Preheater:

Within large steam systems, or in any procedure needs high temperatures, the input fluid is commonly preheated in stages; alter of trying to heat it in one step from ambient to the final temperature.

Preheating in stages raises the plant's effectiveness and minimizes thermal shock stress to parts, as compared to injecting ambient temperature liquid within a boiler or other device which operates at high temperatures. In that case of a steam system, a portion of the procedure steam is tapped off and used as a heat source to reheat the feedwater in preheater stages. Figure is an instance of the construction and internals of a U-tube feedwater heat exchanger searched in a large power generation facility within a preheater stage. As the steam enters the heat exchanger and flows over and around the tubes, it transfers its thermal energy and is condensed. Remember that the steam enters from the top into the shell side of the heat exchanger, whereas it not just transfers sensible heat (temperature change) but also provides up its latent heat of vaporization (condenses steam into water). A condensed steam then exits as a liquid on the bottom of the heat exchanger. A feedwater enters in the heat exchanger on the bottom right end and flows within the tubes. Remember that most of these tubes will be below the fluid level on the shell side.

This denote the feedwater is exposed to the condensed steam first and then travels by the tubes and back around to the top right end of the heat exchanger. After making the 180° bend, the partially heated feedwater is after that subjected to the hotter steam entering the shell side.

Figure: U-tube Feedwater Heat Exchanger

The feedwater is additional heated through the hot steam and after that exits the heat exchanger. In this category of heat exchanger, the shell side fluid level is extremely important in determining the effectiveness of the heat exchanger, as the shell side fluid level denotes the number of tubes exposed to the hot steam.