Welcome to the Aspen Plus V8.6 teaching module on Simulation of Steam Engines.
For information on navigating this module, please refer to Navigation Hints located above the slide. Click the Next button on the bottom right hand corner to begin.

Liquid Water is pumped into a boiler, at pressures in excess of 150 bar.

Water is vaporized in the boiler and becomes high temperature/pressure steam.

Steam flows through the turbine and does work, as thermal and potential energy is transformed into mechanical energy to turn a generator. Pressure and Temperature of the stream is reduced and partially condenses.

The Steam is further cooled and condensed completely in the condenser. The far denser water can now be recirculated back to the boiler.

Create a new simulation in Aspen Plus using the Blank Simulation template. The Components | Specification | Selection sheet is displayed. Enter WATER for Component ID.

Since water is the only component, Method assistant should recommend IAPWS-95 property method. Go to the Methods | Specifications | Global sheet and select this for base method. The IAPWS-95 property method uses the 1995 IAPWS steam table correlations for thermodynamic properties. This is the current standard steam table from the International Association for the Properties of Water and Steam.

Click the Analysis | Pure button from the Home tab of the ribbon to create a new pure property analysis. Select PL for Property and enter 460 for Upper limit and 50 for No. points. PL is short for the vapor pressure of a liquid. In the Available Components list, move WATER into the selected components list.

Press the Run analysis button to generate the property plot for water. This plot shows which phase WATER should be in for a given temperature and pressure. The region to the right of the curve is superheated steam. The region to the left of the curve up the critical temperature is liquid water. For this simulation, steam will be generated at 460˚ at a pressure of 40 Bars. Since 40 bars is far less than the vapor pressure of water at 460˚ this is well within the superheated vapor region. For this design, we will be considering a non-condensing turbine so it is important to ensure that the steam stays above its dew point temperature as it moves through the turbine.

Enter the simulation environment. Add two Heaters, a Pump and a Compressor. Connect the four units with Material Streams. You may have noticed that we are adding a compressor in place of a Turbine. In Aspen Plus, the COMPR block doubles as a turbine and a compressor. Relabel your material streams as shown.

Go to the COOLH2O input form. Specify the water as 98˚ 1 Bar, at a flowrate of 10,000 kg/hr.

Go to the boiler input sheet. Specify the boiler at 460 ˚ and 40 bar. This will be the conditions of steam entering the turbine.

Go to the condenser input sheet. Specify the output as 98˚ and 1 Bar.

Go to the pump specification sheet. Water is discharged into the boiler at the boiler pressure, 40 Bar.

Go to the turbine input sheet and select the turbine radio button. In Aspen Plus, the same model with work for a turbine and a compressor. The turbine will discharge water at 1 Bar pressure. Go to the Convergence sheet and select Vapor-Liquid from the drop-down list for Valid phases.

Now that the steam engine is fully specified, we can run the simulation.

Note that there are two warnings regarding lack of feed and outlet streams for this flowsheet. These appear because water circulates within the flowsheet. Therefore, the warnings can be ignored.

Let’s examine the stream results. The stream conditions can be confirmed with the vapor pressure vs temperature plot we created earlier. For instance, stream COOLH2O is a subcooled liquid according to the plot, yielding a liquid fraction of 1.

As expected, the vapor stream leaving the turbine is only 142˚ significantly cooler than the temperature it enters the turbine due to a loss in energy.
Now let’s examine the model results of the other equipment to determine the overall efficiency. The duty of the boiler is 8,150 KW, The pump requires 26.43 KW of power. This totals 8,176 KW power input assuming a well-insulated furnace. The work of the turbine is 1,648 KW. This is the total work out of the process. Taking the fraction of output power divided by input power we get that the efficiency of this steam engine is only 20%. Much of the energy is lost condensing the water. Modern power plants use much higher temperatures and pressures to achieve efficiencies closer to 40%, this way the energy generated by the turbine is much closer to the energy lost condensing the water.

This flowsheet can be developed further by adding heat and work streams. By selecting and clicking the Heat Stream or Work Stream icon, the user can determine where heat streams and work streams can be added to the flowsheet. Possible connection for heat and work streams will automatically be shown when these options are selected.