Environmental Engineering Reference
In-Depth Information
The CP(FCp) table
Identification of the essential matches in the pinch region can be clarified using what we
call CP(FCp) table [1,2,3]. In a CP(FCp) table as is shown in the graphs below, the CP(FCp)
values of the hot and the cold streams at the pinch are listed in descending order.
It is important to note here that the CP(FCp) inequality constraint applies only when a
match is made between two streams that are both at the pinch. Away from the pinch,
temperature differences increase and it is no longer essential to obey the CP(FCp)
inequalities.
In conclusion there are some essential matches at the pinch or the region of minimum
choice (ROMC) that need to be made around the pinch or the ROMC.
The next task is to design a network that exhibit minimum number of units. In other words
we need to decide the matched heat loads which minimize the HEN number of units.
The “tick-off” heuristic
Once the matches around the pinch have been chosen to satisfy the criteria for minimum
energy, the design should be continued in such a way to keep capital cost to a minimum. One
important criterion in the capital cost is the number of units since more heat exchangers mean
more skids; instrumentation, control; space; concrete and so on.
Keeping the number of units at a minimum can be achieved using the “tick-off” heuristic
[1,2,3]. To tick off a stream, individual units are made as large as possible. In other words the
smaller of the two heat duties on the streams being matched shall be taken completely.
Cooling water must not be used above the pinch to avoid unwarranted excessive use of
utilities, therefore if there are hot streams above the pinch where the duties are not specified
by pinch matches, additional process-to-process matches for more heat recovery shall be
explored; even if it leads to more number of units in the network than the estimated
minimum.
Same logic is correct for heating utilities application below the pinch. Hot utilitites must
not be used below the pinch to avoid unwarranted excessive use of utilities; both heating and
cooling. Therefore if there are cold streams below the pinch where the duties are not specified
by pinch matches, additional process-to-process matches for more heat recovery shall be
identified; even if it leads to more number of units in the network than the estimated
minimum.
Streams Splitting
Stream splitting is sometimes necessary to overcome the CP(FCp) constraints mentioned
above and/or to avoid using cold utility above the pinch or hot utility below the pinch.
Cooling utilities should not be used above the pinch. It means that all hot streams must be
cooled to pinch temperature by heat recovery. If we have a number of hot streams greater
than the number of cold streams (Three hot streams and two cold streams for instance) a
problem will then arise. Since regardless of the CP(FCp) values of the streams, there will be
one of the hot streams that will not be cooled to pinch temperature without some violation of
the ∆Tmin constraint. This problem can only be resolved by splitting a cold stream into two
