solved problem 4.1 - steam power plant for athens_ ohio (4!21!10

7/31/2019 Solved Problem 4.1 - Steam Power Plant for Athens_ Ohio (4!21!10

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olved Problem 4.1 - Supercritical Steam Power Plant with Reheat for

Athens, Ohio

an effort to decentralize the national power distribution grid, the following supercritical (25 MPa), coal

red steam power plant (modeled after the Gavin Power Plant in Cheshire, Ohio) has been proposed to

rvice about 10,000 households in Athens, Ohio. It is to be placed close to the sewage plant on the east

de of Athens and cooled by water from the Hocking river. We consider first a simplified system as shown

elow. Notice that we have replaced the "Boiler" with a "Steam Generator", since at supercritical pressures

e concept of boiling water is undefined. Furthermore we have specifically split the turbine into a High

ressure (HP) turbine and a Low Pressure (LP) turbine since we will find that having a single turbine to

xpand from 25MPa to 10kPa is totally impractical. Thus for example the Gavin Power Plant has a turbine

t consisting of 6 turbines - a High Pressure Turbine, an Intermediate Pressure (Reheat) turbine, and 4

rge Low Pressure turbines operating in parallel.

ote that prior to doing any analysis we always first sketch the complete cycle on a P-h diagram based on

e pressure, temperature, and quality data presented. This leads to the following diagram:


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n examining the P-h diagram plot we notice that the system suffers from two major flaws:

The outlet pressure of the LP turbine at port (3) is 10 kPa, which is well below atmospheric pressure.

The extremely low pressure in the condenser will allow air to leak into the system and ultimately lead

to a deteriorated performance.

The quality of the steam at port (3) is 80%. This is unacceptable. The condensed water will causeerosion of the turbine blades, and we should always try to maintain a quality of above 90%. One

example of the effects of this erosion can be seen on the blade tips of the final stage of the Gavin LP

turbine. During 2000, all four LP turbines needed to be replaced because of the reduced performance

resulting from this erosion. (Refer: Tour of the Gavin Power Plant - Feb. 2000)

he following revised system diagram corrects both flaws. The steam at the outlet of the HP turbine (port

)) is reheated to 550 C before entering the LP turbine at port (3). Also the low pressure liquid condensate

port (5) is pumped to a pressure of 800 kPa and passed through a de-aerator prior to being pumped by the

edwater pump to the high pressure of 25 MPa.


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his system is referred to as a Reheat cycle, and based on the data above is plotted on the P-h diagram as

llows:

hus we see that in spite of the complexity of the system, the P-h diagram plot enables an intuitive and

ualitative initial understanding of the system. Using the methods described in Chapter 4b for analysis of

ach component, as well as the steam tables, determine the following:

1) Assuming that both turbines are adiabatic and neglecting kinetic energy effects determine the

combined output power of both turbines [10.6 MW].


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2) Assuming that both the condensate pump and the feedwater pump are adiabatic, determine the

power required to drive the two pumps [-204 kW].

3) Determine the total heat transfer to the steam generator, including the reheat system [26.1 MW].

4) Determine the overall thermal efficiency of this power plant. (Thermal efficiency (th

) is defined as

the net work done (turbines, pumps) divided by the total heat supplied externally to the steam

generator and reheat system) [40 %].


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5) Determine the heat rejected to the cooling water in the condenser [-15.7 MW].

6) Assume that all the heat rejected in the condenser is absorbed by cooling water from the Hocking

River. To prevent thermal pollution the cooling water is not allowed to experience a temperature rise

above 10C. If the steam leaves the condenser as saturated liquid at 40C, determine the required

minimum volumetric flow rate of the cooling water [22.6 cubic meters/minute].

7) Discuss whether you think that the proposed system can be cooled by the Hocking river. You will

need to do some research to determine the minimal seasonal flow in the river in order to validate your

decision. (Hint-Google: Hocking River Flow)

_____________________________________________________________________________________

ngineering Thermodynamics by Israel Urieli is licensed under a Creative Commons Attribution-

oncommercial-Share Alike 3.0 United States License

solved problem 4.1 - steam power plant for athens_ ohio (4!21!10

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