FIRST LAW OF THERMODYNAMICS
The hot fluid from the heat source passes through the primary side of the steam generator where
its energy is passed to the secondary side of the heat exchanger in such a manner as to create
steam. The fluid, with its energy removed from the primary side, leaves the steam generator at
a lower temperature, and is pumped back to the heat source to be "re-heated." Each major
component of a steam plant can be treated as a separate open system problem. A thermodynamic
analysis, using the various forms of energies discussed, can be applied to the particular
component in studying its behavior. A simplified example of the thermodynamics involved in
the steam generator is shown below.
Example 2: Open System - Steam Plant Component
Primary fluid enters the heat exchanger of a nuclear facility at 610°F and leaves at 540°F.
The flow rate is approximately 1.38 x 108 lbm/hr. If the specific heat of the fluid is taken
as 1.5 Btu/lbm°F, what is the heat transferred out of the steam generator?
1) Neglecting pe and ke and assuming no work is done on the system.
where cp = specific heat capacity (Btu/lbm-°F).
(cp) (Tout - Tin)
1.38 x 108 lbm/hr (1.5 Btu/lbm-oF) (540 - 610oF)
-1.45 x 1010 Btu/hr
The minus sign indicating heat out of the heat exchanger, which is consistent with the
physical case. This example demonstrates that for a heat exchanger, the heat transfer rate
can be calculated using the equation
It is important
to note that the later equation can only be used when no phase change occurs since DT
= 0 during a phase change. The first equation can be used for a phase change heat
transfer process as well as for latent heat calculations.