Environmental Engineering Reference
In-Depth Information
matching at some point and again to avoid using hot utility below the pinch at the
pinch that results in consuming more than originally needed energy target
The CP(FCp) inequality for individual matches
In summary for hot and cold streams matching above the pinch at the pinch; if a hot
steam with CP(FCp) greater than a CP(FCp) of a cold stream, moving away from the pinch,
the temperature difference must increase. At the pinch the match starts with a temperature
difference equal to selected ∆Tmin. The relative slopes (1/FCp) of the temperature-enthalpy
profiles of the two streams mean that the temperature differences become smaller while we
are moving away from the pinch, which is infeasible. Hence we should not consider such
match. On the other hand if we match this hot stream with another cold stream having a
greater CP(FCp), in such case the relative slopes (1/FCp) of the temperature-enthalpy profiles
now cause the temperature differences to become larger moving away from the pinch, which
is feasible. Thus starting with ∆Tmin at the pinch, for temperature difference to increase
while moving away from the pinch, we have to have this inequality achieved.
CP H(FCpH) is less than or equal to CP C(FCpC)
(Above the pinch for streams at the pinch)
So a very simple explaination for you to remember:
Above the pinch at the pinch the CP (FCp) of the hot stream shall be less than ( lighter in
load ) or equal the cold stream matched with to enable it reach its pinch target tempetraturte
without aid of cold utility above the pinch. Otherwise; the matching will be infeasible and to
avoid such infeasibility we have to use cold utility above the pinch to enable the hot stream
reach its pinch target temperature.
Below the pinch at the pinch the rules are the opposite. If a cold stream is matched with a
hot that has a smaller CP(FCp) , in such case steeper slope(1/FCp) will result, then the
temperature differences away from the pinch will become smaller which is infeasible. If the
same cold stream is matched with a hot stream with a larger CP(FCp) different situation will
arise. A less steep slope(1/FCp) will be obtained resulting in temperature differences that
become larger away of the pinch which is feasible. Thus starting with ∆Tmin at the pinch, for
temperature difference to increase while we are moving away from the pinch we have to have
the following inequality achieved
CP H(FCpH) is greater than or equal to CP C(FCpC)
( below the pinch for streams at the pinch)
Again a very simple words for you to remember, below the pinch at the pinch the CP
(FCp) of the cold stream shall be less than ( lighter in load ) or equal to the hot stream
matched with to enable it reach its pinch target tempetraturte without aid of hot utility below
the pinch. Otherwise; the matching will be infeasible and to avoid infeasibility we have to use
hot utility below the pinch to enable the cold stream reach its pinch target temperature that
results in both heating and cooling utilities increase than originally obtained in the energy
targeting step.
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