A U-tube heat exchanger is a shell and tube heat exchange device, whose core working principle is based on the basic physical process of heat exchange, that is, heat transfer is achieved through the flow of two fluids at different temperatures (tube side fluid and shell side fluid) inside and outside the heat exchange tube.
1、 Basic Structure and Fluid Path
U-shaped tube bundle
The heat exchange tube is bent into a U-shape and fixed at both ends on the same tube plate, forming a double or multi pass flow channel.
Pipe side fluids (such as high-temperature steam and hot oil) enter from one end of the head, flow through the interior of the U-shaped pipe, and exit from the other end of the head.
The shell side fluid (such as cooling water and air) flows inside the shell, surrounds the outside of the U-shaped tube bundle, and exchanges heat with the tube side fluid through the tube wall.
Shell and baffle
The shell is a cylindrical structure with internal baffles (such as bow shaped baffles) to guide the fluid in the shell side to flow in a spiral or S-shape, enhance turbulence, and improve heat transfer efficiency.
The spacing between baffles is usually 1.5-2 times the diameter of the pipe to ensure sufficient fluid contact with the tube bundle and reduce short circuits.
Head and guide tube
The head is divided into two chambers with a partition to achieve two-way flow of the fluid in the tube side (i.e. the fluid travels back and forth once inside the tube), extending the heat transfer path.
The guide tube is installed at the inlet of the shell side to guide the fluid evenly into the tube bundle area, avoiding erosion, corrosion or vibration caused by high local flow velocity.
2、 Heat exchange process
Heat transfer mode
Convective heat transfer: The fluid in the tube side and the fluid in the shell side transfer heat to the tube wall (or absorb heat from the tube wall) through flow.
Thermal conductivity: Heat is transferred from the high temperature side to the low temperature side through the tube wall (made of metal materials such as stainless steel and copper alloy).
Convective heat transfer: The low-temperature side fluid absorbs heat from the pipe wall and completes heat exchange.
Enhancement of heat transfer coefficient
The curved design of the U-shaped tube creates a secondary flow of fluid inside the tube, enhances turbulence, and improves the heat transfer coefficient inside the tube.
The shell side baffle forces the fluid to change direction, increases the length of the flow path, and improves the heat transfer efficiency of the shell side.
Typical heat transfer coefficient range: 2000-3500 W/(m ² ·℃), which is 30% -50% higher than that of fixed tube plate heat exchangers.
Thermal stress compensation
The U-shaped tube can expand and contract freely. When there is a large temperature difference between the tube side and the shell side fluid (such as steam heating cooling water, the temperature difference can reach over 300 ℃), the tube bundle automatically compensates for thermal expansion through bending deformation, avoiding damage to the tube plate or tube due to thermal stress.