Environmental Engineering Reference
In-Depth Information
It is instructive to mention here that the hot stream H1 and the cold stream C2 can be
called “near” pinch streams and in some commercial software another near pinch vertical line
will be drawn at 330 ºC-320 ºC as shown in figure 21 below.
The Grid Diagram for the Step -By-Step HEN Design
Near
Pinch
Pinch
CP
(kW/ ºC)
310 ºC
QH_min = 2870 kW
520 ºC
330 ºC
H1=10
380 ºC
300 ºC
H2=5
550 ºC
300ºC
C1=20
380 ºC
320 ºC
C2=2
QC_min = 50 kW
300 ºC
Figure 21. Grid Diagram with near pinch representation
We will start now the design above the pinch at the pinch. Since we have only one
possibility for hot stream matching with cold stream at the pinch, we will check the CP(FCp)
matching rule. Since stream H2 has CP(FCp) equal to 5 while the cold stream C1 has a
CP(FCp) equal to 20, the CP(FCp) matching rule here rule is satisfied, we do not need to
make any split and we can match the hot stream H2 and the cold stream C1 as shown in the
graph below, figure 21. Upon matching H2-C1 in a heat exchanger uint we need to tick-off
one of the streams to minimize the number of heat exchangers, hence we let the stream with
lower heat load ticked-off. In this example it is the hot stream H2. The heat exchganger heat
duty/load can now be calculated by simply using Q= FCp (Ts-Tt) formula. This load (Q) in
this example is equal to 350 kW. The hot stream H2 has now been cooled down to its pinch
target temperature of 310 ºC and the cold stream C1 has been heated up from its pinch
temperature at 300 ºC to 317.5 ºC as shown in figure 22.
It is also important to note here that in the pinch design method, away from the pinch there is
no systematic technique to complete the HEN. The designer can use his/her common sense to
complete the design to reach feasible network rendering the exact minimum utilities
requirements with minimum number of units.
It is clear in figure 22 above that the hot stream H1 and the cold stream C2 can also have
a good match since hot stream H1 can be ticked-off completely, upon such matching with C2
resulting in a HEN with the desired estimated minimum number of units calculated earlier for
the this example (5 units). The minimum approach temperature between the two streams is
still not violated, greater than or equal to the specified
∆
T_min=10 ºC, and is equal to 12.5 ºC.
