Page 36 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

32 Chapter 2 Heat transfer processes in industrial scale

obtained when the shell-side and tube-side heat transfer coefficients are of the same order of
magnitude. This can be achieved by addition of fins on the fluid side with lower heat transfer coef-
ficient but at the expense of increased capital cost. Such considerations may also influence the se-
lection of the shell-side and tube-side fluid as well as the number of passes. In case the pressure drop
gets limiting, parallel sets of exchangers or parallel heat exchanger trains are considered for large flow
rates. Optimisation of such exchanger trains are carried out in practice using Pinch Technology dis-
cussed in Chapter 5.
Heat exchanger design is thus essentially an iterative procedure as the physical layout of the
exchanger can only be decided after the heat transfer area to be provided is known. On the other hand,
the physical/mechanical arrangement of the heat exchanger affects the overall heat transfer coefficient
and consequently the required heat transfer area for delivering the design heat duty.
Based on the problem specification and the design engineer’s experience, an appropriate exchanger
type and flow arrangement is first selected fol-
lowed by its sizing that complies with the design
specification. This calls for thorough knowledge
Selecting Exchanger Type and configuration of the exchanger types and their construction
features along with their suitability for different
applications. Table 2.1 may be referred for this
purpose. Some of the related information is
available in the different codes for Shell and Tube Heat Exchanger design (e.g., IS4503 e 1967
(reaffirmed 2003), BS EN ISO 16,812:2007 and TEMA-R/C/B). As discussed in Chapter 1, it is
desirable that the design develops into a standard configuration and size to the extent possible. This
would not only allow use of established thermohydraulic design equations but also minimise the
delivery time, a major consideration in any project. Use of standard dimensions (e.g., size, rating and
number of tubes, shell id, type and % cut of baffles, etc., for a shell and tube exchanger) also goes a
long way to make the fabrication of equipment easy and enables interchangeability of equipment parts,
which reduces inventory of common spare parts (say spare tube bundle). This is particularly important
in large projects.
Next the exchanger geometry and material are selected. The core geometry is selected for tubular
exchangers (shell type, number of passes, baffle geometry, tube layout, etc., for shell and tube
exchanger) and surface geometry for a plate (plate type and size, number of plates, pass arrangements,
gasket type, etc.), extended surface or regenerative exchanger. Some of the criteria for selecting core
geometry are the desired heat transfer performance meeting the maximum limit of pressure drop,
operating pressure and temperature, thermal/pressure stresses, fluid characteristics and total installed
cost. For compact heat exchangers, one needs to select fin type and geometry.

2.6 Design overview for recuperators

Exchanger design consists of thermal and hydraulic aspects as well as arriving at the complete
mechanical details. The thermal and the hydraulic design of the exchanger are interrelated. Mechanical
design aims at realisation of the thermal design in practice, i.e., the detailed equipment specifications
necessary for fabrication/manufacturing and also considerations like stress and vibration analysis.
Appropriate considerations and analysis during the mechanical design is essential to ensure equipment
integrity under steady state, transient, start-up, shutdown, upset, part load operation as well as maintenance.