Consider a steam plant operating on a Rankine Cycle with two open feed water hea
ID: 1856930 • Letter: C
Question
Consider a steam plant operating on a Rankine Cycle with two open feed water heaters as shown in the figure. steam exiting the boiler is at 18 MPa and 670 C. The high pressure open feedwater heater operates at 4 MPa and the low pressure feed water heater operates at 4 bar while the condenser is at 4 kPa. Determine the
a) Thermal efficiencey of the plant
b) Mass flow rate of steam leaving the boiler to produce 50 MW of power
C) Calculate the efficiency if a reheat loop was added at such that the steam leaves at 1 MPa and reenters the turbine at 450 C
Explanation / Answer
Afeedwater heateris apower plantcomponent used to pre-heat water delivered to asteamgeneratingboiler.Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves thethermodynamic efficiencyof the system.This reduces plant operating costs and also helps to avoidthermal shockto the boiler metal when the feedwater is introduced back into the steam cycle.
In a steam power plant (usually modeled as a modifiedRankine cycle), feedwater heaters allow the feedwater to be brought up to the saturation temperature very gradually. This minimizes the inevitable irreversibilities associated with heat transfer to the working fluid (water). See the article on theSecond Law of Thermodynamicsfor a further discussion of such irreversibilities.
The energy used to heat the feedwater is usually derived from steam extracted between the stages of thesteam turbine. Therefore, the steam thatwould be usedto perform expansion work in the turbine (and therefore generate power) is not utilized for that purpose. The percentage of the total cycle steam mass flow used for the feedwater heater is termed theextraction fractionand must be carefully optimized for maximum power plantthermal efficiencysince increasing this fraction causes a decrease in turbine power output.
Feedwater heaters can also beopenandclosedheat exchangers. An open feedwater heater is merely a direct-contact heat exchanger in which extracted steam is allowed to mix with the feedwater. This kind of heater will normally require a feed pump at both the feed inlet and outlet since the pressure in the heater is between the boiler pressure and thecondenserpressure. Adeaeratoris a special case of the open feedwater heater which is specifically designed to remove non-condensable gases from the feedwater.
Closed feedwater heaters are typicallyshell and tube heat exchangerswhere the feedwater passes throughout the tubes and is heated by turbine extraction steam. These do not require separate pumps before and after the heater to boost the feedwater to the pressure of the extracted steam as with an open heater. However, the extracted steam (which is most likely almost fully condensed after heating the feedwater) must then be throttled to the condenser pressure, anisenthalpicprocess that results in someentropygain with a slight penalty on overall cycle efficiency.
Many power plants incorporate a number of feedwater heaters and may use both open and closed components. Feedwater heaters are used in both fossil- and nuclear-fueled power plants.