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FIRST LAW OF THERMODYNAMICS | Up
Thermodynamics Heat Transfer and Fluid Flow Volume 1 of 3 | Next
Figure 14 First Law of Thermodynamics |
FIRST LAW OF THERMODYNAMICS
Thermodynamics
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.
(all energies in) =
(all energies out) + D(energy stored in system)
S
S
S Ein
S Eout
DE storage
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 flow into the system (Btu/hr)
Q
=
mass flow rate into the system (lbm/hr)
min
uin
=
specific internal energy into the system (Btu/lbm)
Pinnin
=
pressure-specific volume energy into the system (ft-lbf/lbm)
=
kinetic energy into the system (ft-lbf /lbm) where
V2in
2gc
= average velocity of fluid (ft/sec)
Vin
gc
= the gravitational constant (32.17 ft-lbm/lbf-sec2)
= potential energy of the fluid entering the system (ft-lbf/lbm) where
g
gc
Zin
Zin
= height above reference level (ft)
g
= acceleration due to gravity (ft/sec2)
gc
= the gravitational constant (32.17 ft-lbm/lbf-sec2)
HT-01
Page 54
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