feedwater water heater is a power generating component used for pre-heat water delivered to steam generator boilers. The initial heating of feedwater reduces the irreversibilities involved in steam formation and therefore increases the thermodynamic efficiency of the system. This reduces plant operating costs and also helps avoid thermal shocks on boiler metal when feed water is reintroduced into the steam cycle.
In a steam power plant (usually modeled as a modified Rankine cycle), the feed water heater allows the feed water to be brought to a saturation temperature gradually. This minimizes irreversibilities inevitably associated with heat transfer to the working fluid (water). See the article on the second law of thermodynamics for a further discussion of such ineffectiveness.
Video Feedwater heater
Discussion and annotation cycle
The energy used to heat the feed water usually comes from the vapor extracted between the steam turbine stages. Therefore, the vapor that will be used to do the expansion work in the turbine (and therefore generate power) is not used for that purpose. The percentage of the total vapor stream mass flow used for feed water heater is called the extraction fraction and must be carefully optimized for maximum thermal efficiency of the plant because increasing this fraction causes a decrease in turbine power output.
The feed water heaters can also "open" or "closed" heat exchangers. An open heat exchanger is one in which the extracted vapor is allowed to mix with the feed water. This type of heater will usually require a feed pump both in the feed and outlet as the pressure on the heater is between the boiler pressure and the condenser pressure. Deaerator is a special case of open water feed heater specially designed to remove non-condensable gas from feed water.
Closed-feed water heaters are usually heat exchangers of tubes and tubes where the feed passes through the entire tube and is heated by steam turbine extraction. It does not require a separate pump before and after the heater to increase the feed water to the extracted vapor pressure as with an open heater. However, the extracted vapor (which is most likely to be almost fully condensed after heating the feedwater) should then be strangled to the condenser pressure, the isenthalpic process yielding the entropy gain with a slight penalty in the overall cycle efficiency:
Many power plants incorporate a number of bait water heaters and can use open and closed components. Bait water heaters are used in both fossil and nuclear power plants.
Maps Feedwater heater
Economizer
Economizer serves the same purpose as feed water heater, but it is technically different because it does not use steam cycle for heating. In a fossil fuel plant, the economizer uses the lowest exhaust gas from the furnace to heat the water before entering the proper boiler. This allows the heat transfer between the furnace and the feedwater to occur along the smaller average temperature gradient (for whole steam generators). Therefore the efficiency of the system is increasing when viewed in connection with the actual energy content of the fuel.
Most nuclear power plants do not have an economizer. However, the design of 80 NPP Combustion Engineering Systems and its evolutionary successors (eg Korea Electric Power Corporation's APR-1400) combine integral feedback economizers. This saver preheats the steam generator feedwater in the steam generator's inlet using the lowest-temperature primary coolant.
Test
The widely used codes for procedures, directions, and guides for determining thermo-hydraulic performance of closed-feed water heaters are ASME PTC 12.1 Standard Bait Water Heaters.
See also
- Fossil fuel power plants
- Power plant
- Thermal power plant
ASME code
The American Society of Mechanical Engineers (ASME), publishes the following Code:
- PTC 4.4 Heat Recovery Gas Turbine Gas Generator
References
External links
- Power plant diagram
- High pressure feed water heaters
Source of the article : Wikipedia
0 Comments