Pre-Heater Application Heat Exchanger

Air-pre-heater is basically a heat transfer equipment to heat air before another process. In Badrin Industries, we manufacture and export steam heated air-pre-heaters. Steam is passed inside the finned tubes bunks and air outside the tubes.

An air pre heater or air heater is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process.They may be used alone or to replace a recuperative heat system or to replace a steam coil.

one of the purpose of it is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example). These types of air-pre-heaters find application in papers mills, sugar mills and in power plants.

In large steam systems, or in any process requiring high temperature, the input fluid is usually preheated in stages, instead of trying to heat it in one step from ambient to the final temperature. Preheating in stages increases efficiency and minimizes thermal shock stress to components, as compared to injecting ambient temperature liquid into a boiler or other device that operates at high temperatures.

In the case of a steam system, a portion of the process steam is tapped off and used as a heat source to reheat the feedwater in preheater stages. Figure 8 is an example of the construction and internals of a U-tube feedwater heat exchanger found in a large power generation facility in 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. Note that the steam enters from the top into the shell side of the heat exchanger, where it not only transfers sensible heat (temperature change) but also gives up its latent heat of vaporization (condenses steam into water). The condensed steam then exits as a liquid at the bottom of the heat exchanger. The feedwater enters the heat exchanger on the bottom right end and flows into the tubes. Note that most of these tubes will be below the fluid level on the shell side.

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

The feedwater is further heated by the hot steam and then exits the heat exchanger. In this type of heat exchanger, the shell side fluid level is very important in determining the efficiency of the heat exchanger, as the shell side fluid level determines the number of tubes exposed to the hot steam.