Thanks to Kris Dambrink from Imtech.nl for making me aware of this alternative approach to adapting an Open Feedwater Heater to a steam power plant (4 Feb 2010)
This Solved Problem is an alternative extension of Solved Problem 4.1 in which we extend the deaerator by tapping steam from the outlet of the High Pressure turbine and reduce the pressure to 800 kPa by means of a Throttling Control Valve before feeding it into the deaerator. This allows one to conveniently convert the deaerator into an Open Feedwater Heater without requiring a bleed tap from the Low Pressure turbine at exactly the dearator pressure, as shown in the following diagram:
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
Thus we find that for an ideal throttle the enthalpy h9 = h2 independent of the pressure drop, allowing us to conveniently draw the throttling process as a vertical line on the P-h diagram. We now determine the mass fraction y by considering the mixing process in the open feedwater heater as follows:
Notice that we can estimate this value of y directly from the P-h diagram by simply measuring the enthalpy differences (h7 - h6) and (h9 - h6) with a ruler.
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 8.25 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 41%. This may not seem like a significant amount, however all the basic components were already in place, since without a de-aerator the steam power plant will deteriorate and become non-functional within a very short time due to leakage of air into the system. 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. Note that this is a contrived example in order to demonstrate that no matter how complex the system is, we can easily plot the entire system on a P-h diagram and 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.
Engineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License