of all
of these particular energies.
