Environmental Engineering Reference
In-Depth Information
The example given in this chapter for the illustration of the pinch design method for heat
exchangers network design is very simple. Before going through the design of the network for
this simple example in a step-by-step manner, some important rules in pinch design method
need to be mentioned to enable the reader solve more complicated problems.
Start at the Pinch
The pinch is the most constrained region of the problem. At the pinch, ∆Tmin exists
between all the hot and cold streams. As a result, the number of feasible matches in this
region is severely restricted. Quite often there are essential matches to be made. If such
matches are not made, the result will be either using a temperature differences smaller than
∆Tmin or we have to buy more utilities due to heat transfer across the pinch.
Since the pinch point divides the problem into two subproblems we are going to solve
first the above pinch subproblem. We need to start at the pinch. There are some rules for
matches to be both feasible and efficienct for the designated
∆
T_min. With feasible we mean
that the hot stream shall always have a
∆
T_min over the matched cold stream. We will be
elaborating more about feasibility later in this section.
With efficienct we mean that a hot stream shall be matched above the pinch at the pinch in a
way that he/she reaches its target temperature at the pinch. Hence, we reach the target heating
utility calculated beforehand. Otherwise we will be needing to buy cold utility above the
pinch!! Resulting in more heating and cooling utilities than the ones originally calculated
during the energy targeting step.
In order to be able to achive this requirement, during matching above the pinch we have
to have enough number of cold streams above the pinch at the pinch to enable each hot stream
above the pinch at the pinch reaches its pinch temperature via matching with a cold process
stream. Otherwise we end up using cold utility above the pinch. If the number of hot stream
above the pinch at the pinch is less than the number of cold streams in such situation we can
consider splitting cold streams to enable us keep the following two rules intact above the
pinch at the pinch. These rules of the inch design method will be further elaborated in this
section.
But for now we need to keep in mind the following two rules for streams matching above
the pinch at the pinch:
1.
Number of hot streams above the pinch at the pinch shall be less than or equal to
number of cold streams; and
2.
Upon matching hot and cold streams, hot stream CP(FCp) should be less than or
equal to the cold stream matched with it (lighter in load) to avoid infeasiibilty in
matching at some point and again to avoid using cold utility above the pinch at the
pinch that results in consuming more than originally calculated energy target
For streams matching below the pinch at the pinch the reverse is true:
3.
Number of cold streams at the pinch shall be less than or equal to number of hot
streams; and
4.
Upon matching hot and cold streams, cold stream CP(FCp) should be less than or
equal to the hot stream matched with it (lighter in load) to avoid infeasiibilty in
