A feedwater heater is a

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many ...

component used to pre-heat water delivered to a

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizatio ...

generating

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central ...

. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the

thermodynamic efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a ...

of the system.Fundamentals of Steam Power
by Kenneth Weston,

University of Tulsa The University of Tulsa (TU) is a private research university in Tulsa, Oklahoma. It has a historic affiliation with the Presbyterian Church and the campus architectural style is predominantly Collegiate Gothic. The school traces its origin to ...

This reduces plant operating costs and also helps to avoid

thermal shock Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

to the boiler metal when the feedwater is introduced back into the steam cycle. In a steam power plant (usually modeled as a modified

Rankine cycle The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sourc ...

), 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 the

second law of thermodynamics The second law of thermodynamics is a physical law based on universal experience concerning heat and energy interconversions. One simple statement of the law is that heat always moves from hotter objects to colder objects (or "downhill"), unle ...

for a further discussion of such irreversibilities.

Cycle discussion and explanation

The energy used to heat the feedwater is usually derived from steam extracted between the stages of the

steam turbine A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam tu ...

. Therefore, the steam that ''would be used'' to 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 the extraction fraction and must be carefully optimized for maximum power plant

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a ...

since increasing this fraction causes a decrease in turbine power output. Feedwater heaters can also be ''"open"'' or ''"closed"''

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...

s. An open heat exchanger is one 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 the condenser pressure. A deaerator is a special case of the open feedwater heater which is specifically designed to remove non-condensable gases from the feedwater. Closed feedwater heaters are typically

shell and tube heat exchanger A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. As its name implies, this ty ...

s where 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, an

isenthalpic An isenthalpic process or isoenthalpic process is a process that proceeds without any change in enthalpy, ''H''; or specific enthalpy, ''h''. Overview If a steady-state, steady-flow process is analysed using a control volume, everything outside ...

process that results in some

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodyna ...

gain 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.

Economizer

An economizer serves a similar purpose to a feedwater heater, but is technically different as it does not use cycle steam for heating. In fossil-fuel plants, the economizer uses the lowest-temperature

flue gas Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produc ...

from the furnace to heat the water before it enters the boiler proper. This allows for the heat transfer between the furnace and the feedwater to occur across a smaller average temperature gradient (for the steam generator as a whole). System efficiency is therefore further increased when viewed with respect to actual energy content of the fuel. Most nuclear power plants do not have an economizer. However, the Combustion Engineering System 80+ nuclear plant design and its evolutionary successors, (e.g.

Korea Electric Power Corporation Korea Electric Power Corporation, better known as KEPCO (Hangul: 켑코) or Hanjeon (Hangul: 한전), is the largest electric utility in South Korea, responsible for the generation, transmission and distribution of electricity and the developme ...

APR-1400 The APR-1400 (for Advanced Power Reactor 1400 MW electricity) is an advanced pressurized water nuclear reactor designed by the Korea Electric Power Corporation (KEPCO). Originally known as the Korean Next Generation Reactor (KNGR), this ...

) incorporate an integral feedwater economizer. This economizer preheats the steam generator feedwater at the steam generator inlet using the lowest-temperature primary coolant.

Testing

A widely use Code for the procedures, direction, and guidance for determining the thermo-hydraulic performance of a closed feedwater heater is the ASME PTC 12.1 Feedwater Heater Standard.

ASME Codes

The American Society of Mechanical Engineers (ASME), publishes the following Code: * PTC 4.4 Gas Turbine Heat Recovery Steam Generators

References

External links

Power plant diagram

{{Steam engine configurations, state=collapsed Mechanical engineering Chemical process engineering ru:Экономайзер (энергетика)