【Q&A Question 255】2018.09.18

A fixed tube sheet heat exchanger is a shell and tube heat exchanger in which the tube sheets at both ends of the tube bundle are fixedly connected to the shell by welding. After being welded to the shell, the tube sheet can also serve as a flange and be connected to the tube box flange using bolts.

A fixed tube sheet heat exchanger is a shell and tube heat exchanger in which the tube sheets at both ends of the tube bundle are fixedly connected to the shell by welding. After being welded to the shell, the tube sheet can also serve as a flange and be connected to the tube box flange using bolts.

1. No internal leakage; 2. Forgings are used less, resulting in lower costs; 3. The heat transfer area is 20%–30% larger than that of floating-head heat exchangers. 4. The bypass seepage is low

It has a simple structure and low manufacturing costs, but since the tube sheets at both ends are fixed and the tubes are welded or expanded onto these tube sheets, it is only suitable for applications where the pressure in the shell is not high, the fluid is clean, and the temperature difference is small (generally, the temperature difference between the hot and cold streams should not exceed 50°C).

The advantage is its simple structure, which requires the smallest inner diameter of the shell while ensuring equal heat transfer surfaces. The disadvantage is that when there is a large temperature difference between the fluid in the shell side and that in the tube side, significant thermal stress is generated, which can lead to separation and cracking at the junctions between the tubes and the tube sheet, resulting in leaks or tube instability. Additionally, maintenance and cleaning of the shell side are not convenient. Although it is widely used in various applications, its use is limited by temperature differences.

A fixed-tube-sheet heat exchanger is mainly composed of components such as a shell, tube sheet, tube bundle, and top cover (also known as head). Inside the circular shell, a parallel tube bundle is installed; the ends of the tube bundle are fixed to the tube sheets by welding or expansion fitting. The two tube sheets are welded directly to the outer tubes, and the cover equipped with the inlet or outlet pipes is connected to the flanges at both ends of the shell using bolts. It is characterized by a simple structure, no shell-side sealing connections, the maximum number of tubes that can fit within the same shell inner diameter, minimal bypass flow in the presence of baffle plates, and the ability to divide the tube side into any desired number of sections. Since the two tube sheets are supported by the tubes themselves, its tube sheets are the thinnest among various shell-and-tube heat exchangers, resulting in lower costs; for these reasons it is widely used. What are its advantages and disadvantages? The characteristics of fixed-tube-sheet heat exchangers are: 1. Low bypass leakage; 2. Less use of forgings, resulting in lower costs; 3. No internal leakage; 4. A heat transfer area that is 20%~30% larger than that of floating-head heat exchangers. The disadvantages of fixed-tube-sheet heat exchangers are as follows: 1. The temperature difference between the shell and the tube walls is relatively large; generally, this temperature difference t should be ≤50°C. When t ≥50°C, expansion joints must be installed on the shell. 2. Temperature stress is likely to occur, and this stress can cause damage at the junction between the tube sheet and the tube ends. 3. The shell side cannot be mechanically cleaned. 4. When the tubes corrode, the entire unit, including the shell, becomes unusable, resulting in a shorter service life for the equipment.

1. The bypass seepage is low； 2. Forgings are used less frequently and have a lower cost ； 3. No internal leakage ； 4. The heat transfer area is 20%–30% larger than that of a floating-head heat exchanger.

Fixed-tube-sheet heat exchangers, due to the fixed constraints at both ends of the tube bundle which limit expansion, are not suitable for handling high temperatures nor can they accommodate high loads；

It has a simple structure and low manufacturing costs, but since the tube sheets at both ends are fixed and the tubes are welded or expanded onto these tube sheets, it is only suitable for applications where the pressure in the shell is not high, the fluid is clean, and the temperature difference is small (generally, the temperature difference between the hot and cold streams should not exceed 50°C).

Fixed-tube-sheet heat exchangers have the advantages of simple structure, light weight, and easy cleaning of the tube bundle. Since the two tube sheets at both ends of such heat exchangers are directly fixed to the shell, the tube bundle cannot expand or contract freely when heated, which prevents the elimination of thermal stresses; therefore, they are only suitable for medium and low temperature operating conditions.