Page 17 - Design and Operation of Heat Exchangers and their Networks

4 Design and operation of heat exchangers and their networks

be more than two steams) flow either countercurrently (counterflow) or
cocurrently (parallel-flow). Therefore, one-dimensional temperature dis-
tribution can be assumed. In crossflow heat exchangers, the hot and cold
fluids (two steams or multistreams) flow perpendicularly to each other,
and the temperature distribution can be two-dimensional or three-
dimensional. In such a case, if no complete transverse mixing exists, then
the temperatures of fluids in the flowing channels are two-dimensionally
distributed, and therefore, the exit temperature is not uniform. The
so-called exit temperature is the isothermally mixed temperature at the
outside of the exchanger exit.

1.2 Design and simulation methodologies of heat
exchangers

Many well-established methods have been available for thermal design of
simple heat exchangers, such as effectiveness-number of transfer units (ε-
NTU) method and logarithmic mean temperature difference (LMTD)
methods. These are two major methods widely used for design and rating
purposes and will be expounded in Chapter 3.
For complex flow pattern in a heat exchanger, there might be no avail-
able ε-NTU relation. In such a case, the cell method can be applied. We can
divide the heat exchanger into several subregions (cells), which are con-
nected according to the real flow passages. Each cell is considered as a single
heat exchanger of which the type is most closed to the real flow pattern.
Thus, the thermal performance and the dynamic behavior of the whole
apparatus can be simulated by a system of interconnected subunits (cells).
To obtain the transient response of finned crossflow heat exchangers,
Kabelac (1989) divided the heat exchanger into small, geometric simple
basic elements (cells). The dynamics of each cell was solved in the Laplace
domain. Then, these cells were recombined to give the transients of an
entire crossflow heat exchanger. Bonilla et al. (2017) studied several shell-
and-tube heat exchanger models with different degrees of complexity,
including the cell method model, for process simulation and control design.
Simulation results were compared with experimental data, which showed
that the cell method was the most precise but at expense of a higher com-
putation time since the model was more complex than the other models. It
should be pointed out that the outlet fluid temperatures of each cell are the
fluid bulk temperatures after adiabatic mixing. This approximation might
not coincide with the real flow pattern. As an example, for a crossflow heat