First Law of Thermodynamics
The First Law of Thermodynamics is
a balance of the various forms of energy as they pertain to the
specified thermodynamic system (control volume) being studied.
First Law of Thermodynamics - The First Law of Thermodynamics states:
Energy can neither be created nor destroyed, only altered in form. For
any system, energy transfer is associated with mass and energy crossing
the control boundary, external work and/or heat crossing the boundary,
and the change of stored energy within the control volume. The mass flow
of fluid is associated with the kinetic, potential, internal, and
"flow" energies that affect the overall energy balance of the system.
The exchange of external work and/or heat complete the energy balance.
The First Law of Thermodynamics is referred to as the Conservation of
Energy principle, meaning that energy can neither be created nor
destroyed, but rather transformed into various forms as the fluid within
the control volume is being studied. The energy balance spoken of here
is maintained within the system being studied. The system is a region in
space (control volume) through which the fluid passes. The various
energies associated with the fluid are then observed as they cross the
boundaries of the system and the balance is made. As discussed in
previous chapters of this module, a system may be one of three types:
isolated, closed, or open. The open system, the most general of the
three, indicates that mass, heat, and external work are allowed to cross
the control boundary. The balance is expressed in words as: all
energies into the system are equal to all energies leaving the system
plus the change in storage of energies within the system. Recall that
energy in thermodynamic systems is composed of kinetic energy (KE),
potential energy (PE), internal energy (U), and flow energy (PL); as well as heat and work processes. Σ (all energies in) = Σ (all energies out) + Δ(energy stored in system) 
For most industrial plant applications that most frequently use
cycles, there is no change in storage (i.e. heat exchangers do not swell
while in operation). In equation form, the balance appears as indicated
on Figure 14. where:
Heat and/or work can be directed in to or out of the control volume.
But, for convenience and as a standard convention, the net energy
exchange is presented here with
the net heat exchange assumed to be into the system and the net work
assumed to be out of the system. If no mass crosses the boundary, but
work and/or heat
do, then the system is referred to as a "closed" system. If mass,
work and heat do not cross the boundary (that is, the only energy
exchanges
taking place are within the system), then the system is referred to
as an isolated system. Isolated and closed systems are nothing more than
specialized cases of the open system. In this text, the open system
approach to the First Law of Thermodynamics will be emphasized because
it is more general. Also, almost all practical applications of the first
law require an open system analysis.An understanding of the control volume concept is essential in analyzing a thermodynamic problem or constructing an energy balance. Two basic approaches exist in studying Thermodynamics: the control mass approach and the control volume approach. The former is referred to as the Le Grange approach and the latter as the Eulerian approach. In the control mass concept, a "clump" of fluid is studied with its associated energies. The analyzer "rides" with the clump wherever it goes, keeping a balance of all energies affecting the clump.
The control volume approach is one in which a fixed region in space is established with specified control boundaries, as shown in Figure 15. The energies that cross the boundary of this control volume, including those with the mass crossing the boundary, are then studied and the balance performed. The control volume approach is usually used today in analyzing thermodynamic systems. It is more convenient and requires much less work in keeping track of the energy balances. Examples of control volume applications are included in Figures 16-18.
The forms of energy that may cross the control volume boundary include those associated with the mass (m) crossing the boundary. Mass in motion has potential (PE), kinetic (KE), and internal energy (U). In addition, since the flow is normally supplied with some driving power (a pump for example), there is another form of energy associated with the fluid caused by its pressure. This form of energy is referred to as flow energy (Pν-work). The thermodynamic terms thus representing the various forms of energy crossing the control boundary with the mass are given as m (u + Pν + ke + pe).
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