Page 11 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

1.1 Process 5

Engineering design is not just a scientific solution. Merely achieving the functional goal is not
sufficient. The solution must achieve its functional goal economically.
Improved technical performance with the implementation of a “better” or
more efficient design is always associated with the cost to be incurred.
Design
Therefore, the industrial process design has to be an optimum design,
balancing the improved performance and increased cost.
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An example can be designing a heat exchanger to cool a stream of 70 m /hr
of kerosene in a refinery from 90 to 40 C. This is the design objective for the

process and may be achieved by several options:
• use of a water-cooled shell and tube heat exchanger
• use of a water-cooled double pipe heat exchanger
• use of an air-cooled heat exchanger (fin fan cooler)
• direct contact with an immiscible cold fluid (water) followed by gravity separation in a settler.
• utilizing some specific cold process stream that needs to be heated; this can be an attractive design
option as it would save energy
There may be many more exotic options for achieving the same design objective. The acceptable
design alternative has to meet the design objective and strive to be economically optimum.
In addition, the following are also considered:
• Practical deviations in design input parameters
It needs to be appreciated that the design inputs are associated
with a certain level of uncertainty and the design solutions are
Design Considerations worked out based on the best estimates of these inputs. Inputs may
also change over time due to change in operating conditions of other
sections in the plant, variation in market demand or product spec-
ifications and raw material quality. There can be other reasons too. For example, the kerosene stream

known to be available at 90 C in the design stage of the heat exchanger may be available at a slightly

higher temperature (say 94 C) after the equipment has been installed. A “robust design” needs to
accommodate, up to a limit, such uncertainties particularly in quality (temperature, pressure,
composition, etc.) and quantity (flow rates) of the input streams for the process. Safety factors or
design margins take care of this during design calculations but leads to “overdesign.” This is illustrated
in the design problems discussed later in the book. Nevertheless, the extent of overdesign needs to be
optimal as costs increase with a margin of overdesign. Such limits in specific cases are decided based
on industry practice, designers’ experience, and the criticality of the function of the designed process/
equipment. Typically heat exchangers are overdesigned to the extent of 10%e20% of their rated heat
load.
• Compatibility with rest of the plant or the process complex
Compatibility is enforced by clearly defining the specifications, specifying the design codes and
material standards. Such compatibility is warranted in terms of mechanical features, reliability and
process considerations. Mechanical compatibility is essential for connectivity of different equipment.
This is enforced by using the same standards for the flanges and fixtures for “hooking up” the designed
equipment to existing or other equipment that may have been delivered from a different design group.