Sankey Diagrams

A research project at Technical University Dresden in collaboration with a paper manufacturer aims at detecting and implementing energy saving measures for paper machines.

“Better monitoring of process variables, improvement in the operating point (or range) of the process, installation of heat recovery systems (e.g. at heat exchangers), use of heat pumps and replacement of high energy streams (steam) with waste heat streams. Most of these techniques are standard recipes which are readily available in the market. However they are designed without giving any consideration to the specific aspects of a manufacturing facility and also provide no quantitative evidence of the potential benefits. “

This is why they came up with a mass and energy balance model and are using the following Sankey diagram to visualize where energy could be recovered.

Flows are labeled as enthalpy flows, showing the changes in thermodynamic (heat) energy at each process step, hence losses. The Sankey diagram is for a balance period of six hours. No absolute figures are given. Several heat exchangers are installed along the wet section of the paper machine. In the drying section, a large part of the energy is lost as steam/humid air.

This article on “Energy Savings in Tissue Production Process: The Case of the Hayat Tissue Mill in Turkey” by A. Isiklar, L. Aydin, D. Mainardi and O. Lopez was published in July 2008 in the TAPPSA Journal (Technical Association of the Pulp and Paper Industry of the Southern Africa). The article describes how energy can be recovered from process air in a tissue plant in Turkey using a cogeneration hood. It features three beautiful Sankey diagrams, one of which is presented here.

“The exhaust gases coming from the hood are used for the production of the steam needed to feed the YD and the other auxiliary equipment of the mill (wet strength pulper, hall ventilation). The residual energy in exhaust gases in excess from the boiler are used in order to feed a chiller unit, which in turn runs the air conditioning system of the electrical room”.

This almost symmetric top to bottom oriented diagram shows the energy in MW for a certain production capacity (details not given in the article). It is a section of the other Sankey diagram featured in the article (Fig. 3) showing the whole process including the gas turbines plant, the cogeneration hood and the waste heat boilers (omitting only the absorption chillers). Only the latter shows the reduced heat loss (see light blue arrows labeled “to atmosphere”).

As for me, that’s the kind of curves I love…