Role and Principle of Shell & Tube Air Preheaters in High Temperature Furnaces

Role

Shell and tube air preheater plays a vital role in high temperature furnace system. It can effectively utilize the residual heat of the flue gas from the furnace to preheat the air entering the furnace, thus improving the combustion efficiency. For example, in the ironmaking blast furnace or steelmaking heating furnace in the metallurgical industry, the preheated air can make the fuel burn more fully and reduce energy waste. At the same time, the preheated air into the furnace can regulate the temperature distribution in the furnace, improve the process, and improve product quality.

Principle

Shell and tube air preheater is mainly composed of tube bundle and shell. Usually, the flue gas flows inside the tube and the air flows outside the tube (shell and tube). When the flue gas from a high temperature furnace passes through the tube bundle, heat is transferred through the tube walls to the air outside the tubes. This heat transfer is based on the principle of heat conduction. Since the temperature of the flue gas is higher than the air temperature, the heat is naturally transferred from the high temperature flue gas side to the low temperature air side. The presence of tube bundles increases the area over which the heat is transferred and improves the heat exchange efficiency. For example, in an air preheater for a glass melting furnace, where the flue gas temperature is around 1000°C, the air entering the furnace can be preheated from ambient temperature to 300 - 400°C by means of a shell and tube air preheater.

Structural characteristics of the shell and tube air preheater

Tube bundle design

The tube bundle is the core component of the shell-and-tube air preheater, and it can be arranged in various ways, such as square triangles, squares, etc. The tube bundle can be arranged in the same way as the shell-and-tube air preheater. The square triangle arrangement can arrange more tubes in the same space to increase the heat exchange area, but cleaning and maintenance are relatively difficult; the square arrangement is more convenient for maintenance and cleaning. The tube diameter and length of the tube bundle are also determined according to the actual heat exchange demand and space limitations. For example, in some small high-temperature furnace air preheater, in order to facilitate the installation and maintenance, may choose a smaller tube diameter, shorter tube length of the tube bundle, and the square arrangement.

Shell Structure

The shell mainly plays the role of sealing and protecting the tube bundle, and at the same time guides the air flow path outside the tube. Shell generally made of metal materials, such as carbon steel or stainless steel, its shape and size according to the size of the tube bundle and air, flue gas flow to design. The shell also has inlets and outlets for air and flue gas. In order to prevent heat dissipation, the shell usually needs to be insulated, such as wrapping insulation material, to improve the heat exchange efficiency.

Tube Plates and Seals

Tube plates are located at the ends of the tube bundle and are used to hold the tubes in place and separate the tube and shell courses. The design of the tube sheet should ensure a solid connection of the tubes, and at the same time take into account the influence of thermal stress to avoid leakage at the connection between the tubes and the tube sheet due to thermal expansion and contraction. Sealing is also a very important part, good sealing can prevent air and flue gas leakage and ensure the normal operation of the preheater. Materials such as gaskets or sealants are usually used to achieve the seal.