Cequel is an Excel add-in that provides the functionality of NASA's CEA chemical equilibrium code as an Excel spreadsheet function. It allows users to perform chemical equilibrium calculations from within Excel. Cequel includes a graphical wizard interface to help users set up problems and insert the calculation function. The function can be used to perform point calculations, rocket performance analyses, and shock and detonation analyses. Input and output can be specified through cell references, allowing automatic updates when inputs change. Cequel aims to combine the power of an Excel function with the ease of use of a graphical interface.

1. Cequel™:
Chemical Equilibrium in Excel
An Excel Add-In that provides all the
functionality of NASA’s CEA code as
an Excel spreadsheet function
Jonathan French www.praqsys.com

2. “What If”
• Chemical equilibrium codes let us seek answers to a
variety of question
– What combination of reactants will produce the best Isp?
– What is the flame temperature? Will the reaction melt my
device?
– How can I weigh engine performance vs the
concentrations of pollutants generated?
• To get a proper grasp of the answer or the design
space, multiple analyses need to be performed
• Having a quick and inexpensive means to perform
analyses frees and encourages the engineer to consider
ideas “outside the box”

3. Existing Chemical Equilibrium Tools
• Text based tools (CEA, CET, PEP)
– Requires text based input, user has to extract results from output text files
– Very user intensive
• Graphical interface tools (TEP, GUIPEP, RPA, PROPEP)
– Allows for ease of use to set up problem
– Resulting output based on what the developer anticipates is desired, or the
same text file generated by CEA
• Excel chemical equilibrium functions
– The user can incorporate the analysis into their own model
– Usually provides a specific set of inputs and outputs, tied to specific cells
– Not user friendly and not commercially supported
• Text based and GUI results are often copied and pasted into Excel
spreadsheets to document the results, or as part of a larger analysis
– If the analysis changes, then the subsequent chemical equilibrium analyses
must be rerun, and the results copied and pasted again
– The graphical interface approach makes it easier to modify the inputs
– Excel functions allow for automatic updating of the results when input changes

4. Power vs Ease of Use
• The power of an Excel function
– Uses cell references
– Can be copied for parametric analyses
– May be used with GoalSeek
– Allows the user to determine how to solve the
problem
– Established models may be modified and reused
• The ease of use of a GUI
– Define the problem and select reactants
using menus
– Choose the output parameters
– VIew results of an analysis
– Reduces potential for human error
These two approaches are combined in Cequel

5. Multiple Interfaces
• Cequel provides two ways to perform analyses
– Cequel’s GUI (called a “Wizard”) helps the user identify the
desired inputs, and guides the insertion of the function
into the spreadsheet
– The function CEQUEL() may be entered into cells to
perform calculations
• It may use existing cell information as part of it’s input
• Values that it generates may be referenced by other cells
– Cequel provides a ribbon interface to access the GUI, as
well as other menu items that make using Cequel easier

6. The Cequel “Equilibrium Wizard”
• Select the problem type, the
reactants and their
concentrations on a mass or
molar basis
• Problem types:
– Thermodynamic Point
Calculations:
• Pressure-Temperature (TP)
• Pressure-Enthalpy (HP)
• Pressure-Entropy (SP)
• Volume-Temperature (TV)
• Volume-Internal Energy (UV)
• Volume-Entropy (SV)
– Rocket, Shock and Detonation
Analyses
Buttons provide option to use cell references that are later
used when inserting the function in the spreadsheet

7. The Cequel “Equilibrium Wizard”
• Reactant Database
– CEA, PEP, TEP reactant
libraries merged together
– Thermodynamic library’s
species are included as
reactants (setting
temperature sets the
heat of formation)
– User defined reactants
can be incorporated as
additional library files, or
as part of a spreadsheet
for portability

8. The Cequel “Equilibrium Wizard”
• Point Type Problem
– Specify two
thermodynamic values
– Shows calculated results for
each output parameter
– “Checking” a term will
indicate it is to be returned
when inserting the function
in the spreadsheet

9. The Cequel “Equilibrium Wizard”
• Rocket, Shock and Detonation
Problems
– Specify specific inputs for each
problem type
– Multiple exit area or pressure
ratios can be specified
– Select terms to be returned by
the function in the
spreadsheet
– Holding down the control key
allows several different items
in a list to be chosen

10. The Cequel Function
• There are many possible inputs for a single
function call – the wizard helps set up an
instance that can be modified and copied
• Input order:
– Problem type
– Problem specific inputs
– Reactants
– Special options (LJP compressibility model
or altitude for the Isp, for example)
– Output parameters (keywords)
– Output species fractions (mass or molar
based)
• In this example, the each function just
returns the flame temperature. Cell
references are used to vary the tri-
propellant mixture ratios to generate a
surface plot
Single cell function call to
return flame temperature

11. The Cequel Function
• Input can be specified by
– Comma delimited parameters
– A single range of cells containing input parameters (as in
example on the right)
– A combination of parameters and ranges
• Excel functions are limited to 29 comma delimited
parameters, so specifying a cell range that contains
several inputs can help keep you under 29 parameters
• Cell references let the user specify the problem
definition or desired outputs in cells that can be easily
modified by just changing the cell value, rather than
having to edit the function
• Array functions return multiple values over several cells
with just one function call
• Example: Rocket problem
– Input is specified by a single range in the blue box to
embed the entire input in the spreadsheet instead of being
hidden in the function.
– All of the rocket problem outputs are calculated for
multiple area ratios with a single array function
• As for output, Cequel can return one value, several
values, or every value that CEA would yield
ParameterOutputKeywordsSpecies
Problem Definition
Chamber Pressure
Area Ratios
Reactants and Mass Fractions
Special Options
Single Array Function

12. Platforms
• Microsoft Windows XP, 7 and 8 with Excel
2010/2013
• Apple Macintosh (Yosemite) with Excel 2010
• In Development:
– MatLab (both as a function and an interactive GUI)
– Mobile and Web (only an interactive GUI)

13. Fully Functional Trial Version
• While Cequel is not for everyone...
– Heavy CEA users quickly find it to be indispensable
– Ease of use makes it accessible for other engineers, as
it reduces the learning curve for using CEA
• Praqsys provides a 30 day fully-functional trial
version that engineers can use to evaluate the
program’s capabilities
• It helps to get permission from IT departments to
allow engineers to evaluate Cequel, although it
can be installed without administrator privileges

14. Brief History
• In the late 1990’s, NASA JSC contracted the development of an Excel
function that executes a chemical equilibrium code, and given two
reactants returns specific values (Isp, C*, ratio of specific heats)
• The final project was an unholy mix of Excel Visual Basic for Applications
(VBA) and Fortran
• Upon completion of the JSC work, an internal effort was made to expand
the capabilities. The function was changed to include all possible I/O, and
a graphical “Wizard” with reactant databases was developed to make it
easier to place the function in the spreadsheet.
• The resulting product, “Cequel”, was marketed by Software and
Engineering Associates, Inc (SEA) from about 2000-2013
• Microsoft modified Excel’s menu for Excel 2010, breaking Cequel in the
process. SEA could no longer support Cequel, so they sold it to the
original developer, Jonathan French
• Cequel now runs using Microsoft Excel 2010 and 2013 under Windows
XP/7/8, and on the Apple Macintosh with Excel 2011