Custom Calculations Made Simple: 8 Best Practices | AIChE

# Custom Calculations Made Simple: 8 Best Practices

Computational Methods
November
2023

When pre-programmed solutions are not available for solving complex computational problems, develop custom calculations. Employ these best practices to create thorough, accurate, and clear solutions.

For some engineering problems, dedicated software provides well-developed, robust solutions. However, for many problems, a dedicated software solution may not exist. Even when someone develops and distributes a dedicated solution, it may be prohibitively expensive, cumbersome, or lacking a capability that’s key for a given problem. In all such cases, it’s valuable to develop custom calculations.

Custom calculations are any calculations for which the equations haven’t been programmed in advance and must be assembled or reviewed by anyone who prepares, modifies, or reuses the calculation. Custom calculations can be developed using traditional programming languages like Python or symbol-manipulation software such as SymPy. However, such tools are used relatively infrequently by most engineers.

This article discusses how to develop custom calculations using two high-level tools that every engineer can use: Mathcad and Excel. These apps are widely available and simple to use once you work around Excel’s built-in limitations. They are also suitable for calculations of considerable complexity, like those with many steps (1) or multiple steps that are executed in parallel (2).

Like word processors, custom-calculation apps such as Mathcad and Excel make writing legible and easily editable, which increases the efficiency of communication and execution. Unlike word processors, custom-calculation apps allow users to change an input data entry or an equation, and the app will automatically update every subsequent equation and output. These apps’ word-processing and calculation capabilities work together to enable users to progressively improve at solving problems and communicating the solutions’ significance.

Well-executed custom calculations benefit anyone who factors numerical information into his or her decision-making. Experienced engineers benefit from tailoring their calculations to the work at hand; less-experienced engineers benefit from presenting their calculations clearly; graduate and undergraduate students benefit from organizing key new material they are learning. Custom calculations help in business planning, research and development, design, and operations. They are valuable for preparing estimates, summarizing calculations that use dedicated software, independently verifying such calculations, and preparing design bases for various hardware components.

Each person’s use of custom calculations will be different. In process design, I have used custom calculations on a variety of unit operations to solve a variety of problems, including:

• Chemical reactions. I have used custom calculations to size continuous stirred tank and tubular flow reactors. To ensure that a process wouldn’t be operated in the explosive range, I developed a custom calculation that accounted for the electrolyte effect on vapor-liquid equilibrium. In a specialized application, I used a custom calculation to estimate the time required for a minimum dose of a reaction-kill solution to flow from a pre-pressurized tank into the runaway reactor.
• Separations. I have used custom calculations for preliminary sizing of distillation towers, reactive scrubbers, and vented decanters.
• Heat transfer. I have used custom calculations to rate a reactor jacket’s heat transfer and pressure drop, to estimate process condensers’ process flows during vacuum stripping, and to estimate jacketed vessels’ heating and cooling times.
• Fluid flow. Custom calculations can be used to estimate centrifugal pumps’ required heads and positive-displacement pumps’ required pressures. In other applications, I have used custom calculations to estimate a tank-mixing eductor’s mixing time, and a vacuum pump’s required flow, inlet and outlet flammability, and design pressures to prevent permanent deformation or rupture from a deflagration.
• Piping and instrumentation. I have used custom calculations to summarize simulator runs for pressure-relief device sizing, rate a pair of rupture disks to relieve a deflagration, size a stack height to disperse hydrogen safely away from a building, and prevent second-degree burns by estimating the maximum allowable time in contact with hot piping or insulation.
• Utilities. Custom calculations can also be used to determine unit costs for utilities and to estimate a chiller’s load.

Following the pathway described in this article will help engineers prepare custom calculations. This article discusses how to include only the necessary information, ensure that the information is correct, make the calculation’s unfolding story clear to readers, and even make the calculation’s backstory easy to follow. These actions optimize calculations’ accuracy, clarity, and reusability. The following eight best practices will help you prepare in advance, document critical information, and share and retrieve calculations...

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