Sankey Diagram Guide

Instructions are given for the preparation of Sankey diagrams. Sankey diagrams are used for displaying flows through a system. They are especially useful for displaying mass, energy, and cash flows.

Preparation of Diagram

• The first consideration is to determine the basis of the diagram. Typically a basis is selected as the flow rate of the quantity under consideration, such as the fuel input or investor cash flow.
• Then, draw the flow streams. In a Sankey diagram, a stream is represented by two parallel lines whose distance apart is proportional to the flow rate.

Example

To illustrate the concept of a Sankey flow diagram, a simple example is presented.

System

An oil fired heater is used to heat a heat transfer medium. The heat transfer medium is pumped to an evaporator in which steam is generated. Then the heat transfer medium is returned to the fired heater. A block diagram of the example process is shown in Figure.

Basis

The basis of the associated Sankey diagram is 100 units of heat in the fuel per unit of time.

Fired Heater Balance

In addition to the fuel energy input to the heater, are 20 units per unit time of thermal energy of the combustion air. And, the recycled oil has a heat flow rate of 50 units per unit time. Radiant energy losses from the heater are 15% of the fuel energy input and the flue gas effluent heat is 35% of the fuel energy input. Hence, the heater efficiency is 50% based on fuel input.

Evaporator Balance

In addition to the hot transfer medium entering the evaporator, thermal energy of the feed water is 15 units per unit of time. Thermal energy losses from the evaporator via of radiation and blowdown are 15 units per unit of time.

Analysis

As may be noted, 75 units of the heat appears in the steam. This gives an apparent net thermal efficiency of 75% for the overall system. The large recycle stream of 50 units per unit time must be questioned. Should be necessary to recycle so much heat? Or, could some of the this thermal energy be used elsewhere before the heating medium is returned to the heater?