High Temperature Flue Gas Waste Heat Recovery Heat Exchanger

Structural components

Tube bundle: The core component of a shell and tube heat exchanger is the tube bundle. The tube bundle consists of a number of parallel tubes, which are usually light tubes or can be finned tubes (in the case of enhanced heat transfer). The material of the tubes is selected according to the operating conditions, e.g. carbon steel for general working conditions and stainless steel for corrosive high temperature flue gas environments. Both ends of the tube are fixed on the tube plate, which serves to separate the tube and shell fluids and to fix the tube bundle.

Shell: The shell is a cylindrical shell which surrounds the tube bundle. The material of the shell is generally compatible with the working environment, and its diameter and length are determined according to the heat exchanger's heat exchange area and flow rate and other requirements. There are inlets and outlets on the shell for high-temperature flue gases and heated (or cooled) fluids. On the top or side of the shell usually also has an exhaust port and drain port, for the discharge of non-condensable gases and fluid accumulation in the system.

Folding plate (baffle): In order to increase the flow rate of the shell fluid (usually high temperature flue gas), to enhance its heat exchange with the tube bundle, the shell is equipped with folding plate. There are various forms of baffle plates, such as bow-shaped baffle plates, disc - circular baffle plates and so on. Bow-shaped folding plate is the most common one, which makes the shell fluid flow in a folded line, constantly flushing the tube bundle, thus improving the heat transfer efficiency.

Working Principle

High-temperature flue gas enters from one end of the shell and flows along the path guided by the folding plate inside the shell, scouring the outer surface of the tube bundle. Heat is transferred from the high-temperature flue gas through the tube wall to the low-temperature fluid (e.g., air, water, etc.) inside the tube bundle. The fluid within the bundle enters at one end of the tube, absorbs heat as it flows through the tube, and exits at the other end of the tube.

For example, in a shell-and-tube heat exchanger used to preheat air, the high-temperature flue gas carries a large amount of heat into the shell, and when it comes into contact with the outer surface of the tube bundle, due to the temperature difference, the heat passes through the walls of the tubes by means of heat conduction, raising the temperature of the cold air inside the tubes, and achieving waste heat recovery.

Advantages

Wide range of application: Shell and tube heat exchangers are able to handle many different types of fluids, whether they are high temperature, high pressure, or highly corrosive. For example, in chemical production, it can be used to handle the heat exchange between high temperature flue gases containing corrosive chemicals and process fluids that need to be preheated.

Robust structure: Due to its thicker shell and stable tube bundle structure, the shell and tube heat exchanger has high mechanical strength and can withstand large pressure and temperature changes. Shell and tube heat exchanger can work reliably in some occasions that require long-term stable operation and harsh working environment, such as waste heat recovery system of thermal power plant.

Easy maintenance and repair: Compared with some other complex structure of the heat exchanger, shell and tube heat exchanger structure is relatively simple. If problems such as leakage occur in the tube bundle, it can be repaired by replacing individual tubes or tube sheets. Also, it is relatively easy to inspect and clean the interior during routine maintenance.

Application Scenarios

Chemical industry: In the chemical production process, many reactions produce high-temperature flue gases. Shell and tube heat exchangers can be used to recover the waste heat from these flue gases, which can be used to preheat the raw materials, heat the reaction medium or provide thermal energy for other processes. For example, in ammonia plants, shell and tube heat exchangers are used to recover the waste heat of the high temperature flue gases at the exit of the converter to preheat the feed gas entering the converter.

Power industry: In the boiler tail section of a thermal power plant, shell and tube heat exchangers are used to recover flue gas waste heat to heat the feed water or preheat the air. This improves the efficiency of the entire thermal system and reduces fuel consumption. For example, in large thermal power plants, shell-and-tube heat exchangers are used to recover the waste heat from the boiler exhaust, which is used to heat the condensate, thus reducing the amount of steam extracted from the turbine and improving the efficiency of power generation.

Petroleum industry: In the process of petroleum refining, shell and tube heat exchanger can be used to recover the waste heat of flue gas discharged from high temperature reactors, which can be used to heat crude oil or other petroleum products. At the same time, it can also be used to deal with some of the corrosive components of the heat exchange between the high temperature fluid, such as in the process of desulphurisation of crude oil.