Basics of Shell and Tube Heat Exchanger
Basics of Shell and Tube Heat Exchanger
A shell and tube heat exchanger, also known as a tubular heat exchanger, which is a type of indirect heat exchanger that uses the walls of tube bundles enclosed within a shell as the heat transfer surface. This type of heat exchanger features a relatively simple structure, reliable operation, and can be manufactured from various structural materials (primarily metals). It is suitable for use under high temperatures and high pressures, making it the most widely used type of heat exchanger today. In this article, we will explore and learn about the relevant knowledge and details of shell and tube heat exchangers.
Structure and Types of Shell and Tube Heat Exchangers
A shell and tube heat exchanger consists of components such as the shell, heat transfer tube bundle, tube sheets, baffles, and tube boxes. The shell is typically cylindrical, with a tube bundle installed inside. Both ends of the tube bundle are fixed to the tube sheets.
During heat exchange, two fluids—one hot and one cold—are involved. One fluid flows inside the tubes and is referred to as the tube-side fluid, while the other flows outside the tubes and is known as the shell-side fluid. To enhance the heat transfer coefficient of the shell-side fluid, several baffles are usually installed inside the shell.
Baffles increase the velocity of the shell-side fluid, forcing it to pass transversely through the tube bundle multiple times along a prescribed path, thereby enhancing the fluids turbulence intensity.
The heat exchange tubes can be arranged on the tube sheet in either an equilateral triangular or square pattern. The equilateral triangular arrangement is more compact, promotes higher turbulence in the shell-side fluid, and results in a larger heat transfer coefficient. The square arrangement, on the other hand, facilitates easier external cleaning of the tubes, making it suitable for fluids prone to fouling.
In shell and tube heat exchangers, due to the temperature difference between the fluids inside and outside the tubes, the temperature of the shell and that of the tube bundle also differ. If this temperature difference is significant, substantial thermal stress will be generated within the heat exchanger, potentially leading to tube bending, fracture, or detachment from the tube sheet.
Therefore, when the temperature difference between the tube bundle and the shell exceeds 50°C, appropriate compensation measures must be taken to eliminate or reduce thermal stress. Based on the compensation methods employed, shell and tube heat exchangers can be classified into the following main types:
① In a fixed tube sheet heat exchanger, the tube sheets at both ends of the tube bundle are integrated with the shell, resulting in a simple structure. However, this type is only suitable for heat exchange operations where the temperature difference between the hot and cold fluids is small, and mechanical cleaning of the shell side is not required. When the temperature difference is slightly larger but the shell-side pressure is relatively low, an elastic expansion joint can be installed on the shell to reduce thermal stress.
Figure 1 Fixed Tube Sheet Heat Exchanger (BJM)
1 – Impingement Plate; 2 – Tie Rod; 3 – Single Segmental Baffle;
4 – Partition Plate; 5 – Bypass Seal Strips; 6 – Flanged Tube Sheet;
7 – Heat Transfer tube
Figure 2 BEM Vertical Fixed Tube Sheet Heat Exchanger.
② In a floating head heat exchanger, the tube sheet at one end of the tube bundle is free to float, completely eliminating thermal stress. Moreover, the entire tube bundle can be extracted from the shell, facilitating mechanical cleaning and maintenance. Floating head heat exchangers are widely used, but their structure is relatively complex and the cost is higher.
③ U-tube heat exchanger: Each heat exchange tube is bent into a U-shape, with both ends fixed on the upper and lower sections of the same tube sheet. The tube-side flow is divided into inlet and outlet chambers by a partition in the tube box. This type of heat exchanger completely eliminates thermal stress and has a simpler structure compared to the floating head type, but the tube side is difficult to clean.
Figure 4 BIU U tube heat exchanger
Figure 5 Double-Shell-Pass U-Tube Heat Exchanger (AFU)
1 — Annular Baffle Ring Plate; 2 — Annular Baffle Disc Plate; 3 — Longitudinal Baffle;
4 — Heat Transfer Tube; 5 — Tube header; 6 — Pass Partition Plate; 7 — Spacer Tube; 8 — Tie Rod
U-tube heat exchangers are generally used under high temperature and high pressure conditions. Especially in high-pressure applications, the wall thickness of the bend section must be increased to compensate for the thinning of the pipe wall after bending.
If the shell-side tube bundle requires frequent cleaning, a square tube arrangement is usually adopted. In most cases, the tubes are arranged in a triangular pattern, with an even number of tube passes.
Longitudinal baffles can be installed inside the shell as per process requirements to form a two-pass shell-side heat exchanger, thereby increasing the flow velocity of the shell-side medium (Figure 5) and enhancing the heat transfer efficiency of the equipment. The longitudinal baffles are installed parallel to the direction of the heat transfer tubes (the installation of longitudinal baffles depends on process requirements).
④ Packed-Lantern Heat Exchanger
The structural characteristic of the packed-lantern heat exchanger is that one end of the tube sheet is fixed to the shell, while the other end is sealed with a stuffing box. The tube bundle can expand and contract freely, thus avoiding thermal stress caused by the temperature difference between the shell wall and the tube wall.
The advantages of the packed-lantern heat exchanger include a simpler structure compared to the floating-head type, easier manufacturing, less material consumption, and lower cost. The tube bundle can be pulled out from the shell, allowing both the inside and outside of the tubes to be cleaned, which facilitates maintenance.
The disadvantages are that the stuffing box has limited pressure resistance, typically below 4.0 MPa, and shell-side medium may leak through the stuffing box. Therefore, it is not suitable for flammable, explosive, toxic, or valuable media.
The packed-lantern heat exchanger is suitable for applications with significant temperature differences between the tube and shell walls, or where the medium is prone to fouling and requires frequent cleaning, but the operating pressure is relatively low.
⑤ Kettle-Type Heat Exchanger
The structural feature of a kettle-type heat exchanger is that an appropriate evaporation space is provided in the upper part of the shell, which also serves as a steam chamber.
The tube bundle can be of the fixed-tube-sheet type, floating-head type, or U-tube type. The kettle-type heat exchanger offers convenient cleaning and maintenance, can handle unclean or scaling-prone media, and is capable of withstanding high temperatures and high pressures. It is suitable for liquid-vapor heat exchange and can serve as the simplest form of waste heat boiler.
Generally, shell and tube heat exchangers are easy to manufacture, have low production costs, offer a wide range of material selection options, are convenient to clean, possess strong adaptability, handle large capacities, operate reliably, and can withstand high temperatures and pressures. Although they cannot match plate or finned tube heat exchangers in terms of structural compactness, heat transfer efficiency, and metal consumption per unit, their aforementioned advantages still secure them a dominant position in industries such as chemical engineering, petroleum, and energy.
The design and manufacturing of various types of shell-and-tube heat exchangers are based on factors such as the type of medium, pressure, temperature, fouling tendency, and other conditions, as well as the structural characteristics of the connection between the tube sheet and the shell, the shape of the heat transfer tubes and heat transfer conditions, cost considerations, and ease of maintenance and inspection.