A feed water heater is a device used to gradually heat water that is being delivered to a steam boiler. The main reason why feed water heaters are used is because the water needs to be heated gradually until it reaches its boiling point, also known as the saturation temperature. If water is heated too fast or too violently, the boiler metal is damaged because of thermal shock, which occurs when the rapid increase in temperature creates cracks in the metal. A feed water heater also can increase the overall efficiency of the steam cycle, because the primary heat source does not need to be employed. Instead, feed water usually is heated by alternative heat sources naturally found within the steam cycle.
The heat used in a feed water heater can be derived from multiple places within the steam cycle. This can vary depending upon the specific cycle or steam generator, but exhausted steam from a steam turbine typically is used. In most stationary turbine power plants, there are two steam turbines providing energy. First, steam from the steam generator enters into a smaller, high-pressure turbine, and as the steam continues to expand and lose its pressure, it is exhausted into a larger, low-pressure turbine. Typically, the steam used to heat the feed water heater is extracted from the exhaust of the high-pressure steam turbine.
The steam extraction process is very regulated and precise, because the amount of steam extracted has a direct effect on the energy output of the low-pressure turbine. The amount of steam extracted from the exhaust to heat the feed water heater is calculated using what is known as the "extraction fraction." This value must be carefully calculated for maximum optimization of the steam being extracted from the process. A locomotive feed water heater uses the same principle, because exhausted steam also is used to preheat water before entering the steam cycle.
There are two types of feed water heaters: open and closed. Both types heat water with steam, but the way in which steam is applied makes them different. In an open feed water heater, steam is applied directly to the water being heated. This typically is not used, because in many cases, the steam often is combined with vaporized lubricating oil. If oil-rich water enters a steam boiler, small carbon deposits will be left behind, which can result in corrosion.
For this reason, closed feed water heaters are used more often. In a closed feed water heater, the pipe containing flowing water is surrounded by steam. The steam either can be contained within a separate tube or can be directly exposed to the pipe, but the steam does not come into contact with the water being heated. This helps to avoid any other serious corrosion or contamination problems commonly found in most open feed water heaters.
Under some rarer circumstances, feed water is heated by other means. The most common alternative method is a device known as an economizer, which utilizes hot gases from the burner for preheating water, instead of an open flame or exhausted steam. Although they are not typically found, these can be far more efficient, because steam does not have to be extracted from the steam cycle for preheating purposes. Other uncommon methods have involved using the excess heat from cylinders in a reciprocating steam engine.