A feedwater heater is a component of steam powered turbine installations and steam locomotives used to preheat the water in the primary boiler system. These components help to improve the overall thermal efficiency of the boiler and also protect against thermal shock damage. There are two types of feedwater heater, open and closed, both of which use exhaust steam from the turbine or locomotive cylinders to heat the water. Open feed water heaters mix this steam directly with the feedwater which not only serves to heat the water but also removes some of the corrosion-causing oxygen from it. Closed heaters are structured much like the primary boilers with the water circulating through a series of tubes enclosed in a steam filled shell.
Locomotives and turbines draw the steam which supplies their motive power from large, complex boilers. These boilers heat water in a dense bundle of tubes, thereby causing it to rapidly evaporate and form high pressure steam. The water used to replenish the depleted boiler supply is pumped from holding tanks or from a locomotive tender at, or close to, ambient temperature. Introducing this colder water into the far hotter boiler can have a number of negative effects. The first is an obvious drop in boiler water temperature which negatively effects its overall efficiency and the second is potential damage to the internal structure of the boiler by thermal shock.
To prevent these issues, a feedwater heater is used to bring the temperature of the water up to an acceptable level prior to its introduction into the boiler. Exhaust steam from the turbine or the locomotive cylinders is typically used to heat the water which further increases the efficiency of the system. There are two basic types of feedwater heater, i.e., those featuring open or closed steam exposure systems.
An open feedwater heater makes use of a direct exposure system to heat the feedwater. In this system, the water is exposed to direct physical contact with the heating steam. The water is either sprayed or allowed to cascade down horizontal trays into a closed vessel. The steam is introduced at the bottom of the vessel and heats the water as it rises. This system has the added benefit of removing a significant amount of corrosion-causing oxygen from the water before it is pumped to the primary boiler.
Closed feedwater heaters are constructed in a similar fashion to the primary boiler with a tightly packed bundle of tubes positioned within a sealed shell. The feedwater is pumped through the tubes, and steam is circulated through the shell, thus heating the water indirectly. The water formed by the condensing steam within the shell is then piped back to the feedwater supply tank for reuse.