Page 267 - Design and Operation of Heat Exchangers and their Networks
P. 267
256 Design and operation of heat exchangers and their networks
In the pinch design method, Δt min is an important parameter for the bal-
ance between the investment costs and utility costs. A large value of Δt min
would decrease the investment costs but increase the utility costs and vice
versa. Furthermore, the pinch position could also change with Δt min . The
value of Δt min can be optimized by taking the total costs of the network
as the objective function.
The pinch design method focuses on the matches of streams near the
pinch because at that point, the temperature difference is the minimum.
For the matches away from the pinch, the earlier rules must not be fulfilled.
In some cases, there might be multiple pinches or no pinch. A detailed
description of the pinch design method can be found in Linnhoff et al.
(1982).
Example 6.4 Pinch method for H2C2_175R.
We take the problem data of H2C2_175R (Ravagnani et al., 2005. See
Table 6.3) as an example to illustrate how to design the network with
the pinch technology (Luo and Roetzel, 2010, 2013). The problem deals
with two hot streams (N h ¼2) and two cold streams (N c ¼2). Let
Δt min ¼5K, it is easy to calculate the problem table by Eqs. (6.63)–
(6.70), which gives the pinch position at t h ¼125°C, Q HU,min ¼200kW,
Q CU,min ¼120kW. Other results are given in Table 6.4. The composite
curves are shown in Fig. 6.3. The detailed calculation procedure can be
found in the MatLab code for Example 6.4 in Appendix.
To design the network, we divide the problem into two parts at the
pinch, as is shown in Fig. 6.4. In the part above the pinch, there is only
_
one match: H1C1, that is, N h ¼N c ¼1 with C H1 ¼ 10 kW/K,
_
C C1 ¼ 20 kW/K; therefore, Eqs. (6.74), (6.76) are fulfilled, and no
splitting is necessary.
In the part below the pinch, N h ¼N c ¼2, which meets the rule (6.75).
We would like to choose the matches H1C1 and H2C2 due to their
Table 6.3 Problem data for H2C2_175R (Ravagnani et al., 2005).
2
_
Stream T in (°C) T out (°C) C (kW/K) α (kW/m K) Cost ($/kWyr)
H1 175 45 10 2.615
H2 125 65 40 1.333
C1 20 155 20 0.917
C2 40 112 15 0.166
HU 180 179 5 110
CU 15 25 2.5 10
2
Heat exchanger cost¼1200A 0.57 $/yr (A in m )