A Psychrometric Chart in Excel

Excel is my favourite tool to design Pharmaceutical HVAC systems (but not the only). I always like to have a graphical view of the system.  I normally use an Excel file with all calculations in several sheets (air changes, leakages, gains, etc.), and finally, a plot summarizing the calculations. We can plot easily a psychrometric chart in Excel.

Let’s see how.

Download psychrometric functions Addin for Excel

First, download the open-source psychrometric plug-in for Excel. Developed by the Western Cooling Efficiency Center, you can download from here:

http://wcec.ucdavis.edu/wp-https://wcec.ucdavis.edu/resources/software-resource-applications/

You can find the Python file including calculations formulas for your peace of mind here:

https://github.com/remcmurry/Psychropy/blob/master/psychropy.py

This package includes a very well written and concise manual. You can install the package as a plug-in in Microsoft Excel.

Once installed, if you click on ‘insert function’ and select the ‘user-defined’ category.  You can find a list of the available psychrometric functions:

With this fantastic plug-in installed, plotting a psychrometric chart is straightforward.

Humid air properties depend on the altitude (and consequently on the barometric pressure). So first, we must determine this by means of the function STD_Press. In yellow cell H10, we have manually put the site altitude:

A psychrometric chart consists of a plot with:

• Humidity Ratio in the y-axis
• Dry Bulb Temperature in the x-axis

at different Relative Humidity percentages. So, the first thing to do is to prepare a table with increments of temperature. Let’s say 2 °C, with increments of RH, 10%:

Table preparation is almost done. We have to calculate for each Relative Humidity (columns D to M), and corresponding Temperature (Rows from 9 to 34 in the example). The appropriate function is Hum_rat2. As a result:

And entering the function arguments, in this case, Tdb (Dry bulb temperature) and RH (Relative Humidity), at the corresponding Barometric Pressure (P). The result is in g water/g air. For a more comfortable viewing result we can multiply by 1000:

And finally, we only have to extend the formula across the entire table:

Plot psychrometric chart

With all these data collected, the individual lines for relative humidity can be plotted:

As a result, with all relative humidity curves, we’ll see something like this:

I hope you find this article interesting. Remember that you can use this tool, not only for Pharmaceutical HVAC applications!.