Rediscovering nomographs as intuitive tools for complex refrigeration and HVAC calculations.

Shakespeare wrote “What’s in a name? That which we call a rose, by any other name would smell as sweet.” When it comes to selection software, apps and computer-aided design tools, however, there are some that are good, some that are tolerable and some that stink.

Before there were smartphones we had laptops, and before laptops there were personal computers. Even further back, when detailed calculations were required so that competing factors in a design would be balanced, we had scary looking graphs that presented a visual path through complex calculations. These might be showing a pipe sizing calculation given volume flow rate, density, friction factor and pressure drop, or a fan calculation to deliver power requirements and air pressure developed.

It was quite common to have two or three graphs arranged on a page so that a line could be drawn vertically on one graph to a point of intersection on a diagonal on the second graph and then horizontally from there to a second diagonal on the third graph. Great ingenuity was required to create the appropriate layout, and when it was done well, it created a thing of rare beauty. When the system offered the calculation in both I-P and SI units, it got even more convoluted, with axes on all four sides of each graph, and sometimes colors for additional layers of complexity. This was problematic in the days before easy color photocopying because a good nomograph would be passed from trainer to apprentice, from master to novice, from generation to generation until the copies became almost unreadable.

I reminisced about this with a friend at the ASHRAE conference in Chicago last January, and we realized that everyone had their favorite nomograph and would go to great lengths to defend it. My personal gem was for calculating freezing time for different sizes and shapes of foods under different types of wrapping in different styles of plant and at different temperatures. This remarkable Rosetta Stone of freezing served me well until we had to design a plate freezer plant with carbon dioxide as the refrigerant. I learned then that the knowledge of the ancients in the nomograph used some substantial assumptions in the foundations, particularly relating to the drop in saturated temperature caused by the hoses that connect the plates of the freezer to the suction header. The unusual (and beneficial) pressure-temperature characteristic of carbon dioxide produced a freezing performance that was literally “off the chart.”

The advantage of a nomograph over an app is the visual impression created by the drawing exercise shows whether the answer lies in a sensible place or not. These days we are used to “computer says no,” and if we don’t crash the server or get a terse error message, then it is natural to assume that all’s well with the world. This is not always the case.

As another example of the visual feedback that is lacking in many modern selection programs, I used to have on my smartphone a beautiful example of the crossover between these two worlds. It was an app that simulated the function of a traditional slide rule—even to the lovely wood grain of the frame and the ivory of the slider. I held in my hand a computer with more calculating power than the Apollo 11 moon landings, and I used it to do logarithmic approximations on a simulation of 400-year-old technology. Crazy, but fun.