Page 33 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

2.5 Heat exchanger design methodology 29

(optimum) for a given application. Near-optimum heat exchanger designs involve several trade-offs.
For example, a cheaper exchanger can be used at the cost of reduced performance and durability or a
higher performance can be obtained for a heavier or more expensive exchanger. Similarly, a smaller
heat exchanger can be opted at the cost of lower performance or higher pumping power for higher fluid
velocities, etc. The designer needs to arrive at the optimum exchanger for a given application to meet
the design requirements and constraints. Prior experience is the best guide for selection and design of
an exchanger if one or more exchangers are in service for similar applications. Table 2.1 suggests some
selection criteria. It needs to be understood that the values quoted are based on general industrial
practices and information from different sources. These can be altered judiciously for specific design
cases.
To summarise, the most versatile exchanger for a broad range of operating pressure and temper-
atures are shell and tube exchangers for medium to high heat duties and double-pipe exchangers for
lower heat duties (<500 kW). The shell and tube exchanger is most widely used in chemical industries.
Plate-type exchangers are more economical regardless of the heat transfer performance. For high heat
duties, the least desirable solution is the double-pipe exchanger.

2.5 Heat exchanger design methodology
Heat exchanger design involves (1) Firming up the process and design specifications, (2) Thermal and
hydraulic design, (3) Mechanical Design, fabrication detailing and cost estimation. The final design is
a trade-off between several factors and a system-based optimisation.

Process and design specifications
This covers all necessary information to design and optimise an exchanger for a specific application.
The problem specification for operating conditions, exchanger type, flow arrangement, material and
design/manufacturing/operating considerations is included. The design basis requires input on fluid
mass flow rates and their physical properties, inlet temperature and pressure of both fluids, required
heat duty and maximum allowable pressure drop on both fluid sides, fluctuations in inlet temperature
and pressure, corrosiveness and fouling characteristics of the fluids and operating environment.
The exchanger design needs to meet the functional requirement of (i) transferring the specified heat
duty using the available temperature difference and (ii) ensuring that the
pressure drop remains within the maximum permissible limit for both
fluids. The design should also be evaluated from the viewpoints of erection,
Deliverables
commissioning, operation, cleaning, maintenance and alteration/extension
in future. Design problems discussed in Chapters 3 and 4 illustrate the
design procedure.
Optimum design: Arriving at an optimum exchanger design based on an economic parameter is
difficult and complex. In reality, a balance between the heat transfer coefficient and pressure drop of
each fluid is struck while designing the heat exchanger. Usually the cheapest exchanger of a particular
type that satisfies the duty and pressure drop constraints with the smallest area is the chosen optimum.
Higher heat transfer coefficient in case of shell and tube exchanger (tube/shell side) is achieved by
increasing the number of passes that invariably increase the pressure drop. A higher efficiency is