Feedwater heaters gradually heat water for steam boilers to prevent thermal shock. They increase efficiency by using alternative heat sources and are regulated to optimize steam extraction. Closed feedwater heaters are preferred to prevent corrosion and contamination. Alternative methods include economizers and excess heat from steam engines.
A feedwater heater is a device used to gradually heat water that is sent to a steam boiler. The main reason feedwater heaters are used is because the water needs to be gradually heated until it reaches its boiling point, also known as saturation temperature. If the water is heated too quickly or too violently, the metal in the kettle is damaged due to thermal shock, which occurs when the rapid rise in temperature creates cracks in the metal. A feedwater heater can also increase the overall efficiency of the steam cycle, since it is not necessary to use the primary heat source. Instead, feedwater is typically heated by alternative heat sources found naturally within the steam cycle.
The heat used in a feedwater heater can be derived from multiple places within the steam cycle. This can vary depending on the specific cycle or steam generator, but typically steam extracted from a steam turbine is used. In most stationary turbine power plants, there are two steam turbines that provide power. First, the steam from the steam generator enters a smaller high-pressure turbine, and as the steam continues to expand and lose its pressure, it is discharged into a larger low-pressure turbine. Typically, the steam used to heat the feedwater heater is drawn from the exhaust of the high pressure steam turbine.
The steam extraction process is highly regulated and precise, because the amount of steam extracted has a direct effect on the power output of the low pressure turbine. The amount of steam extracted from the exhaust to heat the feedwater heater is calculated using what is known as the ‘extraction fraction’. This value must be carefully calculated to obtain the maximum optimization of the steam that is extracted from the process. A locomotive feedwater heater uses the same principle, because the spent steam is also used to preheat the water before it enters the steam cycle.
There are two types of feedwater heaters: open and closed. Both types heat water with steam, but the way the steam is applied makes them different. In an open feed water heater, steam is applied directly to the water being heated. This is generally not used, because in many cases, steam is often combined with vaporized lubricating oil. If oil-rich water enters a steam boiler, small carbon deposits will be left behind, which can lead to corrosion.
For this reason, enclosed feedwater heaters are more frequently used. In a closed feedwater heater, the pipe containing tap water is surrounded by steam. The steam may be contained within a separate pipe or it may be directly exposed to the pipe, but the steam does not come into contact with the water being heated. This helps prevent other serious corrosion or contamination problems commonly found in most open feedwater heaters.
In some rarer circumstances, the feed water is heated by other means. The most common alternative method is a device known as an economizer, which uses hot gases from the burner to preheat the water, rather than an open flame or exhausted steam. Although not normally found, these can be much more efficient as there is no need to extract steam from the steam cycle for preheating purposes. Other uncommon methods have involved using excess heat from the cylinders in a reciprocating steam engine.
Protect your devices with Threat Protection by NordVPN
