Sankey Diagrams

Found this Sankey-like diagram accompanying an abstract submitted for the 2001 International Conference on Thermal Engineering and Thermogrammetry. Posting it here on the blog before this site eventually vanishes. This Sankey diagram is a good example of how not to draw Sankey diagrams, I think. Or, as a Japanese friend would put it politely: “Maybe… [turn head at 30° degree angle, make slight air-sucking noise by inhaling through open mouth] … maybe not so good”.

Here’s the diagram:

The idea was to display heat losses at a slab furnace in a Turkish steel plant. Heat losses were identified in exhaust gases (22 %), at the cooling pipes (2.90 %), and at the furnace walls (0.47 %).

The fact that the widths of the arrows displaying the heat losses were chosen arbitrarily give a completely wrong idea of the proportions. The powerpointish curved arrows don’t really contribute to a better understanding.

All in all, not a very good one. Adding this to my “Lying with Sankey diagrams” mini series (see part 1, part 2), which has been neglected recently.

The Mexican National Commission on Energy Saving (Comisión Nacional para el Ahorro de Energía (CONAE) present several success stories (casos exitosos) on their website.

One success story dates back to 1997, and describes how an energy efficiency study of fired heaters (i.e. boilers) was carried out in a Nafta producing facility in the Veracruz state of Mexico. As a result of the study, several suggestions for optimization were implemented. Fuel consumption could be reduced by 23-24 %, while the efficiency of the ovens could be raised by 13% (calentador BA-2001 B) and 16% respectively (calentador BA-2001 A).

Para los hispanoparlantes: el título oficial del proyecto fue “estudio técnico económico e ingeniería conceptual realizada a los calentadores a fuego directo BA-2001 A/B de la planta hidrodesulfuradora de naftas, del C.P.Q. “La Cangrejera”, ubicado en Coatzacoalcos, Veracruz” (otro candidato para el concurso Mundial de titulos largos).

The heat losses are shown as Sankey diagrams. The first describes the optimal situation, with an energy efficiency of 82,4 % “as guaranteed” by the maker of the fired heater.

The two other Sankey diagrams show the energy balance of the heaters A and B before the implementation of the measures. They run with an efficiency of 60,6 % and 62,35 %.

The arrows branching off at the top show the heat losses. I like the fancy icons that show how energy is lost through the walls, because of deteriorated or insufficient insulation, and heat energy in the effluent gases. The flows are given in MMBTU/h (millions of BTU per hour).

Unfortunately two of the diagrams are not to scale: The arrow to the right in the second diagram should be roughly 2/3 of the width on the left side. It is about 4/5 (or 80 %) of the width, similar to the width in the first Sankey diagram. This is a visual exaggeration of the inefficiency. However, I refrained from featuring this in my informal “Lying with Sankey diagrams” series.

A few months ago I had found this b/w Sankey diagram on the website of the Institut de Génie Thermique (IGT) de la Haute Ecole d’Ingénierie et de Gestion du Canton de Vaud (HEIG-VD) in Switzerland, showing the energy or heat balance (bilan thermique) of an average family home.

It visualizes the sources of heat as Sankey flows into the building (in MJ per square metre) with the largest chunk being the combustible for the heating system, other inputs are from solar radiation and internal sources. On the right side it shows how and where heat is being lost: windows (fenétres) 122 MJ/m², ventilation (aéreation) 113 MJ/m² or roof (toit) 57 MJ/m². Also, the technical losses from the heating equipment (pertes techniques, shown as Sankey arrow from the heater to the top) are quite significant (57 MJ/m²).

A similar Sankey diagram in German was presented on the e!Sankey forum recently.

This diagram submitted by one of their users is explained as follows:

In the diagram the group of flows in red colors are heat losses due to transmissions through walls, windows, doors, etc. The dark blue arrow shows heat loss through ventilation. The stacked purple/mauve flow represents heat losses at equipment and pipes.

While a little more detailed in the number of flows, it shows the same general situation: In many houses “a lot of the heat gets lost due to heat leaks (thermal bridges) or insufficient external insulation.”

This article on the FAO website shows a comparison of several types of simples stoves and their energy balance using Sankey diagrams.

The Sankey diagrams show how the energy (typically from wood firing) is lost, and that only a small fraction of 12 to 20 % is actually being used as “useful heat”.

More of these “heat flow diagrams” can be found in chapter 4.2. of the article.

A rather special feature of the diagrams shown in this article is that the percentile values given for the flows cover a range (e.g. Ash and Char 5,97% - 12,15%), rather than a specific absolute value. This is rather untypical. Also, it can be noted, that the width of the arrows are not always to scale: compare, for example, the width if the “Surface” arrrow to that of the “Thermal Mass” arrow. It should be roughly four times wider.

The same Sankey diagram created with a Sankey software tool shows the arrow widths correctly.