Solved Problem 4.2 - An Open Feedwater Heater added to the Supercritical Steam Power Plant for Athens, Ohio

This Solved Problem is an extension of Solved Problem 4.1 in which we extend the de-aerator by tapping steam from the Low Pressure turbine at 800 kPa and feeding it into the de-aerator at the same pressure, thus converting it into an Open Feedwater Heater, as shown in the following diagram:

This system is referred to as a Regenerative Reheat cycle, and we will find that this simple extension of our previous sytem will result in an increase in thermal efficiency of the power plant.

Note that prior to doing any analysis we always first sketch the complete cycle on a P-h diagram based on the pressure, temperature, and quality data presented on the system diagram. This leads to the following diagram:

On examining the P-h diagram plot we notice the following:

Thus once more we see that in spite of the complexity of the system, the P-h diagram plot enables an intuitive and qualitative initial understanding of the system. Using the methods described in Chapter 4b for analysis of each component, as well as the steam tables for evaluating the enthalpy at the various stations (shown in red), and neglecting kinetic and potential energy effects, determine the following:

Thus as expected we find that the net power output is slightly less than the previous system without the turbine tap. However power control is normally done by changing the feedwater pump speed, and we normally find a liquid water storage tank associated with the de-aerator in order to accomodate the changes in the water mass flow rate. In our case we simply need to increase the water mass flow rate from 7 kg/s to 8 kg/s in order to regain our original power output.

Note that it is always a good idea to validate ones calculations by evaluating the thermal efficiency using only the heat supplied to the steam generator and that rejected by the condenser.

Discussion: Thus we find that the open feedwater heater did in fact raise the efficiency from 40% to 42%. This may not seem like a significant amount, however all the basic components were already in place, since without a de-aerator the dissolved oxygen and carbon dioxide in the feedwater will cause serious corrosion damage in the boiler. Furthermore, if the reduction in power output is not acceptable, then it can be easily remedied by increasing the mass flow rate in the system design. Notice that no matter how complex the system is, we can easily plot the entire system on a P-h diagram in order to obtain an immediate intuitive understanding and evaluation of the system performance. It is helpful to check each value of enthalpy read or evaluated from the steam tables and compare them to the values on the enthalpy axis of the P-h diagram.